The embryonic (embryonic) development of a person is an early period of development up to 8 weeks. During this time, a body is born from a fertilized egg, which has all the main features of a person. After eight weeks of development, the intrauterine organism is called the fetus, and the development period is fetal.
Human embryonic development is divided into several periods. Consider the stages of development of the human embryo. The first period of a unicellular embryo (a zygote that has all the properties of both germ cells) proceeds from the moment of fertilization of the egg, which contains two nuclei. Each nucleus contains half a set of chromosomes (23 chromosomes of the father and 23 chromosomes of the mother).
The human embryo begins to move slowly through the fallopian tube, driven by the fringes of the tube and the flow of fluid in it. The main goal of his movement is the uterus.
For the first time, the division of a single-celled human embryo occurs 30 hours after fertilization. Then the division occurs one crushing per day.
After four days, the embryo takes the form of a lump, which consists of approximately 8-12 cells. Further, the division of the cells of the human embryo will occur faster and faster.
During this period, the uterus begins to prepare for the adoption of the embryo. The mucous membrane of the uterus thickens and becomes loose. Many additional blood vessels begin to appear in it.
The cells of the human embryo begin to secrete enzymes that destroy the lining of the uterus. Special villi on the surface of the embryo begin to grow rapidly, grow into the tissue of the uterus. It takes 40 hours to implant a human fetus. Appears new organ called the placenta or baby's place. The placenta is an organ that connects the mother's body with the fetus, and also provides nutrition to the embryo.
By the end of the second week, the length of the human embryo is 1.5 mm.
The development of the human embryo occurs in accordance with a well-defined plan.
At the fourth week of development, the rudiments of most organs and tissues of the future person begin to appear (kidneys, intestines, cartilage of the skeletal skeleton, bones, striated muscles, thyroid gland, liver, skin, ears, eyes).
At the fifth week, the length of the human fetus is about 7.5 mm. At this time, with the help of (ultrasound), you can see the contractions of the heart of the embryo.
During the period of 32 days of development, the rudiments of the arms begin to appear in the human embryo, closer to the 40th day - the rudiments of the legs.
At the end of its development, the embryo already becomes about 3-4 cm long (from the top of the head to the coccyx). At this time, the laying of all the main organs of the embryo ends, it acquires all the signs of a person, both in external appearance and in internal organization.
Question 1.
Zygote(from Greek. "zygotos"- connected together) - a fertilized egg. A diploid cell, formed as a result of the fusion of gametes (sperm and egg), is the initial unicellular stage of embryo development.
Zygote- unicellular stage of development of a new organism.
Question 2.
During cleavage, cells divide by mitosis. Mitotic division during crushing differs significantly from the reproduction of cells in an adult organism: the mitotic cycle is very short, cells do not differentiate - they do not use hereditary information. In addition, during crushing, the cytoplasm of cells does not mix and does not move; no cell growth.
Question 3.
Splitting up is the mitotic division of the zygote. There is no interphase between divisions, and DNA duplication begins at the telophase of the previous division. The growth of the embryo also does not occur, that is, the volume of the embryo does not change and is equal in size to the zygote. Cells formed in the process of crushing are called blastomeres, and the embryo is called blastula. The nature of crushing is determined by the type of egg (Fig. 2.).
The simplest and phylogenetically the most ancient type crushing - complete uniform crushing of isolecithal eggs. The blastula formed as a result of complete crushing is called coeloblastula. It is a single layer blastula with a cavity in the center.
The blastula, formed as a result of complete but uneven fragmentation, has a multi-layered blastoderm with a cavity closer to the animal pole and is called amphiblastula.
Incomplete discoidal cleavage ends with the formation of a blastula, in which blastomeres are located only at the animal pole, while the vegetative pole consists of an undivided yolk mass. Blastocoel is located under the layer of blastoderm in the form of a gap. This type of blastula is called discoblastula.
A special type of crushing is the incomplete surface crushing of arthropods. Their development begins with repeated crushing of the nucleus located in the center of the egg among the yolk mass. The nuclei formed in this case move to the periphery, where the cytoplasm is poor in yolk. The latter breaks up into blastomeres, which, with their base, pass into an undivided central mass. Further crushing leads to the formation of a blastula with a single layer of blastomeres on the surface and a yolk inside. Such a blastula is called a periblastula.
Mammalian eggs have little yolk. These are alecithal or oligolecital eggs in terms of the amount of yolk, and in terms of the distribution of yolk throughout the egg, these are homolecital eggs. Their fragmentation is complete, but uneven, already at the early stages of crushing, there is a difference in blastomeres in their size and color: light ones are located along the periphery, dark ones in the center. From the light cells, the trophoblast surrounding the embryo is formed, the cells of which perform an auxiliary function and are not directly involved in the formation of the body of the embryo. Trophoblast cells dissolve tissues, due to which the embryo is introduced into the wall of the uterus. Further, the trophoblast cells exfoliate from the embryo, forming a hollow vesicle. The trophoblast cavity is filled with fluid diffusing into it from the tissues of the uterus. The embryo at this time looks like a nodule located on the inner wall of the trophoblast. The mammalian blastula has a small, centrally located blastocoel and is called a sterroblastula. As a result of further crushing, the embryo has the shape of a disc spread out on the inner surface of the trophoblast.
Thus, the fragmentation of the embryos of various multicellular animals, although proceeding differently, ultimately ends with the fact that the fertilized egg (single-celled stage of development) turns into a multicellular blastula as a result of crushing. The outer layer of the blastula is called the blastoderm, and the inner cavity is called the blastocoel or primary cavity, where the waste products of cells accumulate.
Rice. 2. Types of eggs and their corresponding types of crushing
Irrespective of the peculiarities of cleavage of fertilized eggs in different animals, due to differences in the quantity and nature of the distribution of the yolk in the cytoplasm, this period of embryonic development is characterized by the following common features.
1. As a result of crushing, a multicellular embryo is formed - blastula and cellular material accumulates for further development.
2. All cells in the blastula have a diploid set of chromosomes, are identical in structure and differ from each other mainly in the amount of yolk, i.e., the cells of the blastula are not differentiated.
3. A characteristic feature of cleavage is a very short mitotic cycle compared to its duration in adult animals.
4. During the cleavage period, DNA and proteins are intensively synthesized and there is no RNA synthesis. The genetic information contained in the blastomere nuclei is not used.
5. During crushing, the cytoplasm does not move.
Question 4.
germ layers- these are separate layers of cells that occupy a certain position in the embryo and give rise to the corresponding tissues and organs. They are homologous in all animals, i.e., regardless of the systematic position of the animal, they give rise to the same organs and tissues. The homology of the germ layers of the vast majority of animals is one of the proofs of the unity of the animal world. The germ layers are formed as a result of differentiation of relatively homogeneous blastula cells similar to each other.
Question 5.
Cell differentiation is the process by which a cell becomes specialized, that is, it acquires chemical, morphological, and functional features. An example is the differentiation of cells of the epidermis of human skin, in which cells moving from the basal to the spinous and then to other, more superficial layers accumulate keratohyalin, which turns into eleidin in the cells of the zona pellucida and then into keratin in the stratum corneum. In this case, the shape of cells, the structure of cell membranes and the set of organelles change. It is not a single cell that differentiates, but a group of similar cells. There are about 100 different types of cells in the human body. Fibroblasts synthesize collagen, myoblasts synthesize myosin, epithelial cells of the digestive tract pepsin and trypsin, etc.
The first chemical and morphological differences between cells are found during gastrulation. The process, as a result of which individual tissues acquire a characteristic appearance during differentiation, is called histogenesis. Cell differentiation, histogenesis and organogenesis occur together, and in certain areas of the embryo and at a certain time. This is very important because it indicates the coordination and integration of embryonic development. The question arises of how cells with the same genotype differentiate and participate in histo- and organogenesis in the necessary places and at certain times, according to the integral “image” of a given type of organism. At present, the generally accepted point of view is the point of view of T. Morgan, who, based on the chromosome theory of heredity, suggested that cell differentiation in the process of ontogenesis is the result of successive reciprocal (mutual) influences of the cytoplasm and changing products of the activity of nuclear genes. The idea of differential gene expression as the main mechanism of cytodifferentiation was raised.
At present, a lot of evidence has been collected that in most cases the somatic cells of organisms carry a complete diploid set of chromosomes, and the genetic potencies of the nuclei of somatic cells are also completely preserved, i.e. genes do not lose potential functional activity. The studies of karyotypes of various somatic cells carried out by the cytogenetic method showed their almost complete identity. The cytophotometric method established that the amount of DNA in them does not decrease, and the method of molecular hybridization showed that the cells of different tissues are identical in nucleotide sequences.
The hereditary material of somatic cells is able to remain complete not only quantitatively, but also functionally. Therefore, cytodifferentiation is not a consequence of the insufficiency of hereditary material. The main idea is the selective manifestation of genes in a trait, i.e. in differential gene expression.
The expression of a gene into a trait is a complex step-by-step process, which is studied mainly by the products of gene activity, using an electron microscope, or by the results of the development of an individual.
Question 6.
In different animal species, the same germ layers give rise to the same organs and tissues. This means that the germ layers are homologous. The homology of the germ layers of the vast majority of animals is one of the proofs of the unity of the animal world.
The article is based on the work of prof. Bue.
Stopping the development of the embryo further leads to the expulsion of the fetal egg, which manifests itself in the form of a spontaneous miscarriage. However, in many cases, developmental arrest occurs at a very early stage, and the very fact of conception remains unknown to the woman. In a large percentage of cases, such miscarriages are associated with chromosomal abnormalities in the fetus.
Spontaneous miscarriages
Spontaneous miscarriages, defined as "spontaneous termination of pregnancy between the term of conception and the viability of the fetus", in many cases are very difficult to diagnose: a large number of miscarriages occur at very early dates: there is no delay in menstruation, or this delay is so small that it the woman is unaware of the pregnancy.
Clinical Data
The expulsion of the ovum may occur suddenly, or it may be preceded by clinical symptoms. More often risk of miscarriage manifested by bloody discharge and pain in the lower abdomen, turning into contractions. This is followed by the expulsion of the fetal egg and the disappearance of signs of pregnancy.
Clinical examination may reveal a discrepancy between the estimated gestational age and the size of the uterus. Hormone levels in the blood and urine may be drastically reduced, indicating a lack of viable fetus. Ultrasound examination allows you to clarify the diagnosis, revealing either the absence of an embryo ("empty fetal egg"), or developmental delay and lack of heartbeat
The clinical manifestations of spontaneous miscarriage vary considerably. In some cases, a miscarriage goes unnoticed, in others it is accompanied by bleeding and may require curettage of the uterine cavity. The chronology of symptoms may indirectly indicate the cause of spontaneous miscarriage: spotting from early pregnancy, uterine growth stops, disappearance of signs of pregnancy, a "silent" period for 4-5 weeks, and then expulsion of the fetal egg most often indicate chromosomal abnormalities of the embryo, and the correspondence of the term of the development of the embryo to the term of the miscarriage speaks in favor of the maternal causes of miscarriage.
Anatomical data
Analysis of the material of spontaneous miscarriages, the collection of which was begun at the beginning of the twentieth century at the Carnegie Institution, revealed a huge percentage of developmental anomalies among early abortions.
In 1943, Hertig and Sheldon published a post-mortem study of 1,000 early miscarriages. They ruled out maternal causes of miscarriage in 617 cases. Current data indicate that macerated embryos in apparently normal membranes can also be associated with chromosomal abnormalities, which in total accounts for about 3/4 of all cases in this study.
| Morphological study of 1000 abortions (according to Hertig and Sheldon, 1943) | ||
| Gross pathological disorders of the fetal egg: fertilized egg without embryo or with undifferentiated embryo | 489 | |
| Local anomalies of embryos | 32 | |
| placenta anomalies | 96 | 617 |
| A fertilized egg without gross anomalies | ||
| with macerated germs | 146 | |
| 763 | ||
| with unmacerated embryos | 74 | |
| Uterine anomalies | 64 | |
| Other violations | 99 | |
Further studies by Mikamo and Miller and Polland made it possible to clarify the relationship between the term of miscarriage and the frequency of developmental disorders of the embryo. It turned out that the shorter the miscarriage period, the higher the frequency of anomalies. In the materials of miscarriages that occurred before the 5th week after conception, macroscopic morphological abnormalities of the fetal egg occur in 90% of cases, with a miscarriage period of 5 to 7 weeks after conception - in 60%, with a period of more than 7 weeks after conception - less than 15-20%.
The importance of stopping the development of the embryo in early spontaneous miscarriages was shown primarily by the fundamental research of Arthur Hertig, who in 1959 published the results of a study of human fetuses up to 17 days after conception. It was the fruit of his 25 years of work.
In 210 women under the age of 40 undergoing hysterectomy (removal of the uterus), the date of the operation was compared with the date of ovulation (possible conception). After the operation, the uterus was subjected to the most thorough histological examination in order to identify a possible short-term pregnancy. Of the 210 women, only 107 were retained in the study due to the discovery of signs of ovulation, and the absence of gross violations of the tubes and ovaries, preventing the onset of pregnancy. Thirty-four gestational sacs were found, of which 21 gestational sacs were externally normal, and 13 (38%) had obvious signs of anomalies that, according to Hertig, would necessarily lead to miscarriage either at the stage of implantation or shortly after implantation. Since at that time it was not possible to conduct a genetic study of fetal eggs, the causes of developmental disorders of the embryos remained unknown.
When examining women with confirmed fertility (all patients had several children), it was found that one of the three fetal eggs has anomalies and is subject to miscarriage before the onset of signs of pregnancy.
Epidemiological and demographic data
The fuzzy clinical symptoms of early spontaneous miscarriages leads to the fact that a fairly large percentage of miscarriages in the short term goes unnoticed by women.
In the case of clinically confirmed pregnancies, about 15% of all pregnancies end in miscarriage. Most spontaneous miscarriages (about 80%) occur in the first trimester of pregnancy. However, if we take into account the fact that miscarriages often occur 4-6 weeks after the pregnancy stops, we can say that more than 90% of all spontaneous miscarriages are associated with the first trimester.
Special demographic studies made it possible to clarify the frequency of intrauterine mortality. So, French and Birman in 1953-1956. registered all pregnancies in Kanai women and showed that out of 1000 pregnancies diagnosed after 5 weeks, 237 did not result in a viable baby.
An analysis of the results of several studies allowed Leridon to compile a table of intrauterine mortality, which includes fertilization failures (sexual intercourse in optimal timing during the day after ovulation).
| Complete table within uterine mortality (per 1000 eggs at risk of fertilization) (according to Leridon, 1973) | ||
|---|---|---|
| weeks after conception | Stopping development followed by expulsion | Percentage of continuing pregnancies |
| 16* | 100 | |
| 0 | 15 | 84 |
| 1 | 27 | 69 |
| 2 | 5,0 | 42 |
| 6 | 2,9 | 37 |
| 10 | 1,7 | 34,1 |
| 14 | 0,5 | 32,4 |
| 18 | 0,3 | 31,9 |
| 22 | 0,1 | 31,6 |
| 26 | 0,1 | 31,5 |
| 30 | 0,1 | 31,4 |
| 34 | 0,1 | 31,3 |
| 38 | 0,2 | 31,2 |
| * - failure of conception | ||
All these data indicate a huge frequency of spontaneous miscarriages and the important role of developmental disorders of the fetal egg in this pathology.
These data reflect the overall frequency of developmental disorders, without distinguishing among them specific exogenous and endogenous factors (immunological, infectious, physical, chemical, etc.).
It is important to note that, regardless of the cause of the damaging effect, when examining the material of miscarriages, a very high frequency of genetic disorders (chromosomal aberrations (currently best studied) and gene mutations) and developmental anomalies, such as neural tube defects, is found.
Chromosomal Abnormalities Responsible for Stopping Pregnancy Development
Cytogenetic studies of the material of miscarriages made it possible to clarify the nature and frequency of certain chromosomal abnormalities.
Common Frequency
When evaluating the results of large series of analyzes, the following should be borne in mind. The results of studies of this kind can be significantly influenced by the following factors: the method of collecting material, the relative frequency of earlier and later miscarriages, the proportion of induced abortion material in the study, which is often not amenable to accurate assessment, the success of culturing abortus cell cultures and chromosomal analysis of the material, subtle methods processing of macerated material.
The overall estimate of the frequency of chromosomal aberrations in miscarriage is about 60%, and in the first trimester of pregnancy - from 80 to 90%. As will be shown below, an analysis based on the stages of development of the embryo makes it possible to draw much more accurate conclusions.
Relative frequency
Almost all large studies of chromosomal aberrations in the material of miscarriages have given strikingly similar results regarding the nature of the violations. Quantitative anomalies make up 95% of all aberrations and are distributed as follows:
Quantitative chromosomal abnormalities
Various types of quantitative chromosomal aberrations can result from:
- failure of meiotic division: we are talking about cases of "non-disjunction" (non-separation) of paired chromosomes, which leads to the appearance of either trisomy or monosomy. Non-separation can occur during both the first and second meiotic divisions, and can involve both eggs and sperm.
- failures that occur during fertilization:: cases of fertilization of an egg by two spermatozoa (dyspermia), resulting in a triploid embryo.
- failures that occur during the first mitotic divisions: complete tetraploidy occurs when the first division resulted in a doubling of the chromosomes, but no separation of the cytoplasm. Mosaics arise in the case of such failures at the stage of subsequent divisions.
monosomy
Monosomy X (45,X) is one of the most common anomalies in the material of spontaneous miscarriages. At birth, it corresponds to Shereshevsky-Turner syndrome, and at birth it is less common than other quantitative sex chromosome anomalies. This striking difference between the relatively high incidence of extra X chromosomes in newborns and the relatively rare detection of monosomy X in newborns points to the high mortality rate of monosomy X in the fetus. In addition, the very high frequency of mosaics in patients with Shereshevsky-Turner syndrome attracts attention. In the material of miscarriages, on the contrary, mosaics with monosomy X are extremely rare. Research data have shown that only less than 1% of all X monosomies reach term. Monosomy of autosomes in the material of miscarriages are quite rare. This contrasts greatly with the high frequency of the corresponding trisomies.
Trisomy
In the material of miscarriages, trisomy represent more than half of all quantitative chromosomal aberrations. It is noteworthy that in cases of monosomy, the missing chromosome is usually the X chromosome, and in cases of excess chromosomes, the extra chromosome is most often an autosome.
Accurate identification of the extra chromosome was made possible by the G-banding method. Studies have shown that all autosomes can participate in non-disjunction (see table). It is noteworthy that the three chromosomes most often found in neonatal trisomies (15th, 18th and 21st) are most often found in lethal trisomies in embryos. Variations in the relative frequencies of various trisomies in embryos largely reflect the timing at which the death of the embryos occurs, since the more lethal the combination of chromosomes is, the earlier the development stops, the less often such an aberration will be detected in the materials of miscarriages (the shorter the stop period development, the more difficult it is to detect such an embryo).
| Extra chromosome in lethal trisomy in the fetus (data from 7 studies: Bue (France), Carr (Canada), Creasy (UK), Dill (Canada), Kaji (Switzerland), Takahara (Japan), Terkelsen (Denmark)) | |||
|---|---|---|---|
| Additional autosome | Number of observations | ||
| A | 1 | ||
| 2 | 15 | ||
| 3 | 5 | ||
| B | 4 | 7 | |
| 5 | |||
| C | 6 | 1 | |
| 7 | 19 | ||
| 8 | 17 | ||
| 9 | 15 | ||
| 10 | 11 | ||
| 11 | 1 | ||
| 12 | 3 | ||
| D | 13 | 15 | |
| 14 | 36 | ||
| 15 | 35 | ||
| E | 16 | 128 | |
| 17 | 1 | ||
| 18 | 24 | ||
| F | 19 | 1 | |
| 20 | 5 | ||
| G | 21 | 38 | |
| 22 | 47 | ||
triploidy
Extremely rare in stillbirths, triploidy is the fifth most common chromosomal abnormality in miscarriage. Depending on the ratio of sex chromosomes, there can be 3 variants of triploidy: 69XYY (the rarest), 69, XXX and 69, XXY (the most frequent). Analysis of sex chromatin shows that in configuration 69, XXX, only one chromatin lump is most often detected, and in configuration 69, XXY, sex chromatin is most often not detected.
The figure below illustrates the various mechanisms leading to the development of triploidy (diandry, digyny, dyspermy). By using special methods(chromosomal markers, tissue compatibility antigens) it was possible to establish the relative role of each of these mechanisms in the development of triploidy in the embryo. It turned out that out of 50 cases of observations, triploidy was the result of digyny in 11 cases (22%), deandria or dyspermia in 20 cases (40%), dyspermia in 18 cases (36%).
tetraploidy
Tetraploidy occurs in about 5% of cases of quantitative chromosomal aberrations. The most common tetraploidy 92, XXXX. Such cells always contain 2 clumps of sex chromatin. Cells with tetraploidy 92,XXYY never show sex chromatin, but have 2 fluorescent Y chromosomes.
double aberrations
The high frequency of chromosomal abnormalities in the material of miscarriages explains the high frequency of combined anomalies in the same fetus. In contrast, in newborns, combined anomalies are extremely rare. Usually in such cases there are combinations of anomalies of the sex chromosome and anomalies of the autosome.
Due to the higher frequency of autosomal trisomies in the material of miscarriages, with combined chromosomal abnormalities in abortuses, double autosomal trisomies are most common. It is difficult to say whether such trisomies are due to double non-disjunction in the same gamete, or to the meeting of two abnormal gametes.
The frequency of combinations of different trisomies in the same zygote is random, which suggests that the occurrence of double trisomies is independent of each other.
The combination of two mechanisms leading to the appearance of double anomalies can explain the appearance of other karyotype anomalies that occur in miscarriages. "Non-disjunction" in the formation of one of the gametes in combination with the mechanisms of formation of polyploidy explains the appearance of zygotes with 68 or 70 chromosomes. Failure of the first mitotic division in such a trisomy zygote can result in karyotypes such as 94,XXXX,16+,16+.
Structural chromosomal abnormalities
According to classical studies, the frequency of structural chromosomal aberrations in the material of miscarriages is 4-5%. However, many studies were done prior to the widespread use of the G-banding method. Modern research indicates a higher frequency of structural chromosomal abnormalities in abortuses. The most different types structural anomalies. In about half of the cases, these anomalies are inherited from parents, in about half of the cases they occur de novo.
The influence of chromosomal abnormalities on the development of the zygote
Chromosomal abnormalities of the zygote usually appear already in the first weeks of development. Finding out the specific manifestations of each anomaly is associated with a number of difficulties.
In many cases, it is extremely difficult to determine the gestational age when analyzing the material of miscarriages. Usually, the 14th day of the cycle is considered the term of conception, but women with miscarriage often have cycle delays. In addition, it is very difficult to establish the date of "death" of the fetal egg, since a lot of time can pass from the moment of death to miscarriage. In cases of triploidy, this period can be 10-15 weeks. The use of hormonal drugs can further lengthen this time.
Given these reservations, we can say that the shorter the gestational age at the time of the death of the fetal egg, the higher the frequency of chromosome aberrations. According to studies by Creasy and Loritsen, with miscarriages before 15 weeks of gestation, the frequency of chromosome aberrations is about 50%, with a period of 18-21 weeks - about 15%, with a period of more than 21 weeks - about 5-8%, which approximately corresponds to the frequency of chromosome aberrations in perinatal mortality studies.
Phenotypic manifestations of some lethal chromosomal aberrations
Monosomy X usually stop developing by 6 weeks after conception. In two-thirds of cases, the fetal bladder, 5–8 cm in size, does not contain an embryo, but there is a cord-like formation with elements of embryonic tissue, remnants of the yolk sac, and the placenta contains subamniotic blood clots. In one third of cases, the placenta has the same changes, but a morphologically unchanged embryo is found that died at the age of 40-45 days after conception.
With tetraploidy development stops by 2-3 weeks after conception; morphologically, this anomaly is characterized by an "empty fetal sac".
With trisomy different types of developmental anomalies are observed, depending on which chromosome is superfluous. However, in the overwhelming majority of cases, development stops at a very early stage, and no elements of the embryo are found. This is a classic case of "empty gestational sac" (anembryony).
Trisomy 16, a very common anomaly, is characterized by the presence of a small fetal egg with a diameter of about 2.5 cm, in the cavity of the chorion there is a small amniotic vesicle about 5 mm in diameter and an embryonic germ 1–2 mm in size. Most often, development stops at the stage of the embryonic disc.
With some trisomies, for example, with trisomies 13 and 14, the development of the embryo up to a period of about 6 weeks is possible. The embryos are characterized by a cyclocephalic head shape with defects in the closure of the maxillary hillocks. The placentas are hypoplastic.
Embryos with trisomy 21 (Down's syndrome in newborns) do not always have developmental anomalies, and if they do, they are minor, which cannot cause their death. Placentas in such cases are poor in cells, and appear to have stopped in development at an early stage. The death of the embryo in such cases appears to be a consequence of placental insufficiency.
drifts. Comparative analysis of cytogenetic and morphological data allows us to distinguish two types of moles: classic hydatidiform mole and embryonic triploid mole.
Miscarriages in triploidy have a clear morphological picture. This is expressed in a combination of complete or (more often) partial vesicular degeneration of the placenta and an amniotic vesicle with an embryo, the size of which (the embryo) is very small compared to the relatively large amniotic vesicle. Histological examination shows not hypertrophy, but hypotrophy of the vesicularly altered trophoblast, which forms microcysts as a result of numerous intussusceptions.
Against, classic bubble skid does not affect either the amniotic sac or the fetus. In the vesicles, an excessive formation of syncytiotrophoblast with pronounced vascularization is found. Cytogenetically, most classic hydatidiform moles have a 46,XX karyotype. The conducted studies allowed us to establish chromosomal disruptions involved in the formation of hydatidiform mole. The 2 X chromosomes in classic hydatidiform mole have been shown to be identical and paternally derived. The most likely mechanism for the development of hydatidiform mole is true androgenesis, which occurs as a result of fertilization of the egg by a diploid spermatozoon, resulting from a failure of the second meiotic division and subsequent complete exclusion of the chromosomal material of the egg. From the point of view of pathogenesis, such chromosomal disorders are close to disorders in triploidy.
Assessment of the frequency of chromosomal disorders at the time of conception
You can try to calculate the number of zygotes with chromosomal abnormalities at conception, based on the frequency of chromosomal abnormalities found in the material of miscarriages. However, first of all, it should be noted that the striking similarity of the results of studies of the material of miscarriages, carried out in different parts light, suggests that chromosomal disruptions at the time of conception are a very characteristic phenomenon in human reproduction. In addition, it can be stated that the least common anomalies (for example, trisomies A, B and F) are associated with developmental arrest at very early stages.
An analysis of the relative frequency of various anomalies that occur when chromosomes do not separate during meiosis allows us to draw the following important conclusions:
1. The only monosomy found in the material of miscarriages is monosomy X (15% of all aberrations). On the contrary, autosomal monosomies are practically not found in the material of miscarriages, although theoretically there should be as many of them as autosomal trisomies.
2. In the group of autosomal trisomies, the frequency of trisomies of different chromosomes varies significantly. Studies performed using the G-banding method have shown that all chromosomes can be involved in trisomy, but some trisomies are much more common, for example, trisomy 16 occurs in 15% of all trisomies.
From these observations, we can conclude that, most likely, the frequency of nondisjunction of different chromosomes is approximately the same, and the different frequency of anomalies in the material of miscarriages is due to the fact that individual chromosome aberrations lead to a halt in development at very early stages and therefore are difficult to detect.
These considerations allow us to approximately calculate the actual frequency of chromosomal abnormalities at the time of conception. Bue's calculations showed that every second conception gives a zygote with chromosomal aberrations.
These figures reflect the average frequency of chromosomal aberrations at conception in the population. However, these figures can vary significantly between couples. Some couples are more likely to experience chromosomal aberrations at conception than the average risk in the population. In such couples, miscarriage at short terms occurs much more often than in other couples.
These calculations are confirmed by other studies conducted using other methods:
1. Hertig's classical studies
2. Determination of the level of chorionic hormone (CH) in the blood of women after 10 years after conception. Often this test turns out to be positive, although the menstruation comes on time or with a slight delay, and the woman does not notice the onset of pregnancy subjectively ("biochemical pregnancy")
3. Chromosome analysis of the material obtained during artificial abortions showed that during abortions at a period of 6–9 weeks (4–7 weeks after conception), the frequency of chromosome aberrations is approximately 8%, and during artificial abortions at a period of 5 weeks (3 weeks after conception ), this frequency increases to 25%.
4. It has been shown that chromosome nondisjunction during spermatogenesis is a very common occurrence. So Pearson et al. found that the probability of nondisjunction in the process of spermatogenesis for the 1st chromosome is 3.5%, for the 9th chromosome - 5%, for the Y chromosome - 2%. If other chromosomes have a probability of nondisjunction of about the same order, then only 40% of all spermatozoa have a normal chromosome set.
Experimental models and comparative pathology
Development arrest frequency
Although differences in type of placentation and number of fetuses make it difficult to compare the risk of miscarriage in pets and humans, certain analogies can be seen. In domestic animals, the percentage of lethal conceptions ranges between 20 and 60%.
A study of lethal mutations in primates has yielded figures comparable to those in humans. Of 23 blastocysts isolated from macaques before conception, 10 had gross morphological abnormalities.
Frequency of chromosomal abnormalities
Only experimental studies make it possible to carry out a chromosomal analysis of zygotes at different stages of development and to estimate the frequency of chromosomal aberrations. Ford's classic studies revealed chromosomal aberrations in 2% of mouse fetuses between 8 and 11 days of age after conception. Further studies have shown that this is too advanced stage of embryonic development, and that the frequency of chromosome aberrations is much higher (see below).
The impact of chromosomal aberrations on development
A great contribution to clarifying the scale of the problem was made by the studies of Alfred Gropp from Lübeck and Charles Ford from Oxford, conducted on the so-called "tobacco mice" ( Mus poschiavinus). Crossing such mice with normal mice gives a wide range of triploidies and monosomies, which makes it possible to evaluate the influence of both types of aberrations on development.
The data of Professor Gropp (1973) are given in the table.
| Distribution of euploid and aneuploid embryos in hybrid mice | |||||
|---|---|---|---|---|---|
| Development stage | Day | Karyotype | Total | ||
| monosomy | Euploidy | Trisomy | |||
| Before implantation | 4 | 55 | 74 | 45 | 174 |
| After implantation | 7 | 3 | 81 | 44 | 128 |
| 9—15 | 3 | 239 | 94 | 336 | |
| 19 | 56 | 2 | 58 | ||
| live mice | 58 | 58 | |||
These studies allowed us to confirm the hypothesis that monosomies and trisomies are equally likely to occur during conception: autosomal monosomies occur with the same frequency as trisomies, but zygotes with autosomal monosomies die even before implantation and are not found in the material of miscarriages.
In trisomies, the death of the embryos occurs at later stages, but not a single embryo in autosomal trisomies in mice survives to delivery.
Research by the Gropp group showed that, depending on the type of trisomy, embryos die at different times: with trisomies 8, 11, 15, 17 - up to 12 days after conception, with trisomies 19 - closer to the date of birth.
The pathogenesis of developmental arrest in chromosomal abnormalities
A study of the material of miscarriages shows that in many cases of chromosomal aberrations, embryogenesis is sharply disrupted, so that the elements of the embryo are not detected at all ("empty fetal eggs", anembryony) (development stops before 2-3 weeks after conception). In other cases, it is possible to detect elements of the embryo, often unformed (stopping development for up to 3-4 weeks after conception). In the presence of chromosomal aberrations, embryogenesis is often or completely impossible, or is severely disturbed from the earliest stages of development. The manifestations of such disorders are much more pronounced in the case of autosomal monosomies, when the development of the zygote stops in the first days after conception, but in the case of trisomies of chromosomes, which are of key importance for embryogenesis, development also stops in the first days after conception. So, for example, trisomy 17 is found only in zygotes that have stopped in development at the earliest stages. In addition, many chromosomal abnormalities are generally associated with a reduced ability to divide cells, as shown by the study of cultures of such cells. in vitro.
In other cases, development can continue up to 5-6-7 weeks after conception, in rare cases longer. As Philip's studies have shown, in such cases, the death of the fetus is due not to a violation of embryonic development (detectable defects in themselves cannot be the cause of the death of the embryo), but to a violation of the formation and functioning of the placenta (the stage of development of the fetus is ahead of the stage of placental formation.
Studies of placental cell cultures with various chromosomal abnormalities have shown that in most cases the division of placental cells occurs much more slowly than with a normal karyotype. This largely explains why newborns with chromosomal abnormalities usually have low body weight and reduced placental mass.
It can be assumed that many developmental disorders in chromosomal aberrations are associated precisely with a reduced ability of cells to divide. In this case, there is a sharp dissynchronization of the processes of development of the embryo, development of the placenta and induction of cell differentiation and migration.
Insufficient and delayed formation of the placenta can lead to malnutrition and hypoxia of the fetus, as well as to a decrease in the hormonal production of the placenta, which may be an additional reason for the development of miscarriages.
Studies of cell lines in trisomies 13, 18 and 21 in newborns have shown that cells divide more slowly than in a normal karyotype, which is manifested in a decrease in cell density in most organs.
It is a mystery why, with the only autosomal trisomy compatible with life (trisomy 21, Down's syndrome), in some cases there is a delay in the development of the embryo in the early stages and spontaneous miscarriage, while in others - unimpaired development of pregnancy and the birth of a viable child. Comparison of cell cultures of material from miscarriages and full-term newborns with trisomy 21 showed that differences in the ability of cells to divide in the first and second cases are sharply different, which may explain the different fate of such zygotes.
Causes of quantitative chromosomal aberrations
The study of the causes of chromosomal aberrations is extremely difficult, primarily because of the high frequency, one might say, the universality of this phenomenon. It is very difficult to correctly collect a control group of pregnant women, with great difficulty they lend themselves to the study of disorders of spermatogenesis and oogenesis. Despite this, some etiological factors that increase the risk of chromosomal aberrations have been identified.
Factors directly related to parents
The effect of maternal age on the likelihood of having a child with trisomy 21 suggests a possible effect of maternal age on the likelihood of lethal chromosomal aberrations in the fetus. The table below shows the relationship between the age of the mother and the karyotype of the miscarriage material.
| The average age of the mother with chromosomal aberrations of abortions | ||
|---|---|---|
| Karyotype | Number of observations | Average age |
| Normal | 509 | 27,5 |
| Monosomy X | 134 | 27,6 |
| triploidy | 167 | 27,4 |
| tetraploidy | 53 | 26,8 |
| Autosomal trisomies | 448 | 31,3 |
| Trisomy D | 92 | 32,5 |
| Trisomy E | 157 | 29,6 |
| Trisomy G | 78 | 33,2 |
As can be seen from the table, no relationship was found between maternal age and spontaneous miscarriages associated with monosomy X, triploidy, or tetraploidy. An increase in the average age of the mother was noted for autosomal trisomies in general, but different numbers were obtained for different groups of chromosomes. However total number observations in groups are not enough to confidently judge any patterns.
Maternal age is more associated with an increased risk of miscarriages with trisomies of acrocentric chromosomes of groups D (13, 14, 15) and G (21, 22), which also coincides with the statistics of chromosome aberrations in stillbirths.
For some cases of trisomies (16, 21), the origin of the extra chromosome has been determined. It turned out that maternal age is associated with an increased risk of trisomy only in the case of maternal origin of the extra chromosome. No relationship was found between paternal age and an increased risk of trisomy.
In the light of animal studies, suggestions have been made about a possible link between gamete aging and delayed fertilization and the risk of chromosomal aberrations. Gamete aging is understood as the aging of spermatozoa in the female genital tract, the aging of the egg, either as a result of overmaturity inside the follicle or as a result of a delay in the release of the egg from the follicle, or as a result of tubal overmaturity (late fertilization in the tube). Most likely, similar laws operate in humans, but reliable evidence of this has not yet been received.
environmental factors
It has been shown that the likelihood of chromosomal aberrations at conception is increased in women exposed to ionizing radiation. It is assumed that there is a connection between the risk of chromosomal aberrations and the action of other factors, in particular, chemical ones.
Conclusion
1. Not every pregnancy can be saved for short periods. In a large percentage of cases, miscarriages are due to chromosomal abnormalities in the fetus, and it is impossible to give birth to a live child. Hormonal treatment can delay the moment of miscarriage, but cannot help the fetus survive.
2. Increased instability of the genome of spouses is one of the causative factors of infertility and miscarriage. Cytogenetic examination with analysis for chromosomal aberrations helps to identify such married couples. In some cases of increased genomic instability, specific anti-mutagenic therapy may help increase the chance of conception. healthy child. In other cases, donor insemination or the use of a donor egg is recommended.
3. In case of miscarriage due to chromosomal factors, a woman's body can "remember" an unfavorable immunological response to a fetal egg (immunological imprinting). In such cases, it is possible to develop a rejection reaction to embryos conceived after donor insemination or using a donor egg. In such cases, a special immunological examination is recommended.
The content of the article
BIRTH DEFECTS, violations of the structure, functions and biochemistry of the body due to birth or prenatal causes and leading to physical or mental abnormalities, illness or death. Antenatal causes of such malformations include hereditary factors and/or exposure environment for the development of the embryo. Birth defects can be caused by trauma or infection. Very low birth weight, which reflects either prematurity or insufficiency of fetal development processes and is the main cause of infant mortality and disability, is also considered a congenital malformation.
Historical aspect.
Prehistoric art testifies that birth defects have been known since ancient times. Their appearance inspired fear and gave rise to many myths. Cuneiform tablets of ancient Babylon report that congenital deformities were considered omens of state importance and were deciphered as warnings from angry gods. There was a widespread belief that the impressions of the mother during pregnancy influenced the formation of the child; it was thought that the split (so-called "hare") lip is the result of a fright by a hare, and the deformity of the legs occurs after a meeting with a cripple. Other beliefs were the cause of suffering and death of mother and child, as they claimed, for example, that a monstrous offspring appears as a result of carnal contact with an animal.
One of the first observations revealing the nature of congenital malformations dates back to 1651 and belongs to the English physician William Harvey. He noticed that some of the defects are the result of the preservation of a sign that is normal for the embryo (or fetus), which usually disappears by the time of birth. However, only in the 19th century. malformations have been carefully studied, and the 20th century. was marked by the development of genetic research, and the resulting knowledge replaced the fantastic, often pernicious superstitions of the past; for the first time, methods of preventing and treating some of these severe disorders arose.
CAUSES OF BIRTH DEFECTS
Heredity.
Some birth defects are inherited in the same way as other traits. Hereditary information is passed from parents to children with the help of genes carried on chromosomes. Normally, each sex cell (sperm or egg) contains 23 chromosomes. At fertilization, i.e. the fusion of sperm and egg, a normal genetic set of 46 chromosomes is recreated. 22 out of 23 chromosomes of a reproductive cell are autosomes, i.e. they do not determine sex, but one is either an X- or Y-sex chromosome. The spermatozoon carries either an X-or a Y-chromosome, while the egg carries only the X-chromosome. Fertilization of an egg with a Y-chromosome sperm produces a male offspring, and an X-chromosome sperm produces a female.
Many hereditary traits and their disorders correspond to statistically predictable types of inheritance, called Mendelian - in honor of their discoverer Gregor Mendel. Mendelian inheritance is the most understandable way genetic transmission congenital defects. The latter can be transmitted either by dominant or by recessive type of inheritance.
The genotype of each parent carries two variants (alleles) of the gene that determines this trait, and the child from each parent receives one allele. The manifestation of an abnormal trait as dominant occurs when a child inherits from one of the parents a defective gene that dominates the normal variant from the other parent. A parent with such a dominant gene always has the corresponding disorder (though perhaps in a milder form). A child has a 50% chance of getting this disorder, depending on whether the normal or defective gene is passed on to him by the affected parent. Huttington's disease (progressive lesion of the central nervous system) and achondroplasty dwarfism (stunted bone growth) are examples of dominant inheritance patterns.
Inheritance of a recessive trait results in a severe disorder in a child when both parents carry the same defective gene (along with the normal gene for that trait), but clinical manifestation they have no disease. Each child born will have a 25% chance of not inheriting the defective gene from either parent, a 50% chance of being a carrier (having only one defective gene), and a 25% chance of inheriting it in a "double dose" (two defective gene), thus inheriting the disease. Sickle cell anemia caused by a defect in the hemoglobin molecule (cm. SICKLE CELL ANEMIA), – an example of a recessively inherited disease. Other examples include thalassemia (another form of anemia that occurs mainly in people of Mediterranean and Asian ancestry) and Tay-Sachs disease, a metabolic disorder that leads to early death. childhood and manifested mainly in the families of Jews, immigrants from Eastern Europe.
Disorders such as those discussed above are caused by an autosomal gene (not located on the sex chromosomes) and are therefore called autosomal diseases. Another group consists of the so-called. X-linked or sex-linked disorders; they are determined by a defective gene located on the X chromosome. Since women normally have two X chromosomes, a mother can be a carrier of a defective X-linked recessive gene and still be healthy. Males have only one X chromosome, and due to the lack of a second X chromosome with its compensatory effect, they almost always show the effect of the defective gene. Each child has a 50% chance of inheriting the defective gene from a carrier mother. Women, inheriting such a gene, become carriers, and men develop the disease. A sick father cannot pass on the defective gene to his sons, since they inherit the Y chromosome from him, but all daughters who receive his X chromosome will be carriers. Color blindness and hemophilia (a disease in which blood clotting is impaired) are X-linked recessive disorders. In another X-linked disorder, called fragile X syndrome, there are varying degrees of mental retardation. Men are affected more often and in a more severe form.
Genetically determined birth defects occur accidentally as a result of gene mutations or errors in chromosome replication during the maturation of an egg or sperm. A direct consequence of mutations are molecular, qualitative and quantitative, changes in the gene product. Occasionally there are beneficial mutations, but most of them are harmful. Big number cases of X-linked and dominant diseases arise as a result of new mutations. The two known sources of mutations are ionizing radiation and a series chemical substances. During the development of the sperm and egg, the chromosomes must be very accurately duplicated (doubled) and then distributed in such a way that each mature cell receives only half of the normal set of chromosomes. However, for unclear reasons, when chromosomes separate, errors sometimes occur, as a result of which a mature germ cell may either lack a chromosome or turn out to be superfluous. In addition, chromosomes may inaccurately duplicate or break. Significant chromosomal abnormalities usually result in multiple abnormalities fatal to the embryo, fetus, or newborn, and in particular are found in about 50% of miscarriages. A chromosomal anomaly underlies one of the most common congenital malformations, namely Down syndrome, caused by the presence of an extra 21st chromosome and manifested by mental and physical retardation and a number of other signs (cm. DOWN SYNDROME).
The second most common cause of congenital mental retardation is a chromosomal abnormality known as a fragile X chromosome. A defect in the structure of such X chromosomes is found at the end of the long arm, which takes the form of a stalk with a drop-shaped thickening; a thin stalk often breaks off in preparation for microscopy and is therefore called an unstable region (site), and the chromosome itself is fragile (fragil). It is not known how the fragile chromosome is involved in the development of pathological signs, but it has been shown that a certain sequence of DNA bases (cytosine-guanine-guanine) is repeated with increased frequency in its unstable region. The meaning of such repetitions is unclear.
Fragile X syndrome is inherited as a recessive trait, i.e. its effect may be blocked or obscured by the presence of a normal X chromosome. In men, because they have only one X chromosome, Fragile X Syndrome is fully manifested with mental retardation, enlarged testicles, protruding ears, and a protruding chin. In women, with their two X chromosomes, the presence of one fragile chromosome should not be affected, but, surprisingly, about a third of women who carry the defective chromosome show some mental retardation. But even if they have normal intelligence, female carriers have a 50% chance of passing on the defective chromosome to each of their children.
There are cases when the cells of the embryo have only one X-chromosome and no Y-chromosome; the result is a female child with Turner syndrome. In other cases, the fertilized egg (zygote) contains one (or more) extra X chromosome along with the Y chromosome; this results in the birth of a male child with Klinefelter syndrome. Such chromosomal abnormalities are characterized by sexual underdevelopment, sterility, impaired development and growth processes, and sometimes mental retardation.
Occasionally, an extra chromosome does not occur in a sperm or egg, but in an embryo at an early stage of its development - as a result of an incorrect divergence of a pair of chromosomes in the process of cell division. All cells derived from the resulting defective cell will have an extra chromosome, and the extent to which a given disorder affects an individual depends largely on how early in development the error occurred. Such a deviation from the norm, in which cells have a different number of chromosomes, is called mosaicism. Mosaicism is found in some women with Turner syndrome, but is very rare in Klinefelter syndrome. .
External influences.
After the drug thalidomide was discovered in the 1960s to cause severe birth defects, it became clear that many drugs could cross the placental barrier and affect the embryo or fetus. It is during the early embryonic period that most of the structures of the body are formed (after the eighth week, the embryo is called a fetus). Although the main physical malformations occur from the second to the eighth week of pregnancy, individual anomalies of the eyes, inner ear and nervous system may appear later. Until the second week the impact harmful substances blocks the implantation of the embryo in the uterine wall or affects it so much that development cannot continue .
Babies of mothers who drank heavily during pregnancy show signs of mental and physical defects known as fetal alcohol syndrome. Women who smoke during pregnancy have an increased risk of miscarriage, stillbirth, or low birth weight, which has a significantly higher chance of becoming disabled or dying than a normal weight newborn.
Spontaneous abortions, low birth weight and other problems are also associated with maternal malnutrition. Although the fetus is protected from many infections, some of them can cause severe defects depending on the stage of development during which the infection occurred. Thus, the impact of the rubella virus on the fetus leads to heart defects, blindness, deafness and other disorders. (cm. rubella). Some infections affect the fetus before or during childbirth, causing congenital illness or death. Among them are cytomegalovirus infection and toxoplasmosis (both often mild and unnoticed by the mother), as well as sexually transmitted diseases, in particular gonorrhea, chlamydia, genital herpes and syphilis.
The embryo or fetus may be affected by increased levels of ionizing radiation. In addition to the usual background radiation, the most common source of exposure is X-ray diagnostics. It is believed that modern diagnostic methods are not dangerous for the embryo and fetus. However, when possible, it is necessary to cover the pelvic area in women of reproductive age under fluoroscopy and, unless there is an emergency indication, to order an x-ray examination a week or ten days after menstruation, since pregnancy is unlikely during this period. Concerns have been raised about the safety of non-ionizing radiation from microwave ovens, computer displays, and diagnostic ultrasound. To date, these fears have not been confirmed either from a theoretical point of view or statistical evidence.
multifactorial reasons.
Most birth defects cannot be attributed to any one genetic cause or one environmental factor. It is assumed that they are the result of either the interaction of many genes (polygenic causation), or the combined action of genes and environmental factors (multifactorial causation).
TREATMENT
Very few birth defects can be completely cured, but as a result of therapy, the development of most of them can be slowed down or stopped, and the defect that has arisen is sometimes even partially corrected. Structural defects such as cleft lip and cleft palate (cleft palate), clubfoot, various heart and digestive tract defects are corrected surgically. Various organ transplants are now possible, including kidneys, liver, cornea and, in the treatment of immune deficiency, bone marrow. More than effective methods prosthetics for defective or missing limbs. Rehabilitation and special educational methods can compensate for many mental and physical anomalies and sensory deficiencies. Some inborn metabolic disorders can be treated with diet or medication.
Children with congenital hypothyroidism develop normally if they start the administration of thyroid hormone in time. A special diet can save most children with a severe metabolic disorder such as phenylketonuria from devastating brain damage. (see Phenylketonuria). In hereditary rickets, vitamin D and phosphate supplements have been successfully used. Diseases arising from excess fluid accumulation, in particular hydrocephalus and urinary tract blockade, are amenable to surgical treatment carried out in individual cases even before birth.
Advances have been made in prenatal and non-surgical treatment. Cardiac disorders are corrected with the help of drugs that the mother receives, and in case of metabolic disorders associated with vitamin deficiency, mothers are prescribed large doses of the right vitamin. Vaccines have now been developed to prevent birth defects due to rubella and Rh incompatibility, which occurs when an Rh-negative mother's antibodies destroy redthe blood cells of her Rh-positive fetus (cm. BLOOD) .
IDENTIFICATION AND DIAGNOSIS
With the help of biochemical methods, a number of genetic diseases of newborns are detected. Some of them, including hypothyroidism, phenylketonuria, and galactosemia (a disorder of carbohydrate metabolism), can be determined by testing blood taken from the heel of a newborn. Timely medication or a special diet ensures that sick children develop normally.
For couples who suspect that a child may have a genetic disease, there is a medical genetic counseling service. Typically, couples seek counseling because they already have a child with a birth defect, or because their family history or ethnicity suggests a risk of having a child with a particular condition. However, the greatest risk is associated with the age of the mother - the older she is, the more likely it is that the child will have chromosomal disorders such as Down syndrome. Many birth defects can now be safely and accurately diagnosed in utero. .
The fetal ultrasound image provides insight into developmental and structural defects and provides important information about the course of the pregnancy and the upcoming birth, including gestational age, presence of more than one fetus, position of the placenta, possible fetal heart failure, and position in the uterus . Amniocentesis, i.e. puncture of the fetal bladder and obtaining a sample of amniotic fluid (fluid surrounding the fetus) for analysis, allows you to identify chromosomal abnormalities, some malformations and approx. 100 metabolic disorders. Fetal endoscopy, performed by inserting a fiber-optic endoscope into the uterus, is a more difficult and risky intervention. It allows you to examine the fetus and take blood and tissue samples for diagnostic testing. This procedure is also used for blood transfusions in case of Rh incompatibility.
More than 95% of women undergoing antenatal tests can be sure that the fetus does not have a suspected disease. Information given to parents dramatically reduces the number of abortions. At the same time, information about the presence of certain disorders in the fetus allows doctors to prepare by the time of delivery for the measures necessary to save the life of the newborn and reduce the harmful effects of his defect, as well as notify parents about additional measures that are needed to preserve the health of the child.
|
FREQUENCY OF SOME BIRTH DEFECTS |
|||
|
Disease |
Frequency at birth |
Inheritance type 1 |
|
| hereditary diseases | |||
| Achondroplasty dwarfism | 1/10 000 | ||
| cystic fibrosis | 1/2000, USA, white | ||
| Galactosemia | 1/30 000–1/40 000 | ||
| Hemophilia A | 1/2500 men | ||
| Familial hypercholesterolemia | 1/500 | ||
| sickle cell anemia | 1/625 African American | ||
| Tay-Sachs disease | 1/3600 Jews (Ashkenazi) | ||
| Neurofibromatosis | 1/3000 | ||
| Chromosomal abnormalities | |||
| Klinefelter syndrome | 1/500 men | ||
| Turner syndrome | 1/10,000 women | ||
| Down syndrome | 1/800 | ||
| Congenital malformations | |||
| "Cleft palate" | 1/2000 | ||
| "Hare" lip | 1/1150 | ||
| Clubfoot 2 | 1/400 | ||
| Congenital dislocation of the hip 2 | 1/400 | ||
| Limb underdevelopment | 1/2500 | ||
| Spina bifida 3 | 1/2000 | ||
| Heart defects | 1/200 | ||
| 1 AD - autosomal dominant; AR, autosomal recessive; XP - X-linked recessive. 2 Without defects of the nervous system. 3 No anencephaly, i.e. the absence of all or most of the brain. Spina bifida is an incomplete fusion of the spine. |
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Ministry of Education Russian Federation
Department of Education of the City Administration of Cheboksary
MOU " Cadet School»
Abstract on the topic:
Development of the human embryo
Completed: cadet 9 "A" class
Ivanov K.
Checked by: Nardina S.A.
Cheboksary 2004
What does a child look like at the very beginning of his life - in his mother's belly?
This is an egg, in other words, a cell. It consists, like all cells of the human body, of a drop of matter - protoplasm with a nucleus in the middle. This is a very large cell almost visible to the naked eye, measuring one tenth of a millimeter.
This occurs as a result of the union of two cells: the male cell, or sperm, and the female, egg. The ovum is a large round cell. As for the spermatozoon, it is 30 or 40 times smaller - although not taking into account its long oscillating tail, thanks to which the spermatozoon moves. Upon contact with the egg, the sperm cell loses its tail. And its nucleus penetrates into the egg. Both nuclei merge, fertilization of the egg occurs; henceforth it becomes an egg. Each of the cells that form the egg bears the characteristics of one of the parents. Carriers of these traits are small, rod-like formations contained in the nuclei of all cells and called chromosomes. The nucleus of every cell in the human body contains 46 chromosomes: 23 from the father and 23 from the mother. The same chromosomes of the father and mother form a pair. Each of us in any cell of the body has 23 pairs of chromosomes that are unique to him and determine his individual characteristics; that is why certain features of our appearance, mind or character make us look like a father and mother, grandparents or other relatives.
The sex of the child is the result of a random selection of chromosomes. First, let's pay attention to appearance chromosomes: their size and shape are different, but in every normal cell there are at least 44 chromosomes, each of which has a similar one. Grouped in twos, they form 22 pairs. They are classified by size: the largest is number 1, and the smallest is number 22. 23 - I pair stands apart. In a woman, she, like everyone else, is formed by two similar chromosomes, denoted by the letter X (X). And in men, in the 23rd pair, there is only one X chromosome, along with a smaller one, indicated by the letter Y (Y).
In the body of the parents, the egg or sperm are cells containing only half of the chromosomes, that is, 23 each. Thus, all eggs are of the same type: they all have an X chromosome. Spermatozoa are of two types: one of them has an X-chromosome under number 23, the other has a Y-chromosome. If an egg by chance is paired with a sperm carrying an X chromosome, the egg will develop into a girl, and if by chance the egg is fertilized by a sperm carrying a Y chromosome, the egg will develop into a boy. Thus, sex determination occurs at the time of fertilization.
Theoretically, it would be possible to find out the sex of the child from this very moment, if we had at our disposal the technical means that would allow us to observe the egg without the risk of damaging it. Perhaps the day will come when chance will give way to science and parents will choose the sex of their child, in any case, this will happen only if X- and Y-sperms are separated in the sperm. As soon as it is formed, the egg begins to divide into two, four, eight, sixteen, etc. cells. After a few days, the cells functionally specialize: some - for the formation of sensory organs, others - for the intestines, others - for the genital organs, etc. It is the Y chromosome that tells the sex cells that they will develop in the male pattern. External signs sexes become noticeable by the beginning of the fourth month of pregnancy. But at the chromosomal level, which determines its external manifestations, sex exists from the moment of fertilization. That is why, in some cases, it is possible to find out the sex of the child already at the beginning of pregnancy (in the second or third month), thanks to chromosomal studies of some egg cells (the so-called trophoblast puncture and amniocentesis), or thanks to a kind of radar that, using ultrasound, allows you to see a small penis fetus in the mother's uterus.
A fertilized egg moves along the fallopian tube, simultaneously divides and turns into a multicellular embryo, which enters the uterine cavity after 4-5 days. Within 2 days, the embryo remains free in the uterus, then plunges into its mucous membrane and attaches to it. The gestation period begins prenatal development. Shells are formed from some of the cells. The outer shell has villi with capillaries. Feeding and respiration of the embryo occurs through the villi. Inside the villous shell there is another one, thin and transparent, like cellophane. It forms a bubble. The embryo floats in the fluid of the bubble. This shell protects the embryo from shock and noise.
By the end of the 2nd month of intrauterine development, the villi remain only on the side of the germinal membrane that faces the uterus. These villi grow and branch, plunging into the uterine mucosa, richly supplied with blood vessels. The placenta develops in the form of a disc, firmly fixed in the uterine mucosa. From this moment begins the fetal period of intrauterine development.
Through the wall of the blood capillaries and villi of the placenta, gases and nutrients are exchanged between the body of the mother and the child. Maternal and fetal blood never mixes. From the 4th month of pregnancy, the placenta, acting as an endocrine gland, secretes a hormone. Thanks to him, during pregnancy, the uterine mucosa does not exfoliate, there are no menstrual cycles and the fetus remains in the uterus throughout pregnancy.
Ovulation and fertilization of two or more eggs produce two or more fetuses. These are future twins. They don't look very similar to each other. Sometimes two fetuses develop from the same egg, often they share the same placenta. Such twins are always of the same sex and are very similar to each other.
The embryo in the uterus develops rapidly. By the end of the first month of intrauterine development, the head of the embryo is 1/3 of the body length, the contours of the eyes appear, and fingers can be distinguished at the 7th week. After 2 months, the embryo becomes similar to a person, although its length at this time is 3 cm.
By 3 months of intrauterine development, almost all organs are formed. By this time, you can determine the sex of the unborn child. By 4.5 months, fetal heart contractions are heard, the frequency of which is 2 times greater than that of the mother. During this period, the fetus grows rapidly and weighs about 500 g by 5 months, and 3-3.5 kg by the time of birth.
BIBLIOGRAPHY:
1. Encyclopedia of Blinov I.I. and Karzova S.V. pp.367-369
2. Textbook on biology for grade 9, author Tsuzmer A.M., Petrishina O.L. pp.167-172
The composition of the human embryo in the first days of existence
Fertilization of the egg - page 3
Formation of the placenta - page 3
Development of the embryo - page 4
References - page 5
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