The length of the pregnancy, the overall chronology (pre-embryonic, embryonic, and fetal timeline), the primordial germ cells, and many other issues that we will address later are all covered in this page about embryology. In summary, they are referred to as primordial germ cells, or PGCs. These are sacrococcygeal and craniopharyngeal teratomas.
What is embryology (Foundation for Fellowship in Embryology in India)?
It is the study of embryonic genesis, development, and maturation, all right. It is the study of how embryos form, grow, and mature. Thus, embryology is the term for this. The length of pregnancy is around nine calendar months, so keep that in mind.
According to the calendar, the duration is nine months and seven days, which can also be expressed as 280 days or 40 weeks. The calculation of pregnancy length begins from the first day of the last menstrual period, commonly referred to as the gestational age. Throughout the embryonic stage, the developing fetus is termed an embryo from the third week until the eighth week.
So, the embryonic stage goes from the third week to the eighth week, and after that, we call the developing baby a fetus from the ninth week all the way to delivery. Therefore, the pre-embryonic phase lasts from 0 to 2 weeks, the embryonic period lasts from 3 to 8 weeks, and the fetal period lasts from 9 weeks till delivery. Now, these primordial germ cells are also referred to as primitive sex cells. Another name for them is primordial sex cells.
It's true that these primordial germ cells originate from epiblasts nowadays. What is this epiblast, then? An epiblast is also referred to as the primordial ectoderm. Primitive ectoderm presently indicates that it will eventually produce the ectoderm, but for now, it is not the ectoderm; rather, it is something that will do so, correct? In the second week of embryogenesis, this epiblast will produce primordial germ cells, also known as primitive sex cells. Keep this in mind while embryogenesis is occurring. These early germ cells are now descended from epiblasts. We must thus note that they are descended from epiblasts. We don't have any mesoderm before the third week, thus keep in mind that there isn't a genital ridge in the second week. Since there is currently no mesoderm or genital ridge in the second week of embryogenesis, everything will appear in the third week.
Thus, the testis and ovary will develop from the genital ridge, also known as the mesoderm. Given that these primitive sex cells now lack an ovary, testis, or genital ridge, what will they do? Alright, they will move. They are going to migrate to the yolk sac wall, and by the time that four weeks have passed, they will have done so by the fourth week, correct? Furthermore, during the fifth week, these primordial germ cells, also known as PGCs, will once more move to the genital ridge, also known as the gonadal ridge. Another name for it is the gonadal ridge. The genital ridge or gonadal ridge will now undergo differentiation and eventually give rise to the spermatogonium, oogonium, and oogonium. All right, so either oogonia or oogonium, spermatogonia or spermatogonium, isn't that correct? Absolutely, they will develop into spermatogonium or spermatogonia, as well as oogonium or oogonia. Essentially, these primitive sex cells are the primordial germ cells that will create the sex cells, namely spermatogonia and oogonia, got it? This happens again in the fifth week when they move to the genital ridge, correct? The gonadal ridge.
As of right now, don't worry if you don't know what the gonadal ridge or yolk sac are. You will be able to understand all of these terms in the next video, including what an epiblast, yolk sac, gonadal ridge, and genital ridge are. As of right now, keep in mind that the spermatogonium and oogonium are the primordial germ cells that will develop into the sex cells.
Now, there are two clinical correlations that are crucial in this regard: sacrococcygeal teratoma and craniopharyngeal teratoma. What is this now? We already possess the primordial germ cells, as you are already aware. In the circumstance that these primordial germ cells, referred to as PGCs, do not successfully migrate to the genital ridge, they may display abnormal migration patterns or arrive at the neck region. Upon reaching the neck region, the pluripotency of these primordial germ cells will lead them to either differentiate or form a teratoma.
This is essentially a tumor of the germ cells, which is why there is aberrant migration. These primordial germ cells can now move to the neck area in certain situations. You are already aware of craniopharyngeal, as the name implies, as it is located in the neck, pharynx, and skull. Therefore, these germ cells will essentially cause an outpouching to grow in the neck area.
Pluripotency
Pluripotency means the ability of the germ cells to form all the three germ layers, right? Pluripotent, they have the ability to divide, okay, and that is why they can lead to teratoma or tumor of these germ cells. Apart from that, there could be one more abnormal migration. So, if the primordial germ cells, if the primordial germ cells or PGCs, they reach the, if they reach the area or region between the sacrum and the coccyx.
So, you already know we have the sacral vertebra and the coccyx vertebra. So, there is a region that is in between the sacrum and the coccyx and in that region, if these primordial germ cells will migrate, they can lead to teratoma. So, they reach the area between the sacrum and the coccyx and because of the pluripotency of these germ cells, it can lead to teratoma.
It can lead to teratoma and it is termed as sacrococcygeal teratoma, which you can see over here in this particular picture, how nicely it is given. See this outpouching, it is like outpouching, it is the primordial germ cells which have divided and like they have caused tumor over here. So, this is called as sacrococcygeal teratoma.
Now there are three questions that you have to answer. Now, the very first question says the primordial germ cells migrating into genital ridge are coming from, they are coming from the wall of the yolk sac. I have already told you, yes, they are coming from the wall of the yolk sac.
But again, if the question says the primordial germ cells are derived from, they are derived from epiblast, they are derived from epiblast, that is also called as the primitive ectoderm It is also called as the primitive ectoderm and these primordial germ cells will give rise to spermatogonium and oogonium, spermatogonium and oogonium. Now, coming on to the difference between the mitosis and the meiosis. As you already know, mitosis is the one in which the chromosome number remains the same.
The chromosome number remains the same. For example, this is a cell, right? This is a cell. It is containing 46 chromosomes, right? If mitosis takes place, if mitosis takes place, the resulting cells will also contain the same number of chromosomes, that is 46 and 46, right? Now, in meiosis, we already know the meiosis is of two types, meiosis I and meiosis II.
Now, meiosis I, it is also called as the reductional division. It is also called as the reductional division. Why? Because the chromosome number is reduced to half.
The chromosome number, it is reduced to half. For example, if this cell is containing 46 chromosomes, then if the cell is undergoing meiosis I, if it is undergoing meiosis I, then the resulting cells, okay, the resulting daughter cells will have 23 chromosomes each, right? So, the number of chromosomes is reduced to half. That is why meiosis I, it is also called as the reductional division, okay? Apart from that, if we talk about meiosis II, so meiosis II, it is equational division just like mitosis, equational division just like mitosis.
Now, if again, if again there is mitosis, sorry, if again there is meiosis II in this particular cell, what will happen? The resulting cell will have 23 chromosomes each, right? The resulting cell will have 23 chromosomes each. Why? Because meiosis II, it is the equational division just like mitosis. Apart from that, remember the stages of, yes, remember the stages.
It is prophase, metaphase, anaphase and telophase. We have PMAT like prophase, metaphase, anaphase and telophase. So, these are the four stages that I hope you already know from your plus two knowledge, right? Now, coming on to the next part, that is the non-disjunction, that is one of the clinical aspect related with meiosis, related with meiosis.
Now, in the non-disjunction, remember it is the failure of, let us write down the definition first. It is the failure of two members, right? It is the failure of two members of a homologous or I can say a pair of homologous chromosome, pair of homologous chromosome to separate during first meiotic division, to separate during first meiotic division and thus they pass together, thus they pass together to one of the daughter cells, one of the two daughter cells that are formed, right? One of the two daughter cells. Now, let us understand this, right? Now, what are the results that you have to see over here? See, the results.
What will happen due to this non-disjunction? If these two members of a pair of homologous chromosomes, they do not separate, okay? Now, if they do not separate, the two members will go to one cell directly and they will not go to another cell, right? So, what will happen? One daughter cell, one daughter cell will have extra chromosome, okay? Extra chromosome, that is 24, right? And on the other hand, the other daughter cell, other daughter cell will have one chromosome less, right? It will have one chromosome less. So, you already know 23 minus 1, it will be 22. So, one will have an extra chromosome and one will have one deficient chromosome, right? Now, if the fertilization occurs, now suppose if fertilization occurs with normal gamete, okay? If the fertilization occurs with normal, with normal gamete, right? With the normal gamete, I mean the gamete which is carrying 23 chromosomes.
So, it is a normal gamete, right? Now, what will happen? These 23 chromosomes will combine with this daughter cell, right? This normal gamete will combine with this daughter cell and they will result in the formation of the zygote which will contain 47 chromosomes. It will contain 47 chromosomes. Now, you can easily see that one chromosome is extra.
So, that is something which is referred as trisomy, trisomy, okay? This is something which is referred as the trisomy. Now, one chromosome is present in a triplet, right? Instead of being double, it is in triplet and that is why you see the triplet, it is more commonly seen in the trisomy of chromosome 21, okay? It is the trisomy of the chromosome 21. Yes, it is the trisomy of chromosome 21 and this is something which results in the down syndrome.
This is something which results in the down syndrome. The infant will have 47 chromosomes, right? The down syndrome, right? Apart from that, the other end, right? The other end, you already know the, now let us again, the same situation. If the fertilization occurs with the normal gamete, if fertilization occurs with normal gamete, now normal gamete, it means 23 chromosomes.
What will happen?
These 23 chromosomes and these 22 chromosomes will combine with each other, right? So, the resulting zygote, the resulting zygote will have only 45 chromosomes. Ideally, it should be 46. Now, it will have 45 chromosomes, one chromosome less, right? One chromosome less and this is something which is called as monosomy.
Monosomy - One chromosome is present in single form instead of a duplet, right? So, remember it is the monosomy of the X chromosome that is more relevant, monosomy of X chromosome, right? Now, let us suppose that the ovum, it is having no X chromosome, right? It is having no X chromosome and it is fertilized by, it is fertilized by sperm which is having X chromosome, sperm mein kya hota hai? XY hota hai na? So, if it is fertilized by the sperm carrying X chromosome, then it will result in the formation of XO type, right? XO type, the zygote will be XO type. Kyuki ovum mein toh X tha hi nahi, there is no X chromosome, X is coming only from the father. So, it will result in the formation of XO type, right? Or XO zygote and this is something which is called as the Turner's syndrome.
Turner’s Syndrome
This is something which is called as the Turner's syndrome. So, we have two diseases over here that we have to discuss. One is the down syndrome and one, the other one, it is the Turner's syndrome. Both are resulting from the non-disjunction, non-disjunction of a pair of homologous chromosomes that is during the first meiotic division. That is again an important line that you have to remember. It is during the first meiotic division, right? So, this is like the takeaway point from this slide.
Now, let us discuss these disorders one by one. Now, coming on to the down syndrome, coming on to the down syndrome, you already know that this down syndrome is resulting from the non-disjunction and this non-disjunction, it is occurring at the level of meiotic one, right? So, yes, this is a case of non-disjunction, non-disjunction and this is taking place at the level of first meiotic division or meiosis one, right? When the pair of homologous chromosomes do not separate, right? So, that is you have to remember. Apart from that, yes, it is associated with, it is associated with increased maternal age, it is associated with increased maternal age, right? If the age of the mother is more, more likely the chances that the baby is going to suffer from down syndrome.
Now, remember that this is the most common, most common chromosomal disorder, chromosomal disorder in humans, in humans and it is the most common cause of, it is the most common inheritable cause of mental retardation, mental retardation. Now, you already know mental retardation. It is also known as nowadays intellectual disability, right? So, it is the most common inheritable cause of mental retardation, right? So, the genetic cause you already know, it is the non-disjunction which is taking place during meiotic one, right? Apart from that, you can remember the clinical features of this disease with the help of mnemonic, child has problem, child has problem.
Now, what you will see over here? You will see cardiac defects. So, there are congenital cardiac defects, right? We can see hypotonia, hypotonia means decrease in the muscle tone, right? You can write it also, it is decrease in the muscle tone. Apart from that, you can see there is an increased gap between the great toe and the second toe, right? So, there is an increased gap between the great toe and second toe, right? You can see over here leukemia, the cancers, right? You can also see duodenal atresia.
In patients of down syndrome, you will see duodenal atresia. It simply means that there is an obstruction in the duodenum, which will not allow the food to move down, right? To move distally, atresia, blockage, right? Then you will see one disease that is called as Hirschsprung disease. Hirschsprung disease.
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