For students pursuing Embryology Training in India, mastering the fundamentals of gonad development is essential for understanding human reproductive biology and assisted reproductive technologies. Before we progress even further, we need to ask ourselves what is gonad? We need to know the definition before we even go into the chapter. So, what is gonad? A gonad is a gland in which the sex cells are produced so it would be sperm in males and ovum in females. Now that we know what the gonad is we need to know where it is developed.

So, after fertilisation the gonad glands develop in the urogenital ridge which is derived from the mesoderm. Now during week 4 to 7 the gonads do not have a sex assigned. They are indifferent gonads.

So, we need to determine after fertilisation whether the child will be a male or a female and, in this chapter, we will know how that is possible. Now for the indifferent gonad to develop into either male or female it needs the help of primordial germ cell which is not available in the gonad yet. So where are those primordial germ cells? Those primordial germ cells are available on the lining of yolk sac.

Hang on let me pull up a image. As you can see here the primordial germ cells are available on the lining of yolk sac. From here they will travel all the way from back of the abdomen through as you can see the dark blue portion that is where our indifferent gonads are.

So, the primordial germ cell from the yolk sac will travel from all the way over there to this dark blue spot in the urogenital ridge where they will either determine if the gonad is male which is testis or female which is oval. Now we need to know how the gonad develop into either testis or an ovary. For that we only need to focus on two things TDS and MIF.

It may sound complicated but it's really not. TDS simply stands for testis determining factor. It is self-explanatory.

TDS testis determining factor helps determine the indifferent gonad into testis and MIF is mullerian inhibiting factor which inhibits the production of ovary. Now let's go a little deeper so you can understand it. I can't just brush over this topic and just leave you wondering how it is formed and how it is formed and what happened.

For the gonad to be a testis it needs a Y chromosome. We don't have to dwell into the genetics that much. On the short arm of the Y chromosome, it has an SRY gene.

What does that SRY gene do? It gives you the TDS testis determining factor. Now TDS helps make testis and testis produces sertoli cell which then produces the MIF which is mullerian inhibiting factor. Now this MIF is the key to not only develop testis, this MIF inhibits the production of ovary which prevents the production of the female genital system and leaves the embryo with just one gonad system which is testis.

Now for the embryo to be a male it needs at least one Y chromosome. It can have more than one but if the embryo has at least one Y chromosome then it automatically translates into a male. Let's just say for an example when there is a Y chromosome it gives you SRY which produces TDF, TDF then gives you testis which produces sertoli cell and then which produces MIF but let's say in a certain situation we have all that until the MIF.

We have testis which produces sertoli cell but somehow if it does fail to produce the MIF then the embryo will have both genital system, both the male and the female genital system. This is one kind of abnormality which you should keep in mind when you are reading this chapter. Now the second topic for this chapter is meiosis.

During the process of Embryology Training in India, the importance of concepts such as meiosis increases, as they are required for the formation of gametes, genetic variation, and normal embryonic development. Concepts like these are very important for anyone who wants to pursue a career in reproductive medicine and clinical embryology. In Medline Academics, our Fellowship in Embryology programme helps learners understand complicated embryological concepts using well-explained lectures, visual learning, and clinically relevant discussions. With the help of both theoretical and practical knowledge gained from expert embryologists, learners become more comfortable in dealing with complex developmental concepts, which can be applied in ART laboratory practice.

Before we go any further, we need to know what meiosis is. You might have heard of mitosis but meiosis may be new so I will teach you the difference between mitosis and meiosis before we dwell any further. Now mitosis only involves one cell which divides and produces two cells whereas meiosis requires two cell division and then produces four different cell.

Now in mitosis, in mitosis the daughter cell, the two-daughter cell they are genetically identical whereas in meiosis the four-daughter cell are not genetically identical so they do share their DNA and they mix up which is meiosis not mitosis. Now this is the most important diagram of this chapter. We will go through this diagram three times.

We will teach you how the meiosis one and meiosis two occurs. The second time we will learn the differences between the male and female how it occurs differently in both male and female and on the third time we will learn the time differences when does meiosis one occur when does meiosis two occur in both male and female.

So let's go so let's try the first one how meiosis one and two occur. Now I taught you the difference between mitosis and meiosis. The type B spermatogonia as you can see here the spermatogonia and the oogonia they differentiate by the process of mitosis until they are that differential that they shift to the process of meiosis and that's when they are called primary spermatocyte and primary oocyte.

Spermatocyte in the males and oocyte in the female. One important thing to notice here is that we have 46 chromosomes and it is 2n which is a diploid. Now during meiosis one we have DNA replication.

Two different cells they mix with each other share the DNA and produces four different gametes. Now during meiosis one DNA replication as you can see here before we had 46 chromosome now we still have 46 chromosome but instead of 2n as you can see here we now have 4n. So after replicating that DNA we go through the process of synapses, crossover and as you can notice the difference here this the dark blue one and the light blue one they cross over they mix their DNA and they produce two different secondary spermatocyte or secondary oocyte.

As you can notice here this one has three-fourth of its DNA and the other one-fourth as you can see here the light blue is from the other one and the same here if the three-fourth one is light blue whereas the one-fourth is the dark blue which is the proof that they mix their DNA while producing secondary spermatocyte and secondary oocyte for both male and female. Another thing to notice here before we had 46 chromosome and 4n now we have 23 chromosomes and 2n this is important to notice as a human being a single person either male or female we only need 23 during our lives when the process of fertilisation happens that is when we need the 46 chromosome otherwise a normal single human being have only 23 chromosomes. Now once we have the secondary spermatocyte and secondary oocyte the process of meiosis II helps them divide through the process of cell division and which finally produces gamete, gamete as in either the sperm or the ovum.

We produces four gametes which is not the case in male and female it is different for both but we will go through it when we discuss it on the second time the difference between male and female. So as you can see here the process of meiosis through the cell division produces four gametes now it is 23 and 1n which is called haploid before it was called a diploid now it is a haploid it has 23 chromosomes and a haploid which is a testament that it is either a sperm or an ovum a single cell when it merges with another either a sperm or an ovum the different one from each other then we have 23 plus 23 we have 46 chromosome and 2n then we are back to this so after producing all of this we are back to this when either a sperm and ovum meet we are back to this chap again. Now going through this the second time we will learn how it is different in males and female how the sperm and ovum are different from each other and how the process of meiosis differentiates between this.

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Now let's go through this again we have 46, 2n, diploid, DNA replication, synapses, crossover, cell division is the same until this step until the secondary spermatocyte and secondary oocyte an important thing to notice here is let's just assume the dark blue portion of this cell is the cytoplasm what is the cytoplasm you may ask a cytoplasm in a cell is everything outside the nucleus so whether we have the mitochondria the Golgi bodies the nucleosome and everything outside the nucleus that is what we consider cytoplasm so let's just assume in a cell the dark blue portion is the cytoplasm. Now as you can see here the three-fourth part the cytoplasm the DNA the three-fourth part the retaining part is where the cytoplasm is let's just assume that so so in a female the three-fourth part of a cytoplasm is what we will take and is what we will go through the process of meiosis to with this the one with only less cytoplasm which only has a small amount of cytoplasm this is what we call polar bodies in a female it will be the same in male whether it is this or this the both are sperms and they both will go through the process of meiosis to but in female only this left side the one with majority of the cytoplasm that is the one which will go through the process of meiosis to and the one with only a tiny amount of cytoplasm that is the one which is called polar body which will go on and die off soon enough because it does not have enough cytoplasm and only the female can give the child all the cytoplasm so for the child to develop and for the child to have the mitochondria and produces the energy and everything it needs the cytoplasm which is only given from the mother and not the father so important thing to keep in mind now again in the process of meiosis to this let's just assume the right side this right branch does not exist in female when we go through the process of meiosis to we go through the cell division and then we are produced with two gametes now the same thing the majority of the cytoplasm is retained in one and just a minor amount of cytoplasm is given in the other cells so again the other one which will become a polar body will just die off and only one remaining gamete out of all four before only one remaining gamete is what is left in the female that is what our ovum is our ovum or a egg that is what goes through in the female and that is the only egg remaining from out of all four it will be sperms in all four of them it does not matter the cytoplasm and that that does not correlate in male will all four of them gametes will be sperms but in females only one of those gametes with the most cytoplasm is the one we will take everyone everything else is just polar body and they will just die off now that we have learned the difference between both male and female let's go through the time stems when does process of meiosis one occur and when does the process of meiosis two occurs in both males and females now the process of meiosis one in males only starts when a male hits puberty before that spermatogonia are divided through the process of mitosis and once they reach puberty their spermatogonia will divide through the process of meiosis one which is primary spermatocyte and after puberty is when the process of meiosis one starts in male but in female compared to the male the process of meiosis one starts even before they are born just five months into the fertilisation just five months into an embryo the process of meiosis one starts in a so the primary oocyte they just starts developing straight into the fifth month of embryo so now that we have process of meiosis one in males it is frozen into the first step until they hit puberty but in female one of each primary oocyte will go through the process of synapses each month they will go through the process of synapses crossover cell division and secondary oocyte and then they are arrested in that phase two now in males once the process of meiosis one has started it will just go through the process of meiosis one which continues to meiosis two after they have hit puberty up until their death so in males it is really simple they just go through the process of meiosis one then two they produces four gametes which are sperms and that's it it's done but in females the primary oocyte just goes through the process of synapses crossover one of them each month and they are in the stage secondary oocyte and they are arrested there each month they are arrested in the stage of secondary oocyte they will not progress further to the process of meiosis two until and unless they are fertilised until and unless they are mixed with a sperm they will not go through the process of meiosis two and if they are not fertilised they will just fall off each month which we call just periods in female now once it is fertilised the secondary oocyte is finally allowed to go through the process of meiosis two where it will follow the cell division and everything and produce a gamete which is an embryo so in female meiosis two is completed only and only if the secondary oocyte is fertilised. That is all from chapter one the development of human embryology.