Well, it’s 2011. Know what that means? It means it’s time to get back to SCIENCE!
(Yes, I know, that makes absolutely no sense, but let’s get back to science anyway, shall we?)
Near the end of last year, I made some sort of statement about wanting to write a post about Polycystic Ovary Syndrome in trans men. As I prepared to do just that, I realized that in order to do it well, I would have to explain how ovaries normally function before I can talk about how they abnormally function. And then I realized, that in order to do that, I would need to talk about their structure and development first. And in order to do that, I would need to talk about differentiation.
So, that’s where we are — at the beginning, or, in other words, where ovaries come from.
I’ve talked about this a little bit in another post, but I’m going to go back even earlier in embryonic development this time around, back before the gonads are even noticeable in the embryo.
The whole story is really quite interesting, at least for a geeky reproductive biologist like me anyway. I hope it is for you as well. And so, read on for the details…
In the early embryo, the undifferentiated primordial gonads (ovaries and testes) are said to be “indifferent” because they have no morphology that is different between XX and XY embryos. The primordial gonads are simple, raised streaks of tissue called “genital ridges” and appear in human embryos at 35 to 42 days of embryonic age (Jirásek, 2003).
At this stage, the primordial gonads are bi-potential in that they can become either ovaries or testes, depending on the genetic signals that drive the differentiation pathways of sex determination.
Back in the day, people thought that sex determination was simple. Mammals with a Y sex chromosome would develop into males and those without would develop into females. We know now, however, that things are more complicated than that. I will quote a scientific mini-review as it explains this topic very well (Nef & Vassalli, 2009):
“Mammalian embryos, like those of most mammals, are initially sexually undifferentiated and can develop into either male of female individuals following one of two alternative processes. The paternal transmission of a Y chromosome triggers testicular differentiation, whereas the presence of a paternal X chromosome pushes gonads towards ovarian differentiation.”
I will talk about the underlying complexity of this system in a bit. But first, a little more about the developing gonad.
As mentioned, the embryonic indifferent gonads are raised streaks called genital ridges, appearing at 35 to 42 days of age. These ridges are located medially (i.e. toward the mid-line) of the mesonephric ridges, which will later develop into parts of the reproductive ducts, for example the vas deferens (Tanagho, 2000; Jirásek, 2003). The vas deferens is found in mammalian males and is the duct that delivers the sperm from the testis to the urethra of the penis during ejaculation. It is also the duct that is cut during a vasectomy.
What’s missing from the genital ridges at this stage are germ cells, which are the cells that will eventually become sperm in the testes and eggs (ova) in the ovaries. The story about the germ cells is very interesting in that they do not arise from the embryonic gonads. In fact, they do not come from the embryo at all. They arise outside of the embryo, from the primary yolk sac, at about 13.5 days of embryonic age, or perhaps even earlier (Jirásek, 2003).
Now why in the world would these cells have an extra-embryonic origin, meaning an origin that is outside of the embryo? This answer has two parts.
First, these cells, which are called “primordial germ cells” (PGCs) at this stage, are undifferentiated cells. They must become young sperm or egg cells and maintain their “pluripotent potential.” In other words, they must remain unprogrammed to an extent that allows them, when joining with their counterpart from the opposite sex, to develop into every single cell type in the body. Once sperm and egg unite at fertilization and become the very first cell of the embryo, it has the potential to develop into anything in the body: lung, muscle, brain, even other germ cells. In order to do that, the PGCs must remain undifferentiated.
Second, if the PGCs were to originate within the embryo, within the early gonads, they would be surrounded by developing tissues that are are awash in morphogens. As explained in an earlier post, morphogens are secreted in the developing embryo and orchestrate differentiation and development of tissues. If the PGCs were to arise from somewhere inside the embryo, they would be exposed to these morphogens and would not be able to maintain their undifferentiated state. Instead, they come from outside the embryo and can avoid the effects of the morphogens.
When the PGCs develop in the yolk sac, there are only a handful of cells. They must migrate from outside the embryo through the connecting stalk (where the embryo is connected to the placental membranes), past the developing gut and to the genital ridges which they subsequently colonize. As they make this trek, they proliferate, undergoing multiple rounds of division so that by the time they reach the genital ridges, they are 1000-2000 strong. Those that get lost along the way and take up residence in non-gonadal tissues eventually degenerate from a lack of proper and specialized support (Jirásek, 2003).
The mechanisms behind this migration by the PGCs are discussed in an interesting review by Tarbashevich and Raz (2010) who describe the speeds of the migrating PGCs as reaching 140 microns per hour! That’s zippy fast for a cell, and is comparable to the speeds reached by metastasizing cancer cells. (As a benchmark, the width of a human hair is about 100 microns.) At left is a mobility model for PGC migration taken from studies with zebrafish that shows the theory of how PGCs are able to move around so well and so quickly. (Click image to enlarge).
Development of the Ovaries
Now that all of the components of the ovaries are present when the PGCs arrive at the genital ridges, the ovaries themselves can develop. There are two phases of human ovarian development: pre-follicular and follicular. First, a little information about the terminology.
A follicle is the functional unit of the mature ovary. It consists of an egg (also called an ovum) that is surrounded by specialized cells that nourish the egg, protect it and help it mature. The follicle, after puberty, also has other functions, such as production of steroidal hormones, like estrogen, and ovulation of the egg into the oviduct.
At approximately 5 weeks of age in the human embryo, the PGCs colonize the outer half of the gonal ridges, forming the early ovaries. The PGCs proliferate and increase in number, again, concentrating in the outer half of the embryonic ovary, and form “sex cords” at about week 10. That part of the ovary becomes the “cortex”, whereas the inner part of the ovary, the “medulla,” is where blood vessels grow and forms at about week 15. It’s at that stage that the germ cells proliferate a second time, which causes the sex cords to grow, and there is differentiation of the PGCs into oogonia (reviewed in Jiménez, 2009).
Soon thereafter, at approximately 20 weeks of age, the ability of the germ cells in the ovary to divide and proliferate is exhausted and they differentiate to become oocytes. There are approximately 5-6 million oocytes in the ovaries at this point (Jirásek, 2003; Jiménez, 2009). They enter meiosis, which is the process by which germ cells divide and lose half of their chromosomes. Meiosis only occurs in germ cells, not in somatic cells, and results in two daughter cells that carry one of each of the pairs of the 46 human chromosomes.
That’s one way that genetic diversity occurs, as the pairs of chromosomes, one from the mother and one from the father, split up into the daughter germ cells in an almost infinite number of combinations. However, I should note that the germ cells only enter meiosis in the embryonic ovary. They do not complete it. But more on that later.
So by approximately 28 weeks in the fetal stage, most of the follicles are present and are no longer organized in cords. The follicles form when somatic cells (i.e. cells that are not germ cells) aggregate in a single layer around the oocytes in the embryonic ovaries. At this stage, they’re known as “primordial follicles” and they reside in mostly the cortex, although some can be found deeper in the embryonic ovary, closer to the medulla which has grown and become even more vascularized with blood vessels. In this phase of development, the layer of cells around the oocytes differentiate into granulosa cells which communicate with the oocytes and stop meiosis in the immature eggs.
By the late fetal stage, the human ovary is stocked with 5 to 6 million oocytes that are arrested partway through meiosis and reside in primordial follicles that are made up of a single cell envelope of granulosa cells. By the time of birth, the number of oocytes has dropped to approximately 2 million, falls again to around 400,000 by the age of 20 years and continues to decrease until they are gone, resulting in menopause. The dogma is that at birth, females of most mammalian species are born with a finite number of eggs and cannot generate new ones.
This dogma has been challenged relatively recently by scientists who found that germ stem cells in the mouse ovary not only existed but also contributed to development of new follicles (Johnson et al., 2004), and were derived from bone marrow stem cells that had been delivered to the ovary by the circulation (Johnson et al., 2005). This new theory is controversial, with scientists lining up on either side of the debate as to whether the data from mice are valid (Notarianni, 2011) and can be applied to humans (De Felici, 2010).
Development of the Testes
As mentioned above, development of the embryonic gonad, whether it is to become an ovary or a testis, begins the same way in females and males, with PGCs migrating to the indifferent genital ridge.
In human embryonic males, gonadal development diverges from that of embryonic females at approximately 42 to 45 days when the PGCs come in contact with the somatic cells that will eventually become Sertoli cells of the testis. Just as granulosa cells become the supporting cells for the eggs of the ovary, the Sertoli cells become the “nurse cells” of the developing sperm in the testis.
In the embryonic stage, the undifferentiated somatic cells form sex cords that become populated by the PGCs. When in contact with each other, both cell types differentiate so that the somatic cells become embryonal Sertoli cells which interact with the germ cells, prohibiting them from entering meosis that normally occurs in the embryonic ovary. Instead, the germ cells, now called spermatogonia, acquire the ability for limitless proliferation when supported by the Sertoli cells, a characteristic that allows continual production of sperm cells in the adult male, which is in contrast to the finite development of eggs in the adult female (Jirásek, 2003).
At approximately 60 days of development, Leydig cells appear in the “interstitial” areas between and among the sex cords. The Leydig cells produce testosterone in the fetal testis which is required for sex-based differentiation of the brain and development of the male external genitalia via conversion to dihydrotestosterone (also described in an earlier post). [After birth, the Leydig cells disappear until puberty.] In the fetal testis, the sex cords develop into the seminiferous tubules, which are populated by the slowly dividing spermatogonia and the supportive but not yet mature Sertoli cells.
The Next Chapter
In this post, the focus has been on the origin of the embryonic gonads and their differentiation and development into the ovaries and testes. In the next post, I will talk about the genetic mechanisms behind these developmental steps, which will ultimately lead to a discussion about Polycystic Ovary Syndrome in trans men.
De Felici M, 2010. Germ stem cells in the mammalian adult ovary: considerations by a fan of the primordial germ cells. Mol Hum Reprod 16:632-636.
Jiménez R, 2009. Ovarian organogenesis in mammals: mice cannot tell us everything. Sex Dev 3:291-301.
Jirásek JE, 2003. Normal Sex Differentiation. In: Gynecology and Obstetrics, Sciarra JJ, ed, Vol 5, Chapt 77.
Johnson J, Canning, J, Kaneko T, Pru JK, Tilly JL, 2004. Germline stem cells and follicular nenewal in the postnatal mammalian ovary. Nature 428:145-150.
Johnson J, Bagley J, Skaznik-Wikiel M et al., 2005. Oocyte generation in adult mammalian ovaries by putative germ cells derived from bone marrow and peripheral blood. Cell 122:303-315.
Nef S & Vassalli J-D, 2009. Complementary pathways in mammalian female sex determination. J Biol 8:74.
Notarianni E, 2011. Reinterpretation of evidence advanced for neo-oogenesis in mammals, in terms of a finite oocyte reserve. J Ovarian Res, Jan 6;4(1):1. [Epub ahead of print]
Tanagho, 2000. Embryology of the genitourinary system. IN: Tanagho & McAninch (eds), Smith’s General Urology, 15th Edition, Lange Medical Books/McGraw Hill, New York.
Tarbashevich & Raz, 2010. The nuts and bolts of germ-cell migration. Curr Opin Cell Biol 22:715-721.