Oogenesis

00:01 The process of oogenesis is similar in some ways but very different in others.

00:06 We start off at the same basic process by which primordial germ cells will become an oogonium.

00:13 From there, we're gonna go two further rounds of mitosis to produce a primary oocyte and at that point we will enter meiosis and this is where the process of oogenesis and spermiogenesis are going to diverge.

00:27 As the first round of mitosis proceeds, we're gonna produce a secondary oocyte and a polar body.

00:34 So one primary oocyte will produce one secondary oocyte and a very small residual polar body which is not gonna be useful in further mammalian reproduction.

00:46 The second round of meiosis, when it's completed will produce a mature ovum and another set of polar bodies.

00:54 So the first polar body will produce two and the secondary oocyte will produce one polar body and the ovum.

01:01 The timing of meiosis involved in oogenesis is very complicated and it's tied to development of the egg within a structure called the follicle.

01:11 Early on in development, XX germ cells are gonna begin the phase of meiosis but arrest in prophase.

01:18 So they're not gonna even get done with the first step of meiosis until well later.

01:23 At this time, they are within the ovary and they're surrounded by a group of cells called follicular cells.

01:29 Initially, these follicular cells are very thin and just surround and sustain the ovum, and as they get signaled to mature they're going to become a little bit more cuboidal and transition from a primordial follicle to a primary follicle.

01:44 With a little bit thicker cell lining around it, those follicular cells will sustain the ovum and also anchor it within the ovary.

01:53 Now roughly 400,000 oocytes remain arrested in prophase, the entirety of a young woman's life until puberty, and at that point these primary follicles can begin developing further and be shed one or two at a time.

02:08 Roughly 500 will be ovulated during life.

02:11 As the primary follicles get a little more mature, the follicular cells around them proliferate and become stratified, so in multiple layers of follicular cells and these maturing primary follicles are gonna be those that are responding to the hormonal signals present throughout the menstrual cycle.

02:29 As this is occurring, the follicular cells secrete a protective kind of coating of glycocalyx around the ovum.

02:38 This is called the zona pellucida and will be important in protecting the egg as it migrates down the oviduct and may or may not become fertilized.

02:48 For each menstrual cycle, several primary oocytes will start developing, grow larger, get a more stratified follicular coating and as that proceeds, the follicular coating gets larger and larger and will eventually become called a set of granulosa cells.

03:05 Now a lot of these embryologists keeping track of names changing over time so we're gonna follow these follicular cells to granulosa cells and a few more terms that are applied to them as this process proceeds.

03:17 So here we can see that the granulosa cells have proliferated to the point where they can no longer sustain themselves by simply having nutrients diffused to inform them, so we have small vacuoles starting to form within this secondary follicle.

03:33 As that proceeds, these lacunae or little fluid-filled sacs are going to connect, get bigger and surround the egg on one side.

03:42 And this is a secondary follicle and we can see here a more mature secondary follicle with a large open cavity called the follicular antrum and the coating of cells around the egg.

03:54 Now the coating of cells that surround the egg but keep it close to the follicular antrum are specifically called the cumulus oophorus, so that's one of the specialized terms we're gonna apply to the cells around the egg as they get more and more distinct from one another.

04:10 Prior to ovulation, the follicular antrum is gonna get bigger and bigger and at his point we call the entire assemblage of follicular cells oocyte and connective tissues surrounding it, the Graafian follicle.

04:25 We're gonna have a couple subdivisions of connective tissues surrounding these follicular cells the theca externa, which anchors it to the egg, and the theca interna that's gonna excrete hormones that affect the oocyte's development and make it capable of being fertilized if it is ovulated.

04:43 During each menstrual cycle, several follicles kind of race with each other to develop to this Graafian stage, but typically only one will actually be released from the egg into the oviduct and at that point the follicular antrum ruptures and the egg moves out into the reproductive tract carrying the cumulous oophorus and the zona pellucida with it.

05:06 Thank you very much and we'll follow up a little further along the menstrual cycle and see how the uterus changes to accept the egg if and when it is fertilized.