Spermatogenesis and Oogenesis: Introduction

by John McLachlan, PhD

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    00:01 Hi, there. Welcome to this lecture in which we’ll be looking at the development of the egg, that’s oogenesis, and development of the sperm, that’s spermatogenesis.

    00:10 So we'll start off with the primordial germ cells, and we’ll look at oogenesis and spermatogenesis, and I will touch on fertilization and what happens after fertilization as the fertilized egg is transported down the oviduct towards the uterus. Finally, we’ll briefly indicate some of the key things about in vitro fertilization. In the process of gametogenesis, which is a combined term for oogenesis and spermatogenesis, there are four major phases. The first phase is the origin of the primordial germ cells themselves and the reason which they migrate to the gonads. The second phase is the increase in the number of germ cells by mitosis.

    00:50 And the third is the reduction in chromosomal number by the process of meiosis.

    00:57 Finally, each egg and sperm will have to mature and develop into its final form.

    01:03 Now, of these four phases, the first is essentially identical in male and female embryos.

    01:10 The remaining three are very different between males and females. And when we talk about these processes, we’ll always put the female first because the baseline condition of humankind is female, and males are diverted away from female sex by complex developmental signals.

    01:29 So, let’s begin with the origin of the primordial germ cells. Now, perhaps, unexpectedly, they differentiate quite a long way from the body of the embryo. They first appear in the yolk sac at very early stages in the embryonic period. In this diagram, we can see the amniotic cavity at the top and the yolk sac below and the developing body of the embryo in between the two of them and primordial germ cells are marked by little red dots. At a slightly later stage, we have a cutaway view of the embryo, and we can see the gut in the midline, and the red dots show the path that the primordial germ cells are taking as they make their way towards the gonad. So they’re actually migrating as individual cells following signals within the tissues through which they’re moving to find the gonad itself. There, they go and populate the gonad. In this view, we can also see a section, and we can see how the primordial germ cells are migrating up through the dorsal mesentery of the gut, and then around to populate the gonad. In this diagram, we can also see kidney tubules, and these are part of the mesonephric system. If we follow that tubule around, it terminates into a mesonephric duct.

    02:49 Here’s a stereoscopic view of the same process taking place. So you can see that, of course, it’s identical in both sides of the embryo and also, as when the primordial germ cells reached the developing gonadal primordium, at that time, there are also cords that have grown in from the surface of the gonadal ridge itself. Now, of course, the gonadal ridge is a mesodermal structure, so the coelom is covered in what’s called a “mesothelium”.

    03:19 It’s not a true epithelium. It’s merely a condensed form of mesoderm on the outside.

    03:25 But nonetheless, the cords are still growing into the underlying mesoderm. And the primordial germ cells will take up associations with those primary sex cords.

    03:37 Now, the number of germ cells varies considerably during the course of development. If we look at females, first of all, we can see that the number increases very rapidly up to about seven million or so, and then it falls equally rapidly. Most of the primordial germ cells will, in fact, die in females before birth. Thereafter, the numbers steadily diminish, and at the time of the menopause, there’ll be effectively no normal primordial germ cells left. Here’s a diagram which outlines the process of oogenesis. We can see that the original oogonium will give rise to the primary oocyte, and then it will enter these two meiotic divisions. But it will not complete these divisions until long after birth. Indeed, the second meiotic division is not completed until fertilization.

    04:30 So we can see that studying this in the abstract, looking at this diagram on its own, it’s actually kind of missing the important information. So, first of all, we have to add the background of the developing follicle. In this diagram, this is what we do. We’re looking at the developing primordial germ cell, but we now add it in the cells of the follicle.

    04:53 We can see that it goes through stages such as the primordial follicle, and then after birth, we have the primary follicle. After puberty, we’ll move on to the secondary follicle, and in all of this as it’s not completed the first meiotic division.

    05:10 Following through to later stages, we can see that the tertiary follicle, the third class of follicle, will go on to give rise to the ovulated egg itself, and then fertilization may take place. So the second meiotic division is not complete until this point. But just as we looked at in the context of the follicles, so we also have to look at in the context of the ovary. In this diagram, moving around clockwise from the blood vessels, we can see primary follicles steadily growing, becoming secondary follicles, tertiary follicles becoming mature, becoming ripe, and then the ovum actually hatching from that follicle. If we’re to follow the follicle, in time, they would now become the corpus luteum, which is involved in the signaling about pregnancy. And if we are to look at the fertilized egg which is released, then it may be fertilized up towards the ampulla of the oviduct, and they’d have to make its way down the oviduct subsequently.

    06:14 This is a picture of a mature follicle in scanning electron micrograph. So it’s rather nicely, the arrangement of cells within it. And you can see that the egg cell itself is relatively large and is surrounded by cells derived from the follicle, and these are corona radiata cells. Let us now look at the spermatogenesis, and again, it will form from the spermatogonium, primary spermatocyte, and then enter the two meiotic divisions, which will finally give rise to spermatids, and these would differentiate into spermatozoa. This process is not confined to one particular time in males, and will, in fact, continue through adult life.

    06:58 This illustration shows that process of maturation as it moves to become the definitive spermatozoon.

    07:05 You can see a number of different changes that will take place. So a flagellum will develop.

    07:10 An acrosome will appear at the tip, and the acrosome will be involved in actually mediating the fertilization process until finally, the adult free-swimming sperm is fromed.

    About the Lecture

    The lecture Spermatogenesis and Oogenesis: Introduction by John McLachlan, PhD is from the course Embryology: Early Stages with John McLachlan.

    Author of lecture Spermatogenesis and Oogenesis: Introduction

     John McLachlan, PhD

    John McLachlan, PhD

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    nice use of diagrams
    By opeolouwa o. on 14. July 2018 for Spermatogenesis and Oogenesis: Introduction

    he made mention of certain important words that with a nice aid of diagram