Fertilization – Spermatogenesis and Oogenesis

by John McLachlan, PhD

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    00:01 Now, let’s look at the process of fertilization itself.

    00:04 This is a tricky business for the sperm to accomplish. When it arrives at the egg, as we’ve seen, the egg is surrounded by the corona radiata cells. So the sperm has to push its way between these cells in order to reach the surface of the egg. That surface is represented by an acellular layer called the “zona pellucida”. So the sperm has to eat its way through the zona in order to approach the egg. Once through the zona, it finds itself in the perivitelline space, and only then finally, can attach to the plasma membrane of the egg and make its way inside the egg itself.

    00:45 Now, this demanding process is one that can sometimes go wrong. So in in-vitro fertilization techniques, as part of assisted reproduction, it may be necessary to help this sperm through the corona, indeed through the zona, and perhaps even through the plasma membrane.

    01:03 We’ll illustrate that later. Here we see the process of fertilization having taken place and since we've no [inaudible 00:08:37]to bring together our egg and sperm, the second meiotic division of the egg can also take place. Once the sperm has entered the egg, a series of signals make the zona pellucida impermeable and ideally, what will happen is that if one sperm has reached the egg and fertilized it, it then becomes impossible for other sperm to enter the egg subsequently. Normally, what will happen is that the pronuclei from the sperm and the egg will fuse together restoring the full chromosomal number and cell division and development can take place. However, if two sperms move to fertilize the egg simultaneously, we then have an excess of the number of chromosomes that we need. In fact, we have three half sets as it were. What will normally happen is that the egg will expel one set, and if the surviving set represents a sperm and an egg set, development will continue as normal.

    02:17 However, if the expelled set is the original maternal egg set, then development does not proceed as normal, and a variety of abnormalities can result. Obviously, if both sperms were carrying a Y-chromosome, you would not expect development to continue. But if one sperm is carrying an X and one sperm is carrying a Y, then there would seem to be no obvious reason why you would not have a normal complement of genes.

    02:46 The fact that development does not proceed normally, under those circumstances, indicates that the genes themselves are modified by their passage through the germline.

    02:57 In this process, only a nucleus which has passed through the female germline and the sperm which has passed through the male germline will be capable of giving rise to normal development.

    03:11 Once fertilization has taken place up at the ampulla of the oviduct, then the fertilized egg is wafted down the oviduct towards the uterus, and the cells are dividing as they go. No increase in size is taking place. So at each division, the cells of the developing pre-embryo get smaller, halving in the size of the first division, becoming quarter size at the second division and so on. By the time it reaches the uterus, it has formed first a ball of cells, and then a hollow ball called the “blastocyst”. Inside the blastocyst is a large group of cells called the “inner cell mass”.

    03:52 Once the stage has been reached, implantation can take place into the prepared uterine lining.

    03:59 If the embryo hatches from the zona prematurely, then it may implant in the wrong place, for instance, in the oviduct itself in what’s known as an ectopic pregnancy.

    About the Lecture

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

    Author of lecture Fertilization – Spermatogenesis and Oogenesis

     John McLachlan, PhD

    John McLachlan, PhD

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