Cell Division After Fertilization – Week 1 of Embryogenesis

by John McLachlan, PhD

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    00:01 Hello, and welcome to this lecture on the first week of human development, the pre-embryo stage.

    00:07 We'll be looking at the events from fertilization through to implantation.

    00:11 The first stage is on the fascinating journey of the human from egg to adult. The outcomes we’ll look at include looking at the early division stages, the cleavage stages as early parts of development and then we’ll look at implantation into the uterine wall, the uterus of the mother. We will touch briefly on one of the things that can go wrong, which is an ectopic pregnancy, and we’ll also look briefly at some reproductive technologies.

    00:39 So, let’s begin at the beginning. Here, we have a newly fertilized egg.

    00:44 Fertilization is covered in a separate lecture in this series. Now, this egg is large as human cells go, it is about 80 microns in diameter, about eight to ten times the size of a normal adult cell.

    00:58 When division takes place at these early stages, the cell is half in size at each division.

    01:05 So the two-cell stage is half the size of the egg itself, the four-cell stage is a quarter of the size, and so on until the cells reach something like the normal adult size.

    01:17 It’s important to understand that there’s no growth taking place at this time and the developing egg and the cells are surrounded by a protein coat called the zona pellucida. It’s an acellular, semi-rigid material surrounding the egg, and all of the divisions take place at these early stages inside the zona. At these stages, all the cells are still equivalent.

    01:45 So if I took a four-cell embryo and I divided up the cells into four individual cells, each one of them will be capable of giving rise to a full normal individual. They would be clones of each other, but at this stage, all of the cells are equivalent to each other.

    02:05 This is the same process, but here, illustrated by photographs from left to right. Here, you can clearly see the zona pellucida round the outside and the cells inside as division takes place, getting smaller at each division. After they formed a ball of cells, they become very compact and it becomes harder to see the individual cell boundaries, as you can see on the extreme edge of the picture. This ball of cells is actually becoming hollow and this stage is called the blastocyst.

    02:40 So, looking at this in the slightly larger view, you can see the blastocyst with a hollow space inside, but you can also see that there’s a concentration of cells which are forming inside the ball of cells. So the ball of cells as a whole is called the blastocyst, and that little group of cells on the inside is quite sensibly called the inner cell mass.

    03:06 Now, the interesting thing about the inner cell mass is it is the inner cell mass that will give rise to the baby that’s born. So only a few of those cells are actually going to give rise to the baby, and the other cells will contribute to the extra-embryonic membranes such as the placenta, and that’s also covered in another lecture in this series.

    03:29 The cells which form the shell of the blastocyst are called trophoblast cells. So these form an outer layer with the inner cell mass on the inside. Now, these actually look different from each other, but this difference is highly significant. The cells are divided into two different sorts for the first time. Now, you can recognize trophoblast cells from inner cell-mass cells. What will happen to these cells is very different. So these early distinctions between inner cell mass and trophoblast, as we’ll see, is highly significant.

    04:10 It’s a very important moment in your life as a whole.

    About the Lecture

    The lecture Cell Division After Fertilization – Week 1 of Embryogenesis by John McLachlan, PhD is from the course Embryology: Early Stages with John McLachlan.

    Author of lecture Cell Division After Fertilization – Week 1 of Embryogenesis

     John McLachlan, PhD

    John McLachlan, PhD

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