Early Stages of Development – Embryology

by John McLachlan, PhD

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    00:02 So let us go back to your early stages of development.

    00:05 Cell division takes place after fertilization and in these early stages, there was no growth.

    00:11 So every time the cells divide, they get smaller. So from one cell to two cells, they are half in size, at four cells they are a quarter of the size and they go on dividing until they've reached the normal size for a human cell. As they do so, they form a ball and this is called the morula and then that ball would become hollow and this stage is called the blastocyst. And the blastocyst is not a simple hollow ball like a tennis ball. There is a little cluster of cells when it is inside and that little cluster is called the inner cell mass.

    00:46 So the inner cell mass is marked in the diagram. The outer cells are trophoblast cells and this is not just the difference in naming. Something extremely profound has happened here because the baby that will be born, will come from just the few of the inner cell mass cells and all of the other cells that are present including especially the trophoblast cells will contribute to the extraembryonic membranes such as the placenta. So this first division into two different kinds of cells is actually really important and therefore understanding how this takes place might be a valuable clue to understanding the process of differentiation of one cell from another. So looking at these stages again, two cells to four cells, eight cells and an increasing numbers all the way to the hollow ball of cells, the blastocysts with the inner cell mass pairs to one ball within the hollow mass of the blastocyst. And an extremely clever experiment by Prof. Chris Graham explained how this may happen. What to do? Just to take some labeled cells from an embryo which are most [inaudible 00:10:46] and you took a morula and placed the labeled cells in different positions within the morula and what he found was that the cells he placed in the center of the mass will subsequently contribute to the inner cell mass. In other words, they will be the one that contributed to the baby. Cells that he placed at the edge of the ball cells, will contribute to the trophoblast. In other words, they would contribute to the placenta.

    02:31 And what this means is, the cells that responded to their position by changing what they were turned into. So our question of what informs the DNA, in this case, turns out to be a positional signal reflecting where our cell lies with respect to all the other cells round about it.

    02:55 So that key distinction, that important decision about inner cell mass versus trophoblast whether it's going to be part of the baby or part of the placenta came from a positional signal and that is a valuable clue to understanding the processes that underlie all differentiations.

    About the Lecture

    The lecture Early Stages of Development – Embryology by John McLachlan, PhD is from the course Embryology: Early Stages with John McLachlan.

    Author of lecture Early Stages of Development – Embryology

     John McLachlan, PhD

    John McLachlan, PhD

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    By Shaheen S. on 08. January 2019 for Early Stages of Development – Embryology

    His teaching method is very nice- he makes you think why it is the way it is.

    By Zack M. on 15. August 2018 for Early Stages of Development – Embryology

    Succinct and informative. Explained the importance of positional relationships well.