Formation of the Primitive Streak – Neurulation

by John McLachlan, PhD

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    00:01 Hello, and welcome to our lecture on neurulation, formation of the neural tube and some of its derivatives. What we’re going to look at in this lecture is the formation of the neural tube itself, the early stages of brain development, and some defects that can arise during the formation of the neural tube. As so often, we have to go back to the very early stages of gastrulation, where in the diagram at the top, you can see the amniotic cavity, below that, you can see the yolk sac, and in between the two, is the bilaminar disk. And if you were inside the amniotic cavity looking down at the floor, then you would see a streak beginning to form. That streak is the marker for where the neural tube is eventually going to develop in the embryo. What happens is that the ectoderm will rise up in either side of that primitive streak and begins to fold towards the midline. And as it folds towards the midline, so the cells at the very tip are seen as a neural crest cells and those will be different during subsequent development. This is something all the way along the length of the embryo, as we shall see. And when the folds meet each other, some neural crest cells are left behind as a separate and detached tissue but the neural tube is forming a discrete structure underneath the ectoderm running the whole length of the embryo. If we’re to look at that from above, so again, imagine we’re in the amniotic cavity looking down, you can see in the middle of the 20-day picture on the right hand side that the two edges of the ectoderm are beginning to approach each other, ready to fuse, round about the mid position in the embryo as we see at the moment. The fore part of the neural tube remains wide open, and this is where the brain is going to develop. A little later, we can see that in the middle, the neural folds have actually met and fused and what’s going to happen now is it’s going to zip up, as it were, both towards the head and towards the tail and as it meets, so it will fuse extending forward and backwards towards the tail. Incidentally, the square structures that you can see on either side of the neural tube are in fact mesodermal. These are the somites, and they will grow around the neural tube in time to form the vertebrae which will protect the neural tube as it moves on its way to becoming the spinal cord in the adult. You can see at the fore part of the brain labeled anterior neuropore, that fusion is delayed here, but also that the neural tube is beginning to broaden and become more specialized, marking out where the brain is going to form.

    02:53 It’s always convenient to look at a picture of an embryo. And in this scanning electron micrograph of a human embryo seen from above, we can see the brain at the top end, up at the top, and already fused at the anterior neuropore. The spinal cord is running down the center, and the somites in the mesoderm are in either side. The sac underneath the embryo is the yolk sac. Now let’s look at the same image from this side. What we can

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    The lecture Formation of the Primitive Streak – Neurulation by John McLachlan, PhD is from the course Embryology: Early Stages with John McLachlan.

    Author of lecture Formation of the Primitive Streak – Neurulation

     John McLachlan, PhD

    John McLachlan, PhD

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