Neurulation – Week 2 of Embryogenesis

by John McLachlan, PhD

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    00:01 Once the main body axis had been laid down by the primitive streak, the next process is the formation of the neural tube, what will be the spinal cord in the adult.

    00:11 What happens is that the ectoderm on either side of the midline begins to rise up around the groove in the center. Again, we’re looking at a cross-section across the axis of the body of the embryo. Here, we can see the folds beginning to swing up, and we’ve also marked the cells at the very tip in dark green, and those will be neural crest cells. So as the folds are beginning to approach each other, then the cells at the very tip or the neural crest cells, gradually, the system is beginning to form a tube running the length of the body, hinging about a point in the middle. The folds begin to approach each other.

    00:54 Eventually, they will meet, and where they meet, they will fuse together. The cells that we’ve marked as the neural crest cells in dark green are left as a separate isolated little group of cells and as we’ll see later, these are then extremely interesting developmental history of their own. They will give rise to a whole range of different kinds of tissue. But together, what is done is to form a fusing tube running the length of the embryo, the neural tube or future spinal cord. Neural crest cells marked here, neural tube marked in the diagram as well. Now, looking at that from above, again imagine that we were swimming in the amniotic cavity and looking down at the floor of it, we can see that this process is not quite as simple as the initial diagrams would suggest. In particular, towards the head end, it remains fairly wide and open, and that’s where the brain is going to form. The folds are coming together fast roundabout the middle. They will meet in the middle, and then they’ll begin to, as it were, zip up in either direction. So moving towards the head and moving towards the tail. In the diagram in the bottom right, you can see that this process is largely complete down towards the tail end, but is still taking place up at the head end where the brain is beginning to form. So the folds coming together, first of all in the midline and then zipping up towards the head and towards the tail. So, fusing, as it were, up towards the head and tail in each direction.

    02:31 You can also see little blocks of tissue on either side. Now, these are not actually in the ectoderm itself, they are in the underlying mesoderm. So the little cubes of mesoderm forming on either side of the ectoderm derived neural tube. You can probably guess what these are going to contribute to. They will contribute to the vertebrae. This is the segmental structure of the vertebrae that form the spine, although as we’ll also see in another lecture, they give rise to a wide variety of other kinds of tissue as well. We’ve marked here the posterior neuropore, and that’s just the opening at the tail where fusion is not quite completed, and at the forward end, the developing brain, the anterior neuropore, and that’s where the brain itself will begin to form. Again, there is a separate lecture devoted to the formation of the brain. So, just revising that with the labelling on the diagrams, you can see where the primitive node and primitive streak were, and the folds beginning to rise up on either side coming together to meet in the midline, and then the fusion extending towards the head and extending towards the tail. You can already imagine, I’m sure, that this process might fail, it might not be complete at various points, and that will cause a variety of birth defects, which we’ll look at a little later within this lecture itself.

    About the Lecture

    The lecture Neurulation – Week 2 of Embryogenesis by John McLachlan, PhD is from the course Embryology: Early Stages with John McLachlan.

    Author of lecture Neurulation – Week 2 of Embryogenesis

     John McLachlan, PhD

    John McLachlan, PhD

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