Embryo Folding and Regionalization of the Brain – Skull and Brain Development

by John McLachlan, PhD

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    00:00 Now, let’s look at that process from the side. We can see the developing neural tube in blue, and towards the head end, we can see that this is enlarged where the brain is beginning to form. During this time, the amniotic cavity is enlarging and beginning to sweep around the embryo. Another way of looking at the same thing is to imagine the embryo is rising up into the amniotic cavity. So if we were to look a little later, we can see that the embryo is beginning to flex into a C-shape. And the front part of the neural tube is becoming increasingly specialized as it develops different regions of the brain.

    00:41 Looking a little later still, we can see that this curve is well marked and that the brain is really beginning to show regional specializations which will shadow its future outcomes for parts of the brain.

    00:54 If we were to look at the brain in isolation, and here it’s been left-right reversed, we can see that there are three recognizable regions in the brain, which can be spotted at this time. At the frontend, there is the prosencephalon, little further back is the mesencephalon, and behind that again is the rhombencephalon. And that leads into the spinal which continues on down the length of the embryo. In addition, we can also see some cranial and spinal sensory ganglia beginning to form along the sides of the neural tube in the different regions of the brain. The marked bend is called the cephalic flexure, or if you prefer soft Cs, then the cephalic flexure, but it’s a Greek word in origin, so I think I would stick with cephalic. And this marked bend here is very characteristic of the early stages of brain development. A little later still and we can see that further regionalization has been taking place. So the forebrain, the prosencephalon, has given rise to two regions, the telencephalon from which the cerebral hemispheres will later develop, and the diencephalon which includes the optic cups. These are outgrowths from the brain growing out on each side. They will grow towards the ectoderm. When they meet the ectoderm, they will induce the ectoderm to thicken and form the lenses of the eyes, and the optic cups themselves will become the retina. Little further back, the mesencephalon remains relatively simple and undistinguished as a simple tube. But further back again, the rhombencephalon, the hindmost part of the brain, will give rise to the metencephalon and the myelencephalon. We can also see that the ganglia are continuing to develop on the increasing complexity. Now, we’ve described what happens, but of course it is of interest to know how this actually happens. And if we’re to visualize in this nominal diagram here, a view from the side, we can see that the developing neural tube is underlined by the notochord, by the prechordal plate and by anterior visceral endoderm, and this signals to the neural tube above it in particular ways, and activate particular patterns of gene expression. In the lower diagram of the two, we can see a later stage, not incidentally that this is not colour-coded to match. We can see that the brain is regionalizing.

    03:28 In particular, we can see the rhombomeres in the hindbrain labelled R1 through to R7, and these structures are extremely important in the early development of the brain. If we’re to look at the transverse section through the spinal cord, we can see that not only is the brain regionalizing grossly as seen from the outside, but there are also increasing specializations taking place inside the brain itself. So, for instance, the ventral part will give rise to motor nerves, and the dorsal part will give rise to sensory nerves.

    04:03 This will grow out through the body, generally between the somites to begin with, and the motor nerves will grow out first and the sensory nerves will subsequently piggybank their way along them. And these will grow out to identify their final targets and give rise to the nervous system in the body.

    About the Lecture

    The lecture Embryo Folding and Regionalization of the Brain – Skull and Brain Development by John McLachlan, PhD is from the course System-Specific Embryology with John McLachlan.

    Author of lecture Embryo Folding and Regionalization of the Brain – Skull and Brain Development

     John McLachlan, PhD

    John McLachlan, PhD

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