Embryo Folding and Regionalization of the Brain – Neurulation

by John McLachlan, PhD

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    00:01 Now let’s look at the same image from this side. What we can see is that the embryo is beginning to fold. The head end is beginning to bend over as the brain begins to form, so as the tail. In this process, it’s associated with the amniotic cavity sweeping round the embryo as the embryo rises in to the amniotic cavity, which is of course filled with amniotic fluid. So the brain is developing at the fore end, folding round. At slightly later stage, we can see that regional structures are beginning to develop in that brain at the front. At the hind end, it extends all the way towards the tail bud. And nervous tissue, at the moment, is running all the way down towards the tail.

    00:47 A little later still, we can see that this process of brain specialization is continuing, and distinct regions are recognizable in the brain. We should look at those in a second.

    00:59 So, we can see that the brain could be roughly divided into three major areas. The front part is called the forebrain or prosencephalon. The midbrain is called the mesencephalon, and the hindbrain is called the rhombencephalon. Within each of these regions, additional structures are beginning to develop. So in the prosencephalon, we can see a vesicle beginning to grow out.

    01:23 There’s one on each side of course, growing out towards the overlying ectoderm.

    01:28 This is the optic vesicle which is later going to form the retina. When it reaches the ectoderm, it will induce the ectoderm there to thicken, and eventually, sink under the surface to form the lens of the eye. Then the ectoderm, which remains overlying it, will become transparent and form the cornea. In the hindbrain in the rhombencephalon, there we can see various structures developing, and these are the cranial nerves. These are beginning to grow out from that particular region and that also takes place down in the spinal cord. A little later still, the brain is further subdivided. So, the forebrain, prosencephalon, has given rise to regions called the telencephalon and the diencephalon. Mesencephalon continues as midbrain, as mesencephalon, but the rhombencephalon has given rise to the metencephalon and the myelencephalon. Incidentally, you may hear these words pronounced with a soft C as myelencephalon.

    02:37 But since it’s a Greek word in origin, I preferred the harder sound of the two.

    02:42 But we can also see that the ganglia in the hindbrain and in the spinal cord are beginning to develop, and nerve cell is growing out from those towards various structures in the body. Now, we’ve described what happens, but we haven’t looked at why it happens. What’s taking place during the course of development is that there are signals which are being sent to the developing neural tube, particularly in the frontend, and these signals bring about expression of particular gene patterns, and these gene patterns determine the different regions of the brain.

    03:20 So for instance, in the top image, we can see that the underlying notochord is signaling up to the neural tube, and so is an area called the prechordal plate and the anterior visceral endoderm signaling up towards the forebrain, and bringing about particular specializations in these regions. In the lower image, incidentally, the lower image is not colour coded to the image above it. We can see that this specialization is represented by formation of structures within the brain. So, various areas of gene expression can be detected, and these are influenced by signals from the surrounding tissue. This is also true if we looked at the neural tube or spinal cord cut in section. So it’s well known that there are a variety of different regions within the spinal cord. For instance, the lower part, the ventral part gives rise to motor nerves, and the dorsal or upper part gives rise to sensory nerves and this differentiation between the two is caused by developmental signals again from neighboring tissues. We can see that this is a general principle in development that tissues will signal from one to the other, and respond to signals from neighboring tissues.

    04:40 If we’re to summarize this in a diagram, here we can see the endoderm, which is signaling towards the ventral, the lower part of the neural tube in association with the underlying notochord, and it is this which will induce this to form into motor nerves.

    04:57 Conversely, the ectoderm is signaling to the upper part of the neural tube and inducing that to form the sensory nerves. If you took a piece of notochord and endoderm and placed it above the neural tube and graphed experimentally, what you’d find is that it would then induce the motor nerves that are normally found in the ventral part to appear in the upper part as well. So you can bring about a change in what happens as a result of a grafting of one tissue from its normal position to an additional position. Of course, the neural tube itself will be signaling to the somites, and that helps create the vertebrae, which will grow around the neural tube as it becomes the spinal cord and they will protect the spinal cord within these bony structures, subsequently.

    About the Lecture

    The lecture Embryo Folding and Regionalization of the Brain – Neurulation by John McLachlan, PhD is from the course Embryology: Early Stages with John McLachlan.

    Author of lecture Embryo Folding and Regionalization of the Brain – Neurulation

     John McLachlan, PhD

    John McLachlan, PhD

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