Derivatives of the Germ Layers – Skull and Brain Development

by John McLachlan, PhD

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    00:00 This process takes place under the influence of positional signals from other parts of the embryo. So, for instance, the ectoderm signals to the upper part of the neural tube to encourage it to give rise to the sensory nerves, and the endoderm and notochord signal to the lower part of the neural tube to encourage it to give rise to the motor nerves. The neural tube itself of course also plays a signaling role and it will encourage parts of the somites to grow round and form the vertebrae which will surround the neural tube, and hence later, the spinal cord. Let’s continue looking at the regionalization of the brain. In this cutaway diagram, we can partly see inside the brain because the story of brain development is also one of the spaces inside the brain itself. And these spaces, like the spinal cord, are filled with cerebrospinal fluid. In fact, in many ways, the expansion of the brain is driven by the production of the cerebrospinal fluid.

    00:59 So the spaces inside the brain are particularly important to help understand the nature of this development. But in this one, we can see increasing specialization of different regions, and I’ve noted is the developing cerebral hemispheres towards the frontend of the brain, and we can also note the cerebellum further back. What’s going to happen in the course of development is that the cerebral hemispheres will grow back to cover much of the primitive brain, and that’s the story of the evolutionary development of the brain in the vertebras. As I say, it’s really important to understand the role of cerebrospinal fluid.

    01:38 In this section through the brain along the line of the red line, we can see what this might look like in the inside. And there are plexuses which are producing cerebrospinal fluid, and that is circulating through the brain and being reabsorbed elsewhere.

    01:56 So one can imagine that if there was an excess of production, or if something was to happen to inhibit the removal of cerebrospinal fluid, it would begin to build up in the brain, and we’ll look at the consequences of that later in this lecture. But this production is a key component to the expansion of the brain itself. Seen from the outside, we can see that as time goes by over the foetal period, the brain is steadily expanding. In these images, we can see that the cerebral hemispheres are covering almost all of the brain back to the cerebellum itself. Fissures are just beginning to appear in the brain surface.

    02:38 In this section, we can also see that the interior spaces are also expanding at the same time. This complexity continues, and higher brain processes begin to be present in the developing foetus, probably appearing relatively late during the foetal period.

    02:57 And this is associated with increasing external complexity of the brain including the development of major and minor fissures. Again, seen in section, we can see the spaces of the brain which are helping drive this expansion as it takes place and the relationship of the cerebral hemispheres to the cerebellum.

    About the Lecture

    The lecture Derivatives of the Germ Layers – Skull and Brain Development by John McLachlan, PhD is from the course System-Specific Embryology with John McLachlan.

    Author of lecture Derivatives of the Germ Layers – Skull and Brain Development

     John McLachlan, PhD

    John McLachlan, PhD

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