March 17th lectureEmbryology Biology 441 Spring 2008 Albert Harris
NeurobiologyThe spinal cord, brain, and motor nerves develop from neural tube ectoderm.The neural retina, the pigmented retina and the iris of the eye develop from optic vesicles, which are outfoldings from the sides of the brain. (and therefore also develop from neural tube ectoderm.) Sensory nerves and postganglionic autonomic nerves develop from neural crest ectoderm. The lens, the inner ear, the sensory part of the nose, the lateral line system and several cranial nerves develop from placodes of the somatic ectoderm. &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& The brain and spinal cord are hollow, and contain fluid (water) under pressure. This fluid pressure inflates the various different lobes of the brain. (and the brain fails to develop without this pressure, and also can develop abnormally if this fluid pressure is too strong!)
From what you know about the equation P = C * T By local weakening of Tension of the wall of the brain at special locations, the fluid Pressure (which has the same strength everywhere) will inflate outward those parts of the brain where tension is (temporarily) weakest. (the forebrain, the midbrain, the hind-brain, and the two optic vesicles) But then, as the curvature of these inflated parts gets smaller (in proportion as the sizes of these brain parts gets larger) Large increases in the tension in these parts are needed to limit the amount of inflation, and bring forces back into balance. T gets small at first, to allow C to get smaller, but then T gets stronger than ever to limit the size of each lobe. At these early stages, the walls of the brain are very thin, as you can see in histological sections, or when you dissect a chicken embryo. If you puncture any part of the early brain, all the lobes collapse like a deflated balloon. During each mitotic division, the cells of the wall of the brain and spinal cord move close to the neurocoel cavity. After division, the daughter cells (or at least the nucleus and most of their cytoplasm move outward away from the neurocoel. But when these daughter cells undergo their next mitosis, they move back next to the neurocoel. This (rather odd! But common.) phenomenon is named "interkinetic migration". It occurs in all pseudostratified epithelia. For many years, almost everybody misinterpreted what was going on. They thought that only those cells next to the neurocoel divided, Which is something that really does occur in stratified epithelia, like our skin, which has a basal germinative layer. **********************************************************
Later, the cells of the brain and spinal cord differentiate into
Glial cells (= glia) especially astrocytes and oligodendrocytes.
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