. Analysis of development. Embryology; Embryology. Nervous System GROWTH PATTERNS Proliferative Sources. In the ganglia, cell division is rather ubiquitous. In the CNS, on the other hand, mitoses are, in post-neurula- tion stages, confined to the inner surface, lining central canal and brain ventricles (Fig. 1J9). More peripheral layers are es- sentially devoid of mitotic figures; the ceils there continue to grow in size, but without ensuing divisions. Whether the inner "ger- minal" layer is subject to some active mito- genic stimulation by its exposure to the lumen, or rather divisi
. Analysis of development. Embryology; Embryology. Nervous System GROWTH PATTERNS Proliferative Sources. In the ganglia, cell division is rather ubiquitous. In the CNS, on the other hand, mitoses are, in post-neurula- tion stages, confined to the inner surface, lining central canal and brain ventricles (Fig. 1J9). More peripheral layers are es- sentially devoid of mitotic figures; the ceils there continue to grow in size, but without ensuing divisions. Whether the inner "ger- minal" layer is subject to some active mito- genic stimulation by its exposure to the lumen, or rather division in the outer layers is actively inhibited by local conditions in the mantle, is uncertain. However, upon injury to the early CNS (Hooker, '25), as well as after unilateral ablation (cord: Detwiler, '44; Holtzer, '51; not observed in midbrain: Detwiler, '46b), mitotic cells may appear throughout the mantle, which seems to disprove an early loss of divisory faculty. There is a remote possibility that the con- finement of mitotic figures to the inner surface might not truly express the position of the germinal cells in the resting stage, but that the latter might merely rise to the surface during mitosis (Sauer, '35). The observation that x-irradiation of embryos destroys a cell layer somewhat deeper than the inner lining (Hicks, '52) could be in- terpreted in two ways: either these deeper cells are the true germinal ones, but are im- paired in their normal premitotic centripetal movement, or else they are postmitotic cells defying the general supposition that germinal cells are the most sensitive to radiation (Hicks, '53). At any rate, once neuronal differentiation has become marked, none of the resulting nerve cells would ever divide again under ordinary circumstances. We therefore can confine our consideration of proliferative patterns to the irmer, "ger- minal," layer, whatever its precise delinea- tion may be. In this layer, mitotic density and rate vary characte
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