Postnatal formation of non‐neuronal cells in the rat occipital cerebrum: An autoradiographic study of the time and space pattern of cell division

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Abstract
Rats of the BD III strain were injected with3H‐thymidine (5 μCi per g b. wt.) at postnatal day (PD) 1,3,7,12,16 and 21 and were killed at PD 22‐27 and 27 respectively. The number of cells synthesizing DNA in the visual cortex (areas 17, 18 and 18a) was detected by light microscope autoradiography. It was found that proliferation of non‐Neuronal cells of bothecto‐ (i.e., astrocytes and oligodendrocytes) andmeso‐dermalorigin (endothelial cells, microglia, pericytes, etc.) increases postnatally, reaching a peak at PD 3‐7. After a transient fall of the proliferation rate towards PD 12, another peak in the number of labeled cells was found at PD 16. In ectodermally derived glial cells the latter was evident in the deep cortical layers and the subcortical white matter; in mesodermally derived groups of cells the second peak was clearly evident in all gray matter layers as well as in the adjacent white matter.Another group of animals aged 1,3,7,12,16 and 21 days was injected with3H‐thymidine (5 μCi per g b. wt.) and the animals were killed two hours after the injection. It was shown that towards PD 3‐7 proliferation, of non‐neuronal cells increased in all layers of the hemispheric wall, including leptomeninges. At PD 16 the increase in cell division involved only germinative cells of the periventricular layer. Factors responsible for the cyclic course of proliferation of nonneuronal cells and its relationship to the process of tissue differentiation are not clear. Increased rate of formation ofectodermallyderived glial cells can be related to the (a) intensified growth of surface of nerve cells and vessels – since both must be encased in glial cells – and (b) to the formation of myelin. Changes in the production of non‐neuronal cells ofmesodermalorigin reflect most probably changes in the rate of vasculogenesis because of the high prevalence of endothelial cells in this cell group.

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