An ultrastructural study of the development of the leptomeninx of the mouse spinal cord.

The ultrastructure of the mouse spinal cord leptomeninx was studied from 11 days post-conception to 5 days post natum. At all ages the cells investing the developing cord resembled immature fibroblasts. The most characteristic feature was the rough endoplasmic reticulum, filled with an amorphous ground substance and frequently branched. At first other cytoplasmic organelles were sparse apart from numerous free ribosomes. During development mitochondria became more numerous. From E11 presumptive leptomeningeal processes were joined by tight junctions and the endothelial cells of leptomeningeal vessels were united by zonulae occludentes. Collagen fibrils were found from E11 and in prenatal mice were of small diameter, although the quantity of collagen increased with age. In the postnatal mice collagen was of larger diameter with the characteristic banded appearance.