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An ultrastructural study of the development of leptomeningeal macrophages in the mouse and rabbit.

Macrophages can be identified in the leptomeninges of the mouse and rabbit spinal cord at E11 and E12 respectively. Initially macrophages contained few cytoplasmic organelles and had long, narrow convoluted processes. By E14 in the mouse and E16 in the rabbit the macrophages were well differentiated with short processes; the cytoplasm contained vacuoles of varying sizes, dense bodies, mitochondria, free ribosomes and rough endoplasmic reticulum. In the young postnatal mouse leptomeningeal macrophages contained numerous small cytoplasmic vacuoles. Leptomeningeal fibroblasts containing varying amounts of cell debris were also found in both pial and arachnoid layers in both species examined but there was no evidence of transformation of leptomeningeal fibroblasts into macrophages.