Development of granular pial cells and granular perithelial cells in the spinal cords of mouse and rabbit.

Free cells containing large dense granules first appear in the leptomeninges of spinal cord at E14 in the mouse and at E16 in the rabbit. These ages represent a similar stage of development of the spinal cord and meninges. Despite the early appearance of granular pial cells, granular perithelial cells are not found around blood vessels in the spinal cord until 5 days postnatum in the mouse and E28 in the rabbit. The first appearance of granular perithelial cells coincides with the development of large vessels in the spinal cord. Although there is some correlation between the onset of myelination and the age of appearance of granular perithelial cells in the spinal cord, this appears to be spurious. The early stages of the development of sense granules into honeycomb bodies can be seen as early as 15 days postnatum in the mouse spinal cord.