Proliferation of Schwann cells in a developing feline lumbar ventral spinal root

Schwann cell proliferation in the L6 ventral spinal root of cat fetuses and young kittens was studied by light microscopical autoradiography and electron microscopy. The proliferative activity reached a maximum 35 days after mating. This is about one week before the immature alpha-axons begin to myelinate and about one month before birth (63 days after mating) when the myelination of the immature gamma-axons starts. The proliferative activity was already quite low 45 days after mating and ceased completely after birth. Calculations based on the observed number of Schwann cell nuclei per 100 axons showed that the number of Schwann cells already present 45 days after mating was equal to or even a little higher than the observed adult number. Necrotic nucleated Schwann or endoneurial cells were not observed. I conclude that Schwann cell proliferation in the L6 ventral spinal root is already nearly finished when the first immature alpha-axons begin their myelination and that myelination of the immature gamma-axons begins without a preceding increase in Schwann cell proliferative activity.