OLIGODENDROGLIAL SHEATH LENGTHS IN DEVELOPING RAT VENTRAL FUNICULUS AND CORPUS-CALLOSUM

The lengths of uncompacted and partly compacted oligodendroglial sheat hs in the developing rat spinal cord (SC) ventral funiculus (ages F19 and F21) and corpus callosum (CC; ages P12 and P17) were studied by se rial section electron microscopy. The average newly formed uncompacted sheath is 21 and 33 mu m long in the SC (F19) and CC (P12), respectiv ely, many being less than 10 mu m. In these early series, approximatel y 2/3 of the analysed axon length is unensheathed. The average partly compacted sheath is 102 and 69 mu m long in the SC (F21) and CC (P17), respectively. Here, about 1/3 of the examined axon length is unenshea thed. These results suggest that oligodendroglial sheaths initially ar e very short, and that they elongate actively before and during compac tion. The limited unensheathed space along these axons indicate that s ome early sheaths must be eliminated.