Early myelin formation and glial cell development in the human spinal cord.

In order to identify the onset of myelination as well as to clarify the development of glial cells in human cervical spinal cord 21 embryos and fetuses ranging from 5 weeks of ovulation age (16 mm in crown-rump length) to 13 weeks (110 mm) were examined with the light the electron microscope. The first appearance of definite myelinated fibers occurred in the lateral portion of the ventral marginal layer of 66-mm (10-weeks) fetus. These nascent myelinated axons were always of larger diameter than unmyelinated axons. Although they occurred infrequently, there were, nevertheless, degenerative changes observed in some of these first myelinated axons in the 66-mm fetus. by this time glial cells in the marginal layer rapidly increased in number and substantial differentiation had occurred. Young oligodendrocytes are considered as the only myelin-forming cells in the human spinal cord.