A Confirmation of the Oligodendroglial Origin of Myelin in the Adult Rat

It is widely held that myelin sheaths in the central nervous system are formed by an oligodendroglial process which wraps itself around an axon (1-5, 9-21). We have, in our own work, assumed this to be the case and, indeed, have presented some indirect evidence that the myelin sheath was, at least at times, connected to a glial perikaryon. This evidence consisted of the presence of formed organelles such as mitochondria and dense bodies within the outer, inner, and lateral loops of the myelin sheath (6-8). In addition, we interpreted the presence of unusually long outer loops as indicating a probable connection to a glial perikaryon (8). Furthermore, it is well known that the cytoplasmic portions of the myelin sheath greatly resemble the cytoplasm of the oligodendroglial cell. A distinct connection has been convincingly demonstrated in the young, developing animal by Bunge et al. (2) and by Peters (17). In the adult animal, however, a connection between a mature myelin sheath and a glial cell has not been clearly demonstrated. Only a single micrograph occurs in the literature which purports to demonstrate this connection (Fig. 6 in Kruger and Maxwell, reference 9). If this is the only direct demonstra-