Myelin sheath maturation of intrapulmonary axon bundles in the rabbit fetus.

The development of intrapulmonary myelin sheaths began in the central parts of the lung and proceeded towards the periphery. It started suddenly on the 25th post conceptional (p.c.) day. Early stages of myelination could be demonstrated in the periphery on the 27th day p.c (2) Axons not surrounded by Schwann cells could be seen in large numbers on the 24th and 25th day p.c. but they became fewer on the following days. (3) Myelinated axons was found were often at the periphery of the axon bundle. The maximum number of myelin layers was 7 on the 25th day p.c. and 30 on the 31st day p.c. (4) Axons at early stages of myelination were seen along with axons with more progressive myelin sheath development. Abnormal structures of myelinated fibers could be demonstrated. Their significance is discussed.