GROWTH AND MYELINATION OF CENTRAL AND PERIPHERAL SEGMENTS OF VENTRAL MONONEURONE AXONS - QUANTITATIVE ULTRASTRUCTURAL STUDY

The growth and myelination of those parts of cervical ventral motoneuron axons in the spinal cord (the intramedullary segments) and in the ventral roots of fetal and young rats (up to 21 days postnatal) was compared. The same fiber bundles were examined centrally and peripherally. Myelination began centrally and peripherally at about birth. The peripheral segments of some fibers may begin to become myelinated before the central. Over the first 3 wk after birth the minimum circumference of peripheral segments of myelinated axons remained relatively constant at 3 .mu.m, but that of central segments fell from 2.5 .mu.m to just over 1 .mu.m. Axons within the same fiber bundles tended to be thinner and less heavily myelinated centrally than peripherally. With ageing axon circumference became more strongly correlated with sheath thickness. The thickness of the sheath surrounding an axon of a given circumference did not differ statistically from 1 age to another or between central and peripheral segments. Studies of myelin sheath growth rate showed that in the early stages glial and Schwann cells varied independently of one another in the rates at which they add new turns to sheaths around central and peripheral segments of axons in the same bundles.