Redistribution of Schwann cells in developing feline L7 ventral spinal roots

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Abstract
The length and distribution of Schwann cells along fibres in the ventral root L7 of the developing cat have been studied electron microscopically in serial sections. The average Schwann cell length at the beginning of myelin formation — the ‘initial internodal length’ — was 118 μm (110 μm in α-axons and 124 μm in γ-axons). The number of Schwann cells found in a fully developed root segment was already present at the beginning of the myelination. It showed no systematic age-dependent variation from the beginning of myelination to adulthood. The Schwann cells associated with α-axons increased their length 12.6 times during this period, while the root elongated 5.6 times. About 50% of the Schwann cells had to be eliminated in order to make the elongation of the remaining Schwann cells possible. Corresponding calculations from the mean length of Schwann cells associated with γ-axons, showed that about 50% too few Schwann cells were associated with the γ-axons during the period of initial myelination of the α-axons. At birth, when the myelination of γ-axons had just begun, both the large surplus along α-axons and the deficit along γ-axons had disappeared. We suggest that Schwann cells are eliminated from the α-axons and re-utilized along the γ-axons. During this process of cellular redistribution, affected cells constitute so-called aberrant Schwann cells.