The development of alpha and gamma motoneuron fibres in the rat. I. A comparative ultrastructural study of their central and peripheral axon growth.

The growth of alpha and gamma motoneuron axons was studied from late fetal life to maturity in the fifth lumbar ventral spinal root of the rat. Overall axon circumference distribution is unimodal up to three days postnatum and bimodal subsequently. Alpha and gamma fibre categories can be distinguished from one another before the bimodal distribution appears, since their degrees of segregation differ. By 3 days postnatum, the great majority of presumptive alpha axons are either promyelin or myelinated in form. The two populations follow separate maturation tracks subsequently, alpha axons making up between 60 and 70% of the total. Alpha axons grow most rapidly during the first week, gamma axons during the second and third weeks postnatum. The onset of myelination in both groups coincides with the most rapid period of growth. The delayed rapid growth of gamma fibres is due to a prolongation of their segregated stage; subsequent stages have a similar duration to those of alpha fibres. The periods of most rapid growth in both alpha and gamma fibres may be related to the formation of their respective terminal connections in the muscles. Myelination commences in relation to a smaller axon circumference in gamma than in alpha fibres. The former are therefore more myelinogenic than the latter.