Postnatal growth of medial gastrocnemius motoneurons in the kitten

Cat muscle nerves and ventral roots for the hindlimbs show a unimodal distribution of axon diameter at birth, followed, at about 20 days postnatal (dPN), by a marked change to a bimodal distribution resembling that of the adult. However, volumes calculated for motoneuron somata retrogradely labeled with HRP have been reported to be divided into two size populations at birth in the kitten. In the literature it is suggested that a dissociation between axonal and somal growth appears at a very early age. This apparent dissociation, not present in adults, prompted us to examine the somal growth patterns of kitten lumbar motoneurons. In the present report we have examined somal size development in medial gastrocnemius (MG) motor nuclei in 18 cats aged 2 dPN to adulthood using retrogradely transported horseradish peroxidase to label the motoneurons. Measurements of minimum and maximum diameter somal size, volume calculations and a double circle technique relating the diameters of an estimated spherical volume contained within the soma to that of a second spherical volume enclosing the soma clearly distinguish two subpopulations in the adult, a small and a large population. In contrast, in the kitten we show there is a unimodal distribution of small motoneuron somata at birth which at 19-23 dPN differentiates into a bimodal population. This sudden differentiation of somal size coincides with that reported for MG motoneuron axonal calibre, ruling against a neonatal dissociation of somal and axonal size distributions, and appears to correspond to the time of onset of functional characteristics and the histochemical differentiation of fiber types in the MG muscle.