Spontaneous phagocytosis of C-type synaptic terminals by spinal alpha-motoneurons in newborn kittens. An electron microscopic study.

The cell bodies and proximal dendrites of spinal alpha-motoneurons were studied electron microscopically with the aid of serial sections during the first postnatal week in the cat. The observations suggested that some of the synaptic terminals of the so-called C-type on the cell bodies and dendrites are phagocytosed by the motoneurons during the first few days after birth. This finding is discussed in relation to the earlier demonstrated postnatal loss of synaptic terminals on the motoneurons after birth and differences demonstrated between different functional types of spinal motoneurons with respect to the number and distribution of C-type terminals in the adult cat.