Terminal sprouting in partially denervated muscle of the mouse: a scanning electron microscopic study

Terminal sprouting in the mouse gastrocnemius muscle was examined by scanning electron microscopy after partial denervation. After one of the two muscle nerves in the medial head was cut, terminal sprouts arose from the outer edge of the intact motor endplates and grew out onto a connective tissue layer on the muscle fibres. The sprouts were very slender along their length, although some had a varicose appearance. The terminal tip of the sprouts was swollen in an amoeba-shaped configuration, forming a growth cone. Several shorter projections surrounded the initial part of a long terminal sprout, whilst in the centre the varicosity had a spindle shape like a diamond: the former most likely indicated the processes of a Schwann cell, and the latter may be a Schwann cell soma. Such swelling patterns suggested that the sprout initially grew out from a growth cone and then the Schwann cells migrated into the growing sprout. The sprouts had no synaptic connections with their muscle fibres at least during the growth cone-forming stage. No structural changes to guide the growing terminal sprouts could be recognized on the flat surface of the muscle fibres.