Morphological features of nerve terminal degeneration as part of the remodeling process in the motor endplate in adult muscles

With the exception of signs of retraction and withdrawal, there have been f ew morphological data concerning degenerated neural profiles in adult motor endplates. Here, investigation into the ultrastructure of the soleus motor endplates of adult rats (4 months old) turned up particular axonal degener ation in approximately 3% of the subjects. These axons occur as synaptic de bris in the synaptic matrix of the motor endplate, adjacent to thin process es of the perisynaptic cells occupying the outermost layer of the motor end plate and were devoid of basal lamina. They often possessed dense-cored Ves icles (50-80 nm). Axonal debris released from Schwann cell processes occurr ed during the period of acute sciatic neurectomy, when nerve terminals prog ressively disrupted within the motor endplate-associated Schwann cells. Fin ally, immunohistochemical staining for antibodies to label macrophages (ED1 or ED2) has shown that nerve fiber-associated macrophages are located near the motor endplate. The results suggest that during the course of endplate remodeling, a few parts of the terminal branches are disposed of through s pontaneous collapse, subsequent release from the Schwann cell investment, a nd eventual ingestion by macrophages in the perisynaptic space.