RESUMO
The motor endplates are dynamic structures that present a high degree of plasticity which does not stop with the cessation of development, but lasts throughout life. The present study describes the ultrastructural aspects that characterize this junction renewal process in the oblique abdominal muscles of aged rats (18-24 months). About 50% of the motor endplates studied presented reorganization characteristics such as shallow primary clefts without an axonal terminal, free junctional folds, axon terminals with few synaptic vesicles and presenting pleomorphic structures, large junctional folds containing collagen, and cytoplasmic projections of Schwann cells penetrating the primary synaptic cleft. These aspects are similar to those previously described in adult rats during retraction and degeneration of the axon terminal. Although less frequent, further evidence included the presence of small nerve terminals rich in vesicles, covered by a common Schwann cell and associated with closely packed junctional folds. This last characteristic was associated with nerve sprouting and occupation of the synaptic cleft with new nerve endings. The results of this study are discussed in view of the pertinent literature and we conclude that the plasticity phenomenon of the motor endplate is present throughout life and is more frequent and intense in old animals.