Developmental changes of synaptic endings in the nucleus vestibularis tangentialis of the chicken: degeneration of spoon endings.

The tangential nucleus, a component of the avian lateral vestibular complex, contains a unique class of very large, axosomatic endings, the spoon endings, formed en passant by the largest vestibular afferents, the colossal fibers. In the hatchling, these endings are characterized by numerous gap junctions, attachment plaques, and a few synaptic vesicle complexes on the principal cell. Spoon endings under drastic structural modifications between hatching and 21 months of age in the normal chicken. The spoon endings and their characteristic junctions disappear. However, colossal fibers and principal cells do not degenerate or even atrophy. Synapses are formed by small terminals of unidentified origin on the vacated surfaces of the principal cell bodies. It is concluded that a specific type of synaptic ending may atrophy and degenerate in the normal, young adult brain without the loss of target cells or fibers of origin.