An ultrastructural analysis of neurites in the basal lamina of capillaries in the chinchilla cochlear nucleus.

In an ultrastructural study of the chinchilla anteroventral cochlear nucleus (AVCN), we found innervated capillaries in the layer of granule cells that comprises the dorsolateral portion of the AVCN and forms a cap over the principal portion of the nucleus. In 66% of 215 capillaries we examined in the granule cell layer of various levels of the AVCN, we found structures having ultrastructural features of axons that (1) were within the brain parenchyma and were in direct contact with the pericapillary basal lamina, or (2) were separate fro, the adjacent neuropil and, often in the company of astrocytic processes, were completely enveloped by the pericapillary basal lamina. An analysis of serial sections confirmed that neurites within the pericapillary basal lamina were in continuity with neurites in the brain parenchyma. Most neurites within the basal lamina of capillaries were next to pericytes, but some neurites were next to endothelial cells. None of the neurites adjoining capillaries had the abundance of synaptic vesicles typical of autonomic vasomotor nerve endings and synaptic terminals. Consequently, they may be sensory, responding perhaps to changes in hydrostatic pressure or chemical composition of the blood or cerebrospinal fluid. However, we cannot exclude the possibility that neurites which accompany capillaries in the AVCN terminate elsewhere. In the AVCN the innervation of capillaries is restricted to the superficial layer of granule cells. In none of 177 capillaries of the principal portion of the AVCN did we find an example of a neurite in contact with the pericapillary basal lamina, a pericyte, or an endothelial cell, although it was sometimes necessary to examine specimens at various angles of tilt to confirm that the pericapillary glial sleeve was continuous. Furthermore, we found no innervated blood vessels among 266 capillaries examined in the granular and molecular layers of the cerebellar cortex.