The innervation of the chromaffin cells in the head kidney of the carp, Cyprinus carpio; regional differences of the connections between nerve endings and chromaffin cells.

Nerve fibres and their connections with chromaffin cells in the carp head kidney were studied by light and electron microscopy. Some nerve bundles entered the head kidney from the dorsal aspect along veins. Many unmyelinated axons emerged from the nerve bundles to invade the clusters of chromaffin cells, the distribution of which was restricted to the neighbourhood of the venous trunks and their tributaries. Most of the nerve endings were attached to a chromaffin cell by synaptic junctions and were generally invaginated into the cell. Some nerve endings were flattened in shape and connected with two chromaffin cells. Occasional exocytotic figures of synaptic vesicles opening into the intercellular space, or synaptic junctions along the course of the nerve fibre were observed. The percentage of the chromaffin cells supplied by nerve endings in the head kidney as a whole was similar to that in primitive amphibians. The distribution of the chromaffin cells and the frequency of their innervation suggest that carp chromaffin cells are phylogenetically similar to those of amphibians. The frequencies of synaptic connections in the carp head kidney showed regional differences. The number in dorsal portion was significantly higher than that in two ventral portions. It is suggested that chromaffin cells in the head kidney are separable into two populations: one (in the dorsal portion) shows closer and the other (in the ventral portions) less contact with nerve fibres. The fine structure of the nerve endings indicates that catecholamine secretion of carp chromaffin cells is partially modulated by nerve fibres (probably preganglionic cholinergic fibres). However, the low frequency of synaptic connections on the chromaffin cells and their distribution suggest that carp chromaffin cells are mainly modulated by the endocrine system via the bloodstream.