Clustering of intensely fluorescent sympathetic cells in embryonal and postnatal rats

A fluorescence microscopic study has been performed on the ontogenetic appearance of clusters of small intensely fluorescent (SIF) cells in the superior cervical ganglion of the rat. Small SIF cell clusters were observed after the 13th embryonal day. Postnatally, the number of clusters first reversibly decreased, while after the 2nd week, the adult level was reached. Since the total number of SIF cells greatly increased after the 3rd week, the mean size of the clusters increased. From the 1st postnatal week, SIF cell clusters were present also in the external carotid nerve, and from the 2nd postnatal week in the internal carotid nerve. In adult rats, SIF cell clusters were constantly observed in the main postganglionic nerve trunks or near their outlet. The fine structure of the SIF cell clusters in the ganglion and in the nerve trunks was essentially similar. Widened intercellular spaces between adjoining SIF cells, sometimes closed by mutual membrane thickenings, were separated from the pericapillary space often only by a basal lamina. Opening of coated pits into the intercellular spaces was a common occurrence, suggesting that catecholamines may be secreted by the SIF cells into intercellular canaliculi, possibly then diffusing into the pericapillary space. Coated pits were occasionally observed also in the immediate vicinity of synapsing nerve endings that contained small agranular vesicles, suggesting a reciprocal synaptic mechanism. Occurrence of presumable postganglionic axons and SIF cell processes within the same sheath cell enwrapment provides indirect evidence for the idea that the SIF cells may affect the axon, in addition to the cell soma, of the postganglionic neuron.