Histofluorescence characteristics and quantification of small intensely fluorescent (SIF) cells in sympathetic ganglia of several species.

Small intensily fluorescent (SIF) cells in the superior cervical ganglion of the cow, cat, rabbit, rat, guinea pig and monkey were studied, using the glyoxylic acid monoamine fluorescence method. SIF cell populations per mg ganglion tissue showed great species variation. The greatest numbers of SIF cells per mg were found in the rat (380+/-30 per mg). Intermediate numbers (76+/-20 per mg) were found in the guinea pig; and SIF cells in other species were much more sparsely distributed (less than 10 per mg). Two types of SIF cell were identified. Type I cells have long (up to 200 mu) processes which ramify among the principal ganglionic neurons, and this type often occurs singly; whereas type II cells tend to occur in clusters near blood vessels in the interstitial or subcapsular regions of the ganglion. As a general hypothesis we propose that type I SIF cells are interneurons whereas type II SIF cells operate through a neurosecretory mechanism.