Fine structure of the cells containing large granular vesicles in the superior cervical ganglia of rat embryos.

An electron microscopic study has been performed on the superior cervical ganglia of 11- to 21-day-old rat embryos after fixation with glutaraldehyde and osmium tetroxide. The ganglia of 11-day-old and older embryos are known to consist of cells which show catecholamine fluorescence, the intensity of which varies from weak to bright. Electron microscopically, the ganglia of 11- and 12-day-old embryos were observed to consist of a dense group of cells among the loose mesenchyme. The cells showed a great variation in nuclear morphology and only rarely contained granular vesicles. Only few cells represented the undifferentiated, primitive sympathetic cell type described in previous studies. Therefore, in the rat superior cervical ganglion the ultrastuctural differentiation may be initiated even before the cells complete their migration. From embryonic day 14, three types of cell containing varying numbers of large granular vesicles were distinguished on the basis of nuclear morphology. Those type 1 cells that contained numerous granular vesicles closely resemble the postnatal SIF cells. Those type 2 cells that possessed a great number of granular vesicles may represent an intermediate form, namely the large intensely fluorescent cell described in prenatal ganglia but only exceptionally found in early postnatal ganglia. The appearance of the type 2 cells with only a few granular vesicles is identical with that of the sympathetic cells representing the early stages of maturation. The type 3 cells correspond with the late maturing principal neuron. These cells were never seen to contain more than a few large granular vesicles.