Ultrastructural Study of the Cardiac Ganglia in Prenatal Rat II. Axon Terminals, Satellite Cells and Small Granular Cells

Abstract

Prenatal development of the preganglionic nerve endings, satellite cells, and small granular cells in the cardiac ganglia of fetal rat were studied by electron microscope. Ganglia from 14-16, 18, and 20-21-day-old fetuses were investigated. In the developing cardiac ganglia, three types of afferent axon terminals could be recognized distinctly. The type I axon terminals contained small clear vesicles only, and appeared earliest in the 14-day-old fetus. Type II axon terminals contained numerous small clear vesicles and few large dense-core vesicles, and these may be cholinergic nerve endings of vagal origin. The type III axon terminals were characterized by the presence of glycogen granules, small clear vesicles, and numerous large dense-core vesicles, which may be afferent adrenergic nerve endings of sympathetic origin. Type II and Type III axon terminals could be observed at approximately day 15 of gestation. The development of axosomatic synapses was earlier in synaptogenesis than that of axodendritic synapses. Young satellite cells were characterized by large round nuclei and thin cytoplasm layers with a paucity of organelles. Few short cytoplasmic processes projected from the cell bodies. In mature satellite cells, the nuclei were semilunar in shape and were surrounded by little cytoplasm. Numerous branched cytoplasmic processes extended from the cell bodies to enwrapp the principal ganglion cells. The maturation of small granular cells could be divided into three stages: early immature, young, and mature. Three different types of granular vesicles appeared in the periphery of their perikaryon. In the developing cardiac ganglion of the prenatal rat, mitotic small granular cells were frequently encoutered, but only a few mitotic satellite cells were observed.