The ultrastructure of the rat paracervical (frankenhäuser) ganglion was studied during postnatal development after immersion or perfusion fixation with the glutaraldehyde, followed by postosmification. Three different neuronal cell types were found in the ganglion: (1) Primitive sympathetic nerve cells. They had a “primitive” structure and contained, in one section, one to two dense cored vesicles (DCV) of 700–1100 Å in diameter. They were not found after the fourth day of postnatal development. (2) Principal neurons grew in size during the whole period of postnatal development. Part of them were already well developed in the ganglia of the newborn rat, and no particular changes in the content of their cytoplasmic organelles occurred thereafter. (3) Small granule-containing (SGC) cells. They usually occurred in small groups, often close to blood capillaries of the fenestrated type and were sometimes devoid of their satellite cell sheath in this region. They became slightly smaller in size as the animal aged. The size of their granulated vesicles (GV) varied mainly between 800–1400 Å in all age groups in most of the cells. In addition, another type of SGC-cell containing larger GV up to 3000 Å in diameter were observed in the ganglia of 32-day old and older rats. Degenerating principal neurons and SGC-cells were encountered, especially in the young animals. Mitotic figures were seen in the SGC-cells up to the 8-day stage. The synapses to the principal neurons were mainly axo-somatic. There were only a few synapses present in the newborn, but their number greatly increased during development. It appeared as if the accumulation of synaptic vesicles preceded or appeared simultaneously with the formation of the pre-and post-synaptic membrane thickenings. Synapses to the SGC-cells were few in all the age groups studied.
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