[Ultrastructure of the sympathetic paravertebral ganglia in rat embryogenesis. II. The development of the neuropil and the interneuronal connections].

Electron microscopy was used to study the process of ingrowth of nerve terminals in the primordia of sympathetic ganglia and the formation of specialized contacts. Nerve terminals appeared first in 12 day old embryos. In the forming ganglia of 13 day old embryos there are many preganglionic nerve terminals and processes of principal neuroblasts. The growth cones of nerve endings are usually distended and with transparent cytoplasm. The plasmalemmas of growth cones are lacking often the trilaminar structure. Synapses were observed first in 16 day old fetuses. They are axo-dendritic and axo-somatic ones. The number of synaptic contacts does not increase much during prenatal period. Presumptive afferent nerve terminals were found in the late fetuses.