On the ontogenetic development of the rat dorsal lateral geniculate nucleus. I. GCR-neurons at postnatal day 7--a Golgi-electron microscopic study.

The morphology and synaptic input of four geniculo-cortical relay neurons of albino rats of postnatal day (PD) 7 has been studied with light and electron microscopy, utilizing the combined Golgi-EM technique. Although the nerve cells investigated did show a characteristic relay-cell appearance of the adult, certain dendritic structures exhibited immature character. These were the dendritic swellings, or "growth buds" occurring throughout the whole length of the dendrites, but particularly along the segments and at bifurcations of thin distal dendritic portions. These dendritic thickenings together with the transient spine-like hairy processes were seen to receive many synaptic endings of various developmental stages. The majority of synaptic junctions both on the dendritic swellings and on dendritic shafts were of the asymmetric type. Contrary to our expectation, no proximo-distal gradient could be demonstrated in the maturation of axodendritic synapses, at least not during the first synaptogenetic period, i.e., at PD 7. Also, all axosomatic synapses in the PD7 rat LGNd were asymmetric, indicating that the replacement of asymmetric synapses by symmetric ones (characteristic for the adult stage) occurs only after the first synaptogenetic period. The rarity of Gray II type, symmetric contacts during the early synaptogenesis (i.e., at PD 7) explains the absence of triadic, or serial synapses which will develop only later, during the second synaptogenetic period.