Synaptogenesis in the stratum griseum superficiale of the rat superior colliculus.

The time course of synaptogenesis in the visual part of the superior colliculus (SC) of pigmented rats has been studied. The number of synaptic profiles per unit area and volume of neuropil in the stratum griseum superficiale (SGS) was estimated in seven groups of animals at ages 3, 9, 15, 21, 30, 49 and 85 days after birth. At 3 days only 1.5 +/- 0.06 synaptic contacts per unit area and 5.5 +/- 0.18 per unit volume were found. Most of them were immature contacts between growing processes. The density of synaptic contacts increased slowly during the first week. By day 9, 4.1 +/- 0.25 and 13.4 +/- 0.66 synaptic contacts were counted per unit of area and volume, respectively. A rapid synaptic proliferation occurred during the next 3 weeks and there were 7.7 +/- 0.27 and 25.5 +/- 1.04 synaptic contacts per unit area and volume at 15 days, 21.1 +/- 1.70 and 86.4 +/- 5.11 at 21 days, and 25.9 +/- 1.20 and 96.7 +/- 3.48 at 30 days. At the same time, the synaptic population gradually acquired more mature morphological characteristics: the pre- and postsynaptic structures became more specialized, the number of synaptic vesicles within presynaptic structures increased, and the synaptic junctional apposition became defined. After 30 days, a decrease in the density of synaptic profiles was recorded: 23.9 +/- 0.44 and 79.8 +/- 1.43 per unit area and volume of neuropil at 49 days and 13.4 +/- 0.53 and 49.7 +/- 2.40 at 85 days, respectively. Thus, after the phase of synaptic proliferation, a significant reduction of synaptic density occurred in the SGS neuropil until it was stabilized at the adult level by the third month of life. Considering the data available on the development of the retinal and visual cortical projections to the SC, the process of synaptic elimination, which takes place after the first postnatal month, does not appear to be directly connected with segregation of these particular projections.