Quantitation of synaptic, neuronal and glial development in the intermediate and medial hyperstriatum ventrale (IMHV) of the chick Gallus domesticus, pre- and post-hatch

A quantitative analysis was made of the development of synapses, neurons and glia in both left and right intermediate and medial hyperstriatum ventrale (IMVH) of the forebrain of the domestic chick, Gallus domesticus from 16 days in ovo to 9 days post-hatch. There was a marked increase in total synapse numerical density (NVsyn), from 10 synapses per 100 microns3 at 16 days in ovo to 50 synapses per 100 microns3 at 9 days post-hatch; no significant left/right hemispheric differences were evident but there were differences between the development profiles for synapses with asymmetric as compared to symmetrical synaptic junctions. In contrast to synaptic development, the number of neurons per unit volume, NVneu, decreased by approximately 50% from the value at 16 days in ovo to that at day 0 (hatching). The neuronal population density remained unchanged to 3 days post-hatch and then, in 9-day-old birds, declined to almost a quarter of the original population size; no significant left/right hemisphere differences were evident. Glial cells declined in number from 16 days in ovo to 19 days in ovo, and then gradually increased to 9 days post-hatch. When the synapse to neuron ratio was examined, a trend was observed of a gradual increase with age, but no hemispheric differences were present. It is concluded that these changes are major events which must be considered in experiments aimed at determining the effects of behavioural, and/or environmental manipulations, on the morphology of the IMHV because they may mask other, more subtle, structural changes that occur as a result of behavioural experiences.