Synaptogenesis in the cerebral hemisphere (accessory hyperstriatum) of the chick embryo

Axosomatic and axodendritic synapses, containing almost exclusively spherical clear vesicles, were first detected in the embryonic chick hyperstriatal complex at about 10 days of incubation. Axodendritic synapses gradually become more plentiful during the subsequent week in ovo, while axosomatic synapses remain scarce at all stages. An explosive formation of new synapses, all of them packed with spherical clear vesicles, occurs shortly before hatching. This event coincides with rapid maturation of the dendritic branches, especially with respect to spine formation.