Synaptogenesis in retino-receptive layers of the superior colliculus of the opossum Didelphis marsupialis

The maturation of the neuropil and synapse formation were examined in the r etino-receptive layers of the superior colliculus (SCr-r) in the opossum fr om a period prior to the onset of arborization of retinocollicular fibers ( postnatal day 22 - P22), at 44% of the coecal period (CP), to the end of th e fast phase of optic fiber myelination and weaning time (P81 - 118% CP). D evelopment of the SCr-r neuropil follows a protracted time course and can b e divided into three broad stages, which are characterized by (I) Large ext racellular spaces, numerous growth cones that participate rarely in synapti c junctions, vesicles-poor immature synapses (P22-P30), (II) Synapses of va ried morphology with abundant synaptic vesicles, and small terminals with d ark mitochondria and round synaptic vesicles (RSD terminals) synapsing most ly onto dendritic shafts, flat-vesicles (F) terminals (P40-P56), (II) Seque ntial appearance of retinal (R) and pleomorphic-vesicles (P) terminals and of RSD terminals synapsing onto spine or spine-like processes, appearance o f glomerulus-like synaptic arrays (synaptic islets) (P61-P81). The advancem ent of synaptogenesis in SCr-r from stage to II and from stage II to III co rrelates closely with the differentiation of astrocytes and oligodendrocyte s, respectively.