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 retino-receptive layers of the superior colliculus (SCr-r) in the opossum from 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 the fast phase of optic fiber myelination and weaning time (P81 - 118% CP). Development of the SCr-r neuropil follows a protracted time course and can be divided into three broad stages, which are characterized by (I) Large extracellular spaces, numerous growth cones that participate rarely in synaptic junctions, vesicles-poor immature synapses (P22-P30), (II) Synapses of varied morphology with abundant synaptic vesicles, and small terminals with dark mitochondria and round synaptic vesicles (RSD terminals) synapsing mostly onto dendritic shafts, flat-vesicles (F) terminals (P40-P56), (III) Sequential appearance of retinal (R) and pleomorphic-vesicles (P) terminals and of RSD terminals synapsing onto spine or spine-like processes, appearance of glomerulus-like synaptic arrays (synaptic islets) (P61-P81). The advancement of synaptogenesis in SCr-r from stage I to II and from stage II to III correlates closely with the differentiation of astrocytes and oligodendrocytes, respectively.