Primordial synaptic structures and synaptogenesis in rat olfactory cortex

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Abstract
Mature synaptic contacts and various primordidal synaptic elements were studied, counted, and analyzed in rat olfactory cortex from birth to 30 days of age. Primordial structures possess one or a few, but not all, of the features of a true mature synapse and have been grouped into two major classes based upon type of apposition: (1) single or (2) partial and multiple, with vacant postsynaptic sites included in the latter. There is a classical fivefold increase in number of mature synapses between birth and 30 days, but different patterns in the primordial appositions are observed. It is suggested that single apposition contribute to early synapse formation, while partial and multiple appositions participate later on during a time of rapid growth of new afferents to the area. The results suggest a clear role for primordial synaptic structures in synaptogenesis; that the sequence may be more divers than originally hypothesized, occurring at different stages; and that competition, synapse elimination, and replacement may be more prevalent in normal synaptogenesis than has been previously suspected.