Electron microscopic study of synaptogenesis and myelination of the olfactory centers in developing rats

Abstract

Development of the central olfactory system was studied in the rat with an electron microscope at three main structures: the olfactory bulb, the lateral olfactory tract, and the primary olfactory cortex (the piriform cortex). As a parameter of development, the synaptic density was examined quantitatively in the bulbar glomerulus and layer Ia (termination of bulbofugal fibers) of the piriform cortex. which are the key stations of the olfactory pathway. The synaptic densities in the glomerulus and those in layer Ia were 5.7% and 4.6% on embryonic day 19, 15.8% and 12.5% on postnatal day (P) 0, and 57.3% and 37.2% on P10, as compared with the adult (100%). As another parameter of development, the density of myelinated axons in the lateral olfactory tract was examined quantitatively. The densities of myelinated axons in the tract were 0% on P5, 15.1% on P10, and 73.5% on P21 of the adult density. Maturation in the tract was still progressing, even at P21, in terms of bundle formation and the thickness of myelin sheaths. The results show that synaptogenesis in the bulbar glomerulus is followed by synaptogenesis in layer Ia of the piriform cortex, and that myelination in the lateral olfactory tract occurs over a prolonged period, even in the stages after P21.

Analysis of spontaneous regeneration of olfactory structures with emphasis on myelination and re-innervation of cortical areas

“…These results suggest that ectopic myelination occurred after neonatal LOT injuries, irrespective of the lesion sizes. Myelination of LOT occurs between P7 and P21 [13,16];…”

“…The lateral olfactory tract (LOT) is the main fiber tract of the central olfactory system and connects the olfactory bulb to multiple ventrobasal cortical areas, which are collectively known as the olfactory cortices [1,7,13,14,17,18]. Generally, LOT is macroscopically observed as a white myelinated band on the basal surface of the brain in adult rats.…”

Does the brain connect before the periphery can direct?

“…EM observations confirm the absence of synapses between pyramidal cells of olfactory cortex and the LOT at E15, but later ages were not examined (Derer et al, 1977). In the rat, EM examination revealed that piriform layer 1a synapses first appear infrequently at E19, coincident with E19 formation of glomeruli in rat (Schwob and Price, 1984), increasing in density with age to almost 40% of adult levels by P10 (Moriizumi et al, 1995). …”

“…1). Within the CNS a sequential, peripheral-to-central, pattern for connectivity is observed for the olfactory system, with initial connectivity between OSNs and MT cells at E15 (Blanchart et al, 2006; Hinds and Hinds, 1976), and initial connectivity between LOT and primary olfactory cortex at E16 (Moriizumi et al, 1995; Schwob and Price, 1984). Therefore, anatomical evidence is consistent with the possibility that peripheral activity could play an instructive role during synaptogenesis at both glomeruli and primary olfactory cortex.…”

“…EM data demonstrate that synapses are present in glomeruli at E15 (Blanchart et al, 2008; Hinds and Hinds, 1976). Extrapolating from rat EM data, the LOT forms synapses with piriform pyramidal cells at E16–E17 (Moriizumi et al, 1995). Because anatomical evidence of connectivity includes ultrastructural descriptions of synapses, dating for complete functional maturation of the olfactory system could be moved to E16, provided OSNs and MTs are capable of action potential generation and conduction.…”