Analysis of spontaneous regeneration of olfactory structures with emphasis on myelination and re-innervation of cortical areas
Highlights
► We analyzed the re-innervated cortical areas in neonatally LOT-transected rats. ► We demonstrated ectopic LOT myelination in neonatally LOT-transected rats. ► Regenerated fibers were confined to the regions of the anterior olfactory cortices. ► Ectopic myelination of LOT occurred only at a specific site near the rhinal sulcus.
Introduction
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. Recently, we reported spontaneous LOT regeneration and functional recovery in developing rats [15]. The most striking feature of the regenerated olfactory structures was an almost total lack of myelination in the regenerated LOT, which we initially misinterpreted as absence of LOT regeneration. To demonstrate the unequivocal LOT regeneration, we analyzed completely LOT-transected rats using a retrograde fluorescent tracer that was simultaneously injected at the LOT transection to exclude the incompletely LOT-transected rats. However, we observed that the newly formed myelinated tract was located outside the original LOT in the LOT-transected rats. Therefore, the relation between the extent of LOT injury and myelination in the original and ectopically regenerated LOTs was the focus of the current study.
Previously, we provided a brief description of the extent of the original distribution of regenerated bulbar projections in the olfactory cortices [15]. However, the precise extent of re-innervated olfactory cortices that receive afferent fibers from the olfactory bulb still requires clarification. Therefore, the present study analyzed the spontaneously regenerated olfactory structures in greater detail with an emphasis on myelination and the extent of re-innervated cortical areas, demonstrating that neonatally LOT-transected rats exhibited varying degrees of LOT damage at the adult stage.
Section snippets
Animals
This study used 15 newborn Wistar rat pups of both sexes (Japan SLC Inc., Hamamatsu, Japan). Postnatal day (P) 0 refers to the first 24 h after birth. All procedures were conducted in accordance with the National Institute of Health Guide for the Care and Use of Laboratory Animals, and the protocols were approved by our Institutional Animal Care and Use Committee. All efforts were made to minimize animal numbers and their suffering. Surgical procedures were performed under general anesthesia
Results
In the completely LOT-transected cases (n = 5) where no FB-positive mitral cells were found in the olfactory bulb on the left transected side, macroscopic examination revealed no white myelinated bands at the original LOT position (Fig. 1A–C). However, microscopic examination revealed the presence of thin MBP-positive myelinated components in the outer superficial layers of the olfactory tubercle and the rostral part of the piriform cortex. Of particular interest was the constant occurrence of
Discussion
LOT injuries were classified as complete or incomplete transection based on neuronal labeling of the bulbar projection neurons (mitral cells) with a retrograde fluorescent tracer (FB) that was injected immediately after LOT transection. In the completely LOT-transected rats, BDA-positive bulbar fibers and terminals were observed in approximately anterior two-thirds of the olfactory cortex and they contained truly regenerated axonal elements; FB-positive mitral cells were absent in the olfactory
Acknowledgment
This study was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 22791339.
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