The glial cells in the nerve fiber layer of the rat olfactory bulb

Abstract

In mammals the olfactory receptor neurons are the only ones that are known to undergo continuous cell renewal in the adult animal. This means that the axon of each newly formed neuron must grow into the olfactory bulb to find its appropriate target cell. It is presumed that astrocytes ensheath the olfactory axons as they course through the nerve fiber layer of the bulb even though the cells in question differ ultrastructurally from typical astrocytes. The purpose of the present study was to examine the glial cells in the nerve fiber layer of the rat olfactory bulb in an effort to resolve this apparent discrepancy. Two morphologically distinct types of glial cell were found in the nerve fiber layer. One type, which resembled the typical astrocytes that are present in other areas of the central nervous system, contained bundles of filaments in an electron-lucent cytoplasm. These cells also formed endfeet on blood vessels and formed part of the external glial limiting membrane. They did not, however, ensheath the olfactory axons. The cytoplasm of the other type of glial cell was denser than that of typical astrocytes and contained fewer filaments, which were seldom grouped into bundles. These cells also formed part of the glial limiting membrane at the surface of the bulb and were the only ones that ensheathed the olfactory axons. It is concluded that the cell ensheathing the olfactory axons in the nerve fiber layer of the rat olfactory bulb is a morphological variant of the typical astrocyte.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of a glial network in the olfactory nerve: role of calcium and neuronal activity

“…Glial processes in the sections were labeled with the lectin lycopersicon, which recognizes N-linked glycoproteins. The sequence shows the gradual increase in glial As is the case for their vertebrate counterparts (Kreutzberg and Gross, 1977;Doucette, 1984;Barber and Lindsay, 1982), olfactory receptor axons in the moth are small-diameter (0.1-0.2 µm) unmyelinated axons. In the mature nerve, these glial processes typically envelope groups of 50-80 axons.…”

“…The axonal bundles coursing along the mature antennal nerve are ensheathed by the processes of the peripherally derived AN glial cells, though we do not know to how many bundles a single glial cell might contribute. The glia enwrap bundles of 50-80 small-diameter axons, giving the antennal nerve a cross-sectional appearance similar to that of the olfactory nerve in vertebrates (Kreutzberg and Gross, 1977;Doucette, 1984;Barber and Lindsay, 1982;Burd, 1991). This pattern of glial ensheathment of bundles of small-diameter axons is also seen in the vertebrate central nervous system, for example in the cerebellar array of parallel fibers (Peters et al, 1991) and in Lissauer's tract in the spinal cord (Chung and Coggeshall, 1977).…”

“…The final spatial distribution of the peripherally derived glial cells in the M. sexta olfactory pathway is well understood , the morphological organization of the nerve vis-à-vis axonal bundling closely resembling the vertebrate olfactory nerve (Kreutzberg and Gross, 1977;Doucette, 1984;Barber and Lindsay, 1982). Although our approach includes examination of network development using several markers in fixed tissue, a major objective was to probe the intact nerve electrophysiologically in order to examine glial-neuronal interactions in the three-dimensional and signaling context characteristic of the normal environment.…”

Olfactory ensheathing cells: Historical perspective and therapeutic potential

“…OECs (Figure 1) provide ensheathment for olfactory axons, all of which are unmyelinated (Doucette, 1984(Doucette, , 1993Raisman, 1985). Due to their ability both to support CNS axon regeneration and to remyelinate the regenerated axons, these cells have attracted attention as potential therapeutic agents for the treatment of demyelinating diseases and for repairing the damaged spinal cord.…”

“…These glial cells, which are referred to as olfactory ensheathing cells (OECs), provide ensheathment in vivo for the unmyelinated olfactory axons within both the CNS and PNS portions of the olfactory nerve (Doucette, 1984(Doucette, , 1986(Doucette, , 1993Raisman, 1985). Two reasons why these cells have become so popular are their ability to promote long distance growth in vivo of regenerating axons (Ramon-Cueto and NietoSampedro, 1994;Smale et al, 1996;Li et al, 1997;Perez-Bouza et al, 1998;Nash et al, 2002;Pascual et al, 2002;) and to remyelinate spinal cord axons (Franklin et al, 1996;Imaizumi et al, 1998) in the adult mammalian CNS.…”