Developmental history of nervus terminalis in embryos of insectivorous bats

Abstract

Insectivorous bat embryos (Tadarida and Myotis) ranging from 6- to 16-mm C-R length were examined for the presence of the nervus terminalis. These embryos have no vomeronasal nerve with which the nervus terminalis could be confused. The nerve and associated ganglion cells first appear in the 7-mm embryo. As the embryo ages, a gradual increase in nerve size and ganglion cell numbers occurs. In the 13-mm embryo, nerve size and ganglion cell numbers are reduced, and in older embryos both nerve and cells are absent, as in the adult. The ganglion cells arise as clusters from the nasal septal epithelium. The largest number of cell clusters occurs in the 10.5-mm embryo. Their number then decreases and none are present in embryos of 13-mm and longer. These cells migrate centrally along the course of the nerve which accompanies the olfactory nerve from the nasal cavity roof to a level just caudal to the olfactory bulb, where the nervus terminalis turns dorsalward along the medial telencephalic wall surface. Except in the youngest and oldest embryos the nervus terminalis, where present, divides into two or three branches to pierce the hemispheric wall, one usually entering the region of the nucleus olfactorious anterior, and the other(s), the region of the medial septal nucleus. In some cases, several ganglion cells are present along the intrahemispheric course of the nerve fibers. All ganglion cells resemble those in various sensory ganglia, and so, are probably also sensory neurons.

Terminal nerve in the mouse‐eared bat (Myotis myotis): Ontogenetic aspects

“…The data of this investigation confirm the high variability of the terminal nerve (Huber and Guild, 1913;McCotter, 1913McCotter, , 1915Brookover, 1914Brookover, , 1917Johnston, 1914;Larsell, 1918Larsell, , 1950Humphrey, 1940;Pearson, 1941;Sinclair, 1951aSinclair, , 1951bSinclair, , 1951cBojsen-Møller, 1975;Cooper and Bhatnagar, 1976;Brown, 1980Brown, , 1987Haymaker et al, 1982;Wirsig, 1985;Buhl and Oelschlä ger, 1986;Leonard, 1986a, 1986b;Jastrow, 1995), including the fact that the TN may be unilateral (Jastrow et al, 1998). Analysis of many ontogenetic stages shows a concentration of neurons in some ganglia and along fiber bundles and the persistence of a considerable number of TN neurons up to the adult mouse-eared bat.…”

“…In bats, initial terminal nerve neuroblasts (TNn) and Schwann cells, ensheathing the olfactory and TN fibers later, leave the medial wall of the OP in stage 4. Some immature neurons penetrate the internal limiting membrane of the OP in the two youngest specimens investigated by Brown (1980). The smallest embryo of this study (stage 4; 6.5 mm CRL) already shows an aggregation of TNn near the surface of the telencephalic vesicle; thus, the onset of TNn migration must have happened earlier in ontogenesis, probably early in stage 4.…”

“…Some of them are still on their way to the ganglion situated medial and rostroventral to the telencephalic vesicle. For the first time, the TN is traceable as a distinct fiber bundle in the 11 mm CRL embryo, whereas the formation of the main nasal ganglia begins at 10 mm CRL with the association of cell clusters (Brown, 1980;Oelschlä ger, 1988; this study). Additional smaller ganglia (S) are present on both sides of the nasal septum in the region where a vomeronasal organ normally would be expected.…”

“…In the next stage (5), when the first fila olfactoria can be detected (Brown, 1980;Oelschlä ger, 1988;Jastrow, 1995; this study), outgrowing TN fibers gather in the mesenchyme rostral, ventral, and medial to the location where the primordial olfactory bulb will protrude from the telencephalic wall after being induced by olfactory fibers (Oelschlä ger and Buhl, 1985aBuhl, , 1985bOelschlä ger, 1989). At small pit is found close to the rostral end of the OP (Fig.…”

“…The smallest embryo of this study (stage 4; 6.5 mm CRL) already shows an aggregation of TNn near the surface of the telencephalic vesicle; thus, the onset of TNn migration must have happened earlier in ontogenesis, probably early in stage 4. This is confirmed by Brown (1980), who could not detect any TN material in smaller/younger embryos of other bats (Tadarida brasiliensis, Myotis velifer, and Myotis lucifugus) of 6 mm CRL (stage 3), whereas bats of more than 7 mm CRL (stage 4 and later) exhibit large numbers of TNn adjacent to the OP (Brown, 1980;Oelschlä ger, 1988). In case neural crest material would be an important source of the TN cells in bats, the relevant neuroblasts should have appeared before stage 4 and Brown should have encountered them; therefore, a placodal origin of the TN material seems more convincing for the moment.…”