Development of the Olfactory and Terminalis Systems in Whales and Dolphins

Abstract

In the early ontogenesis of mammals, the chemoreceptor systems of the nose (olfactory and accessory olfactory complex) originate from the olfactory placode together with the terminalis system (Oelschläger and Buhl, 1985a, b; Buhl and Oelschläger, 1986; Oelschläger, 1988, 1989). The embryonal chemoreceptor epithelia send axons and possibly Schwann cells to the rostrobasal part of the telencephalon. Here they induce the formation of an olfactory bulb, which is apparent in an evagination of the telencephalic wall, and of an accessory olfactory bulb, which may be reduced later together with the vomeronasal nerve and Jacobson’s organ. The terminalis system, however, the functional implications of which are only imperfectly known, is represented by neuroblasts which migrate from the olfactory placode to the rostromedial extremity of the basal telencephalic hemisphere, adjacent to the olfactory bulb. While the terminalis neuroblasts send out central processes to the brain wall, they obviously retain connections with the dorsomedial part of the olfactory placode. In the adult stage of mammals the terminalis system, which consists of a ganglionated fiber plexus, connects the olfactory and respiratory mucosa of the nasal septum with rostral and basal parts of the prosencephalon. The neurons belong to at least two subpopulations of cells which differ in the size of their perikarya and nuclei. Whether these two types of neurons are identical with cell populations characterized with histochemical and immunocytochemical methods (Bojsen-Møller, 1975; Wirsig and Leonard, 1986; Silverman and Krey, 1978; Oelschläger and Burda, this volume; Oelschläger and Northcutt, 1991, submitted), however, is not clear. Taken as a whole, the terminalis system may innervate arteries of the basal forebrain and of glands and blood vessels in the mucous membrane lining the septal wall of the nasal cavity (Johnston, 1914; Larsell, 1918, 1950). A fraction of mostly bipolar termina-lis neurons immunoreactive for luteinizing hormone-releasing hormone (LHRH) obviously are involved in the establishment and maturation of the brainpituitary-gonadal circuit (Jennes and Stumpf, 1980; Schwanzel-Fukuda et al., 1981, 1985; Oelschläger and Northcutt, 1991, submitted). In the ontogenesis of rats, LHRH immunoreactivity first appears in peripheral (nasal) and central (meningeal) ganglion cells of the nervus terminalis (Jennes and Stumpf, 1980; Schwanzel-Fukuda and Silverman, 1980; Schwanzel-Fukuda et al., 1985, 1988; Jennes, 1989). Correspondingly, after the establishment of the hypothalamohypophyseal-gonadal axis in rat and mouse, the number of LHRH neurons seems to decrease from late fetal stages onward (Schwanzel-Fukuda et al., 1987; Schwanzel-Fukuda and Pfaff, 1989).