Vomeronasal organ development

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It is well known that the chemical senses play a critical role in the behavior of snakes (Halpern, 1987, 1992 Mason 1992 and Schwenk 1995). Tongue-flicking, a chemosensory behavior pattern unique to snakes and lizards (Gove 1979 Schwenk 1993), serves as the primary vehicle for transfer of chemical substances to the vomeronasal organ (Burghardt and Pruitt 1975 Graves and Halpern 1989 Halpern and Kubie 1980 Kahmann 1932 Wilde 1938). Snakes have well-developed vomeronasal systems and flick their tongues in response to odorants perceived in their environment. 

Herrada G, Dulac C (1997) A novel family of putative pheromone receptors in mammals with a topographically organized and sexually dimorphic distribution. Cell 90 763-773 Huh GS, Boulanger LM, Du H, Riquelme PA, Brotz TM, Shatz CJ (2000) Functional requirement for class I MHC in CNS development and plasticity. Science 290 2155-2159 Inamura K, Kashiwayanagi M, Kurihara K (1999) Regionalization of Fos immunostaining in rat accessory olfactory bulb when the vomeronasal organ was exposed to urine. Eur J Neurosci 11 2254-2260... 

For all these chemical compounds to function as signals, developed chemosensory systems must be available. Extensive olfactory and vomeronasal organ systems and several auxiliary chemosensory systems provide the elephant with an extraordinarily sensitive and fine-tuned chemosensory detection system. Not only does the physiology (especially hormone levels and their actions) of both sexes affect the release of substances, these factors also influence the reception of chemical signals (Schulte et al., 1997 Slade et al., 1997 Rasmussen Schulte, 1998). 

The external sensory organs are drastically reduced, the eyes reduced to subcutaneous lenses (Sweet, 1906) while the ears are represented externally only by small holes surrounded by dense hair. In contrast the olfactory bulbs and tubercula olfactoria are enormous (Johnson Walton, 1989). Furthermore the vomeronasal organ is well developed and displays extensive associated glandular development and vascularization (Sweet, 1904). Generally N. typhlops shows an extremely simple cerebellar cortex for a mammal (Burkitt, 1938). 

Members of the family Iguanidae have traditionally been considered to be primarily visual (e.g., Evans, 1959) they typically possess a fleshy nonserpent-like tongue with only moderate development of the vomeronasal organ and the associated accessory olfactory bulb. Contrary to expectations, however, some iguanid lizards have recently been demonstrated to... 

The subject of the present work is the highly developed chemosensory system of the elephant this system includes both the primary olfactory and the vomeronasal organ (VNO) systems. In such an intelligent mammal, separation of the chemosensory responses from auditory, tactile, and visual responses is difficult. A chemosensory response by the elephant depends on reception of a chemical signal, but its expression may be modified by an intricate combination of other cues, visual, tactile, and auditory, and by the physiological state and experiences of the animal. 

The vasculature is established by the 18th day of gestation in rats, and comes from the arterial supply as the anterior cerebral vessel, eventually entering the basal lamina via septal tributaries of the olfactory artery (Szabo, 1988). Unlike the MOE, the organ s capillaries penetrate in loops into the neuroepithelium. Blood from the vomeronasal complex arrives for collection in the vomeronasal vein, as described earlier [Fig. 2.11(a)]. The establishment of the highly vascular columnar complexes seen in the ophidian organ has not been correlated with functional development (c.f. Wang and Halpem, 1980 Holtzman and Halpem, 1990). 

The vomeronasal system, also known as the accessory olfactory system, consists of chemoreceptors, organized into the VNO, the vomeronasal nerve, its terminal, the accessory olfactory bulb, and more central pathways. First described by Jacobson in 1811, the VNO has been studied intensely. We now know how stimuli reach it and what behaviors it mediates. The VNO occurs in amphibians, reptiles, and mammals. Among mammals, it is best developed in marsupials and monotremes. In birds it only appears during embryogenesis. The VNO and its function are best known for squamate reptiles, particularly snakes, and rodents and ungulates among the mammals. 

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