Histological study of nasal organs ofCrocodylus niloticus laurenti andCaiman crocodilus (Linnaeus) (reptilia, crocodylidae)

Abstract

The vestibulum is very short and lined by stratified squamous epithelium which contains many “alveolar” cells. The cavum nasi proprium is exceedingly complex, with three conchal formations and a series of six recesses and sinuses. Olfactory epithelium lines the whole dorsal or dorso-medial half of the cavum, but not the deep sinuses. Non sensory respiratory epithelium lines the ventral or ventro-lateral half of the cavum, all the caviconchal recess, the posterolateral recess, the postturbinal sinus, the postconchal cavity, and the nasopharyngeal duct. In olfactory epithelium the proportion of sensory cells is about 61 % inCrocodylus and 59 % inCaiman; the ratio of sensory cells to supporting cells is about 2.6/1 in the former and 1.8/1 in the latter. Bowman's glands are sero-mucous and normally developed. As in other reptiles, the respiratory epithelium is composed with mucous and ciliated cells; but, in ventrolateral part of the cavum, there are also sero-mucous cells forming small multicellular glands. The hypertrophied lachrymal duct constitutes a very large naso-lachrymal not previously described gland. The lumen is lined by mucous and ciliated cells, the collet of each branched tubular gland by mucous cells and the glandular tubes by sero-mucous cells. Adult crocodilians lack a Jacobson's organ and there are no vomeronasal sensory epithelium in the cavum. In reptiles, aquatic way of life generally involves regression in olfactory epithelium, while Jacobson's organ (or, in Testudines, vomeronasal epithelium) persists and occasionally increases. In crocodilians, things are exactly reverse. After comparing with other Tetrapods, it seems likely that Jacobson's organ has been lost by terrestrial ancestor of crocodilians and birds. Now, only crocodilians posses olfactory epithelium, and naso-lachrymal gland gives them a supplementary protection, necessary in semiaquatic environment.