Survey of the oviduct of salamandrids with special reference to the viviparous species.

Urodeles include oviparous and a few obligately viviparous species that belong to one family, the Salamandridae. Oviducts of both groups have basically the same design, but some modifications became necessary as viviparous species evolved. The entire oviduct of urodeles is lined by a monolayered epithelium, which is regionally differentiated into large glands and smaller secretory cells rich in glycoproteins. Secretory products important for fertilization form the egg jelly, which also is present in viviparous species. In the latter species, however, there appear to be fewer oviductal convolutions and subdivisions of the glandular part of the organ, as well as fewer gland cells. Viscous, acidic secretory products predominate in viviparous species, whereas resistant neutral mucins predominate in oviparous species. In viviparous species, the caudal portion of the oviduct became altered to retain the developing offspring. This "uterus" lacks glands, but epithelial cells produce carbohydrate-rich material. Offspring remain in the uterus anywhere from several months to five years, depending on the species and climatic conditions. The Na(+)-K(+)-AT-Pase system, common in vertebrate epithelia, is used to regulate the intrauterine milieu. Subepithelial vessels mediate gas exchange and the removal of waste products, mainly urea. Secretions of the uterine epithelium may contribute to the fluid surrounding the young, but do not seem to support nutrition. Sources of nutrition for the young vary from one (sub)species to the next. Some feed on yolk reserves (Salamandra salamandra and certain of its subspecies), others on unfertilized eggs and siblings (other S. salamandra subspecies and Mertensiella luschani with subspecies), and yet others on degrading embryotrophic eggs as well as on cells derived from a specialized uterine trophic zone (S. atra and subspecies). Certain basic alterations in the uterine epithelium during pregnancy are most likely under endocrine control. Others, such as the flattening of cells, the discharge of secretory products, and the development of the trophic zone, may be induced by interactions with the offspring.