Uterine specializations in elasmobranchs

Abstract

The uterus of the elasmobranch demonstrates a variety of structural and physiological specializations associated with the mode of reproduction utilized by a particular species. Reproductive modes include (1) oviparity (Raja erinacea and Raja eglanteria), (2) aplacental viviparity, yolk sac variety (Squalus acanthias), with oophagy and intrauterine cannibalism (Carcharias taurus), and with trophonemata (Urolophus jamaicensis), and (3) placental viviparity (Rhizoprionodon terraenovae and Carcharhinus plumbeus). In oviparous species, the uterus harbors the egg capsule during capsule sclerotization and thereafter until oviposition, which in skates can be several days. The uterus is specialized for synthesis and secretion of materials that may contribute to capsule surface structure or chemistry or that may facilitate biochemical events associated with capsule polymerization, including provision of oxygen and elimination of water generated by the tanning reaction. In aplacental yolk sac species, as well as those species with oophagy and intrauterine cannibalism, the uterus functions as a respiratory membrane and a regulator of the ionic and osmotic environment (Squalus) but provides no supplemental nutrients to augment the yolk sac contents. In stingrays and placental sharks, however, the uterus assumes the additional function of providing nutrients to the developing embryos after the yolk stores have been depleted. Nutrient provision varies from uterine secretions of epithelial origin in the form of histotroph or uterine milk in stingrays to the transfer of material from the maternal vascular system in some placental sharks. Common features shared by all elasmobranchs include (1) increased vascularity of the uterine mucosa, (2) physiological maintenance of specific intrauterine ionic and osmotic environments in some species, (3) provision of secretory products, (4) increase in surface area for respiratory and metabolic exchange, (5) structural specializations of junctional complexes and intercellular spaces for fluid transport in some species, (6) thinning of the uterine epithelium to reduce the diffusion distance for respiratory exchange in stingrays, and (7) provision of nutrients in some viviparous species. J. Exp. Zool. 282:438–459, 1998. © 1998 Wiley-Liss, Inc.