UTERINE SPECIALIZATIONS IN ELASMOBRANCHS

The uterus of the elasmobranch demonstrates a variety of structural an d physiological specializations associated with the mode of reproducti on utilized by a particular species. Reproductive modes include (1) ov iparity (Raja erinacea and Raja eglanteria), (2) aplacental viviparity , yolk sac variety (Squalus acanthias), with oophagy and intrauterine cannibalism (Carcharias taurus), and with trophonemata (Urolophus jama icensis), and (3) placental viviparity (Rhizoprionodon terraenovae and Carcharhinus plumbeus). In oviparous species, the uterus harbors the egg capsule during capsule sclerotization and thereafter until oviposi tion, which in skates can be several days. The uterus is specialized f or synthesis and secretion of materials that may contribute to capsule surface structure or chemistry or that may facilitate biochemical eve nts associated with capsule polymerization, including provision of oxy gen and elimination of water generated by the tanning reaction. In apl acental yolk sac species, as well as those species with oophagy and in trauterine cannibalism, the uterus functions as a respiratory membrane and a regulator of the ionic and osmotic environment (Squalus) but pr ovides no supplemental nutrients to augment the yolk sac contents. In stingrays and placental sharks, however, the uterus assumes the additi onal function of providing nutrients to the developing embryos after t he yolk stores have been depleted. Nutrient provision varies from uter ine secretions of epithelial origin in the form of histotroph or uteri ne milk in stingrays to the transfer of material from the maternal vas cular system in some placental sharks. Common features shared by all e lasmobranchs include (1) increased vascularity of the uterine mucosa, (2) physiological maintenance of specific intrauterine ionic and osmot ic 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 intercellula r spaces for fluid transport in some species, (6) thinning of the uter ine epithelium to reduce the diffusion distance for respiratory exchan ge in stingrays, and (7) provision of nutrients in some viviparous spe cies. J. Exp. Zool. 282:438-459, 1998. (C) 1998 Wiley-Liss, Inc.