ABSTRACT. To study the luteal and placental function of pinnipeds, we analyzed the localization of steroidogenic enzymes (P450scc, 3 β HSD and P450arom) in the corpus luteum and the placenta of ribbon seals (Phoca fasciata) and Steller sea lions (Eumetopias jubatus) immunohistochemically. P450scc and 3 β HSD were present in all luteal cells of both species. Almost all of the luteal cells were immunostained for P450arom, while P450scc and 3 β HSD were negatively immunostained in placentae and P450arom was present in the syncytiotrophoblast of placentae. These findings suggest that 1) corpora lutea of both species synthesize pregnenolone, progesterone and estrogen during the entire pregnancy period, and 2) like other terrestrial carnivores in the suborder Caniformia, placentae of both species do not have the capability for synthesizing progesterone in the latter half of active pregnancy period. KEY WORDS: corpus luteum, placenta, ribbon seal, Steller sea lion, steroidogenic enzyme.J. Vet. Med. Sci. 63(9): 955-959, 2001 Both the ribbon seal (Phoca fasciata) and the Steller sea lion (Eumetopias jubatus) are highly adapted for an aquatic lifestyle, and they are classified in the order Carnivora [13,20]. In the past, seals (family Phocidae), sea lions (family Otariidae) and walrus (family Odobenidae) were usually classified as a separate order, the Pinnipedia, or as a suborder of the order Carnivora. However, many morphological and molecular studies have recently revealed that affinities of the Pinnipedia lie within the suborder Caniformia of the order Carnivora, which includes Ursidae (bears) and Mustelidae (weasels, martens, etc.) [20].From the viewpoint of reproduction, delayed implantation occurs in most pinnipeds as it does in bears and martens [1,16,17]. Although the total pregnancy periods of ribbon seals and Steller sea lions last almost one year, their postimplantation periods (active pregnancy periods) are shorter due to delayed implantations [14]. Timing of implantation in ribbon seals is assumed to occur in August [2], although details are unknown. The attachment of blastocyst in Steller sea lions occurs in late September and October [19]. The peak of pupping of ribbon seals and Steller sea lions occurs early in April and early in June, respectively [3,19].In some terrestrial carnivorous animals, the corpus luteum is the most important source of progesterone during the entire gestation period [6,8,18,22,23]. In seals, however, it is not known whether the corpus luteum can synthesize progesterone in late pregnancy, or if the placenta replaces the corpus luteum as a principal source of progesterone, as occurs in sheep and humans [18]. The aim of this study is to clarify whether the corpus luteum and the placenta of ribbon seals and Steller sea lions in the latter half of an active pregnancy period are capable of steroidogenesis.
MATERIALS AND METHODSOvaries and placentae were collected from 8 wild ribbon seals and 8 wild Steller sea lions. The animals were shot legally by hunters as part of nuisance contr...
