Placental Development in the Mongolian Gerbil

Early gerbil development was studied from days 4 through 12 of gestation. Implantation occurs on day 8 in a shallow antimesometrial implantation crypt. The proamniotic cavity forms from an invagination of basal trophoblast and a folding together of the rim of the cup thus produced. The approximation of this rim gives rise to the ectoplacental cone. The further development and expansion of the ectoplacental cone is much like the rat. Amniogenesis is by folding into the proamniotic cavity. Between large glycogen-filled decidual cells occur many PAS-positive eosinophilic granular cells. A massive subplacental gland forms from proliferating uterine epithelium within the decidua basalis. Epithelial proliferation begins shortly after implantation, and soon short thick cellular processes extend from the antimesometrial side of the lumen whereas long branching cords penetrate the decidua basalis mesometrially. The function of this structure is unknown. The rodent family, Cricetidae, has long been an important group for the study of reproduction, However, until recently one of its subfamilies, Gerbillinae (the jirds and gerbils), has been neglected. Even though the use of these animals in reproductive studies is increasing, surprisingly little has been published on placental development in this group. Basic studies in gerbil reproductive biology have been done by Marston and Chang ('65, '66) in the Mongolian gerbil (Meriones mguicula tz i s ) and by Salzmann ('63) in Merioncs shawi, a North African gerbil. These authors have reported data on estrous cycle lengths, ovulation rates, time of ovulation and mating, preimplantation development, duration of gestation and sex ratios at birth. In addition, histological studies on reproductive organs and implantation have been done in M . shawi (Salzmann, '63). This research is intended to provide a descriptive basis for subsequent investigation of early development in the gerbil, and to compare developmental events in these AM. J. ANAT., 134: 309-320 species with phenomena in the more widely studied rat, mouse, and hamster. MATERIALS AND METHODS Twelve pregnant gerbils, from days 4 through 12 of gestation, were autopsied. Uterine horns of the earlier stages were straightened and pinned flat before fixation in alcohol-formalin-acetic acid, Carnoy's fluid or neutral buffered formalin (10% ). Individual gestation sacs from the later stages were removed from the rest of the uterine horn and fixed in like manner. Serial sections of the tissues were prepared by the paraffin technique and mounted. Staining methods employed were hematoxylin and eosin, periodic acid Schiff (PAS), a stain for specific demonstration of DNA and RNA (A. D. Floyd, unpublished), and naphthol yellow S for demonstration of protein. 1 This investigation was supported in p a t by Institutional Resezrch grants IN-401 and IN-40J to the University of Michigan by the American Cancer Society, by a General Research Support grant to the University of Michigan Medical School, and by Michigan Memorial Phoenix Project 428.