Placentation in the American Beaver (Castor

Implantation in the beaver is superficial and antimesometrial. The embryonic disc occupies one side of the flattened mesometrial hemisphere of the large blastocyst. A n endometrial papilla, unique to the beaver, forms on the mesometrial uterine wall just prior to chorioallantoic contact. The reniform placenta then develops around this papilla. Amniogenesis is by folding. A transient choriovitelline placenta develops very early. Yolk sac inversion is late, and the bilaminar omphalopleure is partially retained throughout gestation. The chorion persists until term as two paraplacental membranes. A large permanent allantoic vesicle is present. The labyrinthine placenta has a lobulate architecture. I t possesses a hemodichorial barrier, the inner layer being syncytial and the outer cellular. Maternal blood is supplied by two arterial channels which do not penetrate the labyrinth, and is drained by a single, central venous channel. A large basal subplacenta develops by trophoblastic invasion of the endometrial papilla, and becomes quite massive at term. Its function is unknown. Except for this specialized subplacenta, all the fetal membrane characteristics are typical of the primitive sciuromorph group, to which the Castoridae seem to be closely related. The mammalian superfamily Castoroidea, which contains only one recent family, Castoridae, is of uncertain relationships among rodents. Characteristics of the skull and dentition rule out affinities with the suborders Myomorpha and Caviomorpha (Romer, '66). Its dentition shows certain similarities to the Sciuromorpha, but skull characters appear to set i t apart from this suborder also. Romer ('66) lists the group as unassigned to any suborder, but indicates a presumed sciuromorph ancestry. In addition to skeletal elements, comparisons of soft tissues of recent groups are valuable in detecting affinities between them. This is especially true of the reproductive system and fetal membranes. Since fetal membranes are relatively sheltered from external environmental forces, they tend to be more evolutionarily conservative than many other systems. Characteristics of fetal membrane development thus can be important in the phylogenetic classification of mammals (Mossman, '37, '53). Therefore, an analysis of placentation in the beaver (Castor canadensis) could be of value in understanding the relationships of the Castoroidea. Two extensive papers (Willey, '12, '14) and a short comparative note (Mossman, '57) comprise the literature on beaver placentation. The early papers are quite comprehensive, but use archaic nomenclature. Willey ('12) also expresses an individualistic definition of terms, which leads to uncertainty and obscures comparison with other species. Mossman ('57) observed two distinct layers between maternal blood and fetal capillaries in the beaver placenta. Because of the obvious separation of layers, he suggested that this placenta may have an endothelio-chorial barrier. This possibility, which has been reported in only one other rodent group (King and Tibbits, '69), required further study for evaluation. The present research was undertaken to describe the development of the fetal membranes in the beaver (Custor canadensis), 1This work was supported in part by General Research Support grant FR-05383-06 of the University of Michigan Medical School, and in part .by Institutipnal Research grant IN-401 to The Unlverslty of Mlchlgan from the American Cancer Society. AM. J. ANAT., 131: 159-184. 159 160 THEODORE V. FISCHER to determine the nature of its placental barentiate into mononuclear giant cells which rier, and to contribute to our understandattract and engulf maternal leucocytes. ing of the phylogenetic relationships of the Large numbers of leucocytes are present in Castoroidea. the antimesometrial decidua, between epiMATERIALS AND METHODS Specimens of beaver preplacental stages and early placentation were made available by the Hubrecht Laboratory, Utrecht, Holland. Reproductite tracts in various stages near midpregnancy were collected and fixed in the ficlld (alcohol/formalin/ acetic acid (AFA) ) and sent to our laboratory for further preparation. Two beavers nearing term were killed and pieces of placenta were fixed in 2% glutaraldehyde (postfixed in osmium tetroxide) for electron microscopy. Other gestation sacs and placentae from these animals were fixed in neutral buffered 10% formalin or AFA. Selected sections of the various stages were stained with all or most of the following methods : hernatoxylin and eosin, periodic acid-Schiff (PAS) reaction, Movai pentachrome (Movat, ’55), Alcian blue or colloidal iron (often with PAS), and silver reticular fiber stain. Gross dissections of the placenta and fetal membranes were made of several of the midterm gestation sacs.