The fetal membranes of Pedetes capensis, and their taxonomic significance

Abstract

Implantation in Pedetes capemis is superficial and antimesometrial with the embryonic disc mesometrially oriented. As the blastocyst expands, the embryonic pole trophoblast proliferates and penetrates the epithelium a t the mesometrial side to establish a massive trophospongial preplacenta. Maternal blood lac inae form in this, and a preplacental maternal circulation is soon established. Amniogenesis is tiy folding. A choriovitelline placenta develops on the preplacental surface. Yolk sac inversion is k t e . The bilaminar omphalopleure is permanent. Syntrophoblastic tongues penetrate the preplacenta and surround maternal spaces lined with an attenuated trophospongial cell layer, thus providing a hemodichorial barrier when the vascular allantoic mesoderm invades these tongues to form the lobular labyrinth. A typical countercurrent bloodflow occurs. A small allantoic diverticulum persists to term. Several fetal membrane characters, known to be evolutionarily "consurvative," are alike in Pedetes and the more primitive sciuromorphs. These are the type of implantation, amniogenesis, yolk sac inversion, and allantois. The massive trophospongial preplacenta (Trager) suggests affinity with the Myomorpha and Hystricomorpha, but this is a h ghly variable character within the Rodentia. Therefore, it is concluded that on the basis of th: fetal membranes the Pedetidae are somewhat specialized sciuromorphs.

Development and phylogenetic significance of the fetal membranes and placenta of the African hystricognathous rodents Bathyergus and Hystrix

“…These include the mesometrial orientation of the embryonic disc a t the time of attachment or implantation, initial attachment effected antimesometrially by the abembryonic pole of the blastocyst, a large and inverted yolk sac, zonary parietal decidua, and a single mesometrially located choriodantoic placental disc (Mossman, 1937;Fischer and Mossman, 1969;Luckett, 1971). These ancestral rodent attributes provide no evidence of the possible phylogenetic relationships within the order Rodentia, but they are extremely valuable in assessing the affinities of rodents to other eutherian mammals.…”

“…(For further discussion of the methods employed for evolutionary analysis of mammalian fetal-membrane characters, see Luckett, 1977). The ancestral pattern of fetal-membrane morphogenesis in rodents appears to be approximated most closely by sciurids and aplodontids (Mossman, 1937;Fischer and Mossman, 1969;Luckett, 1980b). Some of the derived similarities of Old and New World hystricognaths, such as the lack of a choriovitelline placenta, occurrence of yolk-sac splanchnopleuric villi, transitory development of the true chorion, well-developed trophospongium, and absence of an endodermal allantoic diverticulum, are also shared by geomyoids, dipodoids, and muroids (Figs.…”

“…There are differences in the relative developmental stage at which chorioallantoic placentation is initiated among suborders or superfamilies of rodents, but precise data are available for only a few taxa. Fusion of allantoic stalk mesoderm with the preplacenta is initiated in the early limb-bud phase of development in sciurids, aplodontids, geomyids, heteromyids, and probably pedetids (Mossman and Weisfeldt, 1939;Nielson, 1940;Harvey, 1959;Mossman and Straws, 1963;Fischer and Mossman, 1969); and this common pattern may represent the primitive condition among rodents. In contrast, chorioallantoic fusion is established earlier in development (5-15 somite embryos) in murids, cricetids, and dipodids, prior to differentiation of the forelimb buds (Graves, 1945;Theiler, 1972;King and Mossman, 1974;Luckett and Mossman, unpublished).…”

“…Our developmental data are consistent with this viewpoint. Morphogenesis of the fetal membranes in Pedetidae (Fischer and Mossman, 1969), Anomaluridae (Luckett, 1971), and Ctenodactylidae (Luckett, 1980a) bears little resemblance to the hystricognath pattern. Instead, these families have retained many features (including superficial implantation, amniogenesis by folding, transitory choriovitelJine placentation, and a small allantoic vesicle) relatively unchanged from the postulated primitive rodent condition.…”

Higher-level systematics of rodents (Mammalia, Rodentia): Evidence from the mitochondrial 12S rRNA gene

“…If the enamel microstructure data are correct, then hystricomorphy may have arisen independently two or more times throughout rodent evolution as suggested by Patterson and Wood (1982). Even with an examination of other characters, placement of one or more of these families as sister to the Hystricognathi has not been resolved (Simpson, 1945;Wood, 1955;Fischer and Mossman, 1969;Chaline and Mein, 1979;Meng, 1990;Beintema et al, 1991;Otiang'a-Owiti et al, 1992;Martin, 1993). Identification of monophyletic groups within the Sciurognathi becomes more complicated when one considers other rodent families within this group.…”