Trophoblast concept as applied to therian mammals

Abstract

Available evidence provides little support for a recent proposal that the term "trophoblast" be applied solely to eutherian mammals. Arguments for such a restricted usage are based on a dichotomous interpretation of therian reproduction that underestimates the developmental, structural, and functional diversity of trophoblastic tissues occurring within the infraclass Eutherian. The occurrence of developmental patterns that are phenotypically intermediate between those of commonly studied eutherians and metatherians suggests that blastocyst development is not fundamentally different in marsupials and eutherians. The trophoblast of marsupials accomplishes most or all of the major functions of the eutherian trophoblast, including maternal-fetal physiological exchange, implantation, contribution to placental membranes, steroid metabolism, and possibly, immunological protection of the conceptus. Furthermore, application of the term "trophoblast" to marsupials is consistent with present and past usage, as well as with the original definition and etymological derivation of the term. Therefore, we recommend that the term "trophoblast" continue to be applied in a functional-morphological sense to the appropriate extraembryonic tissues of marsupials. Such use of functional (rather than taxonomic) criteria for application of this term avoids biasing interpretations of mammalian reproductive evolution.

The origins of lactation and the evolution of milk: a review with new hypotheses

“…Moreover, such an elaborate mechanism reproductive patterns (e.g. Lillegraven, et al 1987;Tyndale-Biscoe & Renfree, 1987;Blackburn et al, 1988).…”

Specializations of the chorioallantoic placenta in the Brazilian scincid lizard, Mabuya heathi: A new placental morphotype for reptiles

“…Furthermore, regardless of the relative contributions of the various placental components to fetal nutrition, all maternal-fetal transfer is mediated through an extraembryonic ectoderm that is broadly homologous to that of mammals. Consequently, these lizards exhibit specialized tis-sue that meets established criteria for the recognition of "trophoblast," a characteristic that is sometimes said to be confined to eutherian mammals (Lillegraven, 1985;but see Tyndale-Biscoe and Renfree, 1987;Blackburn et al, 1988). Whether the giant binucleate cells of Mabuya play an endocrinological role, as may occur in several eutherians (Meyer, 1973;Osteen et al, 1987;Hoffman and Wooding, 1993), is a distant possibility, but may be worth investigation.…”

Invasive implantation and intimate placental associations in a placentotrophic african lizard, Trachylepis ivensi (scincidae)

“…Marsupials accordingly are sometimes said to be unable to accomplish prolonged gestation because they lack a tissue that can accomplish these functions. In response to this interpretation, other researchers have noted that such functions are accomplished by homologous fetal ectoderm in marsupials andsome squamates, and that the eutherian trophoblast is neither functionally or developmentally unique (Padykula and Taylor,1977,1978,1982; Tyndale‐Biscoe and Renfree,1987; Blackburn et al,1988).…”