The opossum is an ideal animal model in which to study the formation and establishment of endoderm and its subsequent differentiation into fore-, mid- and hindgut structures. The marsupial model is not complicated by the formation of a morula or an inner cell mass as in eutherian species. Only a unilaminar sphere is formed, followed by development of bilaminar and trilaminar spheres due to differentiation of the endoderm and mesoderm, respectively. One region of the trilaminar sphere becomes the embryo while the remainder forms the fetal membranes. Culture methods for studying cleavage and early blastocyst formation in metatherians are available and have been used to study the early developmental biology of Australian marsupials (Selwood and Young 1983; Selwood 1986, 1987, 1989) as well as later developmental stages of Didelphis (New and Mizell 1972).
KeywordsDigestive System Fetal Membrane Late Lactation Short Gestation Period Australian Marsupial
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