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WIREs Dev Biol
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The mammalian blastocyst

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The blastocyst is a mammalian invention that carries the embryo from cleavage to gastrulation. For such a simple structure, it exhibits remarkable diversity in its mode of formation, morphology, longevity, and intimacy with the uterine endometrium. This review explores this diversity in the light of the evolution of viviparity, comparing the three main groups of mammals: monotremes, marsupials, and eutherians. The principal drivers in blastocyst evolution were loss of yolk coupled with evolution of the placenta. An important outcome of blastocyst development is differentiation of two extraembryonic lineages (trophoblast and hypoblast) that contribute to the placenta. While in many species trophoblast segregation is often coupled with blastocyst formation, in marsupials and at least some Afrotherians, these events do not coincide. Thus, many questions regarding the conservation of molecular mechanisms controlling these events are of great interest but currently unresolved. WIREs Dev Biol 2016, 5:210–232. doi: 10.1002/wdev.220 This article is categorized under: Early Embryonic Development > Fertilization to Gastrulation Comparative Development and Evolution > Model Systems Comparative Development and Evolution > Evolutionary Novelties
Marsupial (a, b) and eutherian (c, d) blastocysts. (a) Day 9 (after removal of pouch young) blastocyst of the tammar wallaby. Note the absence of an inner cell mass. The attenuated mucoid coat and zona pellucida (not visible) lie between the blastocyst epithelium and the shell. (b) 2000‐cell diapausing blastocyst of the honey possum. (c) Embryonic day 3.5 mouse blastocyst. (d) Day 8 bovine blastocyst (image courtesy of Marcelo D. Goissis and Jose Cibelli, Michigan State University).
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Mechanisms regulating diapause in the mouse. There is evidence for conserved roles for many of these factors in both tammar and mink, including progesterone, MSX genes, leukemia inhibitory factor, polyamines, and platelet‐activation factor.
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Model for the evolution of a role for Hippo signaling in the mammalian trophoblast. In the large, yolky eggs of ancestral amniotes, rapid proliferation of extraembryonic ectoderm was necessary before the specification of embryonic cells.
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Comparison of early cell lineage specification between tammar, mouse, and cow.
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Comparison of different modes of blastocyst formation among eutherians.
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Comparison of different modes of blastocyst formation among marsupials. All species exhibit early cell–zona adhesion that precedes cell–cell adhesion, which results in the formation of a unilaminar blastocyst. Variation occurs mainly in the degree of conceptus polarity, the number and size of deutoplasts, and the degree to which distinct populations of putative pluriblast and trophoblast precursors can be distinguished. In Didelphis virginiana, it is possible that early conceptus polarity (stages indicated by the boxed area) was overlooked in early studies, since it has only more recently been recognized in another didelphid, Monodelphis domestica.
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Comparison of the relationship between yolk/deutoplasm content, coat deposition, and conceptus expansion in birds, monotremes, and marsupials.
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Comparative Development and Evolution > Model Systems
Comparative Development and Evolution > Evolutionary Novelties
Early Embryonic Development > Fertilization to Gastrulation