Precedence of cell-zona adhesion over cell-cell adhesion during marsupial blastocyst formation prohibits morula formation and ensures that both the pluriblast and trophoblast are superficial.
This study, based on 38 samples taken between the 16-cell stage on day 2.5 of gestation and the expanded 1.0-mm-diameter unilaminar blastocyst on day 6, describes the ultrastructural changes that occur in the conceptus of the marsupial Sminthopsis macroura in relation to cell-zonal adhesion initiated at the zygote stage and cell-cell adhesion initiated at the 16-cell stage, lineage allocation, extracellular matrix (ECM) secretion and embryo coat changes. In S. macroura, rather flattened pluriblast and rounded trophoblast cells appear as different cell types during the fourth division when nucleolar reticulation suggests activation of the zygotic genome in both cell types. The differences disappear nearly completely in blastocysts of 0.6-0.8 mm in diameter, but the two cell types then reappear as two distinct populations. The ECM varies depending on its location within the conceptus up to the stage of the expanding blastocyst. It is of rather granular appearance between the cell lining and zona pellucida and consists of patches of homogeneous material embedded in an electron-lucent substance in the cleavage cavity. Homogeneous ECM coats trophoblast but not pluriblast cells on blastocoelic surfaces. Transient structures such as 'myosin-like' fibrillar arrays, probably associated with exocytosis of ECM, and pearl string-like whorls are still present, but both disappear during further expansion of the 0.6- to 0.8-mm blastocyst. During blastocyst expansion, the patchy homogeneous ECM in the blastocoel changes structure and appears flocculent, while the continuous ECM coating trophoblast cells disappears. Pluriblast cells and yolk mass identify the embryonic pole and hemisphere, and the opposite hemisphere becomes abembryonic and is eventually fully lined by trophoblast cells. An increase in endocytotic, mainly coated vesicles at the apical, zona-orientated surface of both cell types is noticed and is probably responsible for uptake of the mucoid coat. In 1-mm blastocysts, numerous vesicles contain rod-shaped crystalline inclusions.
Department of Anatomy, University of Basel, Basel, Switzerland. Annetrudi.Kress@unibas.ch
Annetrudi
A
Kress
Lynne
L
Selwood
D000818:Animals; D001755:Blastocyst / Q000502:physiology* / Q000648:ultrastructure; D002448:Cell Adhesion; D002454:Cell Differentiation; D005109:Extracellular Matrix / Q000378:metabolism; D005260:Female; D005865:Gestational Age; D008394:Marsupialia / Q000196:embryology*; D009028:Morula / Q000502:physiology* / Q000648:ultrastructure; D011247:Pregnancy; D014327:Trophoblasts / Q000502:physiology* / Q000648:ultrastructure; D015044:Zona Pellucida / Q000378:metabolism*