An ultrastructural study of the role of an extracellular matrix during normal cleavage in a marsupial, the brushtail possum.
In marsupials, the mechanisms of lineage allocation into pluriblast and trophoblast are related to conceptus polarity and polarized discharge of extracellular matrix (ECM). The brushtail possum, Trichosurus vulpecula, a major pest species in New Zealand, is being intensively studied to develop an immunocontraceptive control method. Of 23 specimens examined, 11 were examined by electron microscopy to study the presence and role of the ECM in lineage allocation in the possum. A number of polarized features in the zygote identified the future embryonic and abembryonic poles. Pronuclei, in a broad band of mitochondrion-rich cortical cytoplasm, lay in the embryonic hemisphere, and numerous electron-lucent vesicles characterized the abembryonic cytoplasm. These vesicles seemed to contribute to the ECM. During cleavage, cells lay near the zona in the embryonic hemisphere, and ECM accumulated chiefly in the abembryonic hemisphere. Cell-zona adhesion facilitated by microvillous and club processes occurred at the early 4-cell stage, and cell-cell adhesion commenced at the late 4-cell stage. The first two cleavages were meridional, equal, and accompanied by elimination into the cleavage cavity of much of the electron-lucent vesicular material in the form of several membrane-bound yolk masses. The third cleavage was unequal, with both meridional and latitudinal planes. The first differences between trophoblast and pluriblast lineages appeared at the 8-cell stage. Later cleavage planes were latitudinal or oblique. Conceptus polarity, polarized discharge of ECM, and localized cell-zona adhesion were related to the first lineage allocation in the possum.
Frankenberg
S
S
School of Zoology, La Trobe University, Victoria, Australia.
Selwood
L
L
D000818:Animals; D002970:Cleavage Stage, Ovum; D005109:Extracellular Matrix / Q000502:physiology* / Q000648:ultrastructure; D009893:Opossums / Q000196:embryology*; D015053:Zygote