title: The direct measurement of embryogenic volume and nucleo-cytoplasmic ratio during mouse pre-implantation development.
abstract: After fertilization, the mammalian conceptus undergoes cleavage, a process of cell proliferation in the absence of interphase growth. It is not known when cleavage ends and gives way to fully replicative cell cycles with a stable nucleo-cytoplasmic ratio. We have used two-photon excitation and confocal microscopy to measure directly volumes and nucleo-cytoplasmic ratios of whole murine concepti and their individual constituent blastomeres during pre-implantation development up to the early uterine attachment stage (day 5). We show that the total cytoplasmic volume of the conceptus remains constant during pre-implantation development, and that the average nucleo-cytoplasmic ratio increases exponentially throughout the same period. Data from individual blastomeres show that both volume and nucleo-cytoplasmic ratio diverge in the inner and outer subpopulations evident from the 16-cell stage (fifth developmental cycle) onwards. Cells from emergent outer trophoblast populations are larger and have smaller nucleo-cytoplasmic ratios than those from emergent inner pluriblast populations. Moreover, the nucleo-cytoplasmic ratio of the trophoblast appears to be stabilizing, suggesting that for this subpopulation cleavage may end at the 16-32-cell transition. Putative hypoblast and epiblast cell subpopulations within the pluriblast were not distinguishable by volume or nucleo-cytoplasmic ratio. Embryonic stem cell volume was higher than that of either cell subpopulation of expanded blastocysts, and their nucleo-cytoplasmic ratio was similar to that of trophoblast cells.
authors.affiliation: Department of Anatomy, Downing Street, Cambridge CB2 3DY, UK.
authors.forename: Catherine E M
authors.initials: CE
authors.lastname: Aiken
authors.forename: Peter P L
authors.initials: PP
authors.lastname: Swoboda
authors.forename: Jeremy N
authors.initials: JN
authors.lastname: Skepper
authors.forename: Martin H
authors.initials: MH
authors.lastname: Johnson
mesh_terms: D000818:Animals; D001757:Blastomeres / Q000648:ultrastructure*; D002452:Cell Count; D002467:Cell Nucleus / Q000648:ultrastructure; D049109:Cell Proliferation; D048429:Cell Size; D003593:Cytoplasm / Q000648:ultrastructure; D047108:Embryonic Development / Q000502:physiology*; D051379:Mice; D008815:Mice, Inbred Strains; D018613:Microscopy, Confocal; D014327:Trophoblasts / Q000648:ultrastructure