QUANTITATIVE-ANALYSIS OF MIDGESTATION MOUSE AGGREGATION CHIMERAS - NONRANDOM COMPOSITION OF THE PLACENTA

Mouse chimaeras were produced by aggregating eight-cell embryos from two different F2 matings, abbreviated to AF2 and BF2 respectively: (C57BL/Ola.AKR-Gpi-1s(a), c/Ws female x BALB/c male)F2 and (C57BL/Ws female x CBA/Ca male)F2. Quantitative electrophoresis of glucose phosphate isomerase (GPI-1) was used to estimate the proportions of the two cell populations in different tissues of the 12 1/2 day chimaeric conceptuses, with the % GPI-IA indicating the percentage of cells derived from the AF2 embryos. The % GPI-1A was found to be highly positively correlated within the primitive ectoderm lineage (between the fetus, amnion and yolk sac mesoderm) and within the primitive endoderm lineage (between the yolk sac endoderm and the parietal endoderm) but no correlation (either positive or negative) was seen between the two lineages. This confirms the results of a previous study of chimaeras made between partially congenic strains and suggests the original conclusions have general validity. The % GPI-1A in the placenta was corrected for the expected contribution of maternal GPI-1, based on control experiments involving transfer of homozygous Gpi-1s(b)/Gpi-1s(b) embryos to the uteri of Gpi-1s(a)/Gpi-1s(a) pseudopregnant females. The corrected % GPI-1A in the placenta was positively correlated with that in each of the three primitive ectoderm derivatives. This suggests either (1) exchange of cells between the polar trophectoderm and the underlying part of the inner cell mass that forms the primitive ectoderm or (2) cells are incompletely mixed in the chimaeric blastocyst and patches of AF2 and BF2 cells straddle the boundary between the polar trophectoderm and the underlying primitive ectoderm. The second explanation does not imply the existence of shared developmental lineages between trophectoderm and primitive ectoderm in non-chimaeric embryos. Unlike that of any other tissue, the distribution of placental GPI-1A was U-shaped; in 17/28 placenta samples the proportion of the minor component was 10% or less. This suggests that the placental trophoblast is derived from a small number of coherenct clones of polar trophectoderm cells (either a small number of polar trophectoderm cells or a larger number if the two cell populations are not finely intermingled). Thus, although as a population the placentas of chimaeric conceptuses are balanced with respect to the % GPI-1A (mean close to 50%), individually most placentas are extremely unbalanced in their chimaeric composition (< 10% or > 90% GPI-1A). This non-random composition of the chimaeric placentas is in contrast to the widely held assumption that the distribution of cells in chimaeric conceptuses is normally random.