CELL MIXING DURING THE EARLY DEVELOPMENT OF MOUSE AGGREGATION CHIMERA

Two different inbred strain combinations of mouse aggregation chimeras C3H/HeN (H-2(k)) x C57BL/6N (H-2(b)) and C3H/HeN x BALB/cA (H-2(d)) w ere used for cell mixing analysis at two points in time - 24 h after a ggregation (just prior to transplantation into foster mothers) and 7.5 days post coitum (p.c.). The cell proportion of two H-2 haplotypes at the blastocyst stage was studied using a fluorescence-labeled monoclo nal antibody recognizing a CBH-specific alloantigen - CSA (C3H strain- specific antigen) and laser scanning confocal microscopy. The 7.5-day- old chimeras were sectioned and subsequently processed by sensitive bi otinylated antibody - avidin peroxidase immunohistochemical technique. Our results showed that 24 h after aggregation (blastocyst stage), th ere was equal cell mixing and no mouse strain used in the present stud y was dominant at this time. In 7.5-day-old C3H/HeN x BALB/cA chimeras . cells of both genotypes were intermingled, but the C3H/HeN strain wa s dominant in all cases. In contrast, the combination C3H/HeN x C57BL/ 6N clearly showed reduced numbers of C3H/HeN cells (CSA-positive) in 8 3% of the chimeras evaluated. Generally, CSA positive cells were found only in randomly distributed small distinct areas representing less t han 20% of embryonal cells. Surprisingly, the extraembryonal ectoplace ntal cone was uniformly CSA positive in some C3H/HeN x C57BL/6N chimer as. Furthermore, in 36% of normally implanted chimeras of both strain combinations progressive degeneration was observed. We suggest that th e cell mixing pattern as well as the absolute number of cells derived from each strain in the aggregation chimera can be affected by specifi c immune interactions involving H-2 haplotype combinations of the allo geneic fetus and the fully immune; competent host organism, at later p oints in development.