[Mutant gene expression in mouse aggregation chimeras. 3. The brachypodism-H gene].

The expression of gene brachypodism-H (bpH) was studied on the 12-, 14-, 16-, 17-, 19-day old embryos and 20-day old bpH/bpH C/C----+/+ c + c mice. In 18 chimaeras, the percentage of pigmented cells in the retinal pigment epithelium varied from 2.5 to 84.0%, suggesting a different proportion of the mutant and normal cells in their tissues. The limb development was quite normal in all chimaeras: even in the presence of 84% of mutant cells the lengths of femur, fibula and foot did not differ from those in the control animals. The differentiation and histogenesis of these bones proceeded also quite normally. No increase in cell degeneration was noted and the bpH/bpH cartilage cells were missing, indicating the normalization of the mutant chondrocytes. The absence of the bpH gene expression in the chimaeras appears to be due to a metabolic cooperation between the mutant and normal cells taking place at the early stages of limb development.