Sex chimaerism, fertility and sex determination in the mouse.

Adult intraspecific mouse chimaeras, derived by introducing male embryonal stem cells into unsexed host blastocysts, were examined to determine whether gonadal sex was correlated with the sex chromosome composition of particular cell lineages. The fertility of XX in equilibrium XY and XY in equilibrium XY male chimaeras was also compared. The distribution of XX and XY cells in 34 XX in equilibrium XY ovaries, testes and ovotestes was determined by in situ hybridisation using a Y-chromosome-specific probe. Both XX and XY cells were found in all gonadal somatic tissues but Sertoli cells were predominantly XY and granulosa cells predominantly XX. The sex chromosome composition of the tunica albuginea and testicular surface epithelium could not, in general, be fully resolved, owing to diminished hybridisation efficiency in these tissues, but the ovarian surface epithelium (which like the testicular surface epithelium derives from the coelomic epithelium) was predominantly XX. These findings show that the claim that Sertoli cells were exclusively XY, on which some previous models of gonadal sex determination were based, was incorrect, and indicate instead that in the mechanism of Sertoli cell determination there is a step in which XX cells can be recruited. However, it remains to be established whether the sex chromosome constitution of the coelomic epithelium lineage plays a causal role in gonadal sex determination. Male chimaeras with XX in equilibrium XY testes were either sterile or less fertile than chimaeras with testes composed entirely of XY cells. This impaired fertility was associated with the loss of XY germ cells in atrophic seminiferous tubules. Since this progressive lesion was correlated with a high proportion of XX Leydig cells, we suggest that XX Leydig cells are functionally defective, and unable to support spermatogenesis.