Fine structure of somatotrophs and mammotrophs during development of the dwarf (dw) mutant mouse.

The differentiation of somatotrophs and mammotrophs in the pars distalis of dwarf (dw/dw) and normal (+/+; dw/+) litter mates was studied by means of electron microscopy from the day of birth to 24 days, as well as in adults at 3-4 months of age. On the basis of a combination of growth measurements, including weight, total body length, and tail length, dwarfs were identifiable with certainty as early as 5 days in litters from heterozygous matings (dw/+ x dw/+). Newborn dwarfs were obtained from homozygous matings (dw/dw x dw/dw) of adult dwarfs which had been brought to sexual maturity by hormone supplementation. The results indicate that the dwarf pituitary is already abnormal at birth. In contrast to the pars distalis in the normal newborn mouse, somatotrophs and mammotrophs cannot be distinguished from one another nor can they be distinguished as a group from other cell types in the dwarf. However, as development progresses, the dwarfs show a large population of an ambiguous somatotroph-mammotroph-like cell with fine structural characteristics of both somatotrophs and mammotrophs; these ambiguous cells also occur in the 3-4 months old adult.