This mutation arose in a stock of silver mice obtained from an English fancier (J:13120). Homozygous mutant mice are about one-fourth to one-third normal size and are sterile. The small size is due to a defective anterior pituitary in which there is a great deficiency of GH-producing, PRL-producing, and TSH-producing cells (J:6754, J:7211, J:12161). The anterior pituitary of the Pit1^dw homozygote is already abnormal at birth with no identifiable GH or PRL cells (J:6684). GH and PRL synthesis is not detectable at any stages from birth to 6 weeks of age (J:6589), and there is probably also a deficiency of TSH and corticotropin (J:19241). Adult dwarf mouse pituitaries retain an embryonic, incompletely differentiated form of corticotrophs (J:13323). The defects in growth and fertility may be corrected by pituitary implants (J:13139) or by administration of pituitary hormones (J:30695, J:5085).

Two populations of cells give rise to thyrotrophs in the anterior pituitary in developing mouse embryos. The first population arises at day 12 in the rostral tip of the gland. This population is independent of Pit1, as it appears in Pit1^dw mice, but it disappears by birth. The second population, which arises in the caudomedial portion of the gland at embryonic day 15.5, is Pit1-dependent, and is absent in Snell dwarf mice(J:17223).

Pit1^dw mice have been reported to have a defective immune response that primarily affects the T cell system (J:19990), but other authors (J:6241, J:5638) have been unable to confirm these findings and attribute the previous results to secondary effects of dwarfing on overall vigor and nutritional status. Cross (J:2020) has shown that Pit1^dw homozygous mice do develop normal immunocompetence, but that this development is delayed relative to that in normal littermates. Dwarf homozygotes have a severe deficiency of dopamine in the median eminence (J:6652).