Mammary gland development during pregnancy in the dwarf mouse mutant, little.

Little (lit) is a single gene dwarf mutation described in the C57Bl/6J strain of laboratory mouse. This study was designed to determine the effect of the mutation little on mammary gland development during pregnancy and post-partum periods. Failure of mutant females to lactate at first parturition appears to be due to the endocrine environment of the gland rather than any intrinsic defect of the mammary epithelium. Treatment of primiparous lit/lit females with exogenous growth hormone or prolactin resulted in lactogenesis. Treatment with hydrocortisone resulted in increased amounts of organized rough endoplasmic reticulum but lactation did not occur.