Hormone interactions confer specific proliferative and histomorphogenic responses in the porcine mammary gland

Mammary gland growth and morphogenesis are regulated by interactions between hormones as much as by their individual actions. The effect of these interactions on the mammary gland phenotype in species other than rodents is relatively undefined. We investigated the individual and combined effects of estrogen (E), progestin (P), and prolactin (PRL) on mammary gland development in gilts. Pigs were shown to have a ductal-lobular parenchyma that underwent hormone-stimulated progression of terminal ductal lobular unit (TDLU) morphogenesis similar to that in the human breast. Ovariectomy plus hypoprolactinemia abolished mammary gland growth. Estrogen alone stimulated mammary epithelial cell proliferation, terminal bud formation, and the progression of TDLU1 structures to a TDLU2 morphotype. Maximal epithelial cell proliferation, DNA content, parenchymal area, and morphological development of the porcine mammary gland were realized following treatment with E+PRL or E+P+PRL. In contrast, P alone did not promote epithelial cell proliferation, TDLU type progression, mammary gland growth, or morphogenesis. These data indicate that interactions between E and PRL are the main determinants of growth and morphogenesis in the porcine mammary gland.