Growth and development of the mammary glands of livestock: a veritable barnyard of opportunities.

The mammary glands of all mammals are rich and diverse in their histomorphogenesis, developmental biology, genomics and metabolism. Domesticated livestock comprise a unique population for the analysis of mammary gland and lactation biology, where much of what has been learned about these topics originates from studies of these species. However, with the strong trend toward using rodents as flexible and attractive models for normal mammary biology and cancer, there is a growing void of new information related to biology of the mammary glands in these relevant and informative domestic livestock. In turn, this trend threatens to reduce opportunities to either capitalize on an abundance of pre-existing data or to apply this information to studies of lactation and cancer. Herein we review the unique and discerning features of mammary gland development in several domestic livestock species including cows, sheep and pigs and provide an overview of the factors regulating it. At the same time we discuss some of the key considerations for studying these species, their limitations, and the associated opportunities. From such an analysis it quickly becomes clear that much remains to be learned about the mammary glands of domestic livestock, particularly given their many similarities to the human breast, the unique biological mechanisms they employ, and the phenotypic variation they afford.