Maternal lactocrine programming of porcine reproductive tract development.

The lactocrine hypothesis for maternal programming of female reproductive tract development is based on the idea that non-nutritive, milk-borne bioactive factors (MbFs), delivered from mother to offspring during nursing, play a role in determining the trajectory of development with long-term consequences in the adult. Porcine female reproductive tract development is completed postnatally, and the period during which maternal support of neonatal growth derives exclusively from colostrum/milk defines a window of opportunity for lactocrine programming of reproductive tissues. Beyond nutrition, milk serves as a delivery system for a variety of bioactive factors. Porcine relaxin is a prototypical MbF. Present in colostrum at highest concentrations at birth, relaxin is transmitted into the circulation of nursing piglets where it can act on Relaxin receptors found in neonatal female reproductive tract tissues. This process is facilitated by the physiology of the maternal-neonatal dyad and the fact that the neonatal gastrointestinal tract is open to absorb macromolecules for a period of time postnatally. Age at first nursing and duration of nursing from birth are also important for porcine female reproductive tract development. These parameters affect both the quality and quantity of colostrum consumed. Disruption of lactocrine signaling by feeding milk replacer from birth altered porcine uterine, cervical, and testicular development by postnatal Day 2. Moreover, insufficient colostrum consumption in nursing piglets can impair uterine capacity to support viable litters of optimal size in adulthood. In the pig, lactocrine signaling supports neonatal organizational events associated with normal reproductive development and may program adult uterine capacity.