Molecular mechanism of bovine mammary gland involution

Mammary gland remodelling is related to the cyclic growth and involution of secretor tissues regulated by a dynamic equilibrium between apoptosis and mitosis of mammary epithelial cells (MEC). In bovine mammary glands the loss of MEC populations begins after the peak of lactation, however, the most dynamic induction of MEC apoptosis is associated with cessation of milking at the beginning of the dry period. There are several factors which may affect the rate of MEC apoptosis in dairy animals, including systemic factors (the frequency and efficiency of milking, nutrition, galactopoietic hormones and reproductive status) as well as intra-mammary factors, such as: milk stasis and physical distension of the epithelium, feedback inhibitor of lactation-FIL, IGFBPs (IGFBP-3, IGFBP-5), Fas ligand, detachment from extracellular matrix and TGF-beta(1). It has been shown that TGF-beta(1) is an antiproliferative and apoptogenic agent for bovine MEC, but the regulation of this cytokine expression in the mammary glands of cow is still unknown. Recent studies have revealed that TGF-beta(1),expression in bovine MEC undergoes complex regulation by hormones of somatotropic axis: GH, IGF-I and somatostatin. Understanding the molecular mechanisms of bovine mammary gland proliferation and involution is indispensable in order to control mammary gland remodelling. This knowledge may lead to introducing new strategies for lactation cycle regulation and increase of milk production.