The mammary gland is a dynamic organ that undergoes profound remodeling dependent on proliferation, differentiation, and apoptosis of mammary epithelial cells (MEC) during the cycle of the pregnancy, lactation, and involution. Long-lived populations of stem cells, which have a unique capacity for self-renewal, are responsible for these developmental changes. There is an increasing body of discoveries regarding human and mouse mammary gland stem cells, but the studies on bovine mammary gland stem cells are still very limited. According to morphological criteria bovine MEC are classified into two types: undifferentiated type I stem/progenitor cells assembling small light cells (SLC) and large light cells (LLC), and type II partially differentiated large dark cells (LDC) and terminally differentiated cells. To date there are no identified reliable molecular markers of stem/progenitor cells in bovine mammary glands. The main candidates are membrane transporting proteins of the Adenosine Binding Cassette (ABC) family, including Multi-drug-resistance protein 1 (Mdr1) and Breast cancer resistance protein 1 (Bcrp1). These proteins allow for the isolation of side populations (SP) of MEC assembling stem/progenitor cells by exclusion of dyes. Cytometric analysis of SP revealed from 0.2% to 5% of MEC in human and mouse mammary glands. The knowledge on the number and molecular properties of stem cells in bovine mammary glands would be very useful not only for enhancing milk production but also for explanation of the natural resistance against mammary cancer in this species.
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