Cellular composition and expression of potential stem cell markers in mammary tissue of cows consuming endophyteinfected fescue seed during the dry period and early lactation

We evaluated the effect of consuming endophyte-infected fescue during late pregnancy on parameters of mammary development in Holstein cows. Cows (n = 16) were fed 10% of their ration as tall fescue seed that was free from (CON) or infected with endophyte (INF) from 90 d before expected calving until 10 d of lactation. Mammary tissue was biopsied during dry period (−32 d) and early lactation (+10 d). The percentage of tissue area that was occupied by epithelium, stroma and lumina was quantified. Epithelial proliferation was assessed by nuclear expression of the Ki67 antigen, detected by immunohistochemistry. Staining for putative mammary stem cell markers, nuclear receptor subfamily 5 group A member 2 (NR5A2), fibronectin type III domain containing 3B (FNDC3B) and musashi1 (MSI1), was evaluated and expressed as a percentage (% DAB pixels out of DAB plus hematoxylin pixels). Epithelial content of mammary tissue did not differ between CON and INF cows, nor did stromal and luminal areas differ between treatments in dry cows (P > 0.05). However, in lactating cows, tissue areas reflected greater milk yield in CON than INF cows (luminal area in CON > INF; stromal area in INF > CON; P < 0.05). Proliferation indices did not differ between mammary epithelia of CON and INF cows (P > 0.05). Similarly, nuclear staining of NR5A2, FNDC3B and MSI1 did not differ in INF vs. CON. However, there were differences (P < 0.05) in staining of all 3 markers between dry period and lactation (−32 d vs. +10 d). FNDC3B staining was greater during early lactation than the dry period (P < 0.001) and cytoplasmic staining of myoepithelial cells was observed during lactation. During early lactation, FNDC3B (r = 0.86; P = 0.13) staining tended to correlate with milk yield. Data indicate that fescue toxicity did not alter cellular composition of mammary tissue, epithelial proliferation rate, or expression of mammary stem cell markers. Immediate effects of fescue toxicosis on milk yield are likely mediated by influences on mammary differentiation and secretory activity.