Histological evaluation of the lactating mammary gland.

Multiparous Holstein cows were heat stressed (HT, access to shade only, n = 6) or cooled (CL, access to shade, fans, and water soakers, n = 6) during the entire dry period (~46 d pre-calving). Mammary biopsies were collected at 14, 42, and 84 days in milk (DIM). (A) Hematoxylin and eosin (H&E) stained mammary tissue at 10X, (B) number of alveoli in the mammary gland of CL and HT cows, (C) area of alveoli in mammary gland of CL and HT cows. (D) Masson’s trichrome stained mammary tissue at 20X, (E) proportion of connective tissue in mammary gland of CL and HT cows. Alveoli number was lower in HT compared to CL dams (P = 0.04). Alveoli area was not different between treatments (P = 0.84). The proportion of connective tissue was higher in HT compared to CL dams (P = 0.05) and higher at 14 DIM compared to 42 and 84 DIM (P = 0.04). No interactions between treatment and DIM were observed. All data are presented as LSM ± SE. Disparate letters indicate a significant difference over time (i.e. 14 DIM vs. 42 DIM, 14 DIM vs. 84 DIM, or 42 DIM vs. 84 DIM). An asterisk indicates a significant difference and n.s. indicates a non-significant difference between treatments. AL = alveoli, DUCT = mammary duct. Scale bar is 200 μm for H&E staining and 100 μm for Masson’s trichrome staining.