Alteration of activation, growth, and atresia of bovine preantral follicles by long-term treatment of cows with estradiol and recombinant bovine somatotropin

The hypothesis was that long-term treatment of cattle with estradiol (E-2) and bovine somatotropin (bST) would alter the earliest stages of folliculogenesis. Nonlactating Holstein cows (n = 26) were treated in a 2 x 2 arrangement with E-2 (2 x 24 mg implants, 67.1 +/- 1.4 days) and bST (Posilac, 63.6 +/- 1.5 days). At Day 67 +/- 1.3, one ovary was removed for morphometric and immunohistochemical analysis. For each ovary, 388 +/- 38 microscopic fields (2 x 2 mm) were examined and follicles within each field were classified by histological stage. Fields that contained no follicles were classified as empty. Empty fields (n = 100 per ovary) were further classified as containing no evidence of follicles or containing atretic remnants of follicles. Approximately 30 4-mum sections per ovary were stained for proliferating cell nuclear antigen (PCNA), and 150 fields per ovary were evaluated. Additional sections (n = 10 per ovary) were assessed immunohistochemically for apoptosis, and fluorescence intensity was determined for each follicle. Treatment with bST significantly decreased percentage of empty fields containing atretic remnants. Treatment with E-2 induced activation of follicles as shown by a decrease in percentage of primordial follicles and an increase in percentage of primary follicles as determined by PCNA staining. At the primary follicle stage the combination of bST + E-2 decreased apoptosis as shown by decreased fluorescence intensity. Thus, E-2 induced activation of follicles, bST enhanced survival, and the combination lowered atresia.