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 folliculog enesis. Nonlactating Holstein cows (n = 26) were treated in a 2 x 2 arrange ment 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 fi elds (2 x 2 mm) were examined and follicles within each field were classifi ed by histological stage. Fields that contained no follicles were classifie d as empty. Empty fields (n = 100 per ovary) were further classified as con taining no evidence of follicles or containing atretic remnants of follicle s. Approximately 30 4-mum sections per ovary were stained for proliferating cell nuclear antigen (PCNA), and 150 fields per ovary were evaluated. Addi tional sections (n = 10 per ovary) were assessed immunohistochemically for apoptosis, and fluorescence intensity was determined for each follicle. Tre atment with bST significantly decreased percentage of empty fields containi ng atretic remnants. Treatment with E-2 induced activation of follicles as shown by a decrease in percentage of primordial follicles and an increase i n percentage of primary follicles as determined by PCNA staining. At the pr imary follicle stage the combination of bST + E-2 decreased apoptosis as sh own by decreased fluorescence intensity. Thus, E-2 induced activation of fo llicles, bST enhanced survival, and the combination lowered atresia.