Role of atresia in establishing the follicular hierarchy in the ovary of the domestic hen (Gallus domesticus)

Abstract

At peak laying periods the ovary of the domestic hen contained 30-100 small yolky follicles with diameters varying between 1 and 8 mm. The number of these healthy follicles decreased with increasing size in that there were about 20 follicles with a diameter of 1-2 mm and 1 follicle (mean < 1) with a diameter of 7-8 mm. The number of follicles with diameters > 8 mm (the hierarchy of large, yolky follicles) varied between 4 and 7. By using a dye-marker, growth from 3 to 5 mm was estimated to take 3 days; from 5 to 8 mm; 2 days and from 8 mm to ovulation, 6 days. No information was obtained for growth between 1 and 3 mm because the dye did not enter these smaller follicles. Between 5 and 25 small yolky follicles were atretic. The reduction in the number of follicles with time and the high incidence of atresia suggest that this is a normal fate of small yolky follicles in birds with a high rate of lay. In marked contrast, only 1 large yolky follicle was observed to be atretic throughout the whole experiment. These 2 very different levels of atresia serve to define 2 distinct groups of follicles in the size range studied. Ovulation rate appears to be the product of 2 complementary mechanisms, one for the initiation of growth and the other controlling the rate at which the small yolky follicles are lost through atresia.