Qualitative and Quantitative Micromorphological Investigations of the Tertiary Follicle Population During the Oestrous Cycle in Sheep

Summary. This study describes a quantitative micromorphological investigation of the tertiary follicle population in the ovaries of thirty-three normal cycling Texel ewes. The ovaries were serially sectioned at 10 μm and every section was examined. A pair of ovaries contained an average ±S.E. of 84·2 ± 38·1 (32·2%) normal and 177·1 ± 103·4 (67·8%) atretic tertiary follicles. Each ewe had 3·6 ± 2·8 normal and 8·7 ± 4·9 atretic follicles with a diameter greater than 2 mm. Of the normal tertiary follicles, 82·7% had a diameter of less than 1 mm. During the cycle, two growth waves of follicles ≥1 mm3 (∞-- 2 mm in diameter) occur. The first was seen from Days 1 to 10, the second from Day 6 of one cycle to Day 1 of the next. At the end of the first growth wave, there was only one large follicle per ewe whereas at the end of the second wave, usually two were found. The last follicle of the first wave disappeared by atresia and the last one(s) of the second wave by ovulation. It is suggested that the cyclical changes in progesterone level play an important rôle in the growth pattern of follicles ≥1 mm3 in volume in both growth waves.