Dynamics of ovarian follicle formation in the musk shrew (Suncus murinus L.) from birth to adulthood

Abstract

To understand the kinetics of ovarian follicular development in the musk shrew Suncus murinus L., the growth of follicles was studied from birth to adulthood. Six distinct types of normal follicles are described on the basis of follicular diameter (FD): class 1 (FD < 109 μm, where epitheloid cells of the theca are not differentiated); class 2 (FD between 100 and 150 μm); class 3 (FD between > 150 and 200 μm); class 4 (FD between > 200 and 250 μm); class 5 (FD between > 250 and 300 μm); and class 6 (FD > 300 μm). Folliculogenesis commenced in intrauterine life and was completed by day 10 after birth in this species. Class 1 follicles were present on day 0, whereas follicles of classes 2, 3, 4, and 5 and 6 appeared on days 10, 12, 13, and 20 after birth, respectively. The regression equations relating the growth of the oocyte to the growth of follicles exhibited a biphasic pattern as reported for other mammals. A positive linear relationship was noted between the total number of large follicles (classes 2–6) and the total number of small follicles (class 1), and between the numbers of healthy follicles and atretic follicles. The observed decrease in the total number of large follicles (classes 2–6) in the ovary in relation to age was found to be mainly due to reduction in the population of class 2 follicles. The presence of a greater number of oocytes at the time of birth and the subsequent presence of a greater number of follicles of classes 2–4 in the left ovary than in the right ovary of the musk shrew in all age groups suggests ovarian asymmetry in follicular distribution in this polyovular species.