Effects of short-term in vitro heat stress on bovine preantral follicles

Abstract

Heat stress occurs when heat dissipation mechanisms are overwhelmed by external and internal heat production. Ovarian reserve is an important contributor to reproductive success in cattle. The effects of heat stress on antral follicle development have been well studied; the consequences on preantral follicles are still largely unknown. This study aimed to investigate and characterize the effects of a short in vitro heat stress exposure on bovine preantral follicles, with a special focus on primordial follicles. Bovine ovarian cortex fragments (3 × 3 × 1 mm) were isolated and incubated at different temperatures for 2 h. The incubation methodology was validated using a non-incubated group. Incubation groups consisted in physiological temperature (38.5 °C) or heat stress (41 °C). Data on follicle count, diameter of the primordial follicles, number of granulosa cells, and markers for follicle proliferation and degeneration were recorded. Heat stress caused a significant decrease in the number of primordial follicles (p < 0.0001) compared to control and non-incubated groups. The diameter of these follicles also decreased under heat stress (p < 0.0001), as did the number of granulosa cells. In addition, an increase in the number of degenerated follicles (p < 0.0001) and a decrease in granulosa cell proliferation (p < 0.0001) were found after exposure to heat stress. The results of this in vitro study show that a brief exposure to heat stress can affect the follicular reserve in the ovarian cortex and therefore, provide relevant information of its effects on the reproductive performance of cattle.