Effects of Heat Stress on Follicular Physiology in Dairy Cows

SIMPLE SUMMARY: Environmentally induced hyperthermia, also called heat stress (HS), compromises reproductive physiology in mammals. The number of oocytes is fixed after birth and they are stored in the ovary in a quiescent state (at the stage of the first meiotic prophase) in primordial follicles. There is evidence that HS alters the oocyte quality, the dynamics of follicular growth and ovulation. The dairy cow, submitted to the metabolic stress of high milk production, is a good model for studying the effects of HS on ovarian function. The aim of this review is to describe the influence of HS during the stages of follicular development in dairy cattle, from the activation of primordial follicles to ovulation. Some clinical aspects are also considered. ABSTRACT: Follicular organization starts during mid-to-late fetal life with the formation of primordial follicles. The bilateral interplay between the oocyte and adjoining somatic cells during follicular growth and ovulation may be sensitive to heat stress (HS). Mechanisms giving rise to pre-ovulatory temperature gradients across reproductive tissues are mostly regulated by the pre-ovulatory follicle, and because the cooling of the gonads and genital tract depends on a counter-current transfer system of heat, HS may be considered a major factor impairing ovulation, fertilization and early embryo development. There is evidence of a long-lasting influence of HS on oogenesis and final follicular maturation. Follicular stages that are susceptible to HS have not been precisely determined. Therefore, the aim of this review was to describe the influence of HS during the staged follicular development in dairy cattle, from the activation of primordial follicles to ovulation. Some clinical prospects are also considered.