Physiologic Responses of Mammalian Oocytes and Embryos under Thermal Stress: An Overview

Abstract

Animals respond to environmental stresses by a global adaptation and adjustment to their physiologic homeostasis in order to eliminate most harmful changes and survive from the insults. This adaptation is the summation of all responses from their building blocks, cells with their associated cellular machineries in response to the undesired environmental clues. It has been known that heat shock proteins (Hsps), some of which are molecular chaperones present in all organisms, are a group of universal cellular proteins to oppose environmentally induced denaturation of many other proteins. These proteins function to assist in folding of newly synthesized proteins and maintain preexisting proteins in a stable conformation without aggregation under stress conditions, which are essential for thermal adaptation in prokaryotes and eukaryotes, including mammalian cells. Although thermobiology has been one of the active fields of study in cell biology or cancer therapy, there is less information available for the adaptation or responses to thermal stresses in embryonic cells such as oocytes and embryos. Previous studies have shown that a short-term heat shock (HS) impacts the developmental competence of embryos during the early phase of apoptosis and the alteration of intracellular calcium concentrations of matured porcine oocytes. Cleavage and blastocyst rates declined while the Ca^(2+)-releasing ability of matured oocytes was enhanced by a short duration (2 h) of HS, but declined after prolonged heat exposure. Taken together, the mechanisms of physiologic adaptation in response to thermal stress in oocytes and embryos are complex processes. HS can cause multiple changes to the oocyte and developing embryos such as enzymatic reactions, ionic influxes, DNA structure and cytoskeleton reorganization, as well as changes in the ooplasmic [Ca^(2+)]i after various intensities of HS. These phenomena may be critical parameters to evaluate their developmental competence. Delicate equilibrium between the deleterious effects and thermotolerance thermotolerance of oocytes, embryos and even the whole animal adapting to HS is one of the decisive factors in determining their fate during the course of development.