Preimplantation embryonic development is a dynamic process that involves cell proliferation, differentiation, and death. Those processes are tightly regulated by signaling between the embryo and the maternal environment. The ability of an embryo to respond to changes in its environment is limited during the first cleavage divisions, when most of the embryonic genome is still inactive and when systems for regulating osmotic balance are not completely functional. This period of low transcriptional activity creates a window in which embryos are particularly sensitive to certain forms of stress. One of the alterations in the maternal environment that causes profound effects on embryonic survival is an increase in body temperature due to heat stress or fever. Exposure of embryos to elevated temperature decreases development and reduces protein synthesis. Indeed, exposure of females to heat stress during preimplantation development reduces embryo survival.
In several species, deleterious effects of heat shock decrease as embryos advance in development. This has been shown in vivo in sheep, pigs, cattle, and rabbits and in vitro for cattle. Thus, the embryo acquires one or more thermoprotective responses as embryonic development proceeds.
One of the processes that may be involved in developmental acquisition of resistance to heat shock may be stress-induced apoptosis. Apoptosis plays a role in mammalian development as a quality control mechanism to eliminate cells that are damaged, nonfunctional, abnormal, or misplaced. Cells severely damaged by stress that do not undergo apoptosis often become necrotic.
TUNEL-positive embryos have been demonstrated in mouse, human, and bovine species. The occurrence of apoptosis in bovine embryos as determined by TUNEL staining has been shown to be developmentally regulated. Spontaneous apoptosis was first observed in bovine embryos at the 8- to 16-cell stage. This stage of development in cattle is coincident with the time of the major activation of the embryonic genome, and it is possible therefore that embryonic transcription may be involved in apoptotic responses in embryos.
Although apoptosis is known to occur in preimplantation embryos, there are few studies on extrinsic or intrinsic control systems for activation of apoptosis in preimplantation embryos or the ontogeny of such systems. Moreover, it is not known whether agents such as heat shock, which can induce apoptosis in many cell lines through activation of acid sphingomyelinase, also induce apoptosis in preimplantation embryos. The working hypothesis of this series of experiments was that heat-induced apoptosis is a developmentally regulated process. Experiments were performed to 1) determine whether heat shock can induce ap-optosis in preimplantation embryos, 2) test whether heat-induced apoptosis in embryos depends upon stage of development, 3) evaluate whether heat shock-induced changes in caspase activity parallel patterns of apoptosis, and 4) ascertain whether exposure to a mild heat shock can protect embryos from heat-induced apoptosis. The last objective was conducted because of evidence that apoptosis in heat-shocked cells can be reduced by heat shock protein 70 (HSP70) or mild heat shock that presumably induces HSP70 synthesis.