Stress for stress tolerance? A fundamentally new approach in mammalian embryology

In vitro culture, storage, and manipulation of gametes and embryos require meticulously adjusted conditions to avoid or minimize the harmful effects of uncontrolled stress. However, recent work indicates that a well-defined and properly applied stress may induce general adaptation and increase tolerance to various in vitro procedures. The aim of this review is to summarize reports on the effects of stress on gametes and embryos of several species. Treatment with sublethal doses of high hydrostatic pressure (HHP), or osmotic, heat, or oxidative stress resulted in increased morphological survival, fertilizing ability, or developmental potential after various in vitro or in vivo procedures. HHP treatment of spermatozoa, oocytes, embryos, and embryonic stem cells increased fertilizing ability, developmental competence, and differentiation and improved results after cryopreservation, parthenogenetic activation, intracytoplasmic sperm injection, and somatic cell nuclear transfer. Osmotic stress of oocytes resulted in higher developmental rates after cryopreservation, parthenogenetic activation, and somatic cell nuclear transfer. Heat shock was reported to increase developmental competence of parthenogenetically activated oocytes. Although cellular and subcellular mechanisms supposedly contributing to these processes require further research, the new principle, i.e., to improve the stress tolerance by a defined sublethal stress, may outline a completely new strategy in mammalian embryology, as well as cryopreservation of other cells and tissues with remarkable theoretical and practical consequences.