Molecular, functional, and cellular alterations of oocytes and cumulus cells induced by heat stress and shock in animals.

Global warming is considered as the main environmental stress affecting ecosystems as well as physiological and biochemical characteristics, and survivability of living organisms. High temperature induces various stresses and causes reduction of fertility through reducing the oocyte developmental competence and alteration in surrounding cells' functions. This causes major economic loss to livestock creating a selective pressure on animals to the advantage of better adapted genotypes and to the detriment of others. In this review, a search in Science Direct, Google Scholar, PubMed, Web of Science, Scopus, and SID databases until 2020 was conducted. Keywords which include heat stress, shock, high temperature, oocyte, cumulus, and animals were investigated. Studies have exhibited that heat stress can disturb the development and function of oocyte and cumulus cells (CCs) concerning reproductive efficiency. Heat stress has deleterious consequences on oocyte maturation and development via reduced number of polar body extrusion, adenosine monophosphate, and guanosine monophosphate synthesis. Heat stress caused the alteration of cytoplasmic and nuclear features as well as trans-zonal projections and gap junctions. In addition, heat stress is accompanied with reduced mitochondrial activity (copy mDNA number, distribution, and membrane potential) in cumulus-oocyte complexes. This review targets the description of results in the most recent studies that aimed to call attention to the influences of heat stress on molecular, functional, and cellular changes in oocytes and CCs in animals to design evidence on the acting mechanisms as the core of this problem from a comparative review.