Temperature gradients in female reproductive tissues and their potential significance
Anim Reprod, vol.6, n1, p.7-15, 2009
This review challenges the long-standing dogma that deep-body temperature should be regarded as uniform. Not only may this not be so, but small gradients in temperature in and across reproductive tissues might have assumed functional importance during the evolution of eutherian mammals. Temperature gradients within the Fallopian tubes of estrous animals are interpreted in a context of preovulatory storage of viable spermatozoa and their periovulatory activation and release from the functional reservoir in the caudal isthmus. Proposals concerning the response of potentially-fertilizing spermatozoa to increasing temperature along the isthmus of the Fallopian tube are recalled, and application of the term thermotaxis to this phase of sperm migration is assessed critically. Classical findings on the temperature of Graafian follicles in rabbits and women are highlighted, and more recent work on temperatures in pig preovulatory follicles is considered in detail. Although an experimental approach involving anaesthesia and infrared sensing is open to criticism, the finding that preovulatory follicles are cooler than ovarian stroma cannot be discounted as artifact. Instead, evidence for endothermic reactions that act to lower temperature within pre-ovulatory follicles is presented together with a description of relevant counter-current vascular physiology that enables maintenance of a cooler follicular temperature. As to future experimental work, the possibility is raised that temperatures may not be uniform across the cytoplasm of maturing oocytes nor at different stages of the cell cycle in very young embryos. These proposals lead to speculation that temperature may be exploited at a molecular level to modulate unfolding gene expression in zygotes and early cleavage stage embryos. Modern micro-imaging technology needs to be applied to such concepts.
Fallopian tube, spermatozoa, thermotaxis, Graafian follicle, endothermic reaction, oocyte, zygote, gene expression, cloning