Embryonic diapause: advances in understanding the enigma of seasonal delayed implantation.

Embryonic diapause is an evolutionary strategy by which a reversible arrest in embryo development occurs. In its two forms, facultative and obligate, it assures that offspring are born when optimal maternal and environmental conditions are present to ensure maximal survival. We have explored obligate delayed implantation in the mink (Neovison vison) over four decades: first by evaluation of the environmental regulation, then by determination of the pituitary factors that maintain diapause and provoke implantation followed by exploration of the ovarian contribution to the process. As the uterine environment is the proximal regulator of diapause, we employed a strategy of global gene analysis to discover differentially expressed pathways during embryo arrest and reactivation. These trials revealed that the synthesis of polyamines was increased in the uterus with reactivation of the embryo in vivo. Subsequent experiments demonstrated that the polyamine, putrescine, was capable of inducing escape of the embryo from obligate diapause, providing strong evidence that the paucity of polyamines induces developmental arrest, and reactivation is coupled to renewed uterine and/or embryonic synthesis of these polycations.