Embryonic diapause: development on hold.

Embryonic diapause, the temporary suspension of development of the embryo, is a fascinating reproductive strategy that has been frequently exploited across the animal kingdom. It is characterized by an arrest in development that occurs at the blastocyst stage in over 130 species of mammals. Its presumed function is to uncouple mating from parturition, to ensure that both occur at the most propitious moment for survival of the species. Diapause can be facultative, i.e. induced by physiological conditions, or obligate, i.e. present in every gestation of a species. In the latter case, the proximal signals for regulation are related to photoperiod. Three diverse models, the mouse, the mustelid carnivores and the wallaby have been studied in detail. From these studies it can be discerned that, although the endocrine cues responsible for induction of diapause and re-initiation of development vary widely between species, there are a number of commonalities. Evidence to date indicates that the uterus exercises the proximal regulatory influence over whether an embryo enters into and when it exits from diapause. Some factors have been identified that appear crucial to this regulation, in particular, the polyamines. Recent studies indicate that diapause can be induced in species where it does not exist in nature. This suggests that the potential for diapause in mammals to be due to a single evolutionary event, to which control mechanisms adapted when the trait was beneficial to reproductive success. Further work at the molecular, cellular and organismic levels will be required before the physiological basis of diapause is resolved.