The enigma of embryonic diapause

Abstract

Embryonic diapause - a period of embryonic suspension at the blastocyst stage - is a fascinating phenomenon that occurs in over 130 species of mammals, ranging from bears and badgers to mice and marsupials. It might even occur in humans. During diapause, there is minimal cell division and greatly reduced metabolism, and development is put on hold. Yet there are no ill effects for the pregnancy when it eventually continues. Multiple factors can induce diapause, including seasonal supplies of food, temperature, photoperiod and lactation. The successful reactivation and continuation of pregnancy then requires a viable embryo, a receptive uterus and effective molecular communication between the two. But how do the blastocysts survive and remain viable during this period of time, which can be up to a year in some cases? And what are the signals that bring it out of suspended animation? Here, we provide an overview of the process of diapause and address these questions, focussing on recent molecular data.

The mTOR pathway in reproduction: from gonadal function to developmental coordination

“…Two forms of ED are currently known: obligatory or facultative. Obligatory diapause is species-specific and happens at a well-defined stage of development or in response to environmental stimuli such as photoperiod (Renfree & Fenelon 2017). It occurs in some bats, bears and marsupials (Fenelon et al 2014).…”

“…Facultative ED is frequent in some mammals such as mice. The hormonal regulation during lactation makes the uterus a hostile environment for implantation (Renfree & Fenelon 2017); however because female mice can mate instantaneously after giving birth, ED is a strategic mechanism to guarantee the survival of the next generation without jeopardizing the current litter. Consequently, embryos develop until the blastocyst stage, and when there is contact with the uterine wall (apposition), they become dormant.…”

WOMEN IN REPRODUCTIVE SCIENCE: Reproduction down under

“…In the tammar, over many years, Geoff Shaw and I and others, especially from Hugh Tyndale-Biscoe's lab. demonstrated the environmental and physiological signals for entry into and exit from diapause (reviewed in , Renfree & Fenelon 2017. These include melatonin, mediated by photoperiod beginning on the longest day, prolactin, that inhibits the growth of the corpus luteum, and progesterone that reactivates the secretory endometrium and blastocyst.…”

Homeobox genes for embryo implantation: From mouse to human

“…The blastocyst then attains a state of implantation competency which is known as blastocyst activation. In mice, pre‐implantation embryos can be suspended at the blastocyst stage without further initiation of attachment reaction during lactation, which is known as delayed implantation or embryonic diapause . In addition, embryonic diapause can be induced experimentally through ovariectomy on Day 4 before pre‐implantation estrogen secretion and then with daily injections of progesterone from Day 5, which can be terminated by a single injection of estrogen .…”