In many species, not long after fertilization, the embryo implants into the uterus wall, preparing it for further development. In humans this implantation occurs at around day eight to nine after conception. However, in European roe deer, instead of implanting, the embryo stops developing, hovering in a period of dormancy.

This phenomenon has now been found in over 130 species, including mice and armadillos. But the roe deer have one of the longest known periods of embryonic suspension, lasting up to five months. This period is called diapause. Unlike in many of the other species that can induce diapause, cell division in roe deer embryos do not stop completely, but drastically slows, with cells dividing just once every few weeks.

Until now, the cellular mechanism that regulated the process of extensively slowing down cell replication was unknown. However, a recently published study in PNAS has uncovered it. Researchers discovered that a predominant driver of embryonic diapause is the changing abundance of amino acids in the embryo. One family of amino acids in particular were found to cause a significant increase in a protein called mTORC1, inducing the embryo to activate more of it. In fact, the increase of mTORC1 appeared to immediately coincide with the embryo’s exit from diapause, after which cells start dividing more rapidly, but was not detectable during the previous period of slow cell replication.

The mTOR protein family has been known for many years to be a crucial factor in regulating metabolic pathways, including in humans. In fact, a related protein called mTORC2 thought to be essential for maintaining slow cell divisions remains switched on throughout roe deer diapause. This new study will open up avenues of research into the precise timing of embryo implantation, as well as increasing our understanding about the interplay between the chemical and metabolic pathways of an animal and its embryo.