There are a handful of species, around the world, that can figuratively put a pin in a pregnancy and come back to it at a more convenient time. It's called delayed gestation, or embryonic diapause. But how widespread could it be?
Embryonic Diapause and the Animals Who Love It
What do the sea otter, the wallaby, and the skunk have in common? They can all stop a pregnancy in its tracks - at least for a little while. While there are all kinds of animals that can save unfertilized eggs or packets of sperm, these animals wait until the egg is fertilized and begins multiplying to take a breather. The result is a blastocyst, a tiny clump of cells that will turn into the embryo surrounded by a tiny layer of cells that will turn into the placenta. For most animals, the blastocyst implants automatically. For this special group of animals, including about 100 mammals and marsupials, it floats around until the animal is ready to carry the pregnancy to term.
There are a few things that scientists know about embryonic diapause. It can be facultative, meaning if a female gets knocked up while still nursing a litter of infants the pregnancy is delayed until she's done getting the calories sucked out of her and can fatten up for a new successful pregnancy. The evolutionary advantage of that is fairly easy to see. Embryonic diapause can also be obligate. Obligate embryonic diapause, in the species that display it, is a constant feature of the pregnancy. It's also a source of frustration for many scientists.
The Why and the How
The animals that can do this include rodents, marsupials, pigs, armadillos, and the red panda. They can put their pregnancies on hold for a matter of days or for a year. Sometimes delayed gestation can be turned on and off by something as simple as simulated daylight for extended periods of time. Sometimes scientists can remove a delayed animals' ovaries, and the blastocysts will still implant. There's no reliable relationship between animals that display embryonic diapause. When zoologists examine two types of weasel which live in the same forest and share the same food, they'll find that only one can manage delayed gestation and the other cannot. And yet species as far removed as armadillos and sea otters display the same trick.
And then there's the mystery of why certain species engage in embryonic diapause at all. A female fisher - a member of the weasel family - will mate with a male shortly after she has her pups. Her ova are fertilized and they begin to develop. Then they stop for about nine months, and continue developing the next year. No one knows why the fisher would take on fertilization only to do nothing with it for a year.
Scientists know a few things about the mechanisms of embryonic diapause. Facultative diapause is caused by lactation's suppression of the corpus luteum - a hormone secreting body in the ovary that prepares the uterus for implantation. Obligate diapause is tougher to pin down, but seems to have something to do with the superior cervical ganglion. That's part of the nervous system which regulates the unconscious actions of the body, specifically when the body rests and relaxes.
Embryonic Diapause in Humans?
Delayed gestation only shows up naturally in certain species. It can be induced in others. Recently scientists have found that blastocysts from species which do not usually experience diapause can be put through the process and are none the worse for it. Sheep blastocysts were removed from sheep, and put into mice. Researchers had induced diapause in the mice. As soon as the blastocysts were in the mice, they halted their development. The blastocysts were then implanted back into sheep, and grew up to become healthy lambs.
The researchers believe that embryonic diapause is a trait common to all mammals. Everything mammalian evolved from something that could pause its pregnancy, putting off blastocyst development until a more convenient time. Perhaps other species, including humans, could go back to that. The researchers propose taking a good look at human fertility patients, particularly for pregnancies that extend beyond the due date. Research is in its earliest stages, but it's just possible that humans might someday be able to put a pregnancy on hold.
Image: Torsten Mangner
[Via Kangaroos Embryonic Diapause, Mothers in Waiting, Abolition of Embryonic Diapause in a Wallaby, Embryonic Diapause and its Regulation, Embryonic Diapause is Conserved Across Mammals, Embryonic Diapause in Humans: Time to Consider?]