Marsupials Grow Newborns’ Arms Before Almost Anything Else Forms

A new study has found that marsupials develop their newborns’ arms before almost any other body part, producing functional forelimbs while the embryo remains little more than a flat layer of cells.

In the two marsupial species examined, an indistinct swelling turns into a clawed, usable arm in around four days-well before the embryo develops a head, heart or spine.

The findings call into question a developmental sequence that biologists had regarded as an established rule.

In chickens and mice, a tissue sheet must first divide into two layers and its cells must alter their form before a limb bud can emerge.

Marsupials, however, bypass this stage, potentially reshaping scientists’ understanding of limb development and the birth defects that interfere with it.

The newborn’s urgent crawl

Marsupials are born at an exceptionally incomplete stage following a remarkably brief pregnancy.

Blind, hairless and scarcely beyond the embryonic stage, newborns must make their way over their mother’s abdomen to find a teat. Without latching on, they cannot survive.

Their front legs are vital to this journey. At birth, the hind limbs remain small stumps, whereas the forelimbs already have claws and sufficient strength to pull the newborn forwards.

Dr Axel Newton, a developmental biologist at the University of Melbourne, set out to discover how an embryo can create usable arms so far in advance of the rest of the body.

Studying two distant relatives

Newton’s team investigated two marsupials that are only distantly related. The first was the fat-tailed dunnart (Sminthopsis crassicaudata), a mouse-sized Australian insect-eater.

The second species was South America’s grey short-tailed opossum (Monodelphis domestica).

Professor Karen Sears of the University of California, Los Angeles (UCLA), provided the opossum embryos.

As the evolutionary lines of these species separated roughly 80 million years ago, features they construct in the same manner are likely to be widespread among marsupials.

Skipping a crucial step

For most animals, limb development begins with a strip of tissue extending along each side of the young embryo. This tissue then divides.

The two layers pull apart around a fluid-filled space: the upper layer produces the body wall and limbs, while the lower layer develops into the internal organs.

A further process follows. Cells in the upper layer change their nature, breaking free from their orderly sheet and becoming mobile, migratory cells before gathering to form a limb bud.

A trigger for limb development

Research involving chick embryos identified this shift in cell state as the trigger that starts limb development.

Marsupials do not complete either process on that timetable. When Newton’s researchers cut through dunnart and opossum embryos as their forelimbs formed, they saw that the tissue had not separated.

Instead, the two layers remained joined in a sheet only one or two cells thick, even as cells in the upper layer multiplied and pushed outward into a limb bud.

No one had previously observed a limb developing in this manner. The entire change from unmarked tissue into an expanding bud occurs in approximately eight hours.

The physical separation and the cell-state transformation are not completed until several days later, when the arm is already substantially developed.

Marsupial limbs built in isolation

The marsupial embryo is particularly valuable because of what does not surround the developing limb.

When its forelimbs emerge, the embryo has neither somites-the blocks of tissue that later form the spine and muscles-nor even a head or a beating heart.

Biologists describe this rearranged developmental timing as heterochrony, in which one body part progresses far more quickly than the usual schedule.

At an equivalent point in limb development, a chicken embryo already possesses a head, a beating heart and over 20 pairs of somites.

This separation allows scientists to investigate which signals are genuinely necessary for a limb. Retinoic acid is one possible signal. Released by somites, this chemical was believed to help activate the master limb gene.

Yet marsupials produce forelimbs without any nearby somites, indicating that this signal is not essential for a mammal to form an arm. Findings from other research support the same conclusion.

A new model for limb formation

A separate single-cell study of opossum embryos found that limb tissue develops much earlier than it does in mice and rabbits, supporting the view that this acceleration starts during the first stages of organ formation.

For Newton, this clear separation is precisely what makes the model valuable. “Our previous understanding on how forelimbs grow has largely come from chicken or mice models, where many tissues grow at the same time.”

“The development of marsupial limbs in isolation from other tissues provides an excellent model to untangle how limbs first form.”

Clues to human defects

The research weakens one assumption that had been accepted for many years.

Dividing the tissue layer, creating the fluid-filled gap and remodelling the cells were all considered necessary to build a limb, but marsupials succeed without completing these stages in that sequence.

Those processes still occur, but only later-after the arm has already taken shape.

This adaptability matters to more than evolutionary biologists. Some human babies are born with absent or malformed arms and hands, and such conditions originate during these earliest stages of limb formation.

A model where a limb develops in isolation, separated from every neighbouring signal, offers researchers a clearer means of identifying which processes may fail.

Implications for conservation

Professor Andrew Pask, who heads the Melbourne laboratory responsible for the study, considers the work crucial for conservation.

“If we don’t understand the biology of how these animals are built, we can’t fully understand their vulnerabilities, their resilience, or how to intervene when a species is in trouble,” said Pask.

Animals once overlooked in favour of mice and chicks are now showing how limbs-and the bodies around them-are truly formed.

The rule was never as rigid as textbooks had suggested.

Image credit: Dr Axel Newton