Human Embryos Have ‘Lizard-Like’ Limb Muscles: Study | Anatomy, Biology

A new study published in the journal Development, confirmed the transient presence of atavistic muscles – present in our ancestors, but normally absent in adult humans – during normal human embryonic development, and revealed the existence of other muscles not previously described in human embryos. Some of these muscles, like the dorsometacarpals, disappeared from our ancestors more than 250 million years ago, during the transition from synapsid reptiles to mammals. In the hand and foot, of the 30 muscles formed at around 7 weeks gestation, a third will become fused or completely absent around 13 weeks gestation.

Dorsal view of the left hand of a 10 week old human embryo. The dorsometacarpals are highlighted: these muscles are present in adults of many other animal members, while in humans they normally disappear or merge with other muscles before birth. Image credit: Diogo et al, doi: 10.1242 / dev.180349.

Ever since Charles Darwin proposed his theory of evolution, scientists have argued that the appearance of atavistic structures strongly supports the idea that species change over time from a common ancestor to “descent with modification.”

For example, ostriches and other flightless birds have residual wings, while whales, dolphins, and porpoises do not have hind limbs, but their embryos initiate and then interrupt hind limb development.

Likewise, small, temporary tail-like structures are found in human embryos and the rest of the lost ancestral tail is preserved as our tailbone.

The researchers also suggested that atavistic muscles and bones can also be seen in human embryos, but it has been difficult to clearly visualize these structures, and the images that appear in modern textbooks are mainly based on analyzes many years old. decades.

In the new study, Dr Rui Diogo and his colleagues at Howard University, CNRS and Sorbonne Universities performed the first detailed analysis of muscle development in human arms and legs using 3D images. high quality human embryos and fetuses.

The unprecedented resolution of the 3D images revealed the transient presence of several of these atavistic muscles.

The team then compared their observations with a few previous studies focused on muscle development in the arms and legs in humans, in order to provide information that summarizes the time of onset, as well as division, fusion and / or the loss of each of these muscles. .

“Previously, we had a better understanding of the early development of fish, frogs, chickens and mice than in our own species, but these new techniques allow us to see human development in much more detail,” said Dr. Diogo. , a researcher in the Department of Anatomy at Howard University College of Medicine.

“What is fascinating is that we have observed various muscles that have never been described in human prenatal development, and that some of these atavistic muscles have been observed even in fetuses as young as 11.5 weeks old, this which is surprisingly late for development atavisms. “

“It is interesting to note that some of the atavistic muscles are found on rare occasions in adults, either in the form of anatomical variations without noticeable effect for the healthy individual, or as a result of congenital malformations”, a- he declared.

“This reinforces the idea that muscle variations and pathologies may be related to delayed or stopped embryonic development, in this case possibly a delay or decrease in muscle apoptosis, and helps explain why these muscles are found. sometimes in adults. “

“It provides a fascinating and powerful example of in-game evolution.”

“We hope that our work will not only contribute to the understanding of limb muscle development in humans and tetrapods in general, but will also pave the way and stimulate other researchers to undertake deeper and broader discussions of the links between the upper part and lower limbs, between atavisms, variations and anomalies, and between phylogeny and evolution ”, the scientists concluded.

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Rui diogo et al. 2019. Development of human limb muscles based on immunostaining of the entire mount and the links between ontogeny and evolution. Development 146: dev180349; doi: 10.1242 / dev.180349

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