Scientists have created a three-dimensional model of developing limbs in the human embryo and found previously unnoticed muscles - atavisms that have survived in embryonic development since the times of the ancestral quadruped animals. The order of the appearance and disappearance of similar muscles in development does not coincide with the current evolution of limbs, and also differs in the hands and legs - and therefore, the similarity of their structure may be secondary, wrote scientists in the journal Development.
Much of the knowledge about the development of human body parts that we now have is obtained at the beginning of the twentieth century. Anatomists then examined stillborn and premature infants, built wax models of their body parts and studied histological tissue sections.
Modern technologies make it possible to examine the structure of embryos in more detail. In 2017, French scientists created a three-dimensional model of human embryo development in the first trimester of pregnancy. For this purpose, they stained 36 embryo bodies with 70 different antibodies, which helped them to restore the position of vessels, nerves, and muscles.
Rui Diogo from Howard University Medical College and his colleagues used this data to create a new, more detailed description of limb development in human embryos. In the process, they confirmed that they have atavistic muscles in their arms and legs that adults do not have, but that there are, for example, reptiles.
For example, the researchers found for the first time in the legs of the embryo a muscle that contrasts the little finger of the hand. It has been preserved in embryos for up to 11.5 weeks. This is an unusually long time for atavism: by comparison, a visually discernible tail is lost in a person's embryo around week eight.
Most of the atavistic muscles disappear without a trace or merge with other muscles at the time of birth. But occasionally they can all be found in adults, such as those with an additional 13, 18 or 21 chromosomes. Atavisms alone has no effect on a person's life, at least not in comparison to other pathologies that bring with them the presence of an extra chromosome. The fact that they are present, however, has allowed the authors to suggest that such congenital anomalies may be associated with stopping or delaying embryonic development.
Researchers have compared the order of appearance and disappearance of muscles in the upper and lower extremities of man. They found that some muscles that are similar in structure and function are formed at different times. For example, the worm-shaped muscle of the hand, which flexes the phalanges of the fingers from the index finger to the little finger, is differentiated before the neighbors, and the similar muscle in the foot - the last, when all other muscles have already formed. Based on their observations, the authors conclude that the similarities between limb muscles can be secondary, and initially, their structure and development differed significantly.
Finally, scientists have compared the development of human limbs with the development of other four-legged animals. They found that the order of differentiation of muscles within the limb does not always coincide with their appearance in evolution. For example, the plantar muscle is the last to develop among the foot benders, although it appeared earlier than some other muscles.
In the 19th century, it was believed that germinal development fully reproduces evolutionary (according to the biogenetic law, "Ontogenesis is the recapitulation of phylogenesis"). By the end of the XX century, it was repeatedly shown that not in all cases it is so, but many researchers continue to look for parallels between development and evolution. Using the example of human embryos as an example, researchers have once again demonstrated that there are no strict parallels between them.
Scientists keep finding new organs in the human body and other mammals - although they usually do not resemble organs in the usual sense of the word, but rather resemble formations inside tissues. For example, a new structure for fluid transport was discovered in connective tissue in 2018, and a new "organ" of pain sensitivity was recently found in bones.