The Evolutionary Origin of Intestines and Muscles

How did intestines, skin and muscles develop during the evolution of the animals? This question has been occupied by scientists for more than 100 years. The development biologists Ulrich Technau and Patrick Steinmetz from the University of Vienna question the 150-year-old "Keimblatt theory" by their work on sea anemones. They found that intestinal gland cells originate not from the endoderm, but from the ectoderm. Thus, they throw a new light on the evolution of intestines and muscle tissue. The results appear in "Nature Ecology & Evolution".

Early embryonic stage (gastrula) of the sea anemone Nematostella vectensis. During gastrulation, two animals (in sea anemones and other completely old animal lines) or three germinal leaves (almost all animal species and man) are applied by invagination or immigration processes in all animals. From these germinal layers, ie, cell layers, all further tissues are formed in the course of the development (Copyright: Sabrina Kaul-Strehlow, Patrick Steinmetz).

Human and animal bodies consist of a variety of tissues and organs, such as intestines, skin, nervous system and musculature. Interestingly, all these tissues are derived from one of the three “germinal leaves”, which are applied very early in embryonic development by invagination processes. According to the “Keimblatttheorie” skin and nervous system arise from the external ectoderm, the intestine and internal organs like pancreas emerge from the internal Entoderm, while musculature and germ-glands arise from the middle Keimblatt, the Mesoderm.

While almost all the more complex animals (mammals, but also insects or worms) and humans are built up from these three germinal leaves, their ancestors seem to have been only two-layered: some very old animal groups such as the nettles (cnidaria such as corals, sea anemones and jellyfish) , which have formed 600 million years ago, consist only of two cell layers. The evolution of the mesoderm as a third microscope was probably one of the “key discoveries” in the evolution of complex animals.

Up to now, science has assumed that the inner gut-forming microbial of nettles and other animals has a common origin, that is, homologous. Ulrich Technau and his team from the Department of Molecular Evolution and Development has now gained a fundamentally new view of the evolution of the cotyledons.They examined the embryonic origin of those cells which form digestive enzymes and were able to show that these cells do not originate from the endoderm, but from a part of the ectoderm, the pharynx.”I was amazed that the actual entoderm did not contain any digestive cells, contrary to the general opinion,” says first author Patrick Steinmetz, who has recently been a group leader at the University of Bergen in Norway.

“These results suggest that the entoderm of the higher animals is not evolutionarily related to the inner cell layer of the sea anemone, but rather has probably emerged from the pharyngeal ectoderm,” added Ulrich Technau. The entoderm of the sea anemone, on the other hand, has strong molecular and cellular similarities with the mesoderm of the higher animals: both activate a large number of homologous regulatory genes and both form tissues such as muscles and germs. The sea anemone thus already has a mesoderm-like microbe, which is not in the intermediate layer between the endoderm and the ectoderm as in other animals.

For the transition from the two-layered to the three-layered, therefore, a new localization and not a new discovery of the mesoderm was obviously the key event in evolution.

Source : University of Vienna