The mesentery (and peritoneum) explained
The findings of this paper confirm the mesentery is the organ in and on which all abdominal digestive organs develop and remain connected to. They support the Mesenteric Model of abdominal anatomy, clarify the anatomical foundation of the abdomen and the fundamental order at foundation level.
Behind the paper
In this paper, we clarify how the mesentery develops in the human fetus, and the shape of the mesentery in the adult. In effect, we clarify the nature of the mesentery, i.e. what it is1. Some of the findings were unexpected in so far as they go beyond being relevant to just the mesentery. We found direct evidence that the mesentery is the organ in and on which all abdominal digestive organs develop, then remain directly connected to throughout life2. In the fetal setting, it is an incubator. In the adult setting it is an integrator.
The finding that all digestive organs are directly connected to the mesentery has significant implications. It means that when the abdomen is considered as a whole, it is not the complex mix of separate organs previously thought. As the abdomen develops, it does so as two discrete compartments and that organisation remains throughout human life (Figure 1)1. One compartment is the mesenteric domain, which includes all digestive organs directly connected to the mesenteric organ. The other compartment is the non-mesenteric domain. This includes all genitourinary organs on the musculoskeletal frame of the abdomen.
The findings led to the identification of a structural foundation in the abdomen1. At the foundation level, the abdomen includes the mesenteric frame, the musculoskeletal frame, and the cavity enclosed. The organisation of those structures corresponds to the order at the foundation level. This means it is the fundamental order of the abdomen1. The distribution of abdominopelvic organs (as well as the vascular, neurologic and lymphatic circuitries of these) follows the order at the foundation level.
Collectively, the findings of the paper support the Mesenteric Model (i.e. description) of abdominal anatomy. Whilst it was suspected this model held, the findings are direct evidence that it does so3,4. The Mesenteric Model is welcome as it is far simpler than the conventional, peritoneal-based description of the abdomen.
Given the findings are relevant from the foundation or root level of the abdomen upwards, it is not surprising that one reviewer commented that the work has “…significant implications for a wide range of readers, including anatomists (human and comparative anatomy), embryologists, cell and molecular biologists and surgeons: note that the list is by no means exhaustive.”
The findings related to development of the mesentery help “complete the story of us” as individual biological entities. By following the development of the mesentery, we can follow that of the abdomen throughout the human life course. That was not possible up to now as there were substantial gaps in the story. Many of these gaps are addressed by taking the mesentery into account. We can also do the reverse, start with the adult shape and organisation and reverse engineer this without break to the earliest days of human life.
The anatomical findings of the paper answer questions that have challenged anatomists since the earliest recordings of anatomy. They clarify the nature of the mesentery and peritoneum. These are conventionally described as synonymous. Heretofore the mesentery was defined as a duplicature of peritoneum. The findings demonstrate the peritoneum corresponds to the surface lining of the non-mesenteric domain (visceral peritoneum), the surface lining of the mesenteric domain (parietal peritoneum), and the junction between both surfaces (the reflection)(Figure 1).
Up to now students of anatomy were required to describe the position of organs using several other organs as reference. Human anatomy became complex and required a considerable amount of learning by its students. Based on the findings of this paper, the position of all abdominal digestive organs can be simplified by referencing one organ, the mesentery. The Mesenteric Model of anatomy also allows us generate a blueprint of the position of all contents of the abdomen (Figure 2). Structures listed in Terminologica Anatomica 2 can be incoprorated and explained by this blueprint (Figure 2)5.
The paper includes a comparative section in which the mesentery of several species is compared with that of the human. Whilst the mesentery of the chimpanzee and human were remarkably similar, marked differences were apparent in the conformation of these, and those of other species examined. A similar foundation and design theme to that of the human abdomen, was apparent in all species examined. This suggests that the anatomical foundation and fundamental order of the human abdomen, may be relevant across the animal kingdom in general. The paper also hints of a similar design in fossilised records of ancestral life, which in turn suggests that the foundation design in the human abdomen may be relevant across time. Although not discussed in the manuscript, the findings of the paper can also be extended to the organ, tissue, cellular and subcellular level of other domains of life.
Finally, the paper describes how many aspects of mesenteric and intestinal shape can be explained by progressive changes in shape from a simple curve/buckle complex of the intestine and adjoining mesentery, to a coil/spiral complex involving both. In turn, these findings led us to question if mathematical modelling may explain observations related to mesenteric shape. Preliminary findings suggest that a conal model could explain mesenteric shape, thus bringing us back to the central importance of simplicity in Nature.
In conclusion, the organ-level findings described in this paper refute the idea that fundamental advances in our understanding of nature, can only be made at cellular, sub-cellular, molecular or sub-molecular levels of investigation.
The findings of this paper confirm the mesentery is the organ in and on which all abdominal digestive organs develop and remain connected to1. They confirm the anatomical foundation of the abdomen, and the fundamental order at that level1. The findings prompt investigation of a similar design across species, domains of life and time.
The findings demonstrate that abdominal anatomy is less complex than previously thought. We now have a significantly improved knowledge of what we are made of, and how we are made up. Such knowledge is essential if we are to systematically address the question as to how we function6. This knowledge is particularly important when it comes to understanding derangements of function, i.e. disease.
1 Byrnes, K. G., Walsh, D, Walsh, L.G., Coffey, D.M., Ullah, M.F., Mirapeix ,R., Hikspoors, J., Lamers, W., Wu, Y., Zhang, X-Q., Zhang, S-X., Brama, P., Dunne, C.P., O’Brien, I.S., Peirce, C.B., Shelly, M.G., Scanlon, T.G, Luther, M.E., Brady, H.D., Dockery P., McDermott K.W., Coffey, J.C. The development and structure of the mesentery. Communications Biology, doi:10.1038/s42003-021-02496-1 (2021).
2 Coffey, J. C., Byrnes, K.G., Walsh, D., Dockery, P. The mesentery and the mesenteric model of abdominal compartmentalisation. Gray's Anatomy (2020).
3 Coffey, J. C. in The 19th Congress of the International Federation of Associations of Anatomists. S090 (London, United Kingdom., 2019).
4 Coffey, J. C., Walsh, D., Byrnes, K. G., Hohenberger, W. & Heald, R. J. Mesentery - a 'New' organ. Emerg Top Life Sci 4, 191-206, doi:10.1042/ETLS20200006 (2020).
5 FIPAT. Terminologia Anatomica. 2nd ed. FIPAT.library.dal.ca. Federative International Programme for Anatomical Terminology, 2019
6. , E., , R. and