The mesenteric-based model of abdominal compartmentalisation

Introduction Recent advances in our understanding of mesenteric anatomy clarify its shape distal to the duodenojejunal flexure. Investigation of mesenteric anatomy has raised several questions about its overall shape. These include the shape of the mesentery proximal to the duodenum, its anatomical relationship with fascia at this level, the distribution of the peritoneal reflection, and how the overall structure of the mesentery arises from its embryological forerunner. This thesis aimed to characterise embryological development and anatomical form of the abdominal mesentery in humans. Methods An appraisal of mesenteric development was performed. As part of this appraisal, three-dimensional models and dynamic models of the mesentery at consecutive stages of development were generated. Next, a dissection was performed using cadavers to comprehensively investigate mesenteric shape in adulthood. This dissection applied a mesenteric-based approach that is commonly used in oncologic surgery rather than a peritoneal-based one. A comprehensive appraisal of the cross sectional appearance of mesenteric anatomy and reconstruction of its in situ anatomical shape was performed. To overcome the innate challenges in communicating the mesenteric model of abdominal anatomy, digital sculpting techniques and three-dimensional printing were used to generate virtual and physical models of the mesentery. Results The mesentery (in its ex vivo and in situ state) is continuous from oesophagogastric junction to mesorectal level. In most cases, abdominal digestive organs are connected to the mesentery, rather than the abdominal wall. Moreover, the peritoneal landscape is explained by the mesenteric landscape. The human mesentery remains a continuous structure throughout development and does not become fragmented. The mesentery develops according to a set developmental program during which a foundation shape is acquired (primary mesenteric organogenesis) and undergoes subsequent alterations at a sub-regional level (secondary mesenteric organogenesis). Hence, the abdomen is organised along mesenteric lines, into two principal compartments: the mesenteric and non-mesenteric domain. This system of organisation is referred to as the mesenteric model of abdominal anatomy. Digital sculpting and three-dimensional printing were used to communicate results relating to mesenteric anatomy and development. Conclusions The findings reported in this thesis identify a new level of organisation of abdominal contents that may reconcile embryological, anatomical and surgical approaches to the abdomen. The classic model of mesenteric anatomy, which is complex and fragmented, may be replaced with a mesenteric-based model in which digestive organs are organised around a single mesentery that attaches to the posterior abdominal wall. Limitations were present in each arm of this study. For example, the anatomical study required excision of the mesentery and so its shape was altered. The embryological study required interpolation between stages. Lastly, some structures were challenging to identify during cross-sections analysis. However, collectively, findings from embryological, foetal, anatomical and cross-sectional analysis of the mesentery provide an opportunity to propose a definition for the mesentery: the organ that, in the embryo, supports development of all abdominal digestive organs, and following birth, maintains all abdominal digestive organs in position and in continuity with other systems. This system, which may be more broadly applied to mammalian species in general, requires further investigation

The development and structure of the mesentery

The position of abdominal organs, and mechanisms by which these are centrally connected, are currently described in peritoneal terms. As part of the peritoneal model of abdominal anatomy, there are multiple mesenteries. Recent findings point to an alternative model in which digestive organs are connected to a single mesentery. Given that direct evidence of this is currently lacking, we investigated the development and shape of the entire mesentery. Here we confirm that, within the abdomen, there is one mesentery in which all abdominal digestive organs develop and remain connected to. We show that all abdominopelvic organs are organised into two, discrete anatomical domains, the mesenteric and non-mesenteric domain. A similar organisation occurs across a range of animal species. The findings clarify the anatomical foundation of the abdomen; at the foundation level, the abdomen comprises a visceral (i.e. mesenteric) and somatic (i.e. musculoskeletal) frame. The organisation at that level is a fundamental order that explains the positional anatomy of all abdominopelvic organs, vasculature and peritoneum. Collectively, the findings provide a novel start point from which to systemically characterise the abdomen and its contents