“The mesentery is a continuous set of tissues which is formed by the double fold of peritoneum that attaches the intestines to the wall of the abdomen. ['..] It has been proposed for reclassification as an organ due to research at the University of Limerick in the 2010s”
There is still considerable debate as to its function(s) , and even its structure :
“Whether the mesentery should be viewed as part of the intestinal, vascular, endocrine, cardiovascular, or immunological systems is so far unclear, as it has important roles in them all. Its effects are being investigated at haematological, immunological, endocrine, metabolic, and other levels.
Many, but not all, organs have a distinct functional unit. The functional unit of the mesentery is unknown, and whether a distinctive cell type is primarily responsible for its functionality should be investigated. Several anatomical questions also remain unanswered. For example, although early data suggest that the mesentery proximal to the duodenojejunal flexure is contiguous with it, this is still unclear.”
Source: (paywalled) The mesentery: structure, function, and role in disease The Lancet Gastroenterology & Hepatology, 2016