Final answer:
The mesentery must have specialized functions, containing essential structures like blood vessels and nerves, be composed of multiple tissue types, and be necessary for physiological processes to be considered an organ.
Step-by-step explanation:
If the mesentery is described as an organ, this implies several criteria must be fulfilled. First, it must have a set of specialized functions in the body, which may involve holding various abdominal organs in place against the dorsal body wall. It should also contain essential structures such as blood vessels, lymphatic vessels, and nerves that provide vital services like blood supply and innervation to adjacent organs. Moreover, it would be composed of multiple tissue types that work together to carry out its specific functions. Lastly, it must be a distinct structure that is necessary for a physiological process. Typically, organs are recognized in the medical community once they meet these criteria and have a defined anatomical and functional significance.