Serosa

Recommended video: Peritoneal relations [27:50]

Peritoneal cavity as seen in a parasagittal section.

The serosa, also known as the serous membrane, is a single layer of simple squamous epithelium called mesothelium. It is supported by a thin underlying layer of loose connective tissue, abundant in blood vessels, lymphatic vessels, nerves and adipose tissue. It lines closed body cavities including the pericardial, peritoneal and pleural cavities and forms the outermost covering of organs within these cavities.

The serosa consists of two parts: a parietal layer that lines the body cavity and a visceral layer that envelops the organs. It secretes a thin, watery transudate called serous fluid, which reduces friction between the contacting surfaces of the parietal and visceral layers. In addition to reducing friction, the serosa serves as a protective barrier and provides structural support to certain organs, helping to anchor them in place.

In the peritoneal cavity, the serosa forms the outermost layer of the gastrointestinal (GI) tract wall, which is continuous with the mesothelium of the mesenteries and the peritoneum. In regions where the GI tract lacks a serosal layer, such as the thoracic part of the esophagus, and in certain retroperitoneal organs including the duodenum, ascending and descending colon, as well as in extraperitoneal organs like the rectum and anal canal, a thick connective tissue layer known as the adventitia blends with the tissues of neighboring structures.