serosa

**Structure/histology:** The serosa consists of a single layer of simple squamous epithelium (mesothelium) resting on a thin layer of loose connective tissue (lamina propria-like). Mesothelial cells have microvilli and secrete a small amount of serous fluid that lubricates surfaces and allows organs to glide against one another. Mesothelial cells stain positive for cytokeratin, calretinin, and WT-1 (useful for diagnosing mesothelioma).

**Serous membranes by body cavity (visceral and parietal layers):** - Pleura: lungs (visceral) and thoracic wall (parietal) - Pericardium: serous pericardium covers the heart (visceral layer = epicardium) and lines the fibrous pericardium (parietal layer) - Peritoneum: abdominal organs (visceral) and abdominopelvic wall (parietal) - Tunica vaginalis: testis (a peritoneal derivative from the processus vaginalis)

**GI tract layers (inside to outside):** mucosa (epithelium, lamina propria, muscularis mucosae) → submucosa (Meissner plexus) → muscularis externa (Auerbach/myenteric plexus) → serosa or adventitia. - **Serosa:** intraperitoneal organs (stomach, jejunum, ileum, transverse and sigmoid colon, liver, spleen, gallbladder, first part of the duodenum). - **Adventitia (no serosa):** retroperitoneal structures and those lacking peritoneal covering (esophagus, most of the duodenum, ascending/descending colon, rectum, pancreas, kidneys). The esophagus has only adventitia, so there is less barrier to the spread of infection or tumor.

**Clinical significance:** - **Peritonitis:** inflammation of the serosa/peritoneum (perforated ulcer, appendicitis, SBP in cirrhosis/ascites). It causes rebound tenderness, guarding, and a rigid abdomen. - **Serosal involvement in tumors:** invasion through the muscularis propria into the serosa (T4a in colorectal/gastric cancer) worsens staging and prognosis and raises the risk of peritoneal carcinomatosis. - **Esophageal rupture (Boerhaave):** the esophagus has no serosa, which contributes to its fragility and to mediastinitis after perforation. - **Effusions:** pleural, pericardial, and peritoneal effusions result from serosal inflammation or altered Starling forces. Transudate vs. exudate is classified using Light criteria. - **Pericarditis:** inflammation of the serous pericardium causes pleuritic chest pain relieved by leaning forward, a friction rub, and diffuse ST elevation with PR depression. - **Serositis:** inflammation of serous membranes (pleuritis, pericarditis) is a classic criterion for SLE. It also occurs in familial Mediterranean fever (recurrent peritonitis/pleuritis) and in uremia. - **Adhesions:** damage to the serosa from surgery or inflammation causes fibrous adhesions, a major cause of small bowel obstruction. - **Mesothelioma:** malignancy of mesothelial cells, linked to asbestos exposure (pleural most common). It shows psammoma bodies rarely and is positive for calretinin. - **Embryology:** the serosa derives from lateral plate mesoderm (splanchnic mesoderm for the visceral layer, somatic mesoderm for the parietal layer).

**High-yield pearl:** Think "serosa = intraperitoneal; adventitia = retroperitoneal (or no peritoneal covering)."