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22.5D: Serosa

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    Serosa consists of a secretory epithelial layer and a thin connective tissue layer that reduce the friction from muscle movements.

    Learning Objectives
    • Diagram the structure of the serosa

    Key Points

    • The serous layer provides a partition between the internal organs and the abdominal cavity.
    • Cells of the serous layer secrete a serous fluid that provides lubrication to reduce friction.
    • The connective tissue layer provides blood vessels and nerves.
    • The three serous cavities within the human body are the pericardial cavity (surrounding the heart ), the pleural cavity (surrounding the lungs), and peritoneal cavity (surrounding most organs of the abdomen).
    • The serous membrane covers the heart; it has an inner layer (the parietal pericardium ) and an outer layer (the visceral pericardium).
    • The serosa of the uterus is called the perimetrium.

    Key Terms

    • pleural cavity: The body cavity that surrounds the lungs and is enclosed by the pleura.
    • coelom: A fluid-filled cavity within the body of an animal. The digestive system is suspended within the cavity that is lined by a tissue called the peritoneum.
    • serosa: A membrane that lines an internal cavity to protect the contents and secretes serum.

    The Serous Membrane

    In anatomy, the serous membrane (or serosa) is a smooth membrane that consists of a thin connective tissue layer and a thin layer of cells that secrete serous fluid. Serous membranes line and enclose several body cavities, known as serous cavities, where they secrete a lubricating fluid to reduce friction from muscle movements.

    Serosa is not to be confused with adventitia, a connective tissue layer that binds together structures rather than reduces friction between them.

    This is a drawing of the layers of the stomach lining.The mucosa, submucosa, muscle layers, and the serosa—the outermost layer—are highlighted.

    Layers of stomach lining: The serosa is labeled at far right, and is colored yellow.

    Each serous membrane is composed of a secretory epithelial layer and a connective tissue layer underneath. The epithelial layer, known as mesothelium, consists of a single layer of avascular flat nucleated cells (simple squamous epithelium) that produce the lubricating serous fluid. This fluid has a consistency similar to thin mucus.

    These cells are bound tightly to the underlying connective tissue. The connective tissue layer provides the blood vessels and nerves for the overlying secretory cells, and also serves as the binding layer that allows the whole serous membrane to adhere to organs and other structures.

    For the heart, the surrounding serous membranes include: the outer, inner, parietal pericardium, and visceral pericardium (epicardium). Other parts of the body may also have specific names for these structures. For example, the serosa of the uterus is called the perimetrium.

    The pericardial cavity (surrounding the heart), pleural cavity (surrounding the lungs) and peritoneal cavity (surrounding most organs of the abdomen) are the three serous cavities within the human body. While serous membranes have a lubricative role to play in all three cavities, in the pleural cavity it has a greater role to play in the function of breathing.

    The serous cavities are formed from the intraembryonic coelom and are basically an empty space within the body surrounded by a serous membrane. Early in embryonic life, visceral organs develop adjacent to a cavity and invaginate into the bag-like coelom.

    Therefore each organ becomes surrounded by a serous membrane—they do not lie within the serous cavity. The layer in contact with the organ is known as the visceral layer, while the parietal layer is in contact with the body wall.

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