Serous membrane

In anatomy, a serous membrane, or serosa, is a smooth membrane consisting of a thin layer of cells which excrete serous fluid. Serous membranes line and enclose several body cavities, known as serous cavities, where they secrete a lubricating fluid which reduces friction from muscle movement. Serosa is not to be confused with adventitia, a connective tissue layer which binds together structures rather than reducing friction between them.

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Structure

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) which produce the lubricating serous fluid. This fluid has a consistency similar to thin mucous. 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 which allows the whole serous membrane to adhere to organs and other structures.

For the heart, the surrounding serous membranes include:

Other parts of the body may also have specific names for these structures. For example, the serosa of the uterus is called the perimetrium.

Serous cavities

  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. It should be noted that 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 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 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.

Embryological origins

All serous membranes found in the human body formed ultimately from the mesoderm of the trilaminar embryo. The trilaminar embryo consists of three relatively flat layers of ectoderm, endoderm (also known as "entoderm") and mesoderm.

As the embryo develops, the mesoderm starts to segment into three main regions: the paraxial mesoderm, the intermediate mesoderm and the lateral plate mesoderm.

The lateral plate mesoderm later splits in half to form two layers bounding a cavity known as the intraembryonic coelom. Collectively, both layers are known as splanchnopleure. Individually, each are known as visceropleure and somatopleure.

• The visceropleure is associated with the underlying endoderm which it is in contact with, and later becomes the serous membrane in contact with visceral organs within the body.
• The somatopleure is associated with the overlying ectoderm and later becomes the serous membrane in contact with the body wall.

The intraembronic coelom can now be seen as a cavity within the body which is covered with serous membrane derived from the splanchnopleure. This cavity is divided and demarcated by the folding and development of the embryo, ultimately forming the serous cavities which house many different organs within the thorax and abdomen.

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