Tissue Membranes

Tissue Membranes

Tissue membranes form a barrier or an interface. There are many different types of anatomical membranes. Epithelial membranes are thin structures made up of epithelium and underlying connective tis-sue. They cover body surfaces and line body cavities. There are four types of membranes: serous, mucous, cutaneous, and synovial.

1. serous

2. mucous

3. cutaneous

4. synovial

Serous Membranes

Serous membranes line body cavities that lack openings to the outside of the body. They consist of simple squamous epithelium (a mesothelium) and loose connective (areolar) tissue and secrete watery serous fluid, which lubricates membrane surfaces. Serous fluid contains enzymes. The serous membranes are extremely thin, yet firmly attached to body walls and to organs that they cover. Every serous membrane is divided into the parietal por-tion, lining the inner surface of a body cavity, and an opposing, moist visceral portion or serosa, covering the visceral organs.

The mesothelium cells mix hyaluronic acid with a fluid from the capillaries of related connective tissue to produce thin, clear serous fluid or transu-date. This lubricates opposing surfaces of the vis-ceral and parietal layers so they can slide across each other with ease. The total amount of transudate in a healthy individual is very small. However, after an injury or due to certain diseases, its volume may increase greatly, resulting in medical complications or even causing new conditions to develop. The sero-sae are named for their locations. Examples include the pericardium (which encloses the heart), perito-neum (which encloses the abdominopelvic viscera), and the pleurae (which line the thoracic wall and cover the lungs) . One example of serous glands is the parotid salivary glands.

Mucous Membranes

Mucous membranes are also known as mucosae. These membranes line body cavities that open to the outside of the body, including the nose and mouth as well as digestive, respiratory, urinary, and repro-ductive tubes. Mucous membranes consist of epithe-lium above the loose connective tissue (the lamina propria), with goblet cells that secrete mucus. In some mucosae, the lamina propria lies over a third, deeper layer of smooth muscle cells. Mucous mem-branes are always wet or moist. The cell composi-tion of mucous membranes actually varies, but most contain either simple columnar epithelia or strati-fied squamous epithelia such as in the oral cavity. Mucous membranes may be adapted for absorption and secretion. Many, but not all, secrete mucus. The urinary tract is an example of mucosae that do not secrete mucus. It utilizes a transitional epithelium. Examples of mucous glands include the submucosal glands of the small intestine and the sublingual sal-ivary glands. There are also mixed exocrine glands that may produce a serous secretion and a mucus secretion. One example is the submandibular sali-vary glands.

Cutaneous Membrane

The cutaneous membrane is the skin, which cov-ers the body surface. It consists of a keratinized strat-ified squamous epithelium, known as the epidermis. This is firmly attached to a thick connective tissue layer known as the dermis. The epidermis differs from other epithelial membranes in that it is dry, thicker, mostly waterproof, and exposed to the air. The cutaneous membrane is reinforced by a layer of areolar tissue underneath its dense irregular connec-tive tissue.

Synovial Membranes

Synovial membranes form an incomplete lin-ing within the cavities of synovial joints. They are entirely made up of loose connective tissues. Syno-vial membranes may be the inner of the two layers of the articular capsule of a synovial joint, with a free smooth surface lining the joint cavity. They may also be either the superior or inferior membranes lin-ing the articular capsule of the temporomandibular joint. Synovial membranes have large areas of the areolar tissue that contains a matrix of glycoproteins, proteoglycans, and interwoven collagen fibers. The areolar tissue is separated from the joint cavity by an incomplete layer of specialized fibroblasts as well as macrophages.

The body’s joints that allow significant move-ments are very complex, and a fibrous capsule sur-rounds each of them. The ends of the articulating bones are inside the joint cavity. The lining of a syno-vial joint is not a true epithelium. It develops within a connective tissue and has no basement membrane. Separate adjacent cells may be separated by gaps of up to one millimeter. Fluid and solutes are continuously exchanged between the synovial fluid and capillaries of underlying connective tissue.

The surfaces of bones must be lubricated so that friction does not damage opposing surfaces. Synovial fluid is the clear, viscid, lubricating fluid secreted by synovial membranes, which fills the joint cavities. It is similar in consistency to that of an egg white. Syno-vial fluid circulates from areolar tissue into joint cavities through the articular cartilages. It provides oxygen and nutrients to the chondrocytes. Synovial membranes often have an outer subintima layer (that may be fibrous, fatty, or loosely areolar) and an inner intima layer that consists of a sheet of cells that is thin-ner than a sheet of paper. When the subintima is loose, the intima sits on a pliable membrane. Joint movement stimulates formation and circulation of synovial fluid. When a synovial joint is immobilized for a lengthy period of time, the articular cartilages and synovial membrane begin to degenerate.

1. List the four types of tissue membranes in the body.

2. Which cavities in the body are covered by serous membranes?

3. Define synovial membranes.