Mucocutaneous Junctions
Mucocutaneous junctions are transition points between dry skin and the wet mucous membranes, the tissues lining body orifices such as the mouth, anus, nostrils, urethra, and vagina.
Mucous membranes consist of epithelial tissue over a basement membrane, attached to a layer of connective tissue called the lamina propria. Epithelium is a type of tissue that is generally moist, often containing mucus-secreting goblet cells, and regenerates very quickly.
The skin also contains epithelial tissue, but in the outer layers, the cells have lost their nuclei and flattened out. They contain a horny substance called keratin, forming a protective covering for the body. The skin has hairs, sebaceous glands, and sweat glands, which are absent from the mucous membranes. At the mucocutaneous junction, the epithelial tissue transitions between the dry keratinized skin and the wet epithelium of the mucous membrane.
At mucocutaneous junctions such as that of the lip, where the mucous membrane is exposed to the air, a type of tissue called parakeratinized epithelium is seen. Parakeratinized epithelium is tougher and drier than the typical mucous membrane, such as is found inside the mouth, but its surface cells still retain their nuclei. It exhibits vascular papillae, elevations containing many tiny blood vessels, or capillaries. Because there are no glands in the lips, they are subject to excessive drying in the air unless they are kept moist by saliva.
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