Cells form tissues and tissues form organs. The several types of tissues in humans include nerve, muscle, blood, connective, reproductive and epithelial. Epithelial tissues are comprised of continuous sheets of cells that cover the body, line the digestive tract and other body cavities, and form glands. Epithelial cells have little intercellular material and because of the close proximity between cells, have strong adhesion. Protection is one function of some epithelia. For example, the epithelium covering the body protects from fluid loss, the entry of pathogens (microbes), and from damage by abrasion and other forms of wear and tear. The multilayered skin epithelium, known as a stratified squamous epithelium, is constantly replacing itself from its basal layer. The loss of surface cells is known as exfoliation and this can be demonstrated by briskly rubbing the skin in a darkened room with bright sun entering a window. The gut from the stomach to the beginning of the rectum is lined with a single layer of columnar shaped cells. The columnar epithelium functions both in absorption (of nutrients and water) and in secretion (of enzymes by the stomach).
Gut cells are subject to abrasion and thus there is rapid cell replacement. The epithelium of the renal glomeruli and capsule are structured of squamous epithelia that is one cell thick and function in excretion. Some epithelium is ciliated such as in the respiratory system. The cilia clean the air passageways and are essential for survival. Epithelial cells are anatomically separated from the underlying connective tissue by a basement membrane.
Because the bulk of the body mass is not epithelial, it may come as a surprise to learn that perhaps 85% of all human cancer is of epithelial origin (such cancer is referred to as carcinoma). It may be less surprising when one remembers that the epithelium is the tissue that has contact the environment. Ultraviolet radiation does not affect muscle, nerves or tissue types other than the skin epithelium where is causes melanoma and other skin cancers. Smoking is unlikely to cause cancer of the nervous system or of the reproductive system. The first tissue type that the carcinogen-bearing smoke encounters is the respiratory epithelium.
A tumor of epithelium that does not have the capacity to transgress (invade) the basement membrane is benign. Malignant epithelium by definition has the competence to invade the basement membrane and to gain access to connective tissue and ultimately the vascular system where dissemination (metastasis) occurs. Metastasis therefore requires that invading epithelial cells be motile and that they release enzymes that digest the basement membrane and components of the extracellular matrix.
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