Human body and Body Tissues

Types of body tissues
Body Tissues 
A tissue is composed of similarly specialized cells that per­form a common function in the body. The tissues of the human body can be categorized into four major types: epithelial tissue, which covers body surfaces and lines body cavities; connective tissue, which binds and supports body parts; muscular tissue, which is specialized for contraction; and nervous tissue, which responds to stimuli and transmits impulses from one body part to another.
Epithelial Tissue 
Epithelial (ep "y -the'le-al) tissue, also called epithelium, forms a continuous layer, or sheet, over the entire body surface and most of the body's inner cavities. On the exter­nal surface, it protects the body from drying out, injury, and bacterial invasion. On internal surfaces, epithelial tissue may be specialized for other functions, in addition to protection. For example, in the respiratory tract, epithe­lial tissue sweeps up impurities by means of cilia, while along the digestive tract, it secretes mucus, which protects the organs of the digestive tract from the digestive en­zymes. Epithelial tissue also efficiently absorbs molecules from kidney tubules because of fine, cellular extensions called microvilli.
The three main types of epithelial tissue are squamous, cuboidal, and columnar. Squamous epithelium is composed of flat cells and lines the lungs and blood ves­sels. Cuboidal epithelium has cube-shaped cells and lines the kidney tubules. Columnar epithelium has pillar- or column-shaped cells, with nuclei usually located near the bottom, and is found lining the digestive tract.
An epithelium may have microvilli (tiny extensions from the cells) or cilia, depending on its particular func­tion. For example, the oviducts are lined by ciliated colum­nar cells that propel the egg toward the uterus, or womb.
An epithelium may also be simple or stratified. Simple means that the cells occur in a single layer. Stratified means that the cells exist as layers piled one over the other. The nose, mouth, esophagus, anal canal, and vagina are all lined by stratified squamous epithelium. The outer layer of skin is also stratified squamous epithelium, but the cells are reinforced by keratin, a protein that provides strength. Pseudostratified epithelium appears to be layered; how­ever, true layers do not exist because each cell touches the baseline. Pseudostratified ciliated columnar epithelium lines the air passages of the respiratory system, includ­ing the nasal cavities and the trachea and its branches. Mucus-secreting goblet cells are scattered among the cili­ated epithelial cells. A surface covering of mucus traps foreign particles, and upward ciliary motion carries the mucus to the back of the throat, where it may be either swallowed or expectorated.
An epithelium like pseudostratified columnar epithe­lium that seaetes a product is described as glandular. A gland can be composed of a single epithelial cell as in the case of the mucus-secreting goblet cells found within the columnar epithelium lining the digestive tract, or it can have many cells. Glands that seaete their products into ducts are called exocrine glands (for exam­ple, salivary glands and sweat glands), and those that se­aete directly into the bloodstream are called endocrine glands (for example, the pituitary gland and the thyroid gland).
Epithelial tissue is classified according to cell shape, vvhich may be squamous, cuboidal, or columnar. The cells may be stratified and/or ciliated, and the tissue may be glandular.


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Posted in Human Anatomy and Physiology