 Definition: groups of cells that are similar in structure and functions  The study of tissues is known as histology  Four types of tissues found in human body:  Epithelial tissue (epithelium), which covers body surfaces, lines body cavities, and forms glands  Connective tissue, which underlies or surrounds and supports the other three basic tissues, both structurally and functionally  Muscular tissue, which is made up of contractile cells and is responsible for movement of the body and its parts  Nervous tissue, which gathers, transmits, and integrates information from outside and inside the body to control the activities of the body and its parts

covers the whole surface of the body. It is made up of cells closely packed and ranged in one or more layers. This tissue is specialized to form the covering or lining of all internal and external body surfaces. Epithelial tissue that occurs on surfaces on the interior of the body is known as endothelium. Epithelial cells are packed tightly together, with almost no intercellular spaces and only a small amount of intercellular substance. Epithelial tissue, regardless of the type, is usually separated from the underlying tissue by a thin sheet of connective tissue; basement membrane. The basement membrane provides structural support for the epithelium and also binds it to neighboring structures.

 Protection Epithelial cells from the skin protect underlying tissue from mechanical injury, harmful chemicals, invading bacteria and from excessive loss of water.  Sensation Sensory stimuli penetrate specialized epithelial cells. Specialized epithelial tissue containing sensory nerve endings is found in the skin, eyes, ears, nose and on the tongue.  Secretion In glands, epithelial tissue is specialized to secrete specific chemical substances such as enzymes, hormones and lubricating fluids.  Absorption Certain epithelial cells lining the small intestine absorb nutrients from the digestion of food.

 Excretion Epithelial tissues in the kidney excrete waste products from the body and reabsorb needed materials from the urine. Sweat is also excreted from the body by epithelial cells in the sweat glands.  Diffusion Simple epithelium promotes the diffusion of gases, liquids and nutrients. Because they form such a thin lining, they are ideal for the diffusion of gases (eg. walls of capillaries and lungs).  Cleaning Ciliated epithelium assists in removing dust particles and foreign bodies which have entered the air passages.  Reduces Friction The smooth, tightly-interlocking, epithelial cells that line the entire circulatory system reduce friction between the blood and the walls of the blood vessels.

Epithelial tissue can be divided into two groups depending on the number of layers of which it is composes.  Epithelial tissue which is only one cell thick is known as simple epithelium.  If it is two or more cells thick such as the skin, it is known as stratified epithelium.

Squamous (pavement) epithelium. Squamous cells have the appearance of thin, flat plates. The shape of the nucleus usually corresponds to the cell form and help to identify the type of epithelium. Squamous cells, for example, tend to have horizontal flattened, elliptical nuclei because of the thin flattened form of the cell. They form the lining of cavities such as the mouth, blood vessels, heart and lungs and make up the outer layers of the skin.

Simple Cuboidal Epithelium. As their name implies, cuboidal cells are roughly square or cuboidal in shape. Each cell has a spherical nucleus in the centre. Cuboidal epithelium is found in glands and in the lining of the kidney tubules as well as in the ducts of the glands. They also constitute the germinal epithelium which produces the egg cells in the female ovary and the sperm cells in the male testes.

Simple Columnar Epithelium Columnar epithelial cells occur in one or more layers. The cells are elongated and column-shaped. The nuclei are elongated and are usually located near the base of the cells. Columnar epithelium forms the lining of the stomach and intestines. Some columnar cells are specialized for sensory reception such as in the nose, ears and the taste buds of the tongue. Goblet cells (unicellular glands) are found between the columnar epithelial cells of the duodenum. They secrete mucus or slime, a lubricating substance which keeps the surface smooth.

Pseudostratified epithelium has appearance of being stratified. Some of the cells do not reach the free surface; however, all of them are resting on the basement membrane. Thus, it is actually a simple epithelium. It lines the trachea and bronchi; ductus deferens and efferent ductules of epididymis. The major functions are secretion absorption and conduit.

Stratified squamous epithelium contains multiple cell layers; only first layer is resting on the basement membrane. The basal cells are cuboidal to columnar; these cells give rise to cells that migrate toward the surface and become squamous. There are two types of stratified squamous epithelium: nonkeratinized and keratinized.  Nonkeratinized epithelium exhibits live surface cells and covers moist cavities, such as the mouth, pharynx, esophagus, vagina and anal canal. The major functions are barrier and protection.  Keratinized epithelium is found on exposed surfaces of the body, such as the skin. The surface layers contain nonliving, keratinized cells that are filled with the protein keratin. The major functions are barrier and protection.

Transitional epithelium lines the minor and major renal calyces, pelvis, ureter and bladder of the urinary system. This type of epithelium changes shape and can resemble either stratified squamous or stratified cuboidal epithelia, depending on whether it is stretched or contracted. When transitional epithelium is contracted, the surface cells appear dome-shaped, when it is stretched, the epithelium appears squamous. The major functions are barrier and distensible property.

Glands are composed of epithelial cells specialized to synthesize and secrete a specific product. Typically, glands are classified into two major groups reflecting how their products are distributed:  -exocrine glands secrete their products into a surface through ducts or tubes. The ducts also composed of epithelial cells  -endocrine glands don’t have a duct system. They secrete their products into the connective tissue from which they enter the bloodstream in order to reach their target cells. The products of endocrine glands are called hormones.

 Unicellular glands are the simplest in structure. In this case the secretory component consists of single cells distributed among other cells that are not secretory. A typical example is the goblet cell.  Multicellular glands are composed of more than one cell and exhibit varying degrees of complexity. The end pieces of the gland contain the secretory cells. The portion of the gland connecting the secretory cells to the surface serves as a duct. The ducts of the multicellular exocrine glands may be non-branching or branching. So, accordingly we have two types of glands:  Simple glands have non-branching ducts.  Compound glands have branching ducts.

 If the secretory potion is shaped like a tube, the gland is tubular.  If it is shaped like a flask, the gland is alveolar or acinar.  If the tube ends in a sac-like dilation the gland is tubuloalveolar. Tubular secretory portions may be straight, coiled or branched. Alveolar secretory potions may be single or branched. Thus, exocrine gland may be described as  Simple coiled tubular  Simple branched tubular  Simple acinar  Simple branched acinar  Compound tubular  Compound acinar  Compound tubuloacinar

Secretory units may be mucous, serous or mixed.  Mucous glands secrete mucinogens, large proteins. Examples of mucous glands include goblet cells and the minor salivary glands of the tongue and palate.  Serous glands such as pancreas secrete an enzyme-rich watery fluid.  Mixed glands contain acini that produce mucous secretions as well as acini that produce serous secretions.

In other words, according to how the secretory cells release their secretion.  A)Merocrine glands; In this type, the membranes of the secretory vesicles fuse with the cell membrane releasing their contents of secretion by exocytosis, without loss of any of the glandular cytoplasm. Nearly, all exocrine glands are of this type.  B)Holocrine glands; In this type, the whole of the cell, together with its accumulated secretion, is shed and liberated out of the secretory unit. A good example of this type is the sebaceous gland of the skin.  C) Apocrine glands: The secretory granules are liberated with some of the cytoplasm of the apical part of the secretor cell. Examples of these glands are the mammary glands and the special types of sweat glands.

 Ducts are relatively simple tubular structures which are (usually) easily distinguished from blood vessels by their conspicuous cuboidal to columnar epithelial lining. Blood vessels, in contrast, are lined by simple squamous endothelium.  The glandular cells which comprise ducts generally receive much less attention than those which actually secrete the gland's product. However, the complete understanding of a gland requires some awareness of and attention to the duct system through which it drains. Ducts are not just passive "plumbing". Some duct segments actively modify the secretory product passing through, concentrating it by removing water).

 In general, there are epithelial tissues deriving from all of the embryological germ layers:  from ectoderm (e.g., the epidermis);  from endoderm (e.g., the lining of the gastrointestinal tract);  from mesoderm (e.g., the inner linings of body cavities).  However, it is important to note that pathologists do not consider endothelium and mesothelium (both derived from mesoderm) to be true epithelium. This is because such tissues present very different pathology. For that reason, pathologists label cancers in endothelium and mesothelium sarcomas, whereas true epithelial cancers are called carcinomas . Also, the filaments that support these mesoderm-derived tissues are very distinct. Outside of the field of pathology, it is, in general, accepted that the epithelium arises from all three germ layers. [