Epithelial tissue

Peroxisomes are membranous sacs that contain powerful oxidase enzymes. These enzymes use oxygen to detoxify harmful or poisonous substances.

Mitochondria a threadlike organelles which can continuously change shape. Mitochondria are the site of cellular respiration, in which oxygen is used to break down foods and produce energy. Because mitochondria provide energy, they are referred to as the powerhouse of the cell.

The centrioles are paired organelles that lie close to the nucleus. They are rod-shaped bodies that lie at right angles to each other. The centrioles play a key role in the division of nuclear elements during cell division.

Epithelial tissue

Epithelial tissue, or epithelium, is the lining, covering and glandular tissue of the body. Glandular epithelium forms various glands in the body. Covering and lining epithelium covers all free body surfaces and contains versatile cells. These tissues form both the outer layer of the skin as well as the lining of body cavities. Figure 4 describes the different types of epithelium:

Figure 4 Since epithelial tissue forms the boundaries between our bodies and the outside world, all substances given off or received by the body must pass through the epithelium. Its functions, therefore, include protection, absorption, filtration, and secretion.

Special characteristics of epithelium

Epithelial cells fit closely together to form continuous sheets of cells. These cells are bound together at several points by cell junctions.

The membranes always have one unattached surface called the apical surface. This surface is exposed to the exterior of the body or to the cavity of an internal organ. Exposed surfaces are modified to suit their environment, and are either smooth or may exhibit microvilli or cilia.

The lower surface of epithelium lies on a basement membrane.

Epithelial tissues are avascular, meaning they have no bloody supply of their own. They rely on diffusion of oxygen and nutrients from the capillaries of the underlying connective tissue.

Epithelial cells are able to regenerate quickly and easily when in a healthy, well-nourished state.

Epithelia are classified according to shape and arrangement. Squamous cells are flattened like fish scales, cuboidal cells are cube shaped, and columnar cells are shaped like columns.

There are two classifications of epithelia by arrangement. Simple epithelia refers to one layer of cells, while stratified epithelium refers to more than one layer of cells.

The terms describing shape and arrangement are combined to describe the epithelium fully. These categories are discussed in the next section.

Simple epithelia

Simple epithelial tissues are very thin, and are thus most useful for absorption, secretion, and filtration.

Simple squamous epithelium is a single layer of squamous cells lying on a basement membrane. This type of epithelium is usually found where filtration or exchanges of substances by rapid diffusion takes place. Simple squamous epithelium is found in the alveoli of the lungs, where gaseous exchange takes place, and along the capillary walls, where nutrients and gases are exchanged with body tissues.

Simple cuboidal epithelium is a single layer of cuboidal cells on a basement membrane. These cells are common in glands and ducts

such as the salivary glands and the pancreas. They also form the walls of kidney tubules and the surface of the ovaries.

Simple columnar epithelium is a single layer of tall, columnar cells lying on a basement membrane. These epithelia often form goblet cells, which are responsible for producing a lubrication mucus. Simple columnar epithelia line the entire digestive tract from the stomach to the anus. Epithelial membranes which are exposed to the body exterior are called mucus membranes.

Stratified epithelia

Stratified epithelial tissues are thicker than simple epithelial tissue due to the layering of cells upon one another. This arrangement is more durable, and thus these tissues provide a protective function.

Stratified squamous epithelium is the most common stratified epithelial tissue. Although the free cells at the edge are squamous cells, those close to the basement membrane are often cuboidal or columnar. Stratified squamous epithelium is found at sites of high friction, such as the esophagus, the mouth and the outer portion of the skin.

Stratified cuboidal and stratified columnar epithelia are rare in the human body. And usually, it is only the surface cells that fit this shape category, while the cells closer to the basement membrane vary in size and shape. These tissues are found mainly in the ducts of large organs.

Transitional epithelium is a modified stratified squamous epithelium that forms the lining of the urinary system. These organs are subject to considerable stretching, and the stratified arrangement of the epithelial tissue facilitates this. The cells are able to slide past each other and change shape, to allow for distension of organs when filled with urine.

Glandular epithelia

A gland consists of one or more cells that make and secrete a particular product, called a secretion. Secretion is an active process whereby glands take in materials from the blood, manufacture them into a usable product, and discharge them.

Two major gland types develop from epithelial tissues.

Endocrine glands, also called ductless glands, lose their connection to the surface ducts. These are hormone producing glands such as the