Epithelial tissue is made up of the layers of cells found on the outside of advanced organisms and internally, lining organs. If there is a path from the organ or from internal body cavities to the outside, epithelial cells line the path. These cells act as a barrier to infection and control what goes into the body and what comes out.

The type of epithelium depends on the number of cell layers. For some areas, a single layer of cells or a simple epithelium is enough to offer adequate protection. In other regions, such as in the case of skin cells, many layers are required because the environment is challenging.

There, the epithelium is made up of stratified epithelial tissue. In the case of skin cells, the outermost layers are made up of dead cells offering additional protection against damage to the organism.

The four types of body tissue are muscle, epithelial, connective and nerve tissue. Muscle tissue includes such organs as the heart while nerve tissue is found in the spinal cord and brain. Connective tissue holds organs in place but also takes on special functions in tendons and ligaments.

Epithelial tissue lines the organs, body cavities and the outside of the organism. It is often specialized depending on the organ with which it is associated.

For example, epithelial tissue lines the veins, arteries and capillaries. These cells are quite different from the epithelial skin cells that cover the outside of the organism. Both have different characteristics from the epithelial cells that line the small intestine, those that make up kidney ducts, and those that form part of the respiratory system.

Epithelial cells can form a simple epithelium in a single layer of cells, or they can make up a stratified epithelium that has several layers. Depending on the function of the organ or cavity, the epithelial cells in different locations often have special absorption or excretion functions.

For example, lung cells absorb oxygen, while kidney cells excrete urine through epithelial cells. Despite such different characteristics, epithelial tissues all have a number of similarities.

Although epithelial tissues are varied in specialized function and purpose, they have several common characteristics as a result of their shared role in protecting the interior of their organism from the outside environment.

• Cells are closely bound. Stratified epithelial cells form closed layers of tightly packed cells that are attached to their neighbors. Epithelial tissues have no intercellular material present.
  
• Epithelial tissues don't contain blood vessels. They are facing the outside environment, and if damaged, they may lose some cellular liquids, but they don't bleed.
• Cells are polarized, having an outside and an inside face. The outside or apical surface faces away from the interior of the organism. The inside or basal surface faces toward the interior.
• The tissues have no nerve cells. Epithelial tissues are barriers and don't sense conditions such as heat, cold or pain. The barriers transmit the relevant conditions to the underlying tissues that have the corresponding nerve cells.
• Epithelial cells are anchored to the underlying tissues. The basal surface of the lowest layer of cells is firmly connected to the basement membrane underneath the epithelial tissues.

These shared characteristics allow the epithelial cells to form a continuous layer around the interior of their organism and protect it from physical, chemical and biological attack or damage. An external attack will always encounter one or several layers of epithelial cells, no matter where it tries to access the organism's interior.

Even if the external attack passes through one of the many organism orifices, the internal cavities are still lined with epithelial cells.

There are four types of cells that can make up a stratified epithelium. The type of cell depends on the location of the tissue and its function. Some tissues are subject to physical wear and tear and have to reproduce quickly. Others are slippery but delicate.

Still others have to secrete hormones or other substances. The role the cell plays determines what type is the most appropriate.

The four types are:

• Squamous epithelia have flattened cells in the top outside layer and several layers of irregular shaped cells underneath. These cells are found in places that are subject to physical stress.
  
• Cuboidal epithelia have cube-shaped cells in the outer layer and are primarily found in glands. They are capable of secreting or transmitting substances while offering protection from harm.
• Columnar epithelial cells are tall, column-shaped outer layer cells that can transmit stimuli to underlying tissues and nerve cells. They sometimes have cilia attached or form finger-like protrusions to increase their surface area.
• Transitional cells can change shape rapidly and can multiply quickly to replace damaged outer layer cells. They are found in organs or structures that expand and contract.

Although they have different shapes and capabilities, all epithelial cells form a solid boundary around the interior of the organism and create a barrier to harmful influences.

Epithelia with several layers of cells and flattened top layers can protect underlying tissues in situations where the cells are subject to constant abrasion, such as the skin. The flattened shape allows cells to glide with the abrasive action. In other locations, squamous epithelia cells line blood vessels and lungs where their flat shape facilitates the exchange of oxygen and carbon dioxide.

Depending on where on the outside of the organism the stratified squamous epithelium is located, it can be strengthened with more or less keratin protein. A keratinized stratified squamous epithelium is tougher and more resistant to physical damage than non-keratinized cells.

Heavily keratinized cells in humans are found on the soles of feet and in the palms of hands. These epithelia also contain glycolipids to keep cells moist and flexible.

Non-keratinized epithelia are found where physical damage is less likely or where the emphasis is also on sensory input through the epithelia. Typical examples of non-keratinized cells are found in the inside of the mouth, the vaginal canal and the colon. Skin in these areas is more delicate than the keratinized skin, and it is kept moist and flexible by locally-produced substances such as saliva.

Cuboidal epithelial cells line the ducts of many glands and other organs involved in the exchange, absorption or secretion of body chemicals. The ducts of glands eventually lead outside the body, and the epithelial layer makes sure that poisons, foreign particles and microorganisms that enter the ducts can't get into the interior tissues.

Simple cuboidal epithelia are found in the tiny ducts and tubules of the kidneys, salivary glands, sweat glands and mammary glands. As the ducts join and become bigger, better protection may be required, and the cuboidal epithelial cells start to form layers to make up stratified cuboidal epithelia.

Due to their length, which results in a thick layer of cells, columnar epithelial cells offer a relatively high degree of protection while still allowing substances to cross their layers.

They are found where larger tubes or organs secreting biological substances need protection, and they can form finger-like shapes to increase the surface area available for absorption.

Columnar cells are found in glands and in the digestive system. Endocrine glands secrete their hormones and other substances directly across the columnar epithelial cells, while exocrine glands secrete into ducts that may themselves be protected by cuboidal epithelia.

The stomach and intestines are lined with columnar epithelial cells that allow the secretion of mucus and digestive juices into the digestive tract while absorbing nutrients from the digested food.

Cells of the transitional epithelium are multi-layered with an ability to stretch. As the cells change shape to accommodate the growing or shrinking of the underlying organ, they may look like columnar, cuboidal or squamous cells, depending on the amount of stretching.

The transitional epithelium is impervious to water and many other chemicals and is used where the contents of an organ should not interact with the neighboring tissues.

The transitional epithelium has three main layers:

• The basal layer is firmly attached to the underlying tissue and is made up of tightly linked undifferentiated stem cells that are not heavily specialized.
  
• Intermediate layer that is made up of one or more layers of cells that can rapidly divide to replace cells lost due to damage or abrasion in the top layer.
• Top layer of tightly interlinked cells that can stretch and are covered by an impermeable layer of hexameric plaques made of uroplakin.

The transitional epithelium is found in organs that need to change shape and size such as the bladder. Although urine contains a high concentration of chemicals such as urea and ammonia, the epithelial cells with their surface plaques keep the chemicals inside the urinary tract and protect the surrounding tissues.

When epithelial cells line inner cavities, they sometimes take on an additional specialized function. Columnar epithelial cells can have many hair-like protrusions called cilia on the surfaces facing the inner cavity. The cilia either move to propel fluids or they may be stationary and act as sensors. Stratified columnar epithelia are found in the respiratory pathways and in the digestive system.

Their cilia help with specialized functions required inside the cavities.

In the case of the respiratory tract, ciliated epithelial cells help spread secreted mucus and then transport the mucus out of the system. Cilia work with a coordinated wave motion that passes the mucus from cell to cell. Inhaled particles, other foreign matter and bacteria are trapped in the mucus and swept out of the trachea.

This function is especially critical when polluted air is taken into the lungs or when bacteria cause an infection.

In the digestive system, cilia also help with mucus production and distribution. The cilial motion helps with digestive functions. Non-motive, stationary cilia can be chemical receptors signaling to other cells what substances are present and what chemicals may be required.

Of the four types of tissue, epithelial cells make up the most varied kind. While connective tissue is relatively simple and nerve and muscle tissue have clearly defined and comparatively narrow functionality, epithelial cells take a wide variety of forms and often have specialized roles depending on their location.

Almost every organ has associated epithelial cells, and for some, such cells are the main component. When they are defective, epithelial cells can cause diseases in organs such as the kidney.

When they don't protect tissues adequately, severe infections can result. They are the part of the body facing the external environment and have to adapt to exterior influences while keeping the body safe.