Epithallium

Epithelium is a specialized complex tissue that lines the outer surfaces of organs and blood vessels, as well as the inner surfaces of cavities in many internal organs. An important function of this tissue is to form a barrier against invasive pathogens while allowing nutrient and waste exchange with the surrounding environment. It is one of the four basic types of animal tissue, along with connective tissue, muscle tissue, and nervous tissue. Epithelial tissues are classified based on the shape of cells and the number of cell layers formed. They can be categorized into simple epithelium (a single cell layer) and stratified epithelium (multiple layers of cells).

Classification

Epithelial tissue is classified into two types:

• Simple epithelium - consists of a single layer of cells, with each cell extending from the basement membrane to the free surface. This type is involved in absorption, secretion, and filtration. Examples include simple squamous epithelium and simple cuboidal epithelium.
• Stratified epithelium - consists of two or more layers of cells, with only the bottom layer resting on the basement membrane. This type is primarily protective, with the number of layers increasing in areas of high abrasion. Examples include stratified squamous epithelium and stratified cuboidal epithelium.

Functions

The primary functions of epithelial tissues include:

• Protection - from physical and chemical injury, as well as pathogenic invasion.
• Absorption - certain epithelial cells are adapted to absorb substances, such as nutrients in the intestines.
• Secretion - glands are made up of epithelial cells that produce secretions, such as sweat or hormones.
• Excretion - the removal of waste products is facilitated by epithelial cells in the kidneys and other organs.
• Sensory reception - certain epithelial cells can detect changes in the environment and convey information to the nervous system.

Specialized Structures

Epithelial tissues may contain specialized structures, including:

• Cilia - hair-like projections that can move substances across the epithelial surface.
• Microvilli - small projections that increase the surface area for absorption.
• Glands - structures formed from epithelial cells that secrete substances.

Pathology

Epithelial tissues can be affected by various diseases, including:

• Cancer - epithelial cells are the most common origin of cancers, known as carcinomas.
• Infections - the epithelium acts as a barrier to infection, but pathogens can sometimes breach this barrier and cause disease.
• Inflammatory conditions - such as dermatitis, affecting the skin's epithelial layer.

See Also

• Connective tissue
• Muscle tissue
• Nervous tissue
• Carcinoma
• Dermatitis