We looked at the cell in a series of six articles and now we are moving into the fascinating world of tissues. A tissue is a group of cells with similar structure and function. The tissue then contributes to the functioning of the organs in which it is found. Epithelial tissue exists at the body surface where it lines the skin and various organs such as the mouth, nose and other body cavities. It is also found at the lining of the respiratory, reproductive and urinary tracts. All blood vessels are also lined with epithelial tissue. Epithelial tissue is divided into different types.
The first type of epithelial tissue is the covering and lining epithelium. It forms the epidermis of the skin and the outer covering of some internal organs as well as the inner lining of blood vessels, ducts and body cavities. It is arranged by layers and cell shapes. The layers are: simple epithelium, stratified epithelium and pseudostratified columnar epithelium. The cell shapes are: Squamous, cuboidal, columnar and transitional.
Simple squamous epithelium: It consists of a single layer of flat cells that resembles a tiled floor. It is located in the lining of the blood vessels, lining of body cavities and part of the kidney tubules. Its functions are protection and absorption.Simple cuboidal epithelium: The cuboidal shape of the cells in this tissue is obvious when the tissue is sectioned and viewed from the side. It is located in the secretory portion and ducts of some glands and part of the kidney tubules. Its functions are secretion and protection.
Simple columnar epithelium: When viewed from the side the cells appear rectangular with oval nuclei near the base of the cells. It lines the gastrointestinal tract and the ducts of some glands. Its functions are absorption, protection and secretion.
Stratified squamous epithelium: In the superficial layers of the epithelium these types of cells are flat, but in the deep layers the cell shape varies from cuboidal to columnar. It is found in the epidermis, lining of the mouth, esophagus and the vagina. Its functions are protection, secretion and some absorption.
Stratified cuboidal epithelium: It sometimes consists of two or more layers of cells and is fairly rare. It can be found in the ducts of sweat glands and functions mainly to protect and in a limited capacity in secretion and absorption.
Stratified columnar epithelium: This is also an uncommon type of tissue. It is mainly found a part of the lining of the male urethra and its main function is protection.
Transitional epithelium: It is variable in appearance, depending on whether the organ it lines is relaxed or stretched. It is found in the lining of the ureter and urinary bladder and its main function is protection.
Transitional columnar epithelium: When viewed from the side these cells give the false impression of a multilayered tissue. It is found in the lining of the trachea, upper respiratory tract and parts of the male reproductive system. Its functions are protection and secretion.
There is a further organization of epithelial cells which is called glandular epithelium. This is the second type of epithelial tissue. It constitutes the secreting portion of the glands, such as the thyroid gland, adrenal glands and sweat glands. We will look at these more closely when we discuss the thyroid and adrenal glands in upcoming posts.
How does this affect you?
Cancers of the epithelial cells are called carcinomas and these make up about 85% of all cancers. We will look
at specific types of cancer in upcoming blogs and then tie the discussion of types of cells in with the different types of carcinomas.
