Most epithelial tissue is described with two names. The first name describes the number of cell layers present and the second describes the shape of the cells. For example, simple squamous epithelial tissue describes a single layer of cells that are flat and scale-like in shape. Epithelial Tissue : There are three principal classifications associated with epithelial cells.
Squamous epithelium has cells that are wider than they are tall. Cuboidal epithelium has cells whose height and width are approximately the same. Columnar epithelium has cells taller than they are wide. Simple epithelium consists of a single layer of cells. They are typically where absorption, secretion and filtration occur.
The thinness of the epithelial barrier facilitates these processes. Simple epithelial tissues are generally classified by the shape of their cells.
The four major classes of simple epithelium are: 1 simple squamous; 2 simple cuboidal; 3 simple columnar; and 4 pseudostratified.
Simple squamous epithelium cells are flat in shape and arranged in a single layer. This single layer is thin enough to form a membrane that compounds can move through via passive diffusion. This epithelial type is found in the walls of capillaries, linings of the pericardium, and the linings of the alveoli of the lungs.
Simple cuboidal epithelium consists of a single layer cells that are as tall as they are wide. The important functions of the simple cuboidal epithelium are secretion and absorption. This epithelial type is found in the small collecting ducts of the kidneys, pancreas, and salivary glands.
Simple columnar epithelium is a single row of tall, closely packed cells, aligned in a row. These cells are found in areas with high secretory function such as the wall of the stomach , or absorptive areas as in small intestine. They possess cellular extensions e.
These are simple columnar epithelial cells whose nuclei appear at different heights, giving the misleading hence pseudo impression that the epithelium is stratified when the cells are viewed in cross section. Pseudostratified epithelium can also possess fine hair-like extensions of their apical luminal membrane called cilia. This type of epithelium is found in the mucosa of the oral cavity and esophagus, as well as the vagina, and the eye corneal epithelium.
Image: Epidermis-structure diagram labeled. The outermost cell layers of the epithelium consist of flattened cells with no nuclei and cytoplasm, converting into scales. They are called the stratum corneum, and their purpose is to mechanically protect underlying tissue from dehydration. The above structure shows an example of the typical skin epithelium—the epidermis. It is the outermost layer of the 3 layers that make up the skin; the inner layers are the dermis and hypodermis.
Image : Mucous membrane of the urinary bladder with urothelium. The urothelium consists of a basal layer, several intermediate cell layers, and an umbrella cell layer.
Umbrella cells superficial cells are large and often have 2 nuclei. The crusta, a very dense network of cytoplasm, is located beneath its apical membrane. The plasma membrane consists predominantly of rigid plaques containing uroplakin transmembrane proteins. Transitional epithelium is primarily found in the efferent urinary tract, i. The important characteristic of this epithelium type is that their cells do touch the basal membrane, but not all of them reach the free surface.
The cells that reach the free surface belong to the columnar type. The cells that do not reach the free surface rest on the basal lamina and have a round nucleus. The term pseudostratified is derived from the appearance of this epithelium. Because the cell nuclei appear at different heights, it conveys the erroneous impression that there is more than 1 layer of cells. The non-ciliated pseudostratified epithelium is found, e.
Images : Summary of epithelial tissue cells. Associations of particularly differentiated and specialized epithelial cells are called glandular epithelium and consequently located in the glandular tissue. They are specialized in producing substances such as enzymes, hormones, sweat, oil, mucus, etc.
The elaboration and release are carried out by external stimuli or constitutively into ducts on the surface of the blood. Based on the specialization of the glandular epithelium, we distinguish between exocrine and endocrine glands. The classification is based, among other factors, on the number and location of secreting cells and the type of secretion.
Image : Exocrine glands are classified by their structure. Exocrine glands secrete their products through a duct onto the inner or outer surface of the body. They have their ducts and extensive blood supply and are in control of hormones and nerves. Examples are salivary glands, sweat glands, tear glands, and mammary glands of the breast. Endocrine glands release their incretions directly into the bloodstream and therefore have no excretory duct.
Incretions are mostly hormones. Examples of endocrine glands are the thyroid gland, as well as the testicles and ovaries. The pancreas has the particular anatomical and physiological characteristics of having both types of glands. Its exocrine portion passes digestive enzymes through the pancreatic duct into the duodenum, while the endocrine portion islets of Langerhans produces the hormones insulin and glucagon and releases them into the body.
Image : Modes of glandular secretion. In addition to physiology and histology, a physician should know, understand and treat the pathology of the epithelium. Below you will see some examples of specific disease types and their symptoms. In simple wounds, like cuts and scrapes, the healing process is done by the proliferation and spread of epithelial cells in 24 hours. Deep burns 3rd degree are largely corrected by epithelial regrowth. Image : Tumor located in left soft palate.
Papillomas are benign epithelial tumors. They form the covering of all body surfaces, line body cavities and hollow organs, and are the major tissue in glands. They perform a variety of functions that include protection, secretion, absorption , excretion, filtration, diffusion , and sensory reception.
The cells in epithelial tissue are tightly packed together with very little intercellular matrix. These terms refer to how many layers are present. Simple tissue has only one layer of epithelial cells, while stratified tissue has many layers stacked on top of each other.
Stratified cells are found in places that need to withstand a lot of wear and tear from their environment. An example of this would be your skin, which is made up of many stratified layers of epithelial cells. As the top layer wears down, cells from the bottom layers constantly grow up to replace them.
Epithelial Cells. By volunteering, or simply sending us feedback on the site. Scientists, teachers, writers, illustrators, and translators are all important to the program. If you are interested in helping with the website we have a Volunteers page to get the process started.
Digging Deeper. Digging Deeper: Depression and the Past. Digging Deeper: Germs and Disease. Digging Deeper: Milk and Immunity.
Stained epithelial cells by Page Baluch. View Citation You may need to edit author's name to meet the style formats, which are in most cases "Last name, First name.
0コメント