(Study Material For DNL Students/ other Paramedical Students)
What is Epithelial Tissue
Epithelial tissue forms protective barriers on body surfaces, lines internal cavities, and comprises glandular structures. It is composed of tightly packed cells with little extracellular matrix, allowing for minimal permeability and maximum protection. The various forms of epithelial tissue, such as simple and stratified, are adapted to different functions and locations, from the skin to the lining of the gastrointestinal tract.
Epithelial tissue serves multiple functions, including protection against pathogens and physical damage, absorption of nutrients and other substances, secretion of hormones, enzymes, and mucus, and excretion of wastes. It also plays a role in sensory reception, contributing to the sense of touch and other sensory modalities.
Types of Epithelium
- Simple Squamous Epithelia
- Simple Cuboidal Epithelia
- Simple Columnar Epithelia
- Stratified Squamous Epithelia
- Stratified Cuboidal Epithelia
- Stratified Columnar Epithelia
- PseudoStratified Columnar Epithelia
Simple Squamous Epithelium
Simple squamous epithelium consists of a single layer of flat, scale-like cells. This type of epithelium is found in areas where rapid diffusion or filtration is necessary, such as the lining of blood vessels (endothelium), the air sacs of the lungs (alveoli), and the serous membranes of body cavities (mesothelium). Its thin, delicate structure allows for efficient exchange of gases, nutrients, and waste products.
Simple Cuboidal Epithelium
Simple cuboidal epithelium is composed of a single layer of cube-shaped cells with centrally located nuclei. This type of epithelium is found in glands and their ducts, such as the thyroid gland, and in the kidney tubules. It functions primarily in secretion and absorption, providing structural support and forming the functional units of many glands.
Simple Columnar Epithelium
Simple columnar epithelium consists of a single layer of tall, column-like cells with nuclei typically located at the basal end. This type of epithelium lines the gastrointestinal tract, from the stomach to the rectum, and is specialized for absorption and secretion. In some areas, such as the intestines, it features microvilli to increase surface area for nutrient absorption, and may contain goblet cells that secrete mucus.
Stratified Squamous Epithelium
Stratified squamous epithelium is composed of multiple layers of cells, with the outermost layers being flat and scale-like. This type of epithelium is found in areas subject to abrasion, such as the skin, mouth, esophagus, and vagina. The multilayered structure provides protection against mechanical stress, dehydration, and pathogen entry, with the outermost cells often being keratinized in the skin to enhance barrier function.
Stratified Cuboidal Epithelium
Stratified cuboidal epithelium consists of two or more layers of cube-shaped cells. It is less common than other epithelial types and is typically found in the ducts of sweat glands, mammary glands, and salivary glands. This epithelium provides a robust structure that supports glandular secretion and protects underlying tissues from damage.
Stratified Columnar Epithelium
Stratified columnar epithelium is composed of several layers of cells, with the surface cells being columnar in shape. This rare type of epithelium is found in parts of the pharynx, the male urethra, and some glandular ducts. It functions in protection and secretion, offering a durable lining in areas exposed to both mechanical and chemical stresses.
Pseudostratified Columnar Epithelium
Pseudostratified columnar epithelium appears to be stratified due to the varying positions of the nuclei, but in reality, all cells are attached to the basement membrane, making it a single layer. This type of epithelium lines the respiratory tract, including the trachea and bronchi, where it often features cilia and goblet cells. The cilia help in moving mucus and trapped particles out of the respiratory passages, playing a crucial role in keeping the airways clear.