Tissue Epithelial PDF

 Histology is the study of Tissues  Epithelial Tissue (Epithelium) Sheet of cells Deep surface rests on the basement membrane Cells form continuous layer/layers Tightly packed together, little or no intercellular matrix in between Epithelium covers the body surface, lines b...

 Histology is the study of Tissues  Epithelial Tissue (Epithelium) Sheet of cells Deep surface rests on the basement membrane Cells form continuous layer/layers Tightly packed together, little or no intercellular matrix in between Epithelium covers the body surface, lines body cavities(e.g. pleural, peritoneal), lines tubes (e.g. gastrointestinal tract, respiratory tract, genitourinary tract, blood vessels) and forms Glands (e.g. exocrine, endocrine). Has high regeneration capacity ranging from a few days(e.g. epithelial lining, small intestine) to a month (e.g. epidermis of the skin)  Classification of Epithelium Two main types: Simple Epithelium and Compound Epithelium If the sheet is made up of a single layer of cells, it is termed as simple epithelium and if it is made up of multiple layers of cells, it is termed as compound or stratified epithelium Simple Epithelium Classified into three types depending upon the shape of the cells 1. Simple squamous epithelium: Made of flattened cells bound together e.g. endothelium of the heart and blood vessels, Mesothelium of serous membrane, lining epithelium of alveoli of lungs 2. Simple cuboidal epithelium: Made of cube-like cells, e.g. lining of thyroid follicles, germinal epithelium of ovary 3. Simple columnar epithelium : Made of tall columnar cells e.g. lining epithelium of gastrointestinal tract and its Glands, gallbladder, bile duct. If the simple columnar epithelium is characterized by the presence of cilia along its free surface, it is termed as simple ciliated columnar epithelium, e.g. cells of nasal epithelium. If the simple columnar epithelium is made up of two types of columnar cells: (a) tall and thin And (b) short and thick, the tall cells reach the surface and the short cells do not reach the Surface. In such a situation nuclei of these cells lie at different levels and gives a stratified appearance. This type of epithelium is termed as pseudostratified ciliated columnar epithelium, e.g. lining Epithelium of upper respiratory passages, air sinuses. Compound/Stratified Epithelium It is also classified into three types depending upon the shape of the cells of the surface layer: 1. Stratified squamous epithelium: surface layer is made up of squamous cells. If the surface layer is moist, the surface cells are not keratinized, it is called stratified squamous non keratinized epithelium, e.g. epithelium lining the oral cavity. If the surface layer is not moist, i.e. exposed to air, the surface cells undergo keratinization(i.e. cells are dead and the cytoplasm is replaced by keratin), it is called stratified squamous Keratinized epithelium, e.g. epidermis of skin 2. Stratified cuboidal epithelium: The surface layer is made up of cube-shaped cells, e.g. duct of sweat glands, seminiferous Tubules. 3. Stratified columnar epithelium: The surface layer is made of columnar cells, e.g. large ducts of glands. Transitional Epithelium aka urothelium. Special variety of stratified epithelium lining the urinary tract, viz. Ureter, urinary bladder, urethra, etc. The transitional epithelium exhibits two types of transitions: 1. Transition in surface cells, i.e. when the organ or the tube is relaxed , they are umbrella Shaped but when the organ or the tube is distended, they become flattened. 2. Transition in the number of layers of cells. When the organ or the tube is relaxed, it is made up of 5-6 layers of cells but when Stretched Or distended, it is made up of only 2 or 3 layers. The transitional epithelium is classically described to consist of 3 layers: Superficial layer of large polyhedral cells(umbrella cells), Intermediate layer of pear shaped cells, and Deeper layer of smaller(low columnar or cuboidal cells) The cells of all the layers are connected to the basement membrane through cell junctions. Hence, they do not get disrupted during stretching