Epithelium Tissue 1 (23) PDF

The human body is composed of four basic types of tissue: epithelial, connective, muscular and nervous.  EPITHELIAL TISSUE  Definition: Tissue cover from outside & line from inside. -Characteristics of epithelial tissue: 1- It is composed of closely aggregated cells with very little interce...

The human body is composed of four basic types of tissue: epithelial, connective, muscular and nervous.  EPITHELIAL TISSUE  Definition: Tissue cover from outside & line from inside. -Characteristics of epithelial tissue: 1- It is composed of closely aggregated cells with very little intercellular substance. 2- Junctional complexes between these cells are strong. 3- Epithelial cells are firmly bound to the underlying connective tissue by basal lamina or basement membrane. 4- Blood vessels do not penetrate the epithelium and its nutrition depends on the diffusion from blood capillaries present in the underlying connective tissue. 5- Epithelial cells are renewed continuously by mitotic activity. 6- Epithelium is derived from all three embryonic germ layers , e.g: Most of the epithelium lining the skin, mouth, nose and anus are ectodermal in origin. The lining of respiratory system, digestive tract and its associated glands (e.g. pancreas and liver) is derived from endoderm. Other epithelia (e.g. endothelial lining blood vessels) originate from mesoderm. Functions of epithelial tissue are: 1. to protect the tissues that lie beneath from radiation, desiccation, toxins, invasion by pathogens, and physical trauma 2. the regulation and exchange of chemicals between the underlying tissues and a body cavity 3. the secretion of hormones into the circulatory system, as well as the secretion of sweat, mucus, enzymes, and other products that are delivered by ducts 4. to provide sensation 2  5. Absorb water and digested food in the lining of digestive canal Types of epithelium In general, epithelial tissues are classified by the number of their layers and by the shape and function of the cells. The basic cell types are squamous, cuboidal, and columnar, classed by their shape Epithelium Classified according to structure and function into: 1) Covering and lining epithelium. 2) Glandular epithelium. I - Covering and lining epithelium They are classified according to number of layer: -Simple epithelium that consist of one layer of cells -Stratified epithelium that consist of more than one layer of cells Simple epithelium can be classified according to the cell shape into: -Simple Squamous. -Simple Cuboidal. -Simple Columnar. -Pseudostratified columnar. 3 4 A-Simple epithelium: 1- Simple squamous epithelium: It is made of one layer of flattened plate cells with flattened nuclei  Sites: 1-Lungs (lining the alveoli). 2-Kidney (lining the Bowman's capsule) 3-Endothelium lining the blood and lymphatic vessels. 4-The mesothelium lining the serous membranes (peritoneum, pericardium, pleura)  Function: Allows passage of materials by passive diffusion and filtration, protection , secretes lubricating substances. 2- Simple cuboidal epithelium: It is composed of a single layer of cuboidal cells with round central nuclei.  Sites: 1-Thyroid follicle 2-Lining Kidney tubules 3- ducts and secretory parts of glands 4 -ovary surface 5   Function: Secretion and absorption function. Kidney showing a single layer of lining renal tubules 3- Simple columnar epithelium: It is made of a single layer of tall cells with oval basal nuclei. It is subdivided into non-ciliated and ciliated types: a) Simple columnar ciliated epithelium: - The cells have cilia on the free surface.  Sites: 1-Bronchioles of lungs. 2-Lining the central canal of spinal cord. 3-Together with the non-ciliated variety lining the uterus and fallopian tubes. 6 b) Simple columnar non-ciliated epithelium:  Sites: 1-The gastrointestinal tract (from the stomach to the rectum). 2-It lines the gallbladder. 4-Pseudostratified columnar epithelium -All the cells are attached to the basal lamina but some of them do not reach the surface. -The nuclei appear to lie in various layers and result in the false appearance of stratified epithelium. -This type may be ciliated or non-ciliated. a)Pseudostratified columnar ciliated epithelium: This type is usually associated with goblet cells.  Sites: Respiratory passages e.g.: 1-Most of nasal cavity 2-Larynx 3-Trachea 4-Bronchi 7 b) Pseudostratified columnar non-ciliated epithelium:  Sites: 1-Large ducts 2-Vas deferens 8 B- Stratified Epithelium -The epithelium is made up of 5-30 layers of cells. -It has a protective function. -The basal layer lying next to the basement membrane is columnar in shape followed by a few layers of polyhedral cells.  It is classified according to the shape of the most superficial layer of cells into: -squamous, -cuboidal, -columnar, -and transitional epithelium. 1- Stratified squamous epithelium -The most superficial 1 -2 layers are squamous or flattened. -It is either non keratinized or keratinized. a) Stratified squamous non keratinized epithelium:  Sites: 1- In eye cornea. 2-Mouth cavity, tongue, esophagus and anal canal. 3-Vagina. 9 b- Stratified Squamous keratinized epithelium -It covers dry surfaces -This type of epithelium contains an additional superficial layer of dead cornified cells , the most apical layers (exterior) of cells are dead and lose their nucleus and cytoplasm, instead contain a tough, resistant protein called keratin. 2- Stratified cuboidal epithelium -The most superficial cells are cuboidal in shape.  Sites Stratified cuboidal epithelium protects areas such as the ducts of sweat glands, mammary glands, and salivary glands mammary glands, and salivary glands 10 3- Stratified columnar epithelium: Multiple layers. basal cells are columnar. intermediate cells are polygonal. the most superficial cells are Columnar. There are two varieties: ciliated and non ciliated. a) Stratified columnar non ciliated epithelium:  Sites: 1-Fornix of conjunctiva , 2-Large ducts of glands. b) Stratified columnar ciliated epithelium: Site: Fetal esophagus 11 4. Transitional epithelium: - Has cells that can change from squamous to cuboidal, depending on the amount of tension on the epithelium. o In the empty or relaxed condition, the epithelium is made of 3-8 layers. The superficial layer is composed of large cuboidal dome shaped cells. Each cell may contain 1-2 nuclei. This is followed by few layers of polyhedral cells and the basal layer is cubical or columnar. o When the organs are distended, the epithelium becomes stretched the superficial layers become squamous. The causes of this change are the thin corrugated basement membrane and the presence of mucous-like intercellular substance which allows the cells to glide on each other. -The free surface of the superficial cells is provided with a thick cuticula r border which prevents the escape of urine between the cells during distension. It also protects the cells from the injurious substances present in the urine.  Sites: It lines the pelvis of the kidney, ureter, urinary bladder, and upper part of the urethra 12