Histology of Epithelium PDF

# The structure of Epithelial Tissue **Anatomy Lectures By Dr Bakare Airat** ## What is a tissue? - A tissue is an ensemble or collection of cells: - Comparable in structure. - They carry out a specific function. - From the same embryonic origin. ## Types of tissue - Four types of...

# The structure of Epithelial Tissue **Anatomy Lectures By Dr Bakare Airat** ## What is a tissue? - A tissue is an ensemble or collection of cells: - Comparable in structure. - They carry out a specific function. - From the same embryonic origin. ## Types of tissue - Four types of tissue: - Epithelium - Connective tissue - Muscle tissue - Nervous tissue ## Epithelial Tissue - Tightly packed cells which form continuous sheets. - Epithelial tissue are found lining cavities and surfaces; the skin, airways, mouth, stomach, glands. - They undergo continuous mitotic division. ## Structure of Epithelial tissue - Epithelium compose of cells tightly-bounded. - They have free/apical surface that is not associated with other cells, a basal surface which is attached to a basement membrane and lateral surfaces attached to other cells with specialized cell to cell junctions. ## Lateral surface (Cell to cell junctions) - **Tight junctions** / occluding junctions / zonula occludens are closely associated areas of two cells whose plasma membranes join together on their lateral surfaces forming a virtually impermeable barrier to fluid serving as barriers of diffusion of molecules across epithelial surfaces. - In organs such as; lungs, stomach, kidney, urinary bladder, to prevent unregulated movement of ions, organic solutes, water. - **Gap junction** is a specialized intercellular connection composed of channels which connect across the intercellular space. - Allows the exchange of nutrients, ions, signals between adjacent cells up-to a certain size and are also able to communicate and coordinate their activity e.g cardiac muscle, the need for ionic, signal. - **Desmosomes** link proteins (keratin) embedded within the cell cytoplasm to the proteins in intercellular junctions, allowing individual cells to function as a solid sheet rather than as isolated cells to resist stretching forces, hence enhance/maintain strength. It is usually seen with tight junctions. - They connect intermediate filaments of adjacent cells together (cytoskeletal linkages) thereby resisting mechanical stress. - The intercellular space is very wide and the adhesive gadget in the intercellular space e.g skin. - **Hemidesmosome** are very small complexes on the inner basal surface of cell connecting cell to basement membrane. ## Functions of the lateral surface or cell to cell junctions 1. ?????? 2. ?????? 3. ????? ## Apical surface - Directed towards the lumen of the tissue. - It forms the interface between the extracellular and intracellular space. ## Microvilli - Infoldings of the Apical plasma membrane. - Embedded with vessels. - Increases surface area for absorption/secretion. ## Cilia - Hair-like projections on the apical surface. - Longer and narrower than the microvilli. - Transportation along the luminal surface/trap foreign agents. ## Flagella - These are long projections that help in movement. ## Functions of the Apical surface 1. ?????? 2. ???? 3. ???? ## Basement membrane - Thin sheet of fibers that underlies the epithelium and separate epithelium from underlying connective tissues - Composed of a special type of collagen and lamina fibers. - The fusion of two sub layers: - **Basal lamina:** It consists of an electron-lucent membrane lamina lucida and electron-dence lamina densa. - **Reticular lamina:** an underlying network of reticular collagen. ## Function of Basement membrane - Structural support for the epithelium. - Serve as orientation platform in relation to other tissues. - Form basis upon which new cells attach themselves during cell division/healing. - The basement membrane acts as a mechanical barrier, preventing malignant cells from invading the deeper tissues. ## Functions of Epithelial Tissues - **Protection:** The coverings/ linings serve as selective barriers between the environment and the organ, prevent invasion of foreign bodies, protection against abrasion. - Features of the cells: - Constantly renewing itself. - The higher their numbers, the more the protective power. - **Absorption:** Cells often have microscopic projections on the apical surface of their plasma membranes (formed by involutions) called microvilli which increase surface area in order to facilitate absorption e.g small intestine. - Features of the cells: - Microvilli - Cell membrane - Mitochondrion - Golgi apparatus - Nucleus - Rough endoplasmic reticulum - **Secretion:** In glands, epithelial tissue is specialized to secrete specific chemical substances such as; enzymes, hormones and lubricating fluids. - Features of the cells: - Active cytoplasmic with numerous secretory vacuoles. - **Sensation:** Converting signals from the environment into impulses which can be processed by the nervous system. - Features of the cells: - Sensitive or specialized cells - **Contractility:** Contain contractile protein the myosin and actin filaments in their cyctoplasm (glands, muscles). ## Classification of the Epithelium - Classification of Epithelium: - Number of layers: - Single/simple - Multiple/stratified - Shapes: - Flat/squamous - Cuboidal - Columnar ## Simple epithelium : Cells arranged in a single layer for covering/lining. : This is where all the cells are in direct contact with the basement membrane. ## (1a) Simple squamous epithelia : Tissues composed of irregular, thin, flat cells with elongated nuclei in one layer (alveoli, endothelium of vessels, glomeruli....). : This type of epithelium is often permeable and occurs where small molecules need to pass quickly through membranes (Absorption and filtration). ## (1b) Simple cuboidal epithelium : Single layer of cube-shaped and hence similar in width & height. : Centrally placed spherical nuclei. : Kidney/renal tubules, ovarian surface, Salivary/ thyroid gland (To facilitate absorption of filtered molecules, provide space for secretions....) ## (1c)Simple columnar : Tall, elongated, rectangular-like cells (where more secreatory activity takes place) with the nucleus displaced towards the base of the cell. : Found in the lining of the stomach, intestine, gall bladder and uterine tubes (where more secreatory action takes place) ## Simple columnar- uterine tube : Ciliated columnar epithelium is composed of simple columnar epithelial cells with cilia on their apical surfaces. : The columnar shape will provide a large space to produce substances that will release in the internal environment (mucus rich in fluid are calcium, sodium, chloride, glucose). : The beating action of the cilia helps in the movement of substance. ## Simple columnar- Small intestine/Stomach : Simple columnar epithelial cells with microvilli on their apical surfaces. : The columnar shape will provide a large space for absorption of nutrients and secretion of gastric juice and HCL in the small intestine and stomach respectively. : The in-foldings will create more space for absorption/screation to take. ## II Stratified epithelium : Cells arranged in two or more layers. : Found primarily on the "wear and tear" surfaces of the body such that layers can be lost without exposing sub-epithelial layers. ## (2a) Stratified Squamous epithelium : Multilayer of flat cells. : Seen in area that are prone to injury/insult e.g skin, anal canal, mouth, vagina. ## Stratified Squamous epithelium-Types : **keratinized Stratified Squamous (protein – keratin that prevent water loss hence the dryness) epidermis of the skin**. : **non - keratinized Stratified Squamous** where water molecules is allowed pass through to keep surface moist e.g mouth, vagina and anal canal. ## (2b) Stratified cuboidal epithelium (rare) : Multiple layer of cuboidal cells. : Seen in glands; Sweat gland, mammary gland. : They protect ducts of the glands (stratification). : Secretion of glandular products (cuboidal). ## (2c) Stratified Columnar epithelium (uncommon) : Not very common, seen in Pancreatic duct, conjunctiva. ## III Pseudostratified epithelium : Intermediary between simple and stratified epithelium. : Though it consists of one layer of irregularly shaped and sized cells with nucleus at different levels, giving the appearance of more than one layer with misleading (hence "pseudo") impression that the epithelium is stratified. : Trachea, bronchi, pharynx, nasal cavity, and urethra. ## IV Transitional epithelium : Unstable shape/size of cells. : Found in elastic tissues. : Can appear to be stratified cuboidal when the tissue is not stretched or stratified squamous when the organ is distended and the tissue stretches. : Urinary bladder and Ureter. ## ASSIGNMENT FOR HISTOLOGY OF EPITHELIUM' 1. 5 Organs, Epithelium, and Justify why the epithelium is living there. : Put each on a different column on a table. : Write your name and matric number.