Epithelial Tissue PDF

Basic tissues of human body Epithelial tissue Vincent Rodrigues  Objectives Types of basic tissues Epithelial tissue – General Features / Characteristics – Functions – Classification and examples Glands – Classification and examples...

Basic tissues of human body Epithelial tissue Vincent Rodrigues  Objectives Types of basic tissues Epithelial tissue – General Features / Characteristics – Functions – Classification and examples Glands – Classification and examples Vincent Rodrigues  Basic tissues of human body Tissue: group of cells, which share structural and functional similarities and intercellular materials. Four basic tissues of human body: - Epithelial tissue (epithelium) - Muscle tissue - Connective tissue - Nervous tissue Vincent Rodrigues  Epithelial tissue One or more layers of closely packed cells (with very little intercellular spaces) lining the – Pathways exposed to/ communicating with the external environment (skin and tubular organs e.g. Gastrointestinal, Respiratory and Urogenital Systems). – Serous body cavities (mesothelium) – Lines the interior surfaces of heart, blood vessels and lymphatic vessels (endothelium) – Ducts and secretory elements of glands  Characteristics / Features of Epithelial Cells Cellularity: Composed almost entirely of cells. It may consist of one(1) or more layers of cells that are closely packed together with minimal amounts of intercellular substance between the cells. Polarity: Different types of electrically charged molecules on different sides of its membrane producing a resting potential in living cells allowing epithelial cells to transport molecules across the surface in a directional manner. Determined by surfaces of cells as follows. - Apical or Luminal Surface: exposed to external surface or internal body space. - Basal Surface: The interface between all epithelia and underlying supporting tissues, marked by a non-cellular structure known as the basement membrane that provides structural support for the epithelium and constitutes a selective barrier to the passage of materials between the epithelium and supporting tissue. The epithelial cells rest on the basement membrane. In case of multilayered epithelia the deepest layer of cells rests on the basement membrane. Basement membrane is made up of fibrous tissue (Basal Lamina and Reticular Lamina) - Lateral Surfaces: Linked by cell junctions (Tight junctions, Zonula adherens, Desomosomes, Communication junctions) so that the epithelium forms a continuous cohesive layer, allows for communication channels which govern functions such as growth and cell division. Attachment: Basal Surface bound to basement membrane (Hemidesmosomes bind the base of the cell to the underlying basement membrane) Avascularity: The epithelium is avascular and is nourished by diffusion from the underlying capillaries. Innervation: Contains many nerve endings.  High Regeneration Capacity: Epithelial Cells are frequently damaged or lost to abrasion, they are replaced quickly Extends to form glands  Membrane Specializations of Epithelia Intercellular surfaces (Lateral Surfaces) - Tight Junctions: block the passage of molecules between adjacent cells - Anchoring Junctions: Zonula adherens and desmosomes: give the epithelium strength by linking the cytoskeletons of adjacent cells. - Communication junctions (gap or nexus junctions): provide a conduit for the passage of small molecules directly between adjacent cells. Luminal or Apical Surfaces - Cilia: actively motile structures - Microvilli: shorter projections of the plasma membrane - Stereocilia: extremely long microvilli (not motile) Basal Surfaces - Non-cellular Basement membrane provides structural support for the epithelium and constitutes a selective barrier to the passage of materials between the epithelium and supporting tissues. Hemidesmosomes bind the base of the cell to the underlying basement membrane by linking to the cell’s intermediate filament network.  Functions of epithelium Protection of underlying tissues of the body from abrasion and injury Secretion of mucous, hormones, enzymes from various glands Absorption of material from the lumen (intestinal tract or certain kidney tubules). Detection of sensations (olfactory epithelium of nose, taste buds and receptors in the internal ear) Vincent Rodrigues  Classification of epithelia Depending on the layers of cells it can be classified into simple and stratified epithelia Simple and stratified epithelia classified into subtypes depending on the type of cells present Vincent Rodrigues  Simple epithelia Simple squamous epithelium Simple cuboidal epithelium Simple columnar epithelium Pseudostratified columnar epithelium  Stratified (compound) epithelia Stratified squamous non-keratinized epithelium Stratified squamous keratinized epithelium Stratified cuboidal epithelium Stratified columnar epithelium Transitional epithelium  Simple squamous epithelium One layer of flat cells resting on a basement membrane Width of the cell is more than the height Nucleus is flat, seen as a bulge on the cell surface Thin layer of cytoplasm This epithelium allows passage of materials by diffusion and filtration eg. Alveoli of lung, blood vessels, bowman’s capsule of kidney Vincent Rodrigues  Simple Squamous Epithelium – Bowman’s capsule of kidney Bowman’s capsule of kidney Glomerulus Simple squamous Epithelial cells Lung alveoli Simple Squamous Epithelium Eg. Lining of blood vessel  Simple cuboidal epithelium One layer of cells, have equal height and width Centrally placed round nucleus Function – secretion and absorption Eg. follicles of thyroid gland, renal tubules Vincent Rodrigues Simple Cuboidal Epithelium – thyroid gland – slide 2 Thyroid follicle lined by Cuboidal cells Nucleus  Simple Columnar Epithelium One layer of cells that are almost 3 times taller than their width Oval shaped nucleus placed near the basal part of cell Nucleus Function: – protection – lubrication – Absorption – secretion of mucus, enzymes – ciliated type propels reproductive cells Basement membrane  Simple columnar epithelium Based on the nature of free surface of the cell, the columnar epithelium can be classified into:  Simple columnar epithelium – free surface has no particular specialization eg - stomach  Simple columnar epithelium with microvilli or brush border – free surface presents fine projections called microvilli e.g. gall bladder  Simple columnar epithelium with microvilli and goblet cells – free surface presents fine projections called microvilli e.g. small intestine, large intestine  Ciliated columnar epithelium – free surface has projections called cilia – eg uterine tube Vincent Rodrigues Simple columnar epithelium – stomach Nucleus Simple columnar epithelium – gall bladder Nucleus Columnar cells Simple columnar epithelium with microvilli and goblet cells (small intestine) – slide 61 Goblet cell Vincent Rodrigues Small intestine Simple columnar epithelium with goblet cells (large intestine) Goblet cell Simple ciliated columnar epithelium (uterine / fallopian tube) Vincent Rodrigues Pseudo stratified ciliated columnar epithelium Single layer of cells with different heights All the cells lie on the basement membrane Some cells are short, only some cells reach the surface of the epithelium Their nuclei are located at different levels, giving the impression of a stratified epithelium even though it is composed of single layer of cells Function: Protection, Secretion, propulsion of mucus by ciliary action. E.g. trachea Vincent Rodrigues  Cilia Goblet cell Pseudo stratified ciliated columnar epithelium (trachea)  Stratified (compound) epithelium 1) Stratified squamous epithelium (stratified squamous keratinized and stratified squamous non-keratinized epithelia) 2) Transitional epithelium 3) Stratified columnar epithelium 4) Stratified cuboidal epithelium 1) Stratified squamous epithelium: Seen in areas where there is maximum friction, and cells are constantly replaced Thick, several layers of cells, only the deeper layer of cells are in contact with the basement membrane Two types – stratified squamous keratinized and stratified squamous non-keratinized epithelia Stratified squamous non-keratinized epithelium: Contains 3 types of cells Deepest single layer of cells resting on basement membrane are columnar cells Intermediate layers of polyhedral cells Superficial layers of squamous cells E.g. oesophagus Function: protection, secretion, prevents water loss Vincent Rodrigues Stratified squamous nonkeratinised epithelium Squamous cells Polyhedral cells Polyhedral cells Columnar cells Vincent Rodrigues Stratified squamous nonkeratinised epithelium– esophagus Polyhedral cell Columnar cell Stratified squamous keratinized epithelium: Keratin Contains 3 types of cells Deepest single layer of columnar cells resting on the basement membrane Middle layers of polyhedral cells Superficial layers of squamous cells Superficial part contains keratin E.g. skin Function: protection, prevents water loss Vincent Rodrigues 2) Transitional epithelium (urothelium): Capable of considerable distension Epithelium is 4 to 6 layers in thickness Deepest cells are cuboidal or columnar cells located on the basement membrane Middle 2-4 layers of polyhedral or pear shaped cells Superficial layer of umbrella shaped cells Function: stretches readily and permits distension of urinary organ (distensibility), protection E.g. ureter, urinary bladder  Umbrella shaped cells Pear shaped and polyhedral cells Transitional Cuboidal cells epithelium Ureter Transitional epithelium- ureter (75) urinary bladder (76) Umbrella shaped cells Polyhedral and pear shaped cells Basal cuboidal cells 3) Stratified columnar epithelium Usually made up of 2 layers of cells Deeper layer of cuboidal cells in contact with the basement membrane Superficial layer of columnar cells. Eg. Conjunctiva Function: protection 4) Stratified cuboidal epithelium Two layers of cuboidal cells. E.g. Ducts of sweat glands, large excretory ducts of the salivary glands Function: protection  Glands A cell, group of cells, or organ of endothelial origin that selectively removes materials from the blood, concentrates or alters them, and secrets them for further use in the body or for elimination from the body The simplest glands are invaginations of the surface epithelium. Function is secretion. Classified into two types – Exocrine glands – Endocrine glands  Endocrine glands These glands have no duct system and discharge their secretion directly into the bloodstream and lymph vessels to act on distant tissues. Are composed of groups of cells lying close to blood and lymph vessels The secreting cells mostly occur in clusters or cords of secretory cells surrounded by a rich network of small blood vessels. Each cluster is surrounded by a basement membrane. These release hormones into the intercellular spaces from which they diffuse into surrounding blood vessels. eg. thyroid gland, pituitary gland etc.  Exocrine glands Exocrine glands discharge their secretions onto an epithelial surface either directly or via a duct. Exocrine gland secretions include saliva, perspiration, oil, earwax, milk, mucus, and digestive enzymes. These are classified in different ways: 1) Classification according to the branching of ducts: Simple glands: Secretion of these glands is discharged through single or unbranched duct eg. gastric glands of stomach, sweat glands Compound glands: The ducts of these glands divide into many branches eg. Parotid salivary gland 2) Classification according to the shape of the secretory unit: - Tubular glands: Secretory unit is tubular in shape eg. gastric glands of stomach - Acinar glands: Secretory unit is rounded or oval in shape – which is called acinus eg. parotid salivary gland, pancreas - Alveolar glands: Secretory unit is in the form of flask- shaped sac called alveolus Acinar gland – eg. mammary gland Tubular gland pancreas (eg gastric gland) 3) Classification according to method of discharge of secretion from the secreting cells: - Holocrine glands: eg. sebaceous glands These glands accumulate their secretions in each cell's cytoplasm and release the whole cell into the duct The entire secreting cell forms a part of secretion - Apocrine glands: eg. mammary glands Only a portion (apical part) of secreting cell is lost during secretion and becomes a part of secretion - Merocrine gland: eg. salivary glands The secretion passes through the free surface of the cells into the lumen of the acinus. Wall of the secreting cell remains intact  Sebaceous gland Alveolus of mammary gland 4) Classification according to the type of secretion: - Serous glands: eg. Parotid salivary gland Secretion of these glands is a watery clear fluid The secreting cells have rounded nuclei close to their base - Mucous glands: eg. Sublingual salivary gland Secrete the glycoprotein mucin, which by the uptake of water is converted into a slippery secretion called mucous The secreting cells have flat, peripheral nucleus lying against the base of the cell - Mixed glands: eg. Submandibular salivary gland These have both serous and mucous secretory units Serous gland Mucous gland Mixed gland 5) Classification according to number of secreting cells: - Unicellular glands: eg. goblet cells Gland consisting of single cell that functions as a gland These are situated in the epithelium of trachea and intestine along with other epithelial cells and secrete mucous - Multicellular glands: eg. parotid salivary gland, gastric glands. These glands consists of clusters of secreting cells Most of the glands in the body are multicellular glands