Epithelium & Glands PDF

 The human body is composed of only four basic types of tissue: Tissue Main Functions Nervous Transmission of nervous impulses Nervous tissue Epithelial Lining of surface or body cavities, glandular...

 The human body is composed of only four basic types of tissue: Tissue Main Functions Nervous Transmission of nervous impulses Nervous tissue Epithelial Lining of surface or body cavities, glandular secretion Muscle Movement Connective Support and protection Tissues are groups of cells that have a similar structure and act together to perform a specific function  Objectives describe the characteristic describe the characteristic features and functions of features and functions of epithelial tissues glandular tissue describe the classification of describe the classification epithelial tissues of glandular tissue describe the surface describe the mode of modifications of epithelial secretion of glandular tissue tissue identify the different types describe the junctional of epithelial & glandular complexes of epithelial tissue tissues  Epithelium is present in two forms: 1 1. as sheets of continuous cells (epithelia) that cover the body on its external surface and line 2 the body on its internal surface 2. as glands, which originate from invaginated epithelial cells are derived from all three embryonic germ layers Detection of sensations via taste buds, retina of Protection of underlying the eye and specialized tissues of body from hair cells in the ear abrasion and injury Absorption of material from a lumen (e.g., intestinal tract or certain kidney The principal tubules) functions of epithelial tissues Transcellular transport of molecules across epithelial layers Control of movement of Secretion of mucus, materials between body hormones, enzymes, compartments via selective and etc., from permeability of intercellular various glands junctions between epithelial cells  Characteristic Features of Epithelial Cells The forms and dimensions of epithelial cells range from high columnar to cuboidal to low squamous cells The nuclear form often corresponds roughly to the cell shape eg; cuboidal cells have spherical nuclei squamous cells have flattened nuclei The long axis of the nucleus is always parallel to the main axis of the cell  Epithelia can be divided Types of Epithelia into two main groups according to their structure and function: 1. covering (or lining) epithelia 2. glandular epithelia  Covering or Lining Epithelia cover the external surface or line the surface layer cavities of the body are classified according to the number of cell layers and the morphologic features of the cells in surface layer Simple epithelia contain only one layer of cells Stratified epithelia contain more than one layer Based on cell shape, simple epithelia are classified as 1. squamous (thin cells) 2. cuboidal (cells roughly as thick as they are wide) 3. columnar (cells taller than they are wide)  Simple squamous epithelium cells of the single layer flat and very thin nucleus appear as a bulge are typically specialized as lining of vessels and cavities regulate substances which can enter underlying tissue from vessel or cavity often exhibit transcytosis Epithelium present in/as 1. renal loops of Henle 2. mesothelium lining a mesentery 3. endothelium lining the inner surface of the cornea Simple cuboidal epithelium vary in their height roughly as tall as they are wide thickness is cause by ❑ cytoplasm rich in mitochondria ▪ providing energy for a high level of active transport of substances across the epithelium renal collecting tubule Epithelium present in/as 1. renal collecting tubule 2. pancreatic duct 3. mesothelium covering an ovary mesothelium covering an ovary  Simple columnar epithelium cells are taller are usually highly specialized for absorption, with microvilli, and often have interspersed secretory cells or ciliated cells always have tight and adherent junctional complexes for rapid transfer of absorbed material to the space between the cells additional cytoplasm allows additional mitochondria and other organelles needed for absorption and processing renal collecting duct Epithelium present in/as 1. renal collecting duct 2. oviduct lining, with both secretory and ciliated cells 3. lining of the gall bladder lining of the gall bladder Stratified epithelia Transitional are classified according to the cell shape of the superficial layer(s) Squamous Cuboidal Columnar Transitional  Stratified squamous epithelia keratinized have protective functions: protection against easy invasion of underlying tissue by microorganisms and protection against water loss In the skin, protection against water loss and desiccation is particularly important and the epithelium is keratinized When skin differentiate they will be filled with keratin and other substances lose their nuclei and other organelles The superficial flattened squamous cells lose their nuclei form a layer which prevents water loss slough off and are replaced from below Stratified squamous epithelia non keratinized lining many internal surfaces such as esophagus and cornea cells layers are retain their nuclei nonkeratinized because the differentiating cells accumulate less keratin and retain their nuclei Provide protection against microorganisms Do not fill with keratin because water loss is not an issue  Stratified cuboidal and stratified columnar epithelia are rare Stratified columnar epithelium can be found in the conjunctiva lining the eyelids is both protective and mucus secreting organ or tissue Stratified cuboidal epithelium is restricted to large excretory ducts of sweat and salivary glands provides a stronger lining than that of a simple epithelium ducts of sweat gland Transitional epithelium or urothelium lines only the urinary bladder, ureter and upper part of the urethra is characterized by a superficial layer of domelike cells also called umbrella cells Essential in protection against hypertonic and potentially cytotoxic effects of urine form of the surface cells changes according to the degree of distention of the bladder wall Pseudostratified Columnar Epithelium It is so called because 1. all cells are attached to the basal lamina 2. their nuclei lie at different levels in the epithelium give the false appearance of cellular stratification 3. height of some cells does not extend to the surface epithelium is lining the passages of the upper respiratory tract – heavily ciliated Number of Cell Layers Cell Form Examples of Main Function Distribution Simple (one layer) Squamous Lining of vessels Facilitates the movement (endothelium). Serous of the viscera lining of cavities; (mesothelium), active pericardium, pleura, transport by pinocytosis peritoneum (mesothelium and (mesothelium). endothelium), secretion of biologically active molecules (mesothelium). Cuboidal Covering the ovary, Covering, secretion. thyroid. Columnar Lining of intestine, Protection, lubrication, gallbladder. absorption, secretion. Pseudostratified (layers Lining of trachea, bronchi, Protection, secretion; cilia- of cells with nuclei at nasal cavity. mediated transport of different levels; not all cells particles trapped in mucus reach surface but all out of the air passages. adhere to basal lamina) Number of Cell Layers Cell Form Examples of Main Function Distribution Stratified (two or more Squamous keratinized Epidermis. Protection; prevents water layers) (dry) loss. Squamous nonkeratinized Mouth, esophagus, larynx, Protection, secretion; (moist) vagina, anal canal. prevents water loss. Cuboidal Sweat glands, developing Protection, secretion. ovarian follicles. Columnar Conjunctiva, male urethra Protection. and ducts of some glands Transitional Bladder, ureters, renal Protection, distensibility. calyces. Glandular Epithelia  Glandular Epithelia are formed by cells specialized to secrete molecules to be secreted are generally stored in the cells in small membrane-bound vesicles called secretory granules its cells may synthesize, store, and secrete 1. proteins (eg, in the pancreas) 2. lipids (eg, adrenal, sebaceous glands) 3. complexes of carbohydrates and proteins (eg, salivary glands). Mammary glands secrete all three substances The cells of some glands have low synthetic activity (eg, sweat glands) and secrete mostly water and electrolytes transferred into the gland from the blood.  Unicellular glands Glandular Epithelia Are classified according to goblet cell various criteria Unicellular glands consist of large isolated secretory cells Multicellular glands Multicellular glands have clusters of cells The classic unicellular gland is the goblet cell in the lining of the small intestine or respiratory tract sebaceous gland  Glands are classified into two major groups on the basis of the method of distribution of their secretory products: Exocrine glands secrete their products via ducts onto the external or internal epithelial surface from which they originated. Endocrine glands are ductless, having lost their connections to the originating epithelium, Exocrine glands and thus secrete their products into the blood or lymphatic vessels for distribution Endocrine glands  Exocrine Glands have ducts that lead to an organ or body surface the duct runs through connecting septa and branches repeatedly Divided into 1. secretory portion, which contains the cells specialized for secretion 2. ducts, which transport the secretion out of the gland This morphology allows the glands to be classified according to the scheme  Exocrine Glands Ducts can be 1. simple (unbranched) 2. compound (with two or more branches) Secretory portions can be 1. tubular (either short or long and coiled) 2. acinar (round or globular) Either type of secretory portion may be branched Compound glands can have tubular, acinar or tubuloacinar secretory portions  simple compound  Exocrine Glands are classified functionally according to the way the secretory products leave the cell Merocrine secretion (eccrine) – involves typical exocytosis of proteins or glycoproteins. – most common mode of secretion Holocrine secretion – Cell will be filled with secretory product before the whole cell being disrupted and shed – seen in sebaceous glands of skin Apocrine secretion – an intermediate type – secretory product is typically a large lipid droplet and is discharged together with some of the apical cytoplasm and plasmalemma eg: mammary gland Merocrine secretion Holocrine secretion Apocrine secretion  Merocrine secretion can be further categorized as serous or mucous – according to the nature of the proteins or glycoproteins secreted – the resulting staining properties of the secretory cells  Serous Cells The acinar cells of the pancreas and parotid salivary glands are examples – secrete digestive enzymes stain intensely Each acinar cell is roughly pyramid- shaped, with its apex at the lumen Apical ends are eosinophilic due to abundant immature and mature secretory granules basal ends contain large rounded nuclei and an abundance of rough ER, making the cells highly basophilic basally filled apically with secretory granules Acinar cells refers to any cluster of cells that resembles a many-lobed “berry"  Mucous Cells Example is the goblet cells Contains more flattened basal nuclei lumens The apical region and most of cytoplasm is filled with secretory granules containing strongly hydrophilic lumens glycoproteins known as mucin which has important water-binding properties The basal region contains the RER, nucleus & well-developed Golgi apparatus The lumens are larger than serous acini  Mixed Seromucous Gland Mucous cells of large glands are organized as secretory tubules and in mixed seromucous salivary glands The submandibular salivary glands have both mucous and serous secretory units, typically shaped as acini and tubules Clumps of serous cells at the ends of some mucous tubules appear as crescent-shaped structures called serous demilunes  Holocrine Secretion The sebaceous gland adjacent to hair follicles is example Entire cells fill with product and are released during secretion Undifferentiated cells deep and peripheral in the gland fill with lipid-rich granules & metabolically inactive As they mature they move upward and toward the gland's center Since sebaceous glands has lack myoepithelial cells; cell When terminally differentiated, the cells proliferation inside a dense, separate and disintegrate to form the secretion inelastic connective tissue capsule which serves to protect and lubricate adjacent continuously forces product into skin and hair the duct  Apocrine secretion Mammary gland demonstrate apocrine secretion Characterized by the discharge of secretory product with a pinched off portion of the apical cytoplasm (arrows) The released portion of cell contains lipid droplet(s) Merocrine secretion also occurs from the same and other cells of the secretory units  Exocrine glands like sweat, lacrymal, salivary and mammary glands contain stellate or spindle-shaped myoepithelial cells located between basal lamina and the basal pole Myoepithelial cells are connected to each other and to the epithelial cells by both gap junctions and desmosome These cells are specialized for contraction, containing myosin and large number of actin filaments function is to contract around the secretory or conducting portion of the gland and help to propel secretory products into the duct Salivary gland: its organization, secretory units, and system of ducts  Endocrine Glands produces hormones, which are generally polypeptide or lipid- derived factors that are released into the interstitial fluid Hormones diffuse into the blood for circulation and bind specific receptors on target cells elsewhere in the body The receptors may also be on cells very close to the hormone- secreting cells or on the secreting cell itself – the cellular signaling is termed paracrine or autocrine Polarity and Cell-Surface Specializations Epithelial cell polarity and cell-surface specializations are related to cellular morphology and function  Epithelial Cells Polarity Epithelial cells generally show polarity 1. basal pole 2. apical pole 3. lateral surfaces The membranes on the lateral surfaces often have numerous infoldings to increase the area of that surface, increasing its functional capacity Basal Surface Specializations (Basal Pole)  Hemidesmosomes adhesive structures that bind the cell to basal lamina the plaques contain abundant integrins – a transmembrane proteins contain receptors for the extracellular macromolecules laminin and collagen type IV  Lamina propria Most epithelia rest on connective tissue is often called lamina propria lining the cavity of internal organs like digestive, respiratory and urinary systems. Function is to 1. supports the epithelium 2. provides nutrition and binds it to underlying structures The area of contact is increased by irregularities in its surface in the form of small evaginations called papillae (singular papilla) Papillae occur most frequently in epithelial tissues subject to friction, such as covering of the skin or tongue  Basal Laminae & Basement Membranes At basal surfaces, all epithelial An electron-lucent layers: laminae lucida cells attach to a sheet of extracellular material known as dense layer or basal lamina lamina densa Basal lamina Basal lamina is often called a basement membrane is used to denote the lamina reticular lamina densa and its adjacent layers structures seen with the *TEM and structures seen with the *TEM Basement membrane is used to denote the structures seen with the light microscope is formed by the combination of a basal lamina & reticular lamina  Basal Laminae & Basement Membranes Nutrients for epithelial cells diffuse across the basal lamina through basolateral surfaces of epithelial cell, usually by an energy-dependent process Nerve fibers normally penetrate this structure Capillaries (being epithelial themselves) never enter an epithelium across a basal lamina Blood vessels do not normally penetrate an epithelium basement membrane Receptors for chemical messengers (eg, hormones, neurotransmitters) that influence the activity of epithelial cells are localized in the basolateral *TEM ; transelectron microcope membranes  Basal Laminae & Basement Membranes Its macromolecular components form precise three-dimensional arrays like 1. Laminin: – are large glycoprotein – they are held in place by transmembrane integrins 2. Type IV collagen: – contain three polypeptide chains Entactin (nidogen) Glycoprotein Perlecan Proteoglycan with heparan sulfate side chains – are glycosylated proteins that serve to link the laminin and type IV collagen sheets – are secreted at basal poles of epithelial cells  Basal laminae are attached to reticular fibers made of type III collagen in underlying connective tissues by anchoring fibrils of type VII collagen. These proteins are produced by cells of connective tissue and form the reticular lamina Basal laminae also found in other cell types muscle cells, adipocytes and Schwann cells  Functions Of Basal Laminae filtering functions contain the information necessary for many cell-to-cell are also able to influence cell polarity interactions, such as the reinnervation of denervated muscle cell regulate cell proliferation and differentiation by binding and concentrating growth factors establishment of new neuromuscular junctions in muscle cell influence cell metabolism and survival organize the proteins in the adjacent plasma membrane (affecting signal transduction) serve as pathways for cell migration  Clinical Importance Goodpasture's syndrome – is an antiglomerular basement antibody disease – is a rare autoimmune disease in which antibodies attack the basement membrane in lungs and kidneys, leading to bleeding from the lungs and kidney failure The Goodpasture antigen – is located on the alpha3 chain of type IV collagen, a major supporting component of the basement membrane Lateral Membrane Specializations (Lateral surfaces)  Intercellular Adhesion & Other Junctions Several membrane-associated structures contribute to adhesion and communication between cells Present in most tissues but are numerous and prominent in epithelia Epithelial cells are extremely cohesive and strong Intercellular adhesion is especially marked in epithelial tissues that are subjected to traction and pressure (eg: skin)  Intercellular Adhesion & Other Junctions The lateral membranes of epithelial cells exhibit several specialized intercellular junctions Various junctions serve to function as: 1. Tight junction/ occluding junctions/zona occludens Seals to prevent the flow of materials between the cells 2. Adhesive or anchoring junctions Sites of adhesion 3. Gap or communicating junctions Channels for communication between adjacent cells In several epithelia such junctions are present in a definite order from the apical to the basal ends of the cells  or zonulae occludens located at the apical of the junctions Zonula indicates that the junctions form bands completely encircling each cell Occludens refers to membrane fusions that close off the space between cells membranes are tightly apposed or fused is because of direct interactions of transmembrane protein claudin  The number of these sealing strands or fusion sites is inversely correlated with the leakiness of the epithelium Its principal function is to form a seal that 1. prevents the flow of materials between epithelial cells (the paracellular pathway) in either direction 2. prevent the integral membrane proteins of the apical surface from being transferred to the basolateral surface and vice versa This allows the two sides of the epithelium to maintain different receptors and function differently  apical compartment It also help to form two functional compartments: apical compartment that is composed of an organ cavity (such as the lumen of a secretory unit or the gut) basal compartment that begins at the junctions and encompasses the underlying tissue basal compartment 2. Adherent Junction or Zonula Adherens provides for the firm adhesion of one cell to its neighbors Adhesion is mediated by transmembrane glycoproteins of each cell, the cadherins, which lose their adhesive properties in the absence of Ca2+ The numerous actin filaments has a role in cytoplasmic motility 2. Desmosome Or Macula Adherens resembles a single "spot-weld" (L. macula, spot) is a disk-shaped structure Between cell membranes at a desmosome are larger members of the cadherin family Cable-like filaments of cytokeratin are very strong and provide firm adhesion In nonepithelial cells, the intermediate filaments attached to desmosomes are composed of other proteins, such as desmin or vimentin  can occur anywhere along lateral membranes of epithelial cells are aggregated transmembrane protein complexes that form circular patches in the plasma membrane A protein known as connexins, form hexameric complexes called connexons that has a central hydrophilic pore Connexons permit rapid exchange of molecules between cells  Some molecules mediating signal transduction, such as cyclic AMP, cyclic GMP, and ions move readily through gap junctions, allowing cells in many tissues to act in a coordinated manner rather than as independent units A good example is heart muscle, where abundant gap junctions are greatly responsible for the heart's coordinated beat Apical Domain  Specializations of the Apical cell Surface The apical surface of epithelial cells has specialized structures 1. Microvilli 2. Stereocillia 3. Cillia to increase the cell surface area or to move substances or particles bound to the epithelium many cells are seen to have cytoplasmic projections that may be short or long fingerlike extensions or folds  Microvilli In absorptive cells, such as the lining epithelium of the small intestine, the apical surface contains hundreds of microvilli The total surface area can be increased as much as 20- or 30-fold The complex of microvilli and glycocalyx is called the brush or striated border  Stereocilia are long apical processes of cells in other absorptive epithelia ✓ Eg: lining the epididymis, ductus deferens longer, less motile than microvilli & branched Increase cells' surface area, facilitating the movement of molecules into and out of the cell  Cilia are elongated, highly motile structures on epithelial cells surface longer and x2 wider than a Apical portions of cells lining the respiratory tract typical microvillus bounded by cell membrane that contains axoneme with a central pair of microtubules surrounded by nine peripheral microtubular pairs inserted into basal bodies, at the apical pole just below the Basal bodies have a structure similar to that of cell membrane centrioles  Cilia It exhibit rapid back-and-forth movements coordinated to propel a current of fluid and suspended matter in one direction with adenosine triphosphate (ATP) as the energy source A ciliated cell of the trachea lining is estimated to have about 250 cilia Flagella, present in the human body only in spermatozoa are similar in structure to cilia but are much longer and are normally limited to one flagellum per cell https://youtube/xWpWn8W5nlA?t=19  Transport Across Epithelia all cells have the ability to actively transport certain ions against a concentration and electrical-potential gradient Example is active extrusion of Na+ by means of Mg2+-activated Na+/K+-ATPase (sodium pump) – The cells maintain the required low intracellular sodium concentration (5-15 mmol/L vs. ~140 mmol/L in extracellular fluid) Some epithelial cells actively transfer ions and fluid across the epithelium, from its apex to its base or vice-versa; this is known The apical surface of proximal as transcellular transport renal tubule cell is freely permeable to Na+ in the lumen. For transport in either direction, the tight junctions play an important role in the transport process, sealing the apical portions of the epithelium and preventing back-diffusion of materials  Transport Across Epithelia To maintain electrical and osmotic balance, equimolar amounts of chloride and water follow the Na+ ion into the cell Interdigitation of membrane folds between adjacent cells, all of which increase the surface area for transport Sodium pumps are localized in basal and lateral plasma membranes Between the folds are vertically oriented mitochondria that supply the ATP for active extrusion of Na+ from the cell basally  Both benign and malignant tumors can arise from most types of epithelial cells A carcinoma is a malignant tumor of epithelial cell origin Malignant tumors derived from glandular epithelial tissue are usually called adenocarcinomas these are the most common tumors in adults In children up to age 10 years, most tumors develop (in decreasing order) from hematopoietic organs, nerve tissues, connective tissues and epithelial tissues. This proportion gradually changes, and after age 45 years, more than 90% of all tumors are of epithelial origin THANK YOU