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Volgograd state medical university Department of histology, embryology, cytology Tissues. Epithelial tissue. Glands. For the 1st year General Medicine Students Volgograd, 2017 THE OBJECTIVES: 1.To define the term “tissue” and to characterize tissue types. 2.To develop classification of the epithelial tissue. 3. To provide morphological and functional characteristics of the epithelial tissue and its subtypes. 4. To understand the difference between the exocrine and endocrine glands and to characterize exocrine glands (classifications, structure, types). DEFINITION AND CLASSIFICATION OF TISSUES. A tissue is a complex assemblage of cells and cell products, having a common function. The approximately 200 distinctly types of cells composing the human body are arranged and cooperatively organized into four tissues: 1. Epithelium. 2. Connective tissue. 3. Muscle tissue. 4. Nervous tissue. Body tissues are grouped according to their cells and cell products into organs. These tissues exist and function in close association with one another. Epithelial tissue is present in the two major forms: as sheets of contiguous cells (epithelia) that cover body on its external surface and as glands which originate from invaginated epithelial cells. Epithelial Tissue. Definition. The epithelial tissue consists of sheets of cells that cover the external surfaces of the body, line the internal cavities, form various organs and glands and line their ducts. The are 4 major types of epithelial tissue: 1.Lining epithelium (it lines or covers all body surfaces and cavities except joint cavities). 2.Glandular epithelium (it comprises glands). 3.Germinative epithelium (it is present in the sex glands). 4.Sensory epithelium (it is present in the sensory organs). ORIGIN OF THE EPITHELIAL TISSUE Epithelia are derived from all three embryonic germ layers, although most of the epithelia are derived from ectoderm and endoderm. ectoderm endoderm mesoderm epithelia: oral and nasal mucosae, cornea, the epidermis of skin, nails, hairs, anal canal, the liver, pancreas, and lining of the respiratory and (larynx, trachea, lungs) digestive tract (including pharynx), eustachian tube, urinary bladder, urethra, covering of posterior 1/3 of the tongue. the uriniferous tubules of the kidney, the lining of the male and reproductive system, the endothelial lining of the circulatory system, and the mesothelium of the body cavities, ureter, joint cavities. glands: the glands of the skin (sweat, sebaceous), mammary glands. and salivary glands, adenohypophysis thymus, thyroid, parathyroids, esophageal, duodenal and intestinal glands. kidneys, gonads, adrenal cortex STRUCTURAL AND FUNCTIONAL CHARACTERISTICS OF THE EPITHELIA Epithelium is a structurally and functionally important component of the complex organs. The histology of the epithelium differs from organ to organ depending on its location and function. Irrespective of location all epithelial tissues possess certain features: 1. Intercellular substance. It is almost absent 2. Derivation: ectoderm, mesoderm and endoderm can give rise to epithelium. Ex.: intermediate mesoderm – epithelium of the genitourinary system. 3. Basement membrane. – all epithelia rest on it. 4. Nutrition. – It is provided through the basement membrane by diffusion as epithelia lack blood vessels. 5. Diversity: epithelium ranges from one to many layers - for protection, secretion, absorption, etc. 6. Intracellular polarity: the apical and basal poles of the cells have different structure. 7. Metaplasia: facing a chronic change of environmental conditions, epithelia undergo metaplasia, i.e. they under transition from one type to other (simple to stratified). 8. Regeneration. Regenerative capacity of epithelial cells is very high. Cells readily divide by mitosis. Simple Squamous Epithelium – Mesothelium, Silver impregnation cell boundaries cytoplasm nuclei 1. Intercellular substance. The epithelial tissue have little intercellular substance. The cells are densely packed or closely apposed to each other connected by the specialized intercellular junctions. Intercellular space may be only 25 nm. On the slide cell boundaries are revealed by silver impregnation. Intestinal Villus lined with Simple Columnar Epithelium with Striated Border 3. Basement membrane. Epithelium rests on the basement membrane which binds it to the underlying tissue hence polarity of the epithelial cells: basal surface is connected to the basement membrane while villus apical surface is free. The basement membrane provides scaffolding and is composed of a selective mortar which connects the epithelium to the underlying connective tissue. The basement membrane includes basal lamina composed of the type IV collagen and extracellular matrix molecules (fibronectin and laminin). Basal lamina components are secreted by epithelium. simple columnar epithelium lacteal blood capillary network intestinal gland goblet cell arteriole venule lymph vessel BM C L D FR a b a) Basement membrane may not be seen under LM as it is only 0.05 mcm thick, however a high content of glycoprotein renders it stainable with PAS, when ir appears as a faint magenta stained line (BM). b) By EM basement membrane resolves into several layers (laminae). The lamina densa (D) is a dark staining band 30-100 nm thick. Between this and the attached cell (C) is a lucents zone, the lamina lucida (L), which is usually 60 nm wide. On the other side of the lamina densa is a rarefield layer of variable thickness, the fibroreticular lamina (also included in the basement membrane) which merges with fibrous proteins of the extracellular matrix. Though usually the terms basal lamina and basement membrane are interchangable, the term basal lamina should refer to the lamina densa only. BASAL LAMINA = LAMINA DENSA + LAMINA RARA BASEMENT MEMBRANE = BASAL LAMINA + FIBRORETICULAR LAYER Stratified Squamous Non-keratinized Epithelium of of the Esophagus squampous cells polyhedral cells epithelium basal cells basement membrane connective tissue capillary capillary arteiole connective tissue collagen fibers fibroblasts 4. Nutrition. There are no direct contact between blood vessels and epithelium. All epithelia lack blood vessels. All nutrients and oxygen pass through basement membrane from the capillaries of the adjacent connective tissue because blood vessels do not penetrate the epithelium. Metabolic waste products and carbon dioxide also diffuse from the epitheliocytes to the capillaries in the connective tissue. To shorten the diffusion distance basal surface of the epithelia is often folded. SMALL INTESTINE, H & E mitosis mitosis intestinal enteroendoglands crine cells enteroendo Paneth crine cells ells Paneth cells muscularis muscularis mucosae mucosae 5.Intracellular polarity: the nucleus and the organelles are often found in characteristic regions of epithelium due to intracellular polarity (nuclei are located in the basal part of the cell, the secretory granules – in the apical pole). 7. Regeneration. Because most epithelia cover and line surfaces, the cells are subject to mechanical abrasion, therefore it is necessary for the epithelial cells to be capable of replacement. Regenerative capacity of epithelial cells is very high (mitoses are frequent). Ex.: the turnover rate for intestinal epithelial cells in the colon is about 6 days. Specialized cells that can not divide are provided with stem cells that have the capacity to produce their different kinds of specialized cells. Epithelial cells are capable of division to maintain the integrity of the cellular sheet. The entire epithelial lining the interior of the small intestine is replaced over a period of less than 6 days, of the epidermis – around 28 days. goblet cells FUNCTIONS OF EPITHELIA function example protection from stratified epithelia cover surface of body; cover and line abrasion and injury interior of organs absorption simple columnar (absorptive) cells lining small intestine and stomach absorb variety of products selective permeability nephrothelium, enterocyes, et al., control movement of materials via selective permeability of intercellular junctions between epitheliocytes secretion of mucus, hormones, enzymes etc from various glands goblet cells (unicellular mucous gland cells) in epithelial sheets; mucous and serous glands derived from epithelium; other epithelial cells in oviducts, bronchioles, type II pneumocytes, etc, secrete various products. excretion, transcellular transport gallbladder and kidney tubule epithelial cells actively transport various ions, nitrogenous wastes, water, etc contractility myoepithelial cells around secretory units & smallest ducts of glands (salivary glands, mammary glands). sensory reception taste buds, an example of neuroepithelium on papillae in the tongue immunologic function Langerhans cells in epidermis of integument, a type of antigen-presenting cell (macrophage) CLASSIFICATION OF THE EPITHELIAL TUSSUE The epithelium is classified basing on the number of cell layers and the morphology of the surface cells. 1. Simple epithelium 2. Stratified epithelium Simple epithelium consists of a single layer of cells all cells of which are attached to the basement. The stratified epithelium has several layers of cells while only cells of the basal layer contact the basement membrane. 2 Recto-Anal Junction (*), H & E. * 1 The epithelium is classified basing on the number of cell layers and the morphology of the surface cells. EPITHELIUM • Simple epithelium Stratified epithelium nonkeratinized squamous epithelium -squamous cuboidal epithelium -cuboidal columnar epithelium - columnar pseudostratified: -cuboidal -columnar Simple epithelium consists of a single layer of cells all cells of which are attached to the basement. The stratified epithelium has several layers of cells while only cells of the basal layer contact the basement membrane. keratinized -squamous transitional simple Types of Epithelia squamous cuboidal columnar pseudostratified pseudostratified columnar transitional stratified squamous non-keratinized cuboidal keratinized columnar transitional relaxed transitional (distended) Classification of Simple Epithelia Type of epithelium Location Simple squamous endothelium, mesothelium, parietal layer of Bowman’s capsule, thin segment of loop of Henle, rete testis, pulmonary alveoli Simple cuboidal thyroid, choroid plexus, ducts of many glands, inner surface of capsule of lens, covering surface of ovary Simple columnar surface epithelium of stomach, small & large intestine, proximal convoluted tubules & distal convoluted tubules of kidney, gallbladder, excretory ducts of glands, uterus oviduct small bronchi of lungs some paranasal sinuses Pseudostratified columnar large excretory ducts of glands, portions of male urethra, epididymis trachea, bronchi, Eustachian tube portions of tympanic cavity Specialization striated border brush border cilia cilia cilia stereocilia cilia cilia cilia Classification of Stratified Epithelia Type of epithelium Location Specialization stratified squamous nonkeratinized buccal surface, esophagus, epiglottis, conjunctiva, cornea, vagina gingiva, hard palate epidermis of skin keratin keratin keratinized stratified cuboidal ducts of sweat glands stratified columnar cavernous urethra, fornix of conjuctiva, large excretory ducts of glands transitional urinary system: renal calyces to urethra germinal seminiferous tubules of adult testes Types of Simple epithelium A.Simple squamous epithelium lines the external surfaces of the digestive organs, lungs and heart. The lining of the pleural, abdominal and pericardial cavities is called mesothelium. Similarly a simple squamous epithelium that lines the lumen of heat, blood vessels and lymphatic vessels is called endothelium. B. Simple cuboidal epithelium lines small excretory ducts in different organs, like salivary glands, liver, etc. For example, proximal convoluted tubules of kidney are lined with simple cuboidal epithelium, the apical surfaces of the epithelial cells contain a brush border (composed of microvilli). C.Simple columnar epithelium lines the inner surfaces the digestive organs (stomach, small intestine, large intestine and gall bladder). In the small and large intestines, the surfaces of absorptive epithelial cells that line the villi and/or crypts exhibit a striated border (also formed by the microvill) Pseudostratified Columnar Epithelium • The cells are arranged in single layer of cells on a basement membrane. • But they appear as if they are arranged in two or three different layers as the epithelial cells are of different shape and their nuclei form two to three rows. • Hence the name is pseudostratified (columnar) epithelium. • It lines the air-conducting ways, the lumen of the epididymis and vas deferens. • in trachea, bronchi and larger bronchioles and ductuli efferentes of the testes the surface cells contain motile cilia. • In epididymis and in vas deferens, the surface cells are lined by immotile sterocilia. Pseudostraified Columnar Epithelium of the Nasal Mucosa. G The lining epithelial cells are of different shape and their nuclei are located at different levels forming three rows, but all the epithelial cells are attached to the basement membrane (arrow) either by a thin or a thick base hence epithelium is of a simple type. G-goblet cell. Stratified Epithelium • The cells are arranged in more than one layer. Different layers are placed one above the other. There are the following types of the stratified epithelium. • A. TRANSITIONAL EPITHELIUM • Many layers of cells when collapsed. • The surface cells are dome shaped when contracted and become flattened like squamous epithelial cells when stretched. • It lines the excretory system (ureters, urinary bladder, urethra) • B. STRATIFIED SQUAMOUS EPITHELIUM • Contains multiple cell layers. • The basal cells are cuboidal to columnar. • These give rise to cells that migrate toward the surface and become squamous. • It may keratinized and non-keratinized. • • • • • • • TYPES OF THE EPITHELIAL TISSUE 1b.The keratinized epithelium covers the skin (epidermis). It contains nonliving, kerainized cells that are filled with keratin. Thick epidermis is present on the surface of the palms and soles. 2b.The non- keratinized epithelium contains multiple cell layers. It exhibits live surface cells with nuclei. It lines the oral cavity, pharynx, cornea, esophagus, exocervix of the uterus, vagina and anal canal. C. Stratified cuboidal epithelium (always nonkeratinized) and D. Stratified columnar epithelium (always nonkeratinized) have a limited distribution. Both types line the excretory ducts salivary glands and sweat glands. Stratified Squamous Non-keratinized Epithelium of of the Esophagus squampous cells epithelium polyhedral cells basal cells connective tissue capillary 2b.The non- keratinized epithelium contains multiple cell layers. It exhibits live surface cells with nuclei. It lines the oral cavity, pharynx, cornea, esophagus, exocervix of the uterus, vagina and anal canal. Stratified Squamous Keratinized Epithelium of the Skin excretory ducts of the sweat glands stratum corneum epidermis stratum granulosum stratum spinosum papillae stratum basale connective tissue The keratinized epithelium covers the skin (epidermis). It contains nonliving, kerainized cells that are filled with keratin. Thick epidermis is present on the surface of the palms and soles. CLINICAL CORRELATIONS Each epithelium within the body has its own unique characteristics, location, cell morphology and so on, all of which are related to function. In certain pathological conditions, the cell population of an epithelium may undergo metaplasia, transforming into another epithelial type. Pseudostratified ciliated columnar epithelium of the bronchi of heavy smokers may undergo squamous metaplasia, transforming into stratified squamous epithelium. This change impairs function, but the process may be reversed when the pathological insult is removed. Tumors that arise from epithelial cells may be benign or malignant. Malignant tumors arising from epithelia are called carcinomas; those arising from the glandular epithelial cells are called adenocarcinomas. Due to continuous mitotic cell division, genetically and phenotypically atypical cells form cauliflower-like tumors which invade the normal tissues, change the contour of the original structure penetrate into undelying tissues and give metastasis to the distant organs through the blood and lymph vessels. Polarity and Cell-Surface Specializations 1.Epithelial cells possess apical domain facing the lumen and representing the free surface of cells, and basolateral domain including basal and lateral aspects of cells facing basement membrane and lateral surface of adjacent cells accordingly. 2.The apical domain displays surface specializations, like microvilli, cilia, stereocilia, flagella. 3.The apical domains is rich in ion channels, carrier proteins, ATPase and aqueporins (channel-forming proteins regulating water balance). 4.Basolateral domain exhibits many types of junctional specializations. Surface Modifications of the Apical Domain • 1.Cilia are motile structures which project in parallel rows from certain epithelial surfaces. They cleans the inhaled air and conduct mucus and particulate material across the cell surfaces to the exterior of the body (respiratory tract, uterine tubes of the female reproductive tract; in the efferent ductuli of the male reproductive tract they transport sperm out of testis into epididymis). • 2.Sterocilia are immotile, they absorb fluids produced by the testis (epididymis and vas deferens in the male reproductive tract) or function as sensors of cochlear hair cells in the inner ear. • 3.Flagella - long processes - sensor and transportation organ (in humans only with spermatozoa). • 4.Microvilli (in brush border or striated border): the function is to absorb the nutrients and fluid from the filtrate/contents that passes through these tubules/organs (renal tubules, small intestine, etc). STRIATED BORDER IN THE EPITHELIAL CELLS OF THE SMALL INTESTINE villi columnar epithelium central lacteal smooth muscle basement membrane lymphocyte oblique section of epityhelium: apical and basal parts of cells goblet cell capillary connective tissue (lamina propria) smooth muscle fibers goblet cell striated border In the bowel the lining simple columnar epithelium displays striated border at the apical ends of the epithelial cells. The striated border in the lining epithelial cells are formed by the microvilli (visible under EM) increasing the surface of absorption. M AF MICROVILLI of the BRUSH BORDER of the ILEUM, TEM AC Electron microphotograph showing the surface of a cell lining the small bowel. Microvilli (M) form finger-like projections, each having an actin filament (AF) core that enters the cell and merges with with the actin cortex (AC) which is also known as terminal web. MICROVILLUS lateral anchoring protein (myosin) cell membrane linkage to cell membrane actin cortex linked by spectrin amorphous tip region (villin) actin filaments actin-binding proteins (fimbrin & fascin) spectrin cytokeratin filaments Each microvillus is a finger-like extension of cell membrane which is stabilized by a bundle of actin filaments held rigidly 10 nm apart by actin-binding proteins. The actin bundle is bound to the lateral surface of the microvillus by a helical arrangement of myosin molecules, which bind on one side to the actin and on the other to the inner surface of cell membrane. The bundle is also adherent to the apex of the microvillus in an amorphous area of anchoring proteins which may represent capping proteins for the actin filaments to prevent their depolymerization. At the base of the microvillus the entering actin bundle is stabilized by actin/spectrin cell cortex, under which are cytokeratin intermediate filaments. C Cilia (C) form a hair-like layer at the apical cell surface. They are motile, hair-like projections that emanate from the surface of certain epithelial cells. In the epithelia of the respiratory tract and oviduct there are hundreds of cilia in orderly arrays on the luminal surface of the epitheliocytes. Other epithelial cells as the hair cells of the vestibular apparatus in the inner ear, possess only one cilium (stereocilia) which functions as a sensory mechanism. The internal strucrute is consistently conserved throughout the plant and animal kingdom. Cilia of the epithelial cells of the respiratory mucosa, H & E. Microtubular arrangement the axoneme in the cilia of plasma membrane The core of the cilium contains a complex of uniformely arranged microtubules called axoneme. The axoneme is composed of a constant number of longitudinal tubules arranged in a consistent 9+2 organization. Two centrally placed microtubules (singlets) are evenly surrounded by nine doublets of microtubules. The two central microtubules are separated from each other, each displaying a circular profile in crosssection, composed of 13 protofilaments. Each of the 9 doublets are composed of the 2 subunits. Subunit A also displays circular profile composed of 13 protofilaments while subunit B possesses plasma only 10 protofilaments exhibiting memincomplete circular profile and sharing brane 3 protofilaments with subunit A. central microtubular pair peripheral microtubule douplet microtubular triplet basal body dynein arms (every 24 nm) nexin linking protein, every 86 nm central sheath projections (every 14 nm) DIAGRAM OF A CILIUM radial spoke (every 29 nm) cell membrane central pair of microtubules 0.25 mcm outer doublet tubulin microtubule The nine outer doublet tubules are made of tubulin while arms initiating movement are composed of protein dynein (with ATPase activity). Arms occur every 24 nm down the length of the cilium and interact with adjacent doublets as a “molecular motor” to produce bending. Links between neighbouring douplets are composed of another protein, nexin, and they are more widely spaced (every 86 nm) and hold the microtubules in position. Radial spokes extend from each of nine outer doublets toward the central pair of tubules at 29 nm intervals, while the central sheath projections are present every 14 nm. Respiratory epithelium cell with cilia, TEM C The base of each cilium (C) is seen to arise as a specialized derivative of the centriole (the basal body, BB, with triplets of mucrotubules and without a central pair of microtubules). Here the outer doublets of the cilium arise directly from the outer triplet of the centriole. CM – cell membrane; Cy – cytoplasm. CM BB Cy CELL MODIFICATION OF THE BASOLATERAL DOMAIN 1.Basolateral foldings. 2. Specialized structures which link the individual cells together into a functional unit are called cell junctions. Basolateral Folding of Epitheliocytes Striated ducts of the parotis gland, H & E. Proximal convoluted tubule of kidney, TEM Basolateral folds are deep invaginations of the lateral or basal surface of cells. They are particularly evident in cells involved in fluid and ion transport, and are commionly associated with a high concentration of mitochondria which provide the energy for ion and fliud transport. The presence of the basal folds and mitochondria imparts a stripped appearance to the basal cytoplasm of such cells giving rise to the descriptive term “striated epithelial cells”. Basal folds are seen in the renal tubular cells and in the ducts of many glands. Cell surface may be increased by folding of the lateral cell membrane which can be seen in some epithelial cells, like absorptive cells lining the gut. Cell junctions • Epithelial cells form special contacts with each other called cell junctions. There are three major types of cell junctions: • I. Occluding junctions (zonula occludens and fascia occludens). They link cells to form impermeable barrier. The tight or occluding junctions are present only in the epithelium.The contiguous cell membranes are fused. • 2. Adhesive junctions (zonula adherens, desmosome = macula adherens, fascia adherens). They link the cells to provide mechanical strength. • 3. Gap junctions allow movement of molecules between the cells. Gap junction = nexus, it is a narrow intercellular gap between the cells bridged by tiny tubular bonds for communicating junction. OCCLUDING JUNCTIONS 1. They bind cells together and maintain integrity of epithelial cells as a barrier. They separate apical and basolateral domains of the cells. 2. They have two main functions: - prevention of diffusion of molecules between adjacent cells, thereby contributing to the barrier function of the epithelial cells in which they are present, - prevention of lateral migration of specialized cell membrane proteins, thereby deliniating and maintaining speciailized cell membrane domains. 3. They are particulaly well developed in the small bowel, where they: - prevent digested molecules form passing between the cells, - confine specialized areas of the cell membrane involved in absorption or secretion to the luminal site of the cell. 4. They are important in cells that actively transport substances (esp. ions) against concentration gradient where they prevent back diffusion of the transported substances. OCCLUDING JUNCTIONS Intercellular sealing apical space strands membrane CM CM 600 nm a) intermembranous proteins b) Occluding junctions are particularly evident between epithelial cells that have secretory or absorptive roles. A) A collar of occluding junction is present between each cells, sealing individual cells into a tight barrier. The intramembanous proteins that form these junctions are arranged as serpentine intertwining lines (sealing strands) which stitch the membrane of adjacent cells together. At the fusion sites, claudins or occludins, transmembrane junctional proteins, of the two membranes bind to each other, thus forming a seal occluding the intercellular space. On EM (B) it is seen as an area of close apposition of adjacent areas of cell. O A D D A junctional complex between the enterocytes (Stevens) In the occluding junctions (O) the membranes of the adjoining cells approximate each other, their outer leaflets fuse, then diverge, then fuse again several times within a distance of 0.1- 0.3 mcm. The tightness of the occluding junction is related to the number of sealing strands present between the cells. A junctional complex is commonly seen toward the apex of cuboidal and columnar cells. Immediately below the cell apex are occluding junction (O) is followed by an adherent junction (A) and below this by desmosomes (D). Such complexes are well developed in the intestine, in other place they are not common. The classic junctional complex = O + A + D ANCHORING (ADHESIVE) JUNCTION Cytoskeletal filaments of adjacent cells are joined through intercellular link proteins, which attach the filaments to transmembrane link proteins (namely, cadherins). These can then interact with similar proteins on adjacent cells. The extracellular interaction may be mediated by additional extracellular proteins or ions, such as Ca++. Different link proteins and transmembrane proteins operate for the different classes of junction. cell membrane intracellular link protein cytoskeletal filament extracellular accessory link proteins or ions transmembrane link proteins intercellular space cytosol ANCHORING JUNCTIONS: adherens, fascia adherens, adherens = desmosome Zonula macula 1. They link the cytoskeleton of cells both to each other and to underlying tissues. 2 They provide mechanical stability to groups of epithelial cells so that they can function as cohesive unit. 3. They are most common toward the apex of adjacent columnar and cuboidal epithelial cells, where they link submembranous actin bundles into a so called actin belt. They are visible by LM as an eosinophilic band (the terminal bar). 4. Actin fibers in the adjacent cells are linked by actin-binding proteins (alphaactinin and vinculin) to a transmembrane protein, which is one of a group of cell surface glycoproteins mediating cell adhesion (cadherins). The type in adjacent junctions is E-cadherins, which links cells in the presence of Ca++. zonula occludens zonula adherens macula adherens Junctional complex, TEM, large magnification (Fawcett, 1981). 1) 2) 3) 1) 2) Zonulae adherentes: are belt-like junctions that assist adjoining cells to adhere to one another, they are located just basal to the zonulae occludentes and also encircle the cell, the intercellular space of 15-20 nm between the outer leaflets of the two adjacent cell membranes is occupied by the extracellular molecules of cadherins. These Ca++-dependent integral proteins of the cell membrane are transmembrane linker proteins.Their intracytoplasmic aspect binds to a specialized region of the cell web, specially a bundle of actin filaments that run parallel to and along the cytoplasmic aspect of the cell membrane. The actin filaments are attached to each other and to the cell membrane by vinculin and alpha-actinin. The extracellular region of the cadherins of one cell forms bonds with those of the adjoining cells participating in the formation of zonula adherens. Then this junction not only joins the cell membranes, but also links the cytoskeleton of the two cells via the transmembrane linker protein. Fascia adherens: is similar to zonula adherens but does not go around the entire circumference of the cell. Instead of being belt-like it is “ribbon-like”. Cardiac muscle cells are attached to each other at their longitudinal terminals via the fascia adherens. Desmosome. TEM, large magnification (Fawcett, 1981). Desmosome (= macula adherens) (arrows) is the last of the three components of the junctional complex. They are weld-like junctions along the lateral cell membrane that help to resist shearing forces. They are presnt in both simple and stratified epithelia (epidermis). intercellular space CM CM P transmembrane proteins (desmogleins) intracellular CF plaque of desmoplakin intermediate filaments anchored to plaque a) cell membrane X CF CM CM b) a) Each desmosome consists of an intracellular plaque composed of several link proteins (the main types being desmoplakins), into which cytokeratin intermediate filaments (tonofilaments) are inserted. The cell adhesion is mediated by transmembrane protein called desmogleins (Stevens). b) The disc-shaped adhesive plaques (P) in adjacent cells are seen as electrondense areas into which cytokeratin filaments (CF) are inserted. The cell membranes (CM) between adhesion plaques are about 30 nm apart and there may be an electron-dense band between cells in some desmosomes (X). Desmosome. TEM, large magnification. (Fawcett, 1981). The dense accumulation of the intracellular intermediate filaments (arrow) inserting into the attachment plaque of each cell is visible. Disk-shaped attachment plaques are located opposite each other on the cytoplasmic aspect of the plasma membranes of adjacent cells. Each plaque is composed of a series of attachment proteins, the best characterized of which are desmoplakins and pakoglobins.Cytokeratin filaments are observed to be insert into the plaque, where they make a hairpin turn, then extend back out into cytoplasm. These filaments are responsible for dispersing of shearing forces on the cell. In the region of the opposing attachment plaques, the intercellular space is up to 30 nm in width and contain filamentous material with a thin dense vertical line located in the middle of the intercellular space. It is formed of desmoglein, the extracellular component of the Ca-dependent transmembrane linker protein of the cadherin family. The cytoplasmic aspects of transmembrane linker proteins bind to desmoplakins and plakoglobins constituting the plaque. 2-4 nm adjacent cell membrane connexon pore 1.5 mcm diameter GAP JUNCTION (Stevens) Communication junction allow direct cell-cell communication. Communication (gap) junctions allow selective diffusion of molecules between adjacent cells and facilitate direct cell to cell communication. Gap junctions are usually present at relatively low density in most adult epithelia, but are found in in large number during embryogenesis, when they probably have a role in the spatial organization of developing cells. Gap junctions are also important in cardiac and smooth muscle cells where they pass signals involved in contraction from one cell to another. Each of the gap junctions is a circular patch studded with several hundred pores, formed by the protein subunits traversing the cell membranes and termed a connexon. Pores on adjacent cells are aligned allowing small molecules to move between cells. Gap junctions: 1) also called nexus of communicating junctions are regions of intercellular communication, 2) they are widespread in the epithelia, as well as in cardiac muscle, smooth muscle and neurons. The intercellular cleft at the gap junction is only 2-3 nm, 3) are built by six closely packed transmembrame proteins (connexins) that assemble to form a structure called connexons, aqueous pores, through the plasma membrane extending into the intercellular space. The two connexons fuse, forming the junctional intercellular communication channel. With a diameter of 1.5-2 nm the hydrophilic channels allow the passage of ions, amino acids, cAMP, molecules smaller than 1 kD in weight, and certain hormones, 4) are controlled by pH and Ca++ concentration. They close in low pH and high Ca++ concentration, and visa versa. Exocrine Glands • When the cells release their secretary products, the membranes of secretory granules fuse with the cell membrane and the granular contents spill out of the cell in the process called exocytosis. • Exocrine glands release their secretions on the surface of the body (mammary glands, sweat glands, sebaceous glands) or in the lumen of the hollow organs (uterus, stomach, trachea, etc). They contain secretory portions and excretory ducts. • On the contrary the endocrine glands secrete their products directly in bloodstream and contain no excretory ducts (thyroid gland, adrenal glands, etc). • In many glands (sweat, lacrimal, mammary glands) discharge of the secretions is facilitated by the contraction of the myoepithelial cells. • Secretions may be serous (parotid gland), mucous (goblet cells), mixed (submandibular gland) and sebaceous. Glands PANCREAS, H & E originate from epithelial cells that leave the surface where they C developed and A penetrate into the underlying tissue, manufacturing a basal lamina around themselves. The D I secretory units, along with their ducts, are the parenchyma of the gland, whereas the stroma of the gland represents the elements of the connective tissue that invade and support the parenchyma and divides parenchyma into lobules. A – acini, D – duct, C – stroma, I – pancreatic islet (endocrine part of the gland). SUBLINGUAL GLAND, H. & E. * * The two large excretory ducts (asterisks) run in the stroma of the gland. They are lined by stratified epithelium indicating the ectodermal origin of the gland. Surface epithelium single secretory cells secretory cells Coiled tubular gland Straight tubular gland surface epithelium surface epithelium single central lumen ionpump ing cells secretory cells Simple Unbranched Glands secretory cells in distal part of gland Exocrine Glands may be classified by the: 1.Number of cells (unicellularmulticellular). 2.Shape of the secretory portion (tubular or acinar or tubulo-acinar). 3.Structure of the secretory portion (branched or nonbranched). 4.Structure of the excretory duct (simplecompound. Unbranched glands are always simple). 5.Mode of secretion (merocrine or apocrine or holocrine). 6.Nature of secretion (serous or mucous or mixed or sebaceous). Unicellular exocrine glands represent the simplest form of exocrine gland. Goblet cells are dispersed individually in the epithelia lining the digestive and respiratory tract. The secretions released by these mucous glands protect the linings of these tracts. When stained by H & E, they remain unstained as mucus is stained neither by hematoxylin nor by eosin. Unicellular glands are always intraepithelial while multicellular glands may be intracellular (urethral Littre’s glands) or extraepithelial (most of them). * * Goblet cells (*) in the epithelial lining of the ileum, H & E. Schematic diagram of the classification of multicellular exocrine glands by structure. Green reperesents secretory portion, lavender represents the duct portion of the gland. secretory portion simple tubular simple branched tubular simple coiled tubular simple acinar simple branched acinar duct compound tubular compound acinar compound tubuloacinar Branched Compound Glands Branched Compound Gland minor ducts lined by ionpumping cells add fluid & electrolytes surface epithelium main excretory duct lined by columnar epithelial cells secretory epithelial cells around small central lumen – acinus myoepithe -lial cells expel secretions Compound glands are always branched while branched glands may be both simple and compound. CLASSIFICATION OF GLANDS Type Example Unicellular glands exocrine goblet cells endocrine APUD-cells of the GIT Multicellular exocrine glands secretory sheet surface epithelium of gastric mucosa intraepithelial glands urethral (Littre’s) glands simple tubular glands intestinal glands simple coiled tubular glands apocrine sweat glands simple branched tubular glands esophageal glands simple branched acinar glands Meibomian gland compound tubular glands glands of gastric cardia compound tubuloacinar glands pancreas compound saccular glands prostate MODE OF SECRETION OF THE EXOCRINE GLANDS exocrine merocrine apocrine endocrine holocrine Secretion of cell produce may occur by: - exocytosis from the cell apex into a lumen (merocrine secretion), - pinching off of apical cell cytoplasm containing cell product (apocrine secretion), -shedding of the whole cell containing the cell product (holocrine secretion); or -endocytosis from the cell base into the blood stream (endocrine secretion). Modes of Secretion merocrine holocrine disintegrating cell and its contents (secretion) apocrine secretion intact cell new cell portion of the cell (secretion) In apocrine glands (lactating mammary glands, some sweat glands) a small portion of apical cytoplasm is released along with a secretory product. In holocrine glands (e.g. sebaceous glands) as a secretory cell matures, it dies and becomes the secretory product. microvilli Substances secreted by most of the exocrine glands may be described as mucous, serous and mixed (both) types. 1) Mucous glands secrete mucinogens, large glycosylated proteins that upon hydration swell to become a thick, viscous, gel-like theca protective lubricant known as mucin, a major component of mucus. Examples: goblet cells mucino -gen (unicellular glands) and the minor salivary dropglands of the tongue and palate. lets 2) Serous glands, such as pancreas and parotid gland, secrete an enzyme-rich watery fluid. 3) Mixed glands contain acini (secretory units) that produce mucous secretions as nucleus well as acini that produce serous secretion. stem In addition, some of their mucous acini possess serous demilunes, a group of cells that secrete a serous fluid. Examples: submandibular and sublingual salivary A goblet cell containing glands. tightly packed secretory 4) Sebaceous glands secrete sebum (skin granules in the theca. sebaceous glands, meibomian glands). They are always holocrine glands. sG BC HF Sebaceous glands Sebaceous glands (sG) are branched, acinar holocrine glands, which produce an oily sebum. The secretion of these glands is delivered into the lumen of a hair follicle (HF) with which sebaceous glands are associated. Basal cells (BC), located at the periphery of the gland, undergo mitotic activity to replenish the dead cells, which, in holocrine glands, become the secretory product. Note that as these cells accumulate sebum in their cytoplasm, they degenerate, as evidenced by the gradual pyknosis of their nuclei (arrow). PAROTID (SEROUS) SALIVARY GLAND intercalated duct serous acini striated duct Parotid gland is a purely serous gland with merocrine type of secretion. All the cells of the secretory portions (acini) are serocytes, they are stained light basophylic. Intercalated ducts are the smallest excretory ducts inserted into the acini. They fuse together to form larger straied ducts. SUBLINGUAL (MIXED) SALIVARY GLAND mucus mucous acini serous acini serous demilunes intercalated duct striated duct Sublingual gland is a compound merocrine gland with the two types of acini: purely serous and mixed (with mucocytes surrounded by the serous demilunes). serous cell myoepithelial cell intercalated duct cell serous acinus mucous acinus intercalated duct striated duct . serous demilunes mucous cell striated duct cell SALIVARY (SUBMANDIBULAR) GLAND Acini of many multicellular exocrine glands possess myoepithelial cells that share the basal lamina of the acinar cells. They are of epithelial origin though have some characteristics of the snooth muscle cells (contractility). ENDOCRINE GLANDS 1. Endocrine glands do not contain ducts, and their secretory products are thus released directly into the bloodstream or lymphatic system. 2. The endocrine system consists of several glands, composed of islands of secretory cells of epithelial origin (like thyroid or adrenal glands with follicular or cluster cell arrangement), as well as of isolated groups of cells within certain organs (like pancreas or gonades), and individual cells scattered among parenchymal cells of the body (like APUD-cells of the gastro-intestinal tract). Parathyroid gland with clusters of endocrine cells around the blood capillaries. C – chief endocrine cells, O – oxyphil cells.