Epithelia and Glands PDF

09/09/2024 EPITHELIUM...

09/09/2024 EPITHELIUM is a basic tissue consisting of closely apposed cells, forming a continuous layer covering an outside surface or lining an internal cavity. Epithelia and Glands simple form: consists of polyhedral cells forming a layer one cell thick. Winston S. Abena, MD., FPASMAP, COSH complex form: multiple layers of cells. Saint Paul University Philippines lowermost cells – rest on a thin, continuous, supporting layer “basal School of Medicine lamina/basement membrane” primary function: to form a boundary layer that can control There are four basic fundamental/primary tissues – all derived from movement of substances between the external environment the 3 primitive germ layers: ectoderm, mesoderm, and entoderm. & the body’s internal milieu or between internal compartments of primary tissue – a group of similar cells specialized in a common the body. direction & able to perform a common function. epithelia of various organs may be made up of cells specialized for 4 Primary Tissues: absorption, secretion, or ion transport a) Epithelium c) Muscle b) Connective tissue d) Nervous 1  09/09/2024 Epithelia Common Characteristics: Epithelium cells are closely apposed with very little intercellular cementing substance between arranged as sheets covering or lining surface or masses of cells in glands; one of their surface is free while the other is attached no blood vessels among the cells; nutrition being thru osmosis & diffusion. There are plenty of nerve fibers. attached to a basement membrane and provided with a tunica propia or corium of connective tissue containing blood vessels, lymphatics and nerves. Epithelia Connective tissue All cells connect together to form a sheet if they are a cells are widely separated by a relatively large amount of intercellular covering or lining epithelia or ground substance All glands that secrete contain fibers in the ground substance hormones and other major contain blood vessels and nerves to their destination proteins for the body are also includes specialized tissues such as blood and blood forming tissues, made of epithelia bone and cartilage 2  09/09/2024 Where it all begins Mesenchyme From the mesoderm Collagen fibers (connective Found in the umbilical cord after delivery tissue) between Source of connective tissue stem cells. human epithelial cells. Connective Tissue includes: Blood The dermis Cartilage Tendons Bone Space fillers in organs Fat 3  09/09/2024 Muscle Muscle Three types composed of elongated cells to adapt to their function of contraction contain cytoplasmic filaments which are relatively closely associated and are separated by fine, vascular connective tissue strongly acidophilic in staining Overview of Muscle Cell Types 4  09/09/2024 Neuromuscular Junction Nervous Tissue Where axons of neurons meet skeletal muscle consists of cells, some of which are very large with elongated processes This connection is blocked by nerve cells possess the characteristic “owl’s eye/fish-eye” nucleus – a BOTOX vesicular or lightly stained nucleus with a chromatic or darkly stained nucleolus contain basophilic staining bodies “Nissl bodies” or chromophilic substance cells are closely aggregated in the form of sheets (membranes) covering body surfaces or in clumps forming glands. connection of the gland to the surface – duct of gland (e.g. exocrine glands) those with no surface connection, the glands secrete internally into the vascular system (e.g. endocrine glands) Nervous Tissue Cells of the nervous system Spinal cord Neurons Brain Peripheral Nervous Tissue 5  09/09/2024 Neurons have various appearances Neuron cell body with large nucleus / nucleolus and numerous mitochondria (central nervous system). Note the surrounding neuropil with myelinated axons (dark outlines). Support cells of the nervous system EPITHELIUM Glial cells I. Simple in the spinal cord and brain a. Squamous – Endothelium, Mesothelium, Mesenchymal, Bowman’s Primarily in the white matter capsule of kidney Schwann cells b. Cuboidal – Small ducts, Collecting tubule of kidney, Thyroid follicles Elsewhere in the body c. Columnar – Gall bladder (non-ciliated); Uterine tube (ciliated) Myelination II. Stratified A special form of insulation a. Squamous – skin (keratinizing); cornea b. Columnar – Male urethra c. Cuboidal – Sweat gland ducts d. Transitional – Urinary tracts e. Pseudostratified Columnar – Male urethra (non-ciliated); Trachea (ciliated) 6  09/09/2024 MUSCLE CONNECTIVE TISSUE I. General A. Loose Mesenchyme – mainly restricted to embryo & fetus I. Smooth (involuntary, non-striated) Mucoid – Wharton’s jelly (umbilical cord) Areolar – “loosely packed” in most organs - blood vessels Adipose – subcutaneous tissue (hypodermis) Reticular – lymph node, bone marrow - intestinal tract B. Dense Irregular – capsules of organs; dermis of skin II. Cardiac (striated, involuntary) Regular – tendon, corneal stroma II. Special - heart A. Cartilage Hyaline – trachea, costal cartilage Elastic – external ear, epiglottis III. Striated (striated, voluntary) B. Bone Cancellous (spongy) – center of long bones -skeletal Compact – shaft of long bone C. Hemopoietic Myeloid – bone marrow D. Blood E. Lymph NERVOUS TISSUE Functions: I. Central Nervous System a) Protection – epidermis of the skin Gray matter – brain, spinal cord b) Absorption – epithelium of the intestine White matter – brain, spinal cord c) Excretion – epithelium of the kidney tubule II. Peripheral Nervous System d) Secretion – glandular epithelium Nerves – Peripheral nerves e) Sensory reception – neuroepithelium Ganglia – Sensory, autonomic f) Lubrication – goblet cells & sebaceous glands Nerve endings – naked, encapsulated g) Reproduction – lining epithelium of the seminiferous tubules & III. Special Receptors germinal epithelium of the ovary - eye, ear, nose h) Contractile – myoepithelial cells 7  09/09/2024 General Characteristics Structures Associated with Epithelium: Composition: A. Basal lamina (basement membrane) 1 – Epithelial cells a continuous sheetlike extracellular structure to which all epithelial its form & dimensions are varied: high columnar, cylindrical, cuboidal, tissues are in contact low squamous cells visible only in the EM: thin, granular layer with very cell nuclei: spherical, elongated fine fibrillar network nucleus: corresponds to cell shape contains the protein collagen & some amorphous protein- polysaccharide complexes 2 – Intercellular Substance its origin is from the epithelial cells itself very scant because of the compact arrangement of the cells thickness: 50 – 80 nm (does not present as a barrier for diffusion) permeable to substances: proper nutrition & function epithelial cells of skin: small fibers of collagen “anchoring fibers” – B. Tunica/lamina propia anchors the lamina to adjacent connective tissues all epithelia possess this structure for most epithelia, fibrils of collagen (reticular fibers) complexed with may be areolar or fibro-reticular connective tissue located amorphous protein-polysaccharides constitute another layer beneath immediately below the basement membrane the basal lamina – fibrous or reticular lamina serves as a support to the epithelium 1 – basal lamina: lamina lucida, lamina densa contains blood vessels, lymph vessels, & nerves 2 – ground substance its outgrowths in the form of connective tissue papillae are found in 3 – reticular fibers “basement membrane” (LM) the deep surface of thick epithelia (stratified squamous epithelium of the skin)  to facilitate nutrition by increasing the surface areas for diffusion 8  09/09/2024 Classification & Distribution of Epithelial How are epithelia classified? Tissues - they are classified according to the number of cell layers, shape of Depends on number of layers the cells, and the specializations of their free surface. One layer Two or more layers Simple Multilayered nuclei Stratified Pseudostratified Classification depends on shape Importance of layering Multilayered epithelia: Single layered epithelia: Flattened, scale-like Cells are cubes Cells are columns Protect against friction and injury Communication/gateway Squamous Cuboidal Columnar Barrier to water, disease lumen some toxins, etc lumen Important in regulated transport of cells/molecules lumen Lower layers regenerate upper layers 9  09/09/2024 Importance of cell shape Flattened, squamous Taller, cuboidal or cells columnar cells I. SIMPLE/NON-STRATIFIED Reduced surface - contains a single layer of cells Can be stacked to for passive transport provide protective often express 1. Simple squamous across cells structural and layers functional polarity a. Endothelium – lining walls of blood vessels, lymph vessels; cavities of the heart b. Mesothelium – lining serous cavities (e.g. secretion pleura, pericardium, peritoneum, tunica vaginalis testis) receptivity Basal/lateral surface Apical surface c. Mesenchymal – lines the anterior chamber of the sensory secretion functions functions eye, perilymph spaces of the internal ear, subdural & subarachnoidal spaces junctions Ion transport d. Flattened cells – lining the pulmonary alveoli; communication Water and ion Enzyme Bowman’s capsule (kidney) Absorption transport actions 2. Simple columnar B. Simple modified columnar Subdivided into: a. Simple plain columnar (1) Simple ciliated columnar with: b. Simple modified - motile cilia: uterus & oviducts or fallopian tubes; ductuli A. Simple plain columnar efferentes in patches (1) Simple plain tall columnar – mucosa of the stomach, - non-motile cilia or stereocilia: epididymis small & large intestines, gallbladder, bigger ducts of glands (2) Pyramidal or glandular epithelium (2) Simple plain low columnar – smaller ducts of glands; some - serous glandular: parotid gland; pancreas excretory tubules of the kidney - mucous glandular: sublingual glands; small salivary (3) Cuboidal – thyroid follicles, choroid plexus, pigmented glands in the oral & tongue epithelium of retina, smaller ducts of glands; germinal epithelium of (3) Goblet cell – found among the cells of simple columnar ovary; inner surface of capsules of the lens epithelium of the small & large intestines and pseudostratified columnar epithelium of the respiratory system 10  09/09/2024 (4) Neuroepithelium SIMPLE EPITHELIA a. Taste buds – tongue (for taste) Simple Squamous Epithelium b. Organ of Corti – internal ear (for hearing) is composed of very thin, flat cells of irregular outline fitted closely c. Retina – eyeball (for sight) together to form a continuous sheet d. Olfactory epithelium – nasal cavity (for smell) on surface view, show a characteristic mosaic pattern especially after staining cell limits with silver nitrate stains; or having the appearance (5) Pigment epithelium – pigment epithelium of the retina; Pars of a tiled floor with serrated, irregular wavy outlines ciliaris and Pars iridica retinae in section, the cells show attenuated cytoplasm with a central bulging nucleus Simple Cuboidal (Cubical) Epithelium its appearance in sections at right angles to the surface of the The epithelium lining membrane, as a row of square or rectangular profiles capillaries is simple nuclei tend to be aligned at the same level in all of the cells squamous. This is from each cell appearing box-like or cube-like the spinal cord slide. from the surface, cells appear as polygons 11  09/09/2024 Simple cuboidal epithelia are like "cubes" and usually there is as much cytoplasm over the nucleus as there is under it. This section kidney shows simple cuboidal epithelia in the thyroid gland. Simple Columnar Epithelium Simple Columnar Epithelium (Plain) a membrane composed of cylindrical cells possessing an appreciable height aside from length & width in sections perpendicular to the surface, the rectangular cellular outlines may be taller than that of cuboidal epithelium nuclei are approximately at the same level & situated nearer to the basal surface than to the apical (luminal) surface is usually associated with secretion or absorption 12  09/09/2024 Simple Ciliated Columnar (Modified) cells of the tall columnar type possess fine hairlike projection termed “cilia” on their distal ends direction of the cilia is always towards the outside attached to a row of basal or centrosomal granules which are fragments of the centrosome or kinetic center of cell Pyramidal or Glandular Epithelium Goblet cells Serous Glandular Cells Mucous Glandular Cells derives its name from its secretory or glan-dular function & considered as a unicellular mucous-secreting gland Acidophilic pyramidal shape of cells Pale, slightly basophilic “goblet” – refers to an expanded cup-shaped rim of cytoplasm Granular cytoplasm (apical); Reticulated, nongranular “theca”, filled with secretory droplets & a thin base like the stem of a striated (basal) cytoplasm goblet organelles are compressed towards the basal end & the golgi Rounded nucleus Flattened nucleus pushed toward complex is well developed the BM this is a columnar cell; its cytoplasm has been converted into a thick Presence of intercellular & Not provided with canaliculi, glairy substance mucinogen; takes a clear blue basophilic stain & is intracellular secretory canaliculi secretion merely passes from PAS positive; basophilia is due to abundant free & attached thru which secretion is carried to distal ends of the cell into the ribosomes lumen of the acinus lumen fully developed nucleus – compressed nucleus of deeply staining Narrow lumen of the acinus Wider lumen surrounded by basophilic cytoplasm surrounded by secreting cells secreting cells 13  09/09/2024 Neuroepithelium each differs in appearance, but fundamentally composed of: a) chief sensory cells – a true nerve cell or a modified epithelial cell for reception of the stimulus b) supporting/sustentacular cells – may be a columnar cell or tall columnar cell lying side by side with chief cells c) basal cell – low columnar cell with irregular processes found beneath the chief & columnar cells immediately above the basal membrane Goblet cell (yellow) in the mucosal lining of the small intestine (part of the digestive tract). A goblet cell secretes the mucus to protect the lining of the intestine and helps neutralize stomach acid. Inside the goblets cells are many mucigen granules (purple). When these are released into the intestine they will combine with water to form mucin, the main constituent of mucus. Surrounding the goblet cell are columnar epithelial cells (green cell membrane outline; light purple nucleus; red mitochondria). The brush-like surface of the columnar epithelial cells consists of microvilli (blue), tiny finger-like projections, which increase the surface area available for absorption of nutrients such as lipids, proteins and fat-soluble vitamins II. COMPLEX/STRATIFIED – composed of more than one layer of cells 3 types of cells: columnar, polyhedral, & flattened *According to shape of cells on free surface, are classified as: cells 1. Squamous epithelia – contains flat or squamous cells on a) tall columnar cells – innermost layer of cells next to the basement the free surface membrane 2. Columnar epithelia – contains columnar cells on the free - single row, with darkly staining oval nuclei located in the surface proximal portion of cells arranged parallel to the long axis of the cells or perpendicular to the BM COMPLEX (STRATIFIED) EPITHELIA - cells are young with mitotic figures for replacement of Stratified Squamous Epithelium superficial worn out cells composed of varying number of layers of cells ranging from 5 – 30 or - pigment granules are present in cytoplasm more thickness is due to increase in the number of layers on the surface of flattened cells 14  09/09/2024 b) polyhedral cells – several layers; with spherical dark staining nuclei becoming vesicular as they reach the periphery - cells gradually become flattened the nearer they are in the surface - in certain skins: cells exhibit projections giving a spiny appearance “Prickle cell layer” (stratum spinosum) - cell division does not occur; mitotic figures are rarely seen c) squamous/flattened cells without nuclei – superficial layer appearing like scales on the outer body surface - cells become cornified or hardened (protective) - keratin (superficial layer) – prevents dehydration of underlying cells; also prevents lower cells from taking in too much water = keratinized stratified squamous epithelium *with nuclei – inner moist surface of the body = non-Keratinized stratified squamous epithelium An example of stratified squamous epithelium is found in the skin. The outer layers of cells have filled with keratin and are no longer nucleated. 15  09/09/2024 the gradual transition in shape from polyhedral to flat shape is cells vary in shape characteristic of this type of epithelium a) tall columnar – extending throughout the epithelial thickness found: epidermis of the skin, lining of the oral cavity, epiglottis, - appear widest near the free surface with a long slender esophagus, and vagina processes (bifid or branched proximal ends) extending downward Pseudostratified Columnar Epithelium b) fusiform/spheroidal – smaller basally situated cells appears to be composed of several layers of cell when in reality, cell nuclei – lie at different levels in a perpendicular section, giving there is only one cell layer the impression that the membrane is composed of more than one all cells making up the epithelium rests upon the basement cell layer membrane “pseudo” – false; not all of the cells reach the surface 16  09/09/2024 Pseudostratified Ciliated Columnar Epithelium lower portion of epithelium: appears darker due to close arrangement of the nuclei distal portion: clear cytoplasmic zone with-out nuclei found: male urethra, duct of parotid gland; trachea (ciliated), primary bronchi, auditory tube, part of the tympanic cavity of the ear Transitional Epithelium c) superficial layer – flattened/umbrella shaped cells originally described as a transition from between stratified squamous - inferior surfaces present two lateral depressions into which the and columnar epithelium rounded distal ends of the pear shaped cells below it giving the consists of many layers (contracted state): somewhat umbrella shaped appearance a) deepest layer – cells are polyhedral or columnar in shape with - nuclei are oval, arranged parallel to the long axis of the cells; spherical nuclei; one or two rows of cells moist b) pyriform/pear shaped cells – irregularly polyhedral cells with a - apical cytoplasm of luminal cells stains more deeply bulging apical end extending the spherical nucleus & a tapering proximal end inserted among the polyhedral cells below; 1 – 3 rows of cells 17  09/09/2024 2 layers (stretched state): a) superficial layer – large flattened or squamous cells b) layer of cuboidal cells thin basal lamina (EM) possesses a certain degree of elasticity because the cells on the surface are not so flat appear very thin when the organ is distended and thicker when the organ is collapsed found: urinary bladder (calyces to urethra) Transitional epithelium contains cells that are flattened and cells that are cuboidal; hence the name "transitional". in the bladder transitional epithelium (shown below) the cells nearest the apex are rounded. This is a diagnostic feature of this epithelium. Stratified Columnar Epithelium 2 layers: a) superficial layer – cells are prismatic & columnar in shape b) basal layer – consists of small irregularly polyhedral or cuboidal cells that do not reach the surface found: only in the fornix of the conjunctiva, cavernous urethra, larger excretory ducts of some glands 18  09/09/2024 Epithelial Tissue Epithelial cells – exhibit a tendency to maintain excessive contact with one another, forming coherent sheets that cover surfaces & line body cavities. Columnar epithelium – exhibits the pheno-menon of “polarity” - the structural & functional differentiation between the two ends of the cell, that is the distal end. - the free surface differs from the proximal end toward the underlying connective tissue Stratified Columnar epithelium is rare. One place you can find it is in the largest ducts of salivary glands (parotid, submandibular, etc). The basal layer of cells are cuboidal cells and the layer nearest the apical surface includes columnar cells. The large droplets are mucus, in Goblet cells. - this is also evident in the arrangement of the organelles in the cell interior – centrosome & Golgi apparatus (being in a supranuclear position) - its long mitochondria are oriented parallel to the cell axis & found in great numbers in the apical cytoplasm - the terminal web consists of a feltwork of fine filaments beneath the free surface that provides for stiffness & mechanical support for the ciliated or striated borders of epithelial tissues 19  09/09/2024 Cell Adhesion in Epithelial Membranes Stratified squamous epithelia – has less evidence of cell polarity Epithelial membrane – cell-to-cell adhesion that can resist - well developed surface specializations for cell attachment mechanical forces that tend to separate the cells as in the stratified squamous non-keratinizing lining epithelium of the mouth & - cytoplasm contains a cytoskeleton composed of filaments esophagus, where relatively hard food material passes over the aggregated in bundles that constitute the tonofibrils (LM) and surface. function as support - extensive contact with one another forms coherent sheets that cover surfaces & line body cavities. - narrow spaces between adjacent cells – 15 – 20 nm (150 – 200 Cell Junctions Ǻ); and this space is occupied by the glycocalyx of adjacent cells. types of specialized junctional regions between cells  the binding action of the exposed carbohydrates of these 2 factors to describe: glycoproteins (collectively called cell adhesion molecules, CAMS) and calcium are important in cell adhesion. 1) shape and extent - “jigsaw” interlockings – grooves formed by plasma membranes 2) nature or relative closeness of the cell contact are taken into of two parallel adjacent cells. account can take the form of a spot or punctuate area of limited extent called macula; or can pass around the entire cell in a belt or crown-like manner called a zonula; or as a sheet or strip-like area called a fascia. 20  09/09/2024 relative closeness & nature of cell contact: a) zonula occludens (occluding/tight junction) – the intercellular substance is virtually obliterated with outer surfaces of the two plasma membranes apparently in contact or even fused b) zonula adherens (adhering junction) – the intercellular space is apparently usually 20 – 25 nm wide & with dense material in the space and associated with the cytoplasmic surfaces of the apposed membranes c) nexus (gap junction) – very slender intercellular gap about 2 nm; it is concerned with intercellular communication rather than cell adhesion Zonula adherens between neurons (central nervous system). Tight/Occluding Junctions usually in the form of a belt completely encircling a cell near its terminal or apical border, zonula occludens EM: three dense lines separated by 2 electron-lucent lines with apparent fusion of the outer leaflets of the two plasma membranes : demonstrate a network of linear ridges & grooves which bar passage of molecules, preventing intercellular transport from lumen to extracellular space through the intercellular space Tight Junctions. The most apical is the tight junction. It involves a fusion of the two adjacent membranes with fibrous connections, something like "strapping tape". The following photograph shows a freeze-fracture/freeze etch view of a tight junction (zonula occludens). 21  09/09/2024 Adhering Junctions the adjacent plasma membranes are separated by an intercellular space these junctions function in cell adhesion 2 types: a) Zonula adherens/belt desmosome – forms a complete band around epithelial cells adjacent to the luminal surface - apposed plasma membranes are parallel with an intercellular space of 15 – 20 nm filled with fine filamentous material b) Zonulae adherentes – function in mechanical attachment and may transmit forces generated within cells e.g. cardiac muscle Two dense regions one on each cell Cadherins connect Adherent junctions membrane (alpha the two plaques via actinin) homophilic binding Nexus/Gap Junctions concerned with cell-to-cell communication adjacent plasma membranes are separated by a space of only 2 – 3 nm Cadherin binding may be mistaken for a zonula occludens, but the apposing requires Calcium membranes are not fused “communicating junction” each membrane is studded with polygonal projections/minute structures connexons that fuse the intercellular space; these projections are made up of intramembranous proteins that encircle a Actin connects with hydrophilic channel between the adjacent cells; this channel allows dense regions on membrane for the passage of ions from one cell to another 22  09/09/2024 adhesive function; area of low electrical resistance, important in cell- to-cell communication for the coordination of cellular activities; transmit electrical impulses (heart, smooth muscles of the intestines), synchronization of activity between cells also found in: between osteocytes, smooth & cardiac muscle cells, & neurons absent in: skeletal muscle, blood cells Gap junction allow cells to communicate with one another. Small molecules can pass through pore-like transmembrane proteins called connexons. The following photograph shows this diagrammatically. Note that ions, molecules like fluorescein, and second messengers can pass from cell to cell. Terminal Bars dark dots or dense bars seen on the boundary between adjacent columnar epithelial cells, immediately subjacent to their free surface assumed to close the intercellular space at the free surface it is called “macula”, or it can pass around the entire cell in a belt or corona-like manner 23  09/09/2024 Juxta-Luminal/Junctional Complex (EM) 1. Zonula occludens found between the luminal border of adjacent simple columnar cells found immediately below the free surface of the epithelium composed of 3 morphologically distinct zones: a region of surface specialization where the membranes of adjoining 1. Zonula occludens (tight/occluding junction) cells converge and appear to fuse 2. Zonula adherens extends in a belt around the perimeter of the cell & serves to close the intercellular space 3. Macula adherens (desmosome) it is especially important in a transporting epithelium – making it in LM: entire structure is called “Terminal Bar” possible for cells to pump solute actively through their lateral membranes into the intercellular cleft below the zonula occludens; an osmotic gradient is created that serves to move the water across the epithelium mechanical role: maintaining the structural integrity of the epithelium 2. Zonula adherens (Intermediate Junction) “terminal web” – serves as a site of insertion for the contractile located below the zonula occludens microfilaments that form the core of the microvilli. adjacent plasma membranes are parallel & the intercellular gap is 15 it keeps the terminal web taut so that the microfilament can use it as – 20 nm wide which is filled with fine, filamentous material of an anchoring substrate to aid in contraction of the microvillar border medium electron density have filaments on the cytoplasm running both along the membrane that appears as flat horizontal band continues with the terminal web; about 7 nm in diameter & appear to contain actin. myosin, alpha-actinin, vinculin – are also found in the cytoplasm of the junction (by immunocyto-chemical staining); involved in cross- linking its integral filaments & binding them to the micro-filaments of the terminal web 24  09/09/2024 Base of Terminal Web microvillus Microvillus actin binds to 3. Macula adherens (Desmosome) more actin running appears as dense dots or fusiform thickenings of the cell boundaries horizontally. (LM) bipartite structures consisting of plaque like local differentiations of the apposing membranes (EM) Terminal web actin cell surfaces are 15 – 20 nm apart & the apposing membranes appear is cross-linked by thickened due to the presence of a thin dense layer consisting of a spectrin. feltwork of fine filaments cytoplasm: tonofilaments converge upon the desmosomes terminating in their inner layer; tonofilaments form “hairpin loops” in the dense plaque & turn back into the cytoplasm Underneath the actin filaments are cytoskeletal filaments, sites of attachment of the cytoskeleton to the cell surface (stability) & also running horizontally sites of cell-to-cell adhesion Two dense plaques: TEM of a Desmosome with tonofibrils one on each cell Cadherins connect Desmosome membrane the two plaques via homophilic binding Cadherin binding requires Calcium Intermediate filaments loop into plaque 25  09/09/2024 Key cell structures at each surface Specializations of the Cell surface in Epithelia: Microvilli, Cilia, 1. Striated or Brush Border Stereocilia delicate vertical striations in a refractile border of columnar epithelium when examined in high magnification significance: prominent in cells whose principal function is absorption Specialized : striated borders contain enzymes in or nearer the junctions membranes of the microvilli : an adaptation to enhance the surface area of membrane exposed to substances to be absorbed (Cellular interdigitations) Basal lamina + receptors Microvilli – intestinal epithelium, epithelial tissues of the kidney Villus of the small - the surface membrane shows a multitude of small, intestine; slender fingerlike projections microvilli are on - the apical extremities are covered with an extracellular the surface of an glycoprotein layer (cell coat/glycocalyx)  important for pinocytotic intestinal processes occurring especially near the base columnar epithelial cell. - it increases the efficiency of absorption & surface area of the cell 26  09/09/2024 Actin filaments Core Actin filaments are held at the tip of microvilli Actin is bound by amorphous to myosin anchoring proteins; along lateral capping prevents walls depolymerization Actin filaments are held in bundles by actin-binding proteins: fimbrin, villin, or fascin 2. Stereocilia long pyriform tuft of slender processes projecting into the lumen from each cell found in: lining epithelium of epididymis composed of thin cilium-like structures cohering so as to form a tuft resembling the hairs of a water color brush individual processes are longer than cilia, but are non-motile function: they promote absorption by amplifying cell surface basal infoldings – plasma membrane may show numerous infoldings at the basal surface of epithelial cells; increases the surface area at the base of a cell; they function similarly to microvilli 27  09/09/2024 3. Cilia numerous motile processes larger than microvilli, 7 – 10 μm in length and 0.3 μm in diameter & arranged in parallel rows projecting from the free surfaces of some epithelial cells beat in a constant direction: *stiffen – rapid forward (effective stroke) bend slowly – recovery stroke may have isochronal rhythm – all beat synchronously more common is metachronal rhythm – beat in sequence (wave) base – is a dense elongated granule basal body Micrograph shows the basal surface of kidney epithelium. The interdigitations of the membranes show the increased surface area available for the ion and water transport. Cilia Nine microtubule doublets surround EM: have a core structure called axoneme, consisting of longitudinal microtubules with a constant number & a precise a central pair arrangement : transverse section – 2 single micro- tubules are located centrally with 9 doublet tubules uniformly spaced around them Doublets connected by Dynein arms that function: walk along adjacent a. to propel fluid or coating of mucus towards the exterior microtubule b. nerve receptors – maculae & cristae (inner ear); retinal rods (eye) c. chemoreceptors Nexin spokes radiate inward, connecting the doublets to the central pair 28  09/09/2024 GLANDULAR EPITHELIUM Renewal and Regeneration of Epithelium epithelial cells specialized for secretion outer body surface & intestinal tract some are located in the lining epithelium; others deep to the surface stratified squamous epithelium (epidermis) – superficial cells these epithelial structures are called glands (organs which synthesize undergo continuous keratinization  death (apoptosis) & substances from raw materials delivered by the bloodstream or desquamation of superficial cells eliminate pre-existing substances) – its parenchyma is the epithelium. intestinal tract – cells are continuously exfoliated at the tips of the an aggregation of these cells into a definite structure for the purpose villi of carrying on secretion or excretion is known as gland cells or respiratory tract – degeneration of epithelium is rare glandular epithelium. Regeneration: *stratified squamous epithelium – mitosis in the stratum germinativum or the deep cell layer e.g. healing wounds parenchyma – usually formed by the invagination of the lining 2 major categories of glands: epithelium from the free surface. a) Exocrine glands – deliver their products into a system of ducts - simple forms: all of the cell lining the lumen are opening on an external or internal surface. secretory cells b) Endocrine glands – release their products into the blood or - highly developed: secretion is by the deeper cells & the lymph for transport to another part of the body. remainder serves to carry the secretion to the surface (duct) stroma – located beneath a definite BM; composed of fine vascular connective tissue. its secretory products is passed into a system of tubes/ducts which transport it to a surface; the secretions consists of: aqueous fluid with secretory products (hormone, enzyme, mucin) 29  09/09/2024 Histogenesis of Glands: Synthesis, Storage & Release of Protein-rich Secretory Products: both glands are derived from a proliferation of cells of an embryonic Ribosomes (site of protein synthesis)  ER  Golgi apparatus  epithelium vesicles & glands for secretion (synthesis requires ATP – provided by they originate as solid epithelial ingrowths from the surface mitochondria) epithelium into the underlying tunica propia; grow with multiplication of the epithelial cells & arrangement following some architectural patterns; the deeper portion develop into the secretory portion of the gland. tubular connections – in the exocrine type, these enlarges & acquire a lumen; in the endocrine type, their connection with the surface disappear thus they become ductless. EXOCRINE GLANDS Glandular secretory products: majority of exocrine glands possess: 1. Exocrine cells secreting pure protein products – “serous” cells 1. a secretory portion of glandular cells grouped as tubules, - acinar cells of pancreas & parotid gland acini, or alveoli - secreted products are enzymatic proteins (trypsin, amylase, 2. an excretory portion composed of an excretory duct, which pepsin, & others) may be unbranched or branched - mechanism of synthesis, intracellular accumulation & release: both portions are invested in a loose connective tissue stroma similar to endocrine cells secreting polypeptide or pure protein containing numerous blood capillaries hormones in compound exocrine glands (branched excretory ducts), the stroma 2. Exocrine cells secreting mucus – “mucous” cells demarcates lobules - goblet cells, mucous neck cells of the gastric glands, & the glands of the respiratory tree 30  09/09/2024 3. Exocrine cells that secrete particular products released through a ENDOCRINE GLANDS particular process of extension: 1. General Structure a) Sebaceous gland cells – secrete sebum (fatty product) on multicellular gland – consists of glandular epithelial cells which are the skin; the cells themselves together with their contents arranged in cords, bundles, or islets form the secretory product (holocrine gland) connective tissue stroma – supports the cells containing numerous b) Mammary gland cells – secrete milk composed of two fenestrated blood capillaries components: protein by exocytosis & lipid globules enveloped by plasmalemma (apocrine gland) arrangement: thyroid – follicular c) Gastric gland parietal cells – secrete HCl as Cl¯ and H⁺ ions, : others – scattered randomly which traverse the plasmalemma 2. Endocrine Gland Secretion the secretory granules, which pinch off the Golgi complex, migrate always secrete hormones directly into the neighboring blood through the cytoplasm toward the plasmalemma, to which they capillaries adhere According to nature of secreted hormone: 4 major groups of finally, their contents are released to the exterior of the cell, by the endocrine cells process of exocytosis, & then enter the blood capillaries. a) Endocrine cells secreting polypeptide/pure protein hormones b) Endocrine cells secreting glycoprotein hormones Proteins (GER) are synthesized from amino acids in the blood mechanism is similar to first during synthesis, proteins enter the cisternae of the GER there is inclusion of glycogen synthesized in the Golgi complex they move to the Golgi complex, where they are concentrated & all endocrine cells involved in the secretion of polypeptides packaged into secretory granules (secretory product invested by (pure protein & glycoprotein), because of their similar mechanism membranes of Golgi complex) possess similar morphological characteristics: a distinct nucleolus, abundant GER, a well-developed Golgi complex, & membrane-bound secretory granules. 31  09/09/2024 c) Endocrine cells secreting steroid hormones d) Endocrine cells secreting biogenic amines enzymes involved in the synthesis of hormones from cholesterol are enzymes involved in the synthesis are located within the cytoplasm localized principally in the mitochondria and SER or within the secondary granules themselves secretory product does not appear as a formed element; mechanism appear as small, dense-cored vesicles (EM) of release are not well understood contents are released by exocytosis characteristic ultrastructural features of steroid-secreting endocrine cells: numerous mitochondria with tubular cristae abundant SER absence of secretory granules liposomes (lipid vacuoles) & lipofuscin pigments are frequently present CLASSIFICATION OF GLANDS 2. Multicellular Gland I. According to the Number of Component cells: composed of many cells 1. Unicellular Gland simplest form: a sheet of epithelium consisting of a one-celled gland (goblet cell) homogenous population of secretory cells (surface epithelium found scattered among the columnar cells of the epithelium on many mucous membranes; described as intramural unicellular mucus-secreting of the gastric mucosa & uterine lining) glands forms the parenchyma of pancreas or liver secretes mucin (protein polysaccharide), which upon hydration, forms a lubricating solution called mucus – lubricates & protects the surface of the epithelium fully developed cell: an expanded apical end, slender basal end containing a compressed nucleus with small amount of deeply staining basophilic cytoplasm “goblet” cell – shape of cell: expanded cup-shaped rim of cytoplasm (theca) filled with secretory droplets 32  09/09/2024 II. According to Morphology or Shape: 2. Saccular – tubular invaginations bear sac-like dilatations at 1. Tubular their terminal ends a. Simple Tubular a. Simple Saccular composed of one straight tube-like structure made up of short tube-like invagination dilated at its terminal portion (e.g. no excretory duct & the terminal portion opens directly onto the epithelial small sebaceous glands) surface (e.g. intestinal glands of Lieberkuhn) b. Branched Saccular b. Convoluted/Coiled Tubular tube-like invagination gives off branches whose terminal portions are dilated terminal portion is a long coiled tubule connected to the surface by a long, (e.g. bigger sebaceous glands) unbranched excretory duct (e.g. sweat glands) c. Compound Saccular c. Branched Tubular made up of several branched saccular glands (e.g. mammary glands) main invagination of the gland gives off several branches which are also tube-like in appearance (e.g. gastric glands) d. Compound Tubulo-alveolar/Tubulo-acinar or Racemose d. Compound Tubular composed of several units whose pattern is a combination of the tubular & composed of several simple or branched tubular glands closely bound together saccular types (e.g. salivary glands, glands of respiratory passages, & (e.g. kidney, testis, liver) pancreas) “alveolus” – refers to a bigger dilatation with a wider lumen III. Histological Classification (histological characteristics of secreting cells): 1. Serous Glands cells possess an acidophilic granular cytoplasm with a round nucleus (e.g. parotid gland & pancreas) 2. Mucous Glands cells possess a basophilic non-granular, reticulated cytoplasm & a flattened nucleus at the BM (e.g. sublingual gland) 3. Mixed Glands cells are composed of both serous & mucous types (e.g. submaxillary glands), with the mucous cells making up the major portion of the gland; and the serous cells forming crescentic caps, called serous demilunes over the ends of the acini “mixed” gland – sometimes refer to glands containing both exocrine & endocrine portions (liver & pancreas) 33  09/09/2024 Mixed Gland with Serous Demilunes (SD) IV. Physiological Classification: 1. Serous Glands secretion is watery & albuminous in content (pancreas & parotid) 2. Mucous Glands secretion is rich in mucin; appears thick & viscid in consistency (goblet cells & some smaller salivary glands) 3. Mixed Glands secretion is a mixture of serous & mucous (submaxillary or submandibular) 4. Glands that are neither serous nor mucous (e.g. ceruminous glands) 5. Cytogenic or Cellular Glands glands that produce cells (testis, ovary) Cytocrine glands – represented by melanocytes (from the neural crest), which manufacture melanin pigment granules 34  09/09/2024 V. According to Integrity of Secretory cells (or Manner of Elaboration 3. Merocrine Glands of secretion): maintain the integrity of their constituent cells throughout the process of secretion (e.g. sweat & salivary gland) 1. Holocrine Glands smaller sweat glands – “eccrine” glands; elaborated product is an excretion cells suffer a more or less complete destruction in the process of secretion, rather than a secretion being transformed into the secretion itself secrete substances thru the process of exocytosis (pancreas) e.g. sebaceous gland – cells break down with outpouring of cytoplasm & lipid seminiferous epithelium (testis) – release of spermatozoa 2. Apocrine Glands cells suffer a partial destruction of their distal borders in the process of secretion (e.g. mammary glands & some axillary sweat glands) VI. According to the Absence or Presence of Excretory Ducts: 2. Exocrine Glands 1. Endocrine/Ductless Glands glands with an excretory duct system glands without any excretory duct system e.g. testes (interstitial cells of Leydig) composed of all secretory portion whose cells are arranged in groups with no 3. Acrine Glands characteristic lumen (except: thyroid gland – where cells form follicles with a glands whose secretion is not discharged from the cells producing it (e.g. wide lumen) granular leukocytes & phagocytes – produce enzymes not released but e.g. pituitary gland/hypophysis, thyroid, para- thyroid, utilized within the cells for phagocytosis) adrenal/suprarenal, pineal gland & some neurons in the hypothalamus of the diencephalon of the brain, ovaries, testes & placenta 35