Epithelium

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A complete lecture on epithelium.

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Presentation Transcript

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EPITHELIUM A tissue composed of one or more layers of continuous cell with scanty intercellular substance that lines a surface which maybe internal or external. It is anchored by a basement membrane or basal lamina to the underlying connective tissue known as lamina propria.

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Functions: Protection Absorption Secretion Sensory reception Excretion Reproduction Lubrication

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General Characteristics: Epithelium 1. Mainly made up of cells that are closely opposed with scanty intercellular substance. 2. Rest on the lamina propria with the basal lamina in between. 3. Avascular 4. Arranged in sheets covering or lining a surface. 5. It undergoes metaplasia.

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B. Epithelial cells Nucleus – conforms the shape of the cell. Cytoplasm Manifest Polarity

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Indications of Polarity

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Classification Basis: A. Number of cellular layers 1. Simple (one layer) 2. Stratified (two or more layers) B. Shape of the cells 1. Squamous 2. Cuboidal 3. Columnar

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Histological Types of Epithelium Simple 1. Simple Squamous 2. Simple Cuboidal 3. Simple Columnar B. Stratified 1. Stratified squamous 2. Transitional 3. Stratified Columnar C. Pseudostratifed Columnar D. Specialized Epithelium 1. Glandular Epithelium 2. Ciliated Epithelium 3. Neuro-Epithelium

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Simple Epithelium Simple squamous - consists of thin, flat, platelike cells arranged in single layer. - they adhere to one another by their thin edges. - On top view, the individual cells have irregularly hexagonal shape with wavy outline, this is the serrated border or interlocking membrane.

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Specific names of simple squamous epithelium: Mesothelium – lines serous cavities. Peritoneum, Pleura, Pericardium B. Endothelium – lines the heart, blood vessels and lymphatic vessels. C. Mesenchymal epithelium – lines cavities that arise as clefts in the embryo. Subdural space, anterior chamber of the eye

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2. Simple Cuboidal Epithelium - Single layer of cells that has equal height and width. - Nucleus is rounded and centrally located - Examples: Thyroid gland, free surface of the ovary Kidney tubules, ducts of glands and choroid plexus

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3. Simple Columnar Epithelium - One layer of cells whose height exceeds the width. - Nuclei are elongated and are located near the base and they lie of the same level.

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B. STRATIFIED EPITHELIUM General Characteristics: 1. Consists of many layers basal row – two rows middle row – three to four rows superficial layer – one to two rows 2. Shape of the cells in the basal and middle layers are the same in all types of stratified epithelium. 3. Shape of the cells in the superficial layer determine the type of stratified epithelium. 4. Serves as a protection because this can withstand more wear and tear than the simple type.

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Types of Stratified Epithelium Stratified squamous > Wet (non-keratinized) - found lining hollow organs and other unexposed parts. > Dry (keratinized) - lining the surface of the body.

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General Characteristics: a.Thick epithelial sheet b. Has prominent connective tissue papilla c. Basal row consists of cuboidal cells d. Middle row is composed by polyhedral cells. e. Superficial layer in: 1. Keratinized stratified squamous lose their nuclei, cytoplasm is replaced by sclero-protein known as keratin. The cells become dry, devitalized scales. 2. Non-keratinizing stratified squamous epithelium – found n most inner surfaces. Superficial cells are viable nucleated cells resembling the inner cells, except for their flattening. Cells also contain keratin but not excess.

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Examples: Wet type Esophagus Epiglottis Conjunctiva Distal part of urethra 2. Dry type Skin Labia minora

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2. Stratified columnar epithelium a. same cells in the basal and middle row. b. superficial cells are tall. Examples: Rare in humans. Seen lining the fornix of the conjunctiva.

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3. Transitional Epithelium a. Superficial cells are bloated b. Cells become bigger as they go towards the surface. c. Cells in the superficial layer retain their nuclei. d. No connective tissue papilla. Examples: lines the urinary passages from the renal pelvis, ureter, urinary bladder, urethra.

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C. PSEUDOSTRATIFIED COLUMNAR EPITHELIUM This appears to be made up of several layer but actually there is only one layer. Made up of 3 types of cells (basal, fusiform, and columnar) All cells touches the basal lamina. Only the columnar cells reach the surface. Nuclei lie at different levels. Example: Respiratory tract (trachea, bronchi, larynx) Male genital tract (ductus epididymis and vas deferens)

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D. Specialized Epithelium Glandular epithelium - basically columnar cells arranged around a lumen forming an acinus. Because of this, the cells will now become pyramidal in shape. These will produce secretions. Types: a. Goblet cells – unicellular gland found among the epithelial cells. Examples: small intestines, large intestines, trachea, larynx, bronchi and bronchiole. b. Serous glandular epithelium c. Mucous glandular epithelium

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2. Ciliated Epithelium - cells have tiny protoplasmic processes known as cilia. They are mostly seen in simple and peudostratified columnar epithelium. 3. Neuroepithelium - basically pseudostratified columnar and specialized for sensory reception. This is made up of: a. sensory cells with sensory hairs b. basal cells c. columnar cells

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VII Functional surfaces A. Free surface B. Lateral surface C. Basal surface D. Basal surface *Only the simple type of epithelium will show all the three functional surfaces.

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VIII Specializations on the functional surfaces: FREE surface 1. Cytoplasmic specializations – part of the cell a. Condensed border – narrow, dense layer of the superficial part of the cytoplasm just beneath the free surface. b. Non-motile pocesses – hair-like processes that projects from the free surface.

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Types based on the function: Those that are related to absorption – microvilli 1a. Striate border (striated cuticular border) - if found in the small intestines. They are slender and uniform in length. 1b. Brush border (found in proximal) - convoluted tubule of the kidneys. They are taller, stouter, and irregular in length.

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2. For EXCRETION (stereocilia) - long, hair-like processes projecting into the lumen. They are found in the male genital tract and in the hair cells of inner ear. 3. For SENSORY reception - sensory hair-modified cilium found in neuroepithelium.

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C. Motile processes - Cilia or Kinocilia - functions primarily to propel fluid or mucus by beating towards the external surface. Found in the female genital tract and respiratory tract. STEREOCILIA - have a core bundle of actin filaments that extends into the apical cytoplasm, but is less organized than the microvilli seen in the intestinal absorptive cells. They are parallel at their base but become sinous and entwined near their tips. Functions: 1. Known to absorb 90% of the original volume of fluid secreted by the testis. 2. It increases the surface and increase the effeciency of the epithelium in concentrating the seminal plasma during its passage thru the epididymal duct.

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Axoneme Components: Central pair of microtubules 9 doublets of microtubules made up of tubulin which makes up the protofilaments. a. Subunit-A - Complete tubule with circular cross-section composed of 13 protofilaments. It has 2 rows of short arms projecting towards the next doublet. Each arm has 3 subunits and contains protein DYNEIN (has adenosine triphosphatase activity). Radial spokes extend inwards from Subunit A to a sheath around the central pair. b. Subunit – B - Incomplete, so has a C-shape on cross section. The defect is completed or closed by subunit- A by sharing three of the latters microtubules. This fuses with subunit A forming a figure of 8. This doublet is responsible for the movement of the cilium. Energy is supplied by ATPase activity of the dynein arm so it’s the motor for ciliary-beat. Movement is interpreted to be brought about by the sliding of the microtubules.

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Movement of Cilia - Mechanism is not fully understood. - Hydrolysis of ATP  dynein arm detach & re-attach at the lower site of the adjacent doublet. Rapid repetition of this cycle of attachment and detachment results to sliding of the doublets with respect to one another  bonding of the axoneme by restraints imposed by the radial spokes. - Dynein on doublets 1-4 are active during effective strokes and those on doublets 6-9 then become active, bending the cilium in the opposite direction, during recovery strokes. This results to a “to and fro” movement of the cilia.

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Two types of Cilia movement: 1. Isochronal rhythm – when all cilia beat at the same time. 2. Metachronal rhythm – cilium beats first then others will follow. This is more common. *Kartagener’s syndrome – a rare congenital disorder in which the dynein arms are lacking on the doublets of the axoneme resulting in immotile cilia and flagella  infertility and inability to clear the upper tract or mucus.

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(second main type of modification. Next to cytoplasmic modification-Free surface) 2. Secreted plate or cuticle - solid, formed substances that is produced by the cell but is no longer a part of the cell. Ex. Ameloblast producing the enamel

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Specialization on the lateral surface: Intercellular bridge – cytoplasmic bridges seen to cross the spaces between cells. Desmosome or bridge corpuscle – localized thickening of the opposed cell membranes of 2 adjacent epithelial cell. Tonofibrils are anchored to it. Intercellular cements – plastic cement substances which promotes adhesions of cells to change shape & wondering cells to invade them. Terminal bar – a ring-like band seen in many cuboidal cells or columnar epith. Lying just beneath the free surface, closing off the intercellular space. Under the E/M it reveals a more complex structure.

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5. Interlocking membrane – lateral projections interlock with similar projections of adjacent cells. 6. Nexus or gap junctions – similar to desmosome & is the site of Low resistance to ion flow  transmission of impulses. Adjacent plasma membranes are closely opposed leaving a narrow intervening gap. Each gap is provided with pores which permit the passage of positively charged ions & other molecules (less than 2nm in diameter) from the cytoplasm of one cell to another. Each pore consists of a minute tubular structure called a connexon which traverses the intercellular gap

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7. Junctional complex 7a. Zonula occludens – just beneath the free surface where there is fussion of the opposing cell membranes. There is a multiple sites where fussion separated by short regions in which the opposing membranes are separated. The fussion extends in a hole around the perimeter on the cell and serves to close the intercellular space. - With freeze fracture method, each membrane appears to contain straight rod-like structures that branch and join to form a rectilinear network of ridges on the P-face layer and a complementary pattern of – grooves on the E-face. This rods coincide with the lines of membrane fussion. Significance: Obliteration of the intercellular space makes it possible for the cells to pump solute actively thru the lateral membranes into the intercellular space below  creating a standing osmotic gradient that moves the water across the epithelium  concentrating the luminal contents. It has a mechanical role in maintaining the structural integrity of the epithelium because cells are more firmly attached to this region of fussion then anywhere else.

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7b. Zonula Adherens - membranes diverge to a distance of 15-20nm. Internally this reinforced by the dense mat of filamentous material that form a continuous band around the cell parallel to the zonula occludens. Identified probably by the freeze-fracture method. 7c. Macula adherens or desmosome - The cell surface are 15-20nm apart. Opposing membranes are of normal dimensions but appear thick – to a thin but dense layer that is closely applicable to their cytoplasmic surface. Immediately next to the dense plaque is a thicker layer and of a feltwork of fine filaments. Those where the Tonofilaments are anchored. The Tonofilaments form hairpin loops in the __ of filaments associated with the dense plaques and turn back into the cytoplasm. A slender intermediate dense line is seen at the middle of the intercellular space between the halves of desmosome in thin. In heavy metal straining – delicate striations traversing the intercellular space. The integrity of desmosome is dependent on the presence of calcium. There maybe more than one desmosome in the lateral surface. *Hemidesmosomes are found in the basal surface of stratified squamous epithelium.

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Specialization on the Basal surface: Infoldings – serve to increase the area of the membrane engaged in pumping ions to generate an osmotic gradient to move H2O across the epithelium. Basal lamina – thin supporting layer with two(2) zones as seen in the electron micrograph. a. Lamina lucida – a pale zone of very low density immediately adjacent to the basal membranes of the epithelium. It is sometimes thicker than… b. Lamina densa – outer zone of greater density exposed to the underlying connective tissue.

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Constituents of Basal lamina (main components) Type IV collagen – exclusively found in the basal lamina and consists of three(3) alpha-chains retaining a terminal peptide that prevents their assembly into cross-striated fibers characteristic of other form of collagen. Large glycoproteins and laminin are found mainly in the lamina lucida. The glycoproteins include Heparan sulfate which gives the lamina a strong anionic charge that may contribute to its function as a selective filter. Fibronectin – maybe involved in its attachment to the reticular fibers of the underlying tissue.

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2. Other component: - Anchoring fiber of type-VII collagen which course downwards looping around collagen fibers in the subjacent connective tissue and terminating in the anchoring plaques – dense bodies in the extracellular matrix composed of both type IV and type VII collagen. It is common in stratified squamous.

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Functions of Basal lamina: Primary function is to support the epithelium type-IV collagen in the lamina densa has tensile strength and flexibility to permit stretching & re--- of epithelia lining hollow organs that are subject to changes in volume. In repair after injury, the persisting basal lamina provides a substrate that guides migration of cells from the margin of the wounds to restore epithelial continuity. It serves as a passive molecular sieve or ultra-filter the charge dependent selectivity of the filter is attributed to the polyioic chains of associated proteoglycans, which have large sphere of hydration, leaving spaced occupied by the water that would permit passage of macromolecules.