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Epithelium Overview
- All epithelia are supported by a basement membrane of variable thickness. Basement membranes separate epithelia from underlying supporting tissues and are never penetrated by blood vessels. (this is regarding outer layers of
- epithelium. There are also inner layers… endothelium, but called epi)
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Epithelium Characteristics
- Form continuous cell sheets
- Thickness: one or more cell layers
- Connected by adhesion plaques/cell junctions
- Attached to basement membrane
- Avascular: never penetrated by blood vessels
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Membrane Junction
Zonula Occludens
Tight junction -impermeable junction encircles cell
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Membrane Junction
Zonula Aderens
- Adhering junctions continuous intermediate junction
- Cadherin
- Actin filaments
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Membrane Junction
Mascula Adherens
- Desmosome - scattered spot-adhering junctions
- Cadherin
- Intermediate filament
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Membrane Junction
Gap Junction
- Communicating Nexus
- -small chemicals pass through pores
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Hemidesmosome
- Epithelial cells are linked to the subcellular matrix by a supportive basement membrane which also acts as a barrier. Anchorage of epithelial cells is through hemidesmosomes.
- strong linkage between epi and basement membrane
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junctional epidermolysis bullosa
Genetic or acquired defect in HD (hemidesmosome)
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Oral EB-mucous membrane pemphigoid
- • An oral autoimmune disease
- • Auto-antibodies against the basement membrane antigens
- • Results in epithelial separation from connective tissue and ulceration.
- Mucous membrane pemphigoid showing ulceration of the maxillary gingiva, erosions
- Lack of brushing-> pitted enamel and tooth carie
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Microvilli
- • Extension of cytoplasmic processes
- • Increase surface area for resorption
- • Prominent in small intestine and proximal renal tubules
- • Appear as “brush borders” in H & E sections
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Cilia
- Pseudostratified ciliated epithelium
- Ciliary action serves to propel mucous and trapped debris out of the respiratory tract.
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Classification of Epithelium
- • Number of layers - Simple vs. stratified
- • Shape of component cells - e.g. flat (squamous), cuboidal, columnar
- • Presence of surface specializations - e.g. ciliated, keratinized
- • Variations - pseudostrafied, transitional
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Simple Squamous Epithelia
Lining:
Function:
Appearance
- • Simple squamous – flat, sometimes hard to see
- • Lines vascular system (endothelium) and cavities (mesothelium)
- • Also found in Bowman’s capsule of kidney and alveoli of lung
- Function: diffusion
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Simple Columnar cells
- Columnar cells are usually found in tissue in which absorption is a major function
- Intestines, gall bladder lining (nucleus polarized to the base of the cell)
- Lines other salivary gland ducts, and inner enamel epithelium
- Can be polarized meaning the top of the cell is different than the bottom of the cell.
- Allows for different type of transport through the cell.
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Simple Cuboidal Cell
- Lines ducts of various glands (salivary gland ducts), kidney tubules, covers ovaries
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Pseudostratified Epithelium
- • Many layers of cells but all cells rest on BM—not all cells reach surface
- • Found in respiratory system (trachea) and male reproductive tract.
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Pseudostratified columnar ciliated epithelium (brochial ET)
- • All cells rest on the basement membrane
- • Nuclei polarized
- • Surface is ciliated
- • Lines the respiratory tract
- • Also known as respiratory epithelium
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Stratified Squamous Epithelia
- • Many layers of cells with basal cells cuboidal; upper layers are squamous
- • Can be keratinized upper layers forming cornified or dry non-nucleated cells
- • In wet mucosal tissue, tissue is non- keratinized
- - Lines Epidermis layer of skin
- - Lines mouth, vagina, anal, esophagus, pharynx
- (oral) Functions to protect and withstand abrasion
- Oral epithelium is several layers thick, but varies in the number of layers in different parts of the mouth.
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Simple Columnar ET
- Lines GI Tract, uterus, gallbladder
- Lines other salivary gland ducts, and inner enamel epithelium
- Unique w/ Goblet cells (secrete mucous)
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Stratified Cuboidal & Columnar
- • Two or more cell layers thick with the surface cuboidal or columnar—rare
- • Usually found at transition of a simple to a strafied epithelium
- • Example of stratified cuboidal epithelium: duct of sweat glands
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Transitional Epithelium
- Transitional – a form of stratified that lines the urinary tract.
- Contains cells are flattened and are cuboidal; hence the name "transitional" = "stretch"
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Distinct phases of wound healing
- a parade of different cell populations become “activated”
- clot formation
- inflammation response
- Re-epithelialization
- Resolution
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Glands
- Develop from invaginations of epithelial surfaces
- Secretory function
- Types (exocrine, endocrine)
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Exocrine
- glands – connect to surface by ducts
- Simple (un-branched duct)
- Compound (branched ductal system)
- Secretion types
- 1. Merocrine – secretory granules leave cell by
- exocytosis (no loss of cellular material)
- o Glands of the intestine, simple coiled sweat glands, complex salivary glands
- 2. Holocrine – secretory products is shed with the entire cell contents (cell is destroyed)
- o Simple sebaceous gland
- 3. Apocrine – secretory product is shed with part of the apical portion of the cell.
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