Bio 143

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epithelium

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sheet of cells that covers an exposed surface or lines an internal cavity or passageway

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what does epithelial tissue include

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epithelial, glands (secretory structures derived from epithelia.

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what does epithelial line

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every exposed surface, skin, digestive, respiratory reproductive, and urinary tracts. internal cavities, passageways that communicate with outside world

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important characteristics of epithelia

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cellularity

polarity

attachment

avascularity

arranged into sheets or layers

regeneration

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characteristic of epithelia...cellularity

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composed of almost entirely cells bound closely together. little or no intracellular space between cells in epithelial tissue

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characteristic of epithelia ...polarity

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epithelium has apical layer (exposed) and basal surface (attached surface). there is an uneven distribution or organelles and cytoplasmic components between apical and basal=polarity. polarity of cell is determined by its function.

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characteristic of epithelia ...attachment

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basal surface is bound to the basal lamina

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basal lamina

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complex structure that is produced by epithelium cells and the underlying connective tissues. this is what the basal surface of an epithelial tissue attaches to.

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characteristic of epithelia...avascularity

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epithelia do not contain blood vessels so they rely on diffusion and absorption via the basal surface and apical surface

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characteristic of epithelia ...regeneration

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epithelial cells damaged or lost at surface are continually replaced through the division of stem cells located within epithelium

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characteristic of epithelia ...arranged into sheets or layers

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all epithelial tissue is composed of a sheet of cells one or more layers thick

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functions of epithelial tissue

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provide physical protection

control permeability

provide sensation

produce specialized secretions

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functions of epithelial tissue ...provide physical protection

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protect exposed surfaces and internal surfaces from abrasion, dehydration and destruction by chemical and biological agents

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functions of epithelial tissue ...control permeability

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some are relatively permeable...some impermeable. contain molecular machinery for selective absorption or secretion. can be regulated and modified through various stimuli (ex hormones, stress)

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functions of epithelial tissue ...provide sensation

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innervated by sensory nerves. specialized cells can detect changes in environment. (ex touch receptors, neuroepithelium)

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neuroepithelium

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specialized sensory epithelial cell found in areas of the body that provide sensation of smell, taste, sight, equilibrium and hearing

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functions of epithelial tissue ...produce specialized secretions

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gland cells

unicellular- individual gland cells scattered among cell types in epithelium.

glandular epithelium-most or all cells produce secretions

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tissues with specialized functions

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epithelial- covers exposed surfaces, lines internal passageways and chambers, produces glandular secretions

connective- fills internal spaces, provides framework and support, stores energy

muscle-contracts to produce movement

neural-conducts electrical impulses,carries info

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basolateral surface

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where epithelium contacts to the basal lamina and neighboring epithelial cells

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stereocillia

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very long microvilli, incapable of movement. found along portions of male reproductive tract and on receptor cells of inner ear.

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ciliated epithelium

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substances are moved over epithelial surface by synchronized beating of cilia. ex ciliated cells of respiratory tract move mucus toward the throat

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three factors involved in maintaining physical integrity of an epithelium

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intracellular connections, attachment to basal lamina, epithelial maintenance and renewal

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three factors involved in maintaining physical integrity of an epithelium...intracellular connections

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bound together by variety of cell junctions, extensive infolding of opposing cell membranes that interlock cells and increase the surface area of cell junctions

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three factors involved in maintaining physical integrity of an epithelium...attachment to basal lamina(superficial and deep)

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epithelial cells hold onto one another and remain firmly attached to rest of the body.

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Basal lamina

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superficial layer:

clear layer-composed of glycoproteins, network fine microfilaments. provides barrier between epithelia & underlying connective tissue

Dense Layer:

secreted by underlying connective tissues. contains course protein fibers giving basal lamina strength. attachment of protein fibers of each layer keep them combined

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three factors involved in maintaining physical integrity of an epithelium...maintenance and renewal

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must continually repair and renew itself. rate of cell division varies on how much is lost. some don't even last a day and rely on stem cells located close to basal lamina to frequently undergo cell division

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Classification of epithelia

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simple-

stratified

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simple

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one layer. thin and fragile, not much protection, used in areas where exchangement of materials must happen rapidly (lessens diffusion distance) ventral body cavities, all blood vessels, lining of intestines)

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stratified

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two or more layers above basal lamina. height and shape of cells may differ form layer to layer. look at superficial. found in places where significant mechanical or chemical stresses happen (mouth, skin)

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where does squamous epithelia occur?

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protected regions where diffusion or other forms of transport take place.

slick slippery surfaces reduce friction (respiratory exchange surfaces-alveoli), lining of ventral body, inner surface of circulatory system

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mesothelium

endothelium

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mesothelium-simple squamous epithelium that lines ventral body cavities.

endothelium-simple squamous epithelium lining the heart and all blood vessels

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stratified squamous epithelium

keratinized vs nonkeratinized

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found where mechanical stresses are severe. oral cavity, vagina, rectum, esophagus.

apical surfaces of ec packed with keratin (protein). meaning superficial layers are tough and water resistant.=keratinized

nonkeratinized-provide resistance to abrasion, but dry out and deteriorate unless kept moist.

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cuboidal epithelium simple and stratified

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simple- limited protection occur in regions where regular secretion or absorption take place (epithelium lining portions of the kidney tubules)

stratified- rare, lining ducts of sweat glands and in larger ducts of some exocrine glands (mammary glands)

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columnar epithelium simple and stratified

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simple- nucleus close to basal lamina, slight more protection that cuboidal. found in areas where absorption or secretion occurs. (lining of stomach, intestinal tract, uterine tubes, excretory ducts)

stratified- rare. protection along pharynx, urethra, anus,

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pseudostratified columnar epithelium

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respiratory system, hollow conducting organs of urinary system. mixture of cell types. nucleus situated at varying distance so it appears stratified but all cells are attached to basal lamina so its simple.

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transitional epithelium

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lines renal pelvis, ureters, urinary bladder. stratified epithelium with special characteristics that allow it to distend or stretch.

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types of secretions

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exocrine- unicellular or multicellular secrete mucins, enzymes, water and waste products. released at apical surface

endocrine- ductless glands release secretions directly into interstitial fluids, lymph or blood

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ducts

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passageway that delivers an exocrine glands secretions to an epithelial surface

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serous glands

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secret watery solution usually containing enzymes such as salivary amylase in saliva

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mucous glands

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secret glycoproteins called mucins that absorb water to form a slippery mucus (mucus in saliva)

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mixed exocrine glands

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contain more than one type of gland cell and may produce two different types of exocrine secretions. such as serous and mucous.

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endocrine

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release secretions by exocytosis from gland cells into fluid surrounding cell (hormone secretion) diffuse into blood for distribution to other parts of body.

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gland structures

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unicellular- individual secretory cells secret mucins

multicellular- include glandular epithelia and other form of gland cells that produce exocrine or endocrine secretions.

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examples of unicellular glands

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goblet cells- scattered among columnar epithelium of large and small intestines.
mucous cells-found in pseudostratified ciliated columnar epithelium that lines trachea

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multicellular exocrine gland

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simplest multicellular exocrine gland- secretory sheet. glandular cells dominate epithelium and release secretions into inner compartment. (ex mucus secreting cells lining stomach)

most multicellular exocrine glands found in pockets set back from epithelial surface.

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2 components of multicellular exocrine glands

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1glandular proportion that produces secretions and:

2 a duct that carries secretions to epithelial surface.

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2 characteristics used to describe organizations of a multicellular gland

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1. shape of secretory portion of gland

2. branching pattern of duct

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1 shaping pattern of duct

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tubular- cells making up gland arranged in tube

alveolar or acinar- made up of cells in a blind pocket

tubuloalveolar or tubuloacinar glands -combination of two arrangements

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2 branching pattern of duct

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simple-does not branch

compound-if it branches repeatedly

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mechanism of secretion...3 methods

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glandular epithelial cell may use three methods:

merocrine- ex serous cells

apocrine- lactiferous cells of mammary glands

holocrine-sebaceous gland cells

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merocrine

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secretory product packaged into secretory vesicles, released through exocytosis onto surface of cell. most common secretion mechanism (ex release of saliva from serous cells in salivary gland)

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Apocrine

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secretory product is released during shedding of the apical portion of cells cytoplasm, which has become packed with secretory vesicles. gland cells then undergoes regrowth and produces additional secretory vesicles.

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holocrine

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destroys gland cell. entire cell becomes packed with secretory products and then burst apart. secretion is released when cell dies. further secretions depends on gland cells being replaced by division of stem cells found deeper within epithelium.