Epithelial specializations and exocrine glands Flashcards
Regional domains of epithelial cells
Apical, basal, lateral
Apical domain
Demonstrates structural specifications. Most apical layer of cells is used to classify cell morphology.
3 modifications to apical cell surface
- Microvilli 2. Stereocilia 3. Cilia
Microvilli
Small finger-like extensions of the apical membrane that project into a lumin and possess a core of actin filaments. Anchored into terminal web. Increase the surface area of the cell. Have bilayer of plasma membrane, glycocalyx (fuzzy coat).
Stereocilia
Long non-motile microvilli which may be branched. Simular to microvilli (cylindrical projections of the apical plasma membrane which contain an actin core) but longer and may be branched. Increase apical surface area. Function in signal generation (ex. in the ear)
Terminal web
Apical array of cytoskeletal filaments and accessory proteins where microvilli are anchored and supported
Cilia
Hair-like projections of the apical cell surface. Much larger than microvilli. Microtubule core called the axoneme composed of tubulin dimers. Easily distinguished from microvilli by their large size and the “9+2” (9 peripheral doublets and 2 center singlets) arrangement of microtubules. Motile-using dynein (ATPase), microtubles slide past each other causing cilium to bend. Move substances across surface of an epithelial layer (ex. respiratory tract uses cilia to move mucus)
Flagella
Contain same microtubule core (9+2 axenome) as cilia. Usually fewer in number and longer than cilia. In humans only found in spermatozoa.
Basal bodies
Where cilia are anchored. Composed of 9 microtubule triplets (structurally the same as centrioles)
Primary ciliary dyskinesia
Immotile cilia syndrome. Group of hereditary disorders including Kartagener’s syndrome and Youngs’s syndrome.
Kartagener’s syndrome
Cilia lack dynein arms resulting in chronic respiratory distress due to impaired mucus transport, male infertility
Youngs’s syndrome
Cilia have malformed radial spokes and dynein arms producing thick mucus secretions, persistent sinusitis, and reduced male fertility or sterility
3 types of terminal bars
Zonula occludens (tight junctions), zonula adherens (adhering junctions), macula adherens (desmosomes)
Terminal bars
Junctional complexes between adjacent cells
Zonula occludens structure
Tight junctions. Belt-like (encircle the circumference of the cell) structures located close to the apical surface. Characterized by system of membrane folds in which outer leaflets of adjacent plasma membranes brought in close contact. Mediated by occludins in association with cadherins.
Zonula occludens functions (2)
- Prevent free flow of water soluble molecules between cells (apical basal) 2. Compartmentalizes integral membrane proteins into apical and basolateral domains
Occludins
Integral plasma membrane protein that mediates zonula occludens along with cadherins
Cadherins
Integral plasma membrane protein that mediates zonula occludens along with occludins
Zonula adherins structure
Adhering junctions. Belt-like structures located adjacent and basal to the zonula occludens. Characterized by uniform distance between adjacent plasma membranes. Mediated by cadherins which interact with catenins that are linked to actin.
Zonula adherins functions (2)
- Cell adhesion 2. Link actin cytoskeletons of adjacent cells (anchor points)
Catenins
Interact with cadherins and actin in zonula adherins
Macula adherens structure
Desmosomes. Located basal to the belt junctions and distributed randomly along lateral membrane. Characterized by paired thickened membrane regions called attachment plaques (containing proteins like desmoplakins and pakoglobins), dense band (desmoglein) in intercellular space, site of intermediate filament cytokeratin insertion
Macula adherens function
Strong spot adhesions between cells (like spot welds)
Gap junctions structure
Located randomly along lateral membranes. Comprised of packed connexons (proteins composed of integral membrane proteins called connexins). Characterized by paired plaques of connexons on adjacent cell membranes and a narrow intercellular cleft.
Gap junctions function
Provide rapid exchange of water, ions, and small molecules between adjacent cells. They are non-specific so regulate by size.
Clostridium perfringes
Bacterial pathogen which produces a toxin that binds to claudin proteins preventing incorporation into tight junctions. This causes junctional breakdown. Diarrhea, abdominal pain.
Heliobacter pylori
Bacterial pathogen which binds to extracellular domains of zonula occludens proteins and translocates one of its own proteins into epithelial cells. This interferes with ZO-1 protein function and causes cytoskeletal rearrangements and junction failure. May lead to gastric ulcers.
Dust mites
Parasitic pathogen from fecal pellets the contain peptidases that cleave occluding and ZO-1 proteins. If fecal pellets inhaled tight junctions in the respiratory system break down and expose lungs to allergens. Leads to asthma attacks.
3 basal specializations
- Hemidesmosomes 2. Basal membrane infoldings 3. Basal lamina
Hemidesmosomes
Junctions that attach basal cell membrane to basal lamina. Like desmosomes, site of keratin tonofilament (intermediate filament) attachment. Linkage to basal lamina is through integrins which link the ell cytoskeleton to ECM providing adhesion, traction, and bidirectional signaling.
Integrins
Link hemidesmosomes to the basal lamina
Basal membrane infoldings
Increase available surface area for transport (ex. Kidney)
Basal lamina
All epithelial sheets rest on basal lamina. Two layers of epithelial cells: 1. lamina lucida-an electron lucent layer directly below basal membrane 2. lamina densa-electron dense layer directly beneath lamina lucida
Basal lamina functions (2)
- Provides support and attachment for epithelial sheet 2. Acts as molecular sieve or filter
Gland definition
Cell, group of cells, or organ that produces a secretion
Two main types of glands
- Endocrine-secrete substances or hormones directly into the blood or lymph 2. Exocrine-secrete substances via a duct
How are glands made?
Develop from infoldings of epithelial sheets that penetrate underlying connective tissue
Endocrine gland
Lose connection to the epithelial surface of origin. Ductless and secrete products into blood or lymphatic vessels.
Exocrine glands
Retain connection to the surface they originated from and consequently secrete product via duct or duct system
Cellular vs. multicellular glands
Most endocrine ducts multicellular, but isolated secretory cells within an epithelium can be unicellular exocrine glands. They secrete their product apically onto the epithelial sheet.
2 criteria for classifying multicellular glands
- Complexity of the duct system (simple or compound) 2. Morphology of secretory proteins-tubular (straight, branched, or coiled), acinar, tubuloacinar
Simple multicellular exocrine glands
One duct (can be straight, branched, or coiled) that opens onto the surface of the epithelial sheet
Compound multicellular exocrine glands
Will be classified as “compound, tubuloacinar glands.” Organs that have a connective tissue capsule, divided into lobes and lobules and have a complex, differetiated duct system. Small ducts–>larger ducts–>main excretory duct(s)–>surface of epithelium.
Parenchyma
Ducts and secretory portions
Stroma
Connective tissue components that surround and support parenchymal tissue
Acininus
Round clusters of epithelial cells
Intralobular duct
Inside a lobule, secretion from acinus and drains into the interlobular ducts
Interlobular duct
Ducts between lobules, where the intralobular duct drains
2 cell populations in secretory regions of exocrine glands
- Secretory cells 2. Myoepithelial cells
2 types of secretory cells
- Mucous-secrete mucinogens (large glycosylated proteins) 2. Serous-secrete solution of protein
Myoepithelial cells
Within the basal lamina surround the acinus. Have contractile function which aids in moving secretions out of glands.
Invasive (inflitrating) ductal carcinoma
Mammary gland duct cells undergo 2 sequential processes: 1. Uncontrolled proliferation 2. Invasion/colonization of stroma
5 factors controlling normal cell proliferation
- Rate of cell proliferation=rate of cell death (rate depends on tissue) 2. Presence of committed transit amplifying daughter cells: get overall increase in number 3. Regulation of signaling pathways 4. Regulation of integrin signaling (bidirectional between cell and environment) 5. Hormone levels in mammory glands (estrogen and progesterone)
What 4 things are carcinoma cell proliferation is affected by?
- Genetic mutations 2. Gene duplications 3. Incorrect signaling 4. Hormonal status One or more of these may precipitate uncontrolled epithelial cell proliferation and result in tumor formation
What times in development are are estrogen and progesterone regulating cell proliferation?
- Initial development at puberty 2. Minor cyclic changes during menstual cycle 3. Development during pregnancy (also affected by prolactin) 4. Apoptosis after weaning 5. Involution (shrinkage or organs) after menopause
What does it mean to say cancer cells are invasive?
Cells have increased migratory capacity and ability to move through or destroy the basal lamina resulting in colonization of adjacent tissues (often connective tissue) and may provide access to blood and/or lymphatic vessels with which to spread (metastasize)
