Epithelial specializations and exocrine glands Flashcards
4 characteristics of epithelial tissue
- Contiguous sheets of cells (more cells than ECM) joined by junctions 2. Polarity 3. Rest on/attached to basal lamina 4. Avascular (receive nutrients via blood vessels)
2 components of tissues
- Cells 2. Extracellular matrix (ECM)
Extracellular matrix (ECM)
Region outside cells including complex of macromolecules produced by cells and exported into extracellular space (dissolved substances and fibers)
4 functions of epithelial tissue
- Covering/protection 2. Absorption/secretion (transcellular transport, absorb liquids and gas, secrete mucus, enzymes, hormones) 3. Barrier (compartmentalize) 4. Sensory
2 ways to classify epithelia
- Number of layers 2. Morphology of apical layer
3 morphologies of epithelia
- Squamous 2. Cuboidal 3. Columnar
2 main categories of layers of epithelia
- Simple (one layer) 2. Stratified (multiple layers) which are always classified by the apical cell layer
Pseudostratified ciliated columnar epithelia
Specialized epithelia where all cells in contact with basal lamina, but not all reach the apical surface of the epithlium. Nuclei appear as 2-3 layers. Usually columnar and have ciliated cells.
Transitional epithelia
Several layers (polyhedral shaped) with most cells in apical layer being dome shaped (relaxed) and some flattened (distended). Variable morphology throughout layers.
Keratinized layers
Seen in stratified squamous epithelium (like skin). Should be classified as “stratified squamous, keratinized (or non-keratinized)”
Which epithelial layer determines structural specializations and morphology?
Apical layer
Microvilli
Small, finger like extensions of apical membrane that project into lumen and have actin filament core and fuzzy coat (glycocalyx). Anchored into the terminal web. Increase surface area and form a brush border.
Glycocalyx
Fuzzy coat found on microvilli seen in cross-section
Terminal web
Apical array of cytoskeletal filaments where microvilli or stereomicrovilli are anchored
Stereocilia
Long, non-motile microvilli which may be branched. Anchored into the terminal web. Increase apical surface area and function in signal generation.
Cilia
Hair-like motile projections of apical cell surface anchored via basal bodies and much larger than microvilli. Contain a microtubule core of tubulin dimers which make the axoneme (9+2 arrangement). Associate with dynein ATPase.
Flagella
Same 9+2 microtubule core as cilia and only found in spermatozoa
Basal bodies
Anchor cilia and contain 9 microtubular triplets (just like centrioles)
Anoneme
9+2 microtubule core (9 peripheral doublets with dynein arms and 2 center singlets) made up of tubulin dimers found in cilia
Tubulin
Dimerize to make microtubules which form axonemes in cilia
Primary cilia dyskinesia
AKA mmotile cilia syndrome. Group of hereditary disorders including Kartagener’s syndrome and Young’s syndrome.
Kartagener’s syndrome
Cilia lack dynein arms which cause chronic respiratory disease due to lack of mucus transport and male sterility
Young’s syndrome
Cilia have malformed radial spokes and dynein arms which make very thick mucous secretions causing persistent sinusitis and reduced fertility/sterility
Terminal bars
Junctional complexes (all 3 types of junctions together) that are visible at the light microscope level
Zonula occludens
AKA tight junctions. Belt-like structures close to apical surface. Mediated by occludens and claudins in association with cadherins.
Occludens
Mediate zonula occludens with claudens
Claudens
Mediate zonula occludens with occludens
Cadherins
Associate with occludens, claudins to mediate zonula occludens and associate with catenins to mediate zonula adherens
Zonula adherens
AKA adhering junctions. Belt-junctions which are adjacent and below zonula occludens. Mediated by catenins which are linked to actin. Anchor actin filaments for cell adhesion.
Catenins
Mediate zonula adherens and link to actin
Macula adherens
AKA desmosomes. Below belt junctions and randomly distributed along lateral membrane. Have attachment plaques (contain desmoplakins and pakoglobins) and associate with cytokeratin. Spot welds between cells.
Attachment plaques
Found in macula adherens and include desmoplakins and pakoglobins
Desmoplakins
Attachment plaque protein in macula adherens
Pakoglobins
Attachment plaque protein in macula adherens
Cytokeratin
Intermediate filaments which associate with macula adherens and insert themselves at the cytoplasmic face
Gap junctions
AKA communicating junctions. Rapid exchange of small molecules between adjacent epithelial cells. Located randomly along lateral membranes. Comprised of paired packed connexons which are formed by connexins) which form aqueous core. Regulated by intracellular levels of Ca2+ and pH.
Connexons
Formed by connexins and make up gap junctions
Connexins
Integral membrane proteins which form connexons
Clostridium perfringens
Bacterial pathogen in gut which produces toxin that binds to claudin proteins and prevent their incorporation into tight junctions. Tight junctions break down. Diarrhea, abdominal pain.
Heliobacter pylori
Binds to extracellular domain of zonula occludins and inserts its’ own protein into epithelial cells interfering with ZO function causing cytoskeleton rearrangements and junction failure. Gastric ulcers.
Dust mites
Parasitic pathogen (fecal pellets) containing peptidases that cleave occluding ZO proteins. If inhaled cause respiratory epithelium breakdown and expose lungs to inhaled allergens. Asthma attacks.
Hemidesmosomes
Junctions that attach basal cell membrane to basal lamina. Site of keratin tonofilament attachment. Linkage to basal lamina is through integrins which link cytoskeleton to ECM for adhesion, traction, and signaling.
Keratin tonofilaments
Intermediate filaments which attach to hemidesmosome
Integrins
In hemidesmosomes link cell cytoskeleton to ECM
Basal membrane infoldings
Multiple invaginations of basal plasma membrane to increase surface area for transport. Many mitochondria are present.
Basal lamina
Acellular structure produced by epithelia where epithelial sheets rest. 2 layers include lamina lucida and lamina densa. Can only be observed at electron microscopic level.
2 functions of basal lamina
- Support/attachment for epithelial sheet 2. Molecular sieve/filter
Lamina lucida (rara)
Electron lucent layer directly below basal membrane
Lamina densa
Electron dense layer directly beneath lamina lucida
Basement membrane
Includes 2 layers of the basal lamina + a third layer called the lamina reticularis (produced by connective tissue cells)
Exocrine glands
Retain connection to surface they originated from and secrete product via ducts or duct systems.
Endocrine glands
Lose connection to epithelial surface of origin. Ductless and secrete products into blood or lymphatic vessels.
2 main types of glands
- Endocrine: secrete substances or hormones directly into blood/lymph 2. Exocrine: secrete substances via ducts Both develop from infoldings of epithelial sheets that penetrate into underlying connective tissue.
Classification of exocrine glands
Unicellular (goblet cells in respiratory and digestive tracts) or multicellular (sweat glands, sebaceous glands in scalp, gastric glands in stomach, salivary gland)
Further classification of multicellular exocrine glands
- Complexity (simple or compound) 2. Morphology (tubular, acinar, tubuloacinar)
Simple multicellular exocrine glands
Part of an organ or tissue. One duct (can be straight, branched, or coiled) which opens to surface of epithelial sheet. Sweat glands, sebaceous glands in scalp, gastric glands in stomach.
How will compound exocrine glands ALWAYS be classified?
Compound tubuloacinar
Compound multicellular exocrine glands
The organ themselves. Complex, branched, differentiated duct system (small ducts to larger and larger). Parotid salivary gland, mammory gland, pancreas.
3 morphologies of secretory portions of glands
- Tubular 2. Acinar or alveolar 3. Tubuloacinar
Tubular
Can be straight, branched, or coiled (tangled ball of yarn cut)
Acinar or alveolar
Ball of cells which can be solid or have space/lumen in the center
Tubuloacinar
Branched with acini
Acinus
Cluster of cells organized around central lumen
Parenchyma
Secretory portions and ducts in compound glands
Stroma
Connective tissue components which surround/support parenchyma in compound glands
Intralobular ducts
Will be found surrounded by acini (dark areas)
Interlobular ducts
Will be found in connective tissue (light areas)
2 cell populations in secretory regions
- Secretory cells 2. Myoepithelial cells
2 main types of secretory cells
- Mucous 2. Serous
Mucous cells
Secrete mucinogens (large glycosylated proteins)
Serous cells
Secrete solutions of proteins
Myoepithelial cells
Located surrounding groups of secretory cells within the basal lamina. Characteristics of smooth muscle cells (contractile filaments) which aids in moving secretions out of secretory portion of glands (like a hand squeezing a stress ball).
How is the type of secretion in exocrine glands determined?
By the kind of secretory cells which predominate (serous or mucous)
Invasive (infiltrating) ductal carcinoma (2 changes to cells)
Mammory gland duct cells undergo 1. Uncontrolled proliferation 2. Invasion/colonization of stroma
4 factors controlling normal epithelial cell proliferation
- Rate of proliferation = rate of apoptosis 2. Presence of committed transit amplifying daughter cells (overall increase in #) 3. Regulation by signal pathways 4. Regulation of integrin signaling (through hemidesmosomes)
How is mammory gland proliferation normally controlled vs in carcinoma?
Normal ways + hormones estrogen and progesterone during development. In carcinoma genetic mutations, gene duplications, incorrect signaling, and hormonal status affect proliferation.
Invasive cancerous epithelial cells
Able to move through or destroy basal lamina and colonize in adjacent tissues
