Lecture 2, Microstructure Of Epithelium Flashcards
Epithelia definition
Sheets of cells that cover or line internal and external body surface, form secretory glands and ducts
Functions of epithelia
Protection - all substances entering or leaving an organ must cross this tissue (ex epidermis)
Absorption (ex intestinal lining)
Secretion ( ex salivary gland)
Characteristics of epithelia
Cells tightly packed together
One free surface not in contact with other cells (faces cavity of the organ)
Attached to underlying connective tissue (CT) by an acellular basement membrane (BM)
Lack of blood vessels (derive nutrition and O2 from vessels in the lamina propria
Basement membrane definition
Mesh work of carbohydrates and proteins
Lamina propria
Connective tissue underneath an epithelium
Types of epithelium
Described by both shape and arrangement
Shape : squamous, cuboidal, columnar
Arrangement: simple, stratified, specialized
Squamous general definition
Flattened cells wider than they are tall
Cuboidal general definition
Cells relatively equal in height and width
Columnar general definition
Taller than they are wide
Simple general definition
Single layer of cells
Stratified general definition
Multiple layers of cells
Specialized general definition
Pseudostratified and transitional epithelium (kinda lumped together looking)
Simple squamous epithelium
Single layer of flattened cells (scanty cytoplasm and flatter nuclei)
Cells specialized for easy diffusion across their cytoplasm
**appear in air was of lung, lining of blood vessels, structures in kidney
Simple cuboidal epithelium
Cells have about equal physical dimensions and appear square in histo section (nucleus appears round)
Function in protection, absorption, synthesis and secretion of substances needed for the function of the organs they line
***line the collecting tubules in kidneys to participate in the reabsorption of water
Simple columnar epithelium
Taller than they are wide, rounded nuclei
Function in protection, transport of materials, absorption and secretion
Found in bronchial tubes of lungs, uterine tubes, lining of stomach and intestinal tract
***cells in lining of the stomach secrete mucus that serves as a protective barrier for nearby cells
Stratified epithelium
Composed of more than one layer of cells - only bottom layer touches BM
Nuclei in each layer aligned with each other
Classified based on the shape of the TOP layer of cells
Stratified cuboidal and columnar epithelium
Rarer type of stratified epithelium found in larger glands and male urethra
**cuboidal line ducts of sweat glands, recycle sodium and chloride ions in sweat
***columnar in male urethra protect tissue from urine by secreting mucus
Stratified squamous epithelium
Piles of flattened cells with many layers
Not as easy to see lines of nuclei (aka rows)
*most common type of stratified
Found in vocal folds, esophagus, epidermis of the skin, vagina
How is stratified squamous adapted for toughness?
**seen in areas that undergo friction
Can undergo keratinization
High turnover rate because of mito activity
Keratinization meaning
Cells synthesize keratin filaments that enhance cell toughness
Cells in upper layers may eject cellular organelles (including nuclei) —— in a histological section, cells closest to lumen lack nuclei
Specialized epithelium
Don’t fit into simple or stratified categories
Pseudostratified epithelium
Has cells of different shapes and sizes
May appear as if cells are in layer, but all cells touch the BM
Transitional epithelium
Urothelium
Found in bladder and ureter
Functions to maintain a urinary barrier to keep urine stored, even when the organ is distended
Epithelial cell polarity domains
Apical - faces lumen
Lateral - attached to other cells
Basal - opposite apical domain, in simple epithelium it is the domain that touches the BM
Apical domain
Surface facing the lumen
Contain specialized structures that add functionality
-proteins (receptors, transporters, channels) allow interactions with lumen
-other specializations increase surface area or help move substances along epithelial surface
Microvilli, cilia, stereo cilia
Microvilli
Short, finger-like projections of uniform length on apical surface
-enclosed by plasma membrane
-significantly increases SA
Appear as individual structures in TEM
**present in cells specialized for absorption
Microvilli in microscopy
In TEM - individual structures
-supported by actin filaments which insert into a network of cytoskeletal proteins (terminal web)
-glycocalyx seen as a fuzzy border
In light - look like bristles of a soft brush (brush border)
Cilia
Longer, hair like apical specializations
-not as uniform in length
-individual strands can be seen in light micro.
**specialized for motility
Ex; resp tract have cilia that flex to move fluid along surface to transport particulate to places where it can be disposed of
Structure of cilia
Internal array of microtubules arranged with one pair at the center and 9 pairs in a ring (9+2)
Dynein motor proteins attached to outer microtubule pairs
Primary ciliary dyskinesia
Recessive genetic disorder where cilia lack dynein arms (=nonmotile)
In resp. System, cilia can’t clear mucus = chronic sinusitis and bronchitis
All cilia in body are affected
Stereocilia
Specializations related to microvilli
Rarer - found in epididyis and inner ear
Not motile
Have core actin filaments that attach to terminal web
Longer, can branch
Basal domain
Area opposite apical domain
Basement membrane (BM)
Hemidesmosomes
Basal unfolding (striations)
Basement membrane
Acellular sheet of macromolecules
Functions :
Structural support for epithelium and attachment site to lamina propria
Semipermeable filter
Collagen fibers, laminins, and intergrin proteins
BM appearance microscopy
H&E - acellular pink area in basal domain of bottom row of epithelium
EM - two layers
-basal lamina - dense line, base of epithelial cells is attached here, contains molecules that act as permeability barrier
-reticular lamina -fibrous mesh that provides structures and attaches basal lamina to lamina propria
BM reticular lamina
fibrous mesh that provides structures and attaches basal lamina to lamina propria
BM basal lamina
dense line, base of epithelial cells is attached here, contains molecules that act as permeability barrier
Hemidesmosome
Junction that functions to:
Attach basal domain of cell to the basal lamina
Distribute force throughout epithelium = increases structural integrity
Consist of protein plaques that anchor to structures
Hemidesmosome protein plaque inside cell
Binds to intermediate filaments (keratin) of cytoskeleton
Hemidesmosome protein plaques outside cell
Extracellular domains of inegrins and laminins within HD bind to molecules in basal lamina
Lateral domain
Site of attachment to other cells in sheet
Avenue of transport for materials (paracellular pathway)
**Proteins here form junctions that attach cells together and regulate transport
Occluding junctions in lateral domain
Seals cells together, control transport of substances via paracellular pathway (tight junctions)
Adhering junctions in lateral domain
Mechanically attaches cells together or to extracellular matrix
Zonula adherents, macula adherens (desmosome)
Communicating junctions in lateral domain
Mediates passage of chemical or electrical signals from one cell to another
Gap junctions
Tight junctions
Zonula occludens
Important to cellular polarity & ability to constrict fluids (selective permeability)
Found in lateral domain adjacent to apical surface
Formed by transmembrane proteins Claudin and occludin
Tight junctions form seals that :
Prevent proteins from migrating to other domains
Limit transcellular transport
Zonula adherens (ZA)
Junctions that attach cells together and stabilize sheet
Forms a continuous belt (Zonula) around cell on lateral surface
Directly apposed in adjacent cells - plasma membranes held together by cadherins
Found just below tight junctions
Desmosomes (macula adherens)
Form a series of attachment points that stabilize
**important in skin
Generally below tight junctions and zona adherens
Hold adjacent plasma membranes together by desmocollin and desmoglein
Gap junctions
Junctions that permit flow of ions and small molecules
Allow for electrochemical communication between cells
N lateral domain below tight junctions and ZA
Consist of transmembrane connexin proteins that form channels between cells (connexons)
2 connexons form a gap junction *****
Order of junctions in lateral (from apical)
Tight, adherens, desmosome, gap
Hemidesmosome basal domain to basal lamina
