1-48 Epithelium Histology Flashcards
Epithelium (3 major characteristics)
- 1 of 4 fundamental body tissues
- covers or lines the free surfaces of the body and provides and interface between various biological compartments
- anchored to a basement membrane that rests upon an underlying connective tissue
- Avascular: rely on diffusion of nutrients and gases from the vascularture in the adjacent connective tissue
- tightly connect to one another at their lateral surface via specialized junctional complexes
- each plasma membrane is subdivided into 3 regions, specializeations of these regions give rise to the function of the specific type of epithelium
- apical: faces lumen, often microvilli
- lateral: side of the cell that touches neighbor epithelium, gap jxns
- basal: on top of basement membrane
- derived from all 3 layers of embryo: ecto, meso, endo derms
Simple squamous epithelium
- parietal layer of bowman’s capsule
- lines blood vessels
- lines body cavities
- pericardium
- pleura
- peritoneum
- capsule of solid organs
- lung alveolar walls
Simple cuboidal epithelium
- renal tubules
- thyroid follicales
- glands
Simple columnar
- small intestine
- terminal/small airways of respiratory system: bronchioles
Pseudostratified epithelium
- trachea (large airways of resp tract)
- epidydimus
stratified squamous epithelium (keratinized vs non keratinized)
Non-keratinized:
- esophagus
- oral cavity
- vagina
- anal canal
- all nucleated (non-keritinized)
Keratinized: flaky dry layers without nuclei, dead
- skin
Transitional epithelium
- two states
- relaxed: many layers, all nucleated, thick
- expands: stretch out to a few layers
- urinary tract (bladder)
Epithelium apical features (3)
- Microvilli: numerous small porcesses that project together from the surface to increase area
- intertinal brush border
- proximal convulted tubule of kidney
- Sterocilia: unusually long, thin, non-motile microvilli
- epididymis
- ductus deferencs
- Cilia: actively motile processes with a complex internal structure, helps move particles along
- respiratory tract
- femal reproductive tract
- 9 outer microtubule doublets and 2 inner microtuble singlets
Apical junction complex
gap junctions
Lateral domain of epithelial cells
- tight junction (zonula occludens): block the movement of extracellular molecules between adjacent cells and maintain the polarity of the cell membrane. For example, the tight junctions located beneath the luminal surface of the simple columnar epithelium of the intestine prevent the contents of the gut from penetrating the cell lining. Tight junctions are composed of proteins known as claudins and occludins.
- adherens junction (zonula adherens): provide strength to the epithelium by linking the actin cytoskeletons of adjacent cells. They are located just beneath the zonula occludens and form a continuous contractile band around the cell. The zonula adherens is composed of transmembrane proteins called cadherins, whose cytoplasmic tails bind to anchor proteins in an intracellular plaque.
- desmosome (macula adherens): typically located deep to adhering junctions and are abundant in stratified epithelia exposed to stress. Cadherins are also the transmembrane proteins of desmosomes, but the intracellular segment binds to intermediate filaments instead of actin.
Gap junctions: provide a direct pore between two closely apposed membranes. They allow for the passage of small molecules and nutrients between adjacent cells in order to coordinate the functions of the epithelium. They are composed of transmembrane connexin proteins that together form connexons. Multiple connexons from two adjacent cells align to form the gap junction.
Apical junction complex: Tight Junctions
Lateral domain of epithelial cells
1. tight junction (zonula occludens): block the movement of extracellular molecules between adjacent cells and maintain the polarity of the cell membrane. For example, the tight junctions located beneath the luminal surface of the simple columnar epithelium of the intestine prevent the contents of the gut from penetrating the cell lining. Tight junctions are composed of proteins known as claudins and occludins.
Apical junction complex: Adherens junction
Lateral domain of epithelial cells
2. adherens junction (zonula adherens): provide strength to the epithelium by linking the actin cytoskeletons of adjacent cells. They are located just beneath the zonula occludens and form a continuous contractile band around the cell. The zonula adherens is composed of transmembrane proteins called cadherins, whose cytoplasmic tails bind to anchor proteins in an intracellular plaque.
Apical junction complex: Desmosome
Lateral domain of epithelial cells
3. desmosome (macula adherens): typically located deep to adhering junctions and are abundant in stratified epithelia exposed to stress. Cadherins are also the transmembrane proteins of desmosomes, but the intracellular segment binds to intermediate filaments instead of actin.
gap junctions
Lateral domain of epithelial cells
Gap junctions: provide a direct pore between two closely apposed membranes. They allow for the passage of small molecules and nutrients between adjacent cells in order to coordinate the functions of the epithelium. They are composed of transmembrane connexin proteins that together form connexons. Multiple connexons from two adjacent cells align to form the gap junction.
Basal domain:
thin non-cellular layer, intervenes between the epithelium and the connective tissue. This membrane is 30 to 60 nanometers thick and made up of collagenous and non-collagenous glycoproteins and proteoglycans. It provides structural support for the epithelium and serves as a selective barrier for the movement of materials to and from the supporting tissue
- Hemidesmosome: A variant of the intercellular desmosomes and anchor the cell to the basement membrane. Each acts as a cell surface attachment site for keratin intermediate filaments. The transmembrane proteins are known as integrins, which bind to a matrix of extracellular laminins (type IV collagen) in the basement membrane.
