Apical Cell Surface Specializations and Cellular Junctions PDF

Apical Cell Surface Specializations Microvilli: o Structure: Finger-like, stable, uniform projections. o Core: Actin filaments with actin-binding proteins attached to terminal web. o Location: Small intestine, kidney tubules. o Function: Increases...

Apical Cell Surface Specializations Microvilli: o Structure: Finger-like, stable, uniform projections. o Core: Actin filaments with actin-binding proteins attached to terminal web. o Location: Small intestine, kidney tubules. o Function: Increases surface area for absorption (20-30 times). o Microscopy: Appears as striations or brush border (L/M); detailed actin core (E/M). Cilia: o Structure: Highly motile, longer than microvilli. o Core: Microtubules (9 doublets+2 arrangement) with dynein arms for movement. o Location: Upper respiratory tract, female genital tract. o Function: Moves fluids or particles along the surface. o Microscopy: Hair-like on cell surface (L/M); basal body and shaft (E/M). Flagella: o Structure: Long, single, similar to cilia. o Core: Similar to cilia's axoneme structure. o Location: Sperm cells (forms tail). o Function: Facilitates cell movement. Cellular Junctions Tight (Zonula Occludens): o Location: Apical position in epithelial cells. o Structure: Bands encircling each cell; no intercellular space. o Proteins: Claudin and occludin. o Function: Prevents material flow between cells, provides a protective barrier. o Medical Significance: Defects may affect the blood-brain barrier, leading to neurological disorders. Adherent (Zonula Adherens): o Location: Below tight junctions, encircling cells. o Structure: Continuous band around cells connected to actin cytoskeleton. o Proteins: Cadherins. o Function: Strengthens and stabilizes nearby tight junctions. Desmosomes (Macula Adherens): o Location: Disk-shaped on adjacent cell surfaces. o Structure: Spot-like, binds intermediate filaments. o Proteins: Desmogleins, desmocollins (cadherin family). o Function: Maintains cell integrity and cohesion. o Medical Significance: Autoimmune reactions can cause skin blistering. Hemidesmosomes: o Location: Binds epithelial cells to basement membrane. o Structure: Half of a desmosome. o Proteins: Integrins. o Function: Anchors cells to the basement membrane. Gap Junctions (Nexuses): o Location: Throughout cell membranes. o Structure: Channels between adjacent cells, formed by connexins. o Function: Allows rapid exchange of small molecules for cell coordination and communication.