Cell Surface

Cell Surface

Basement Membrane

• Sheetlike structure that underlies all the epithelia

• Consists of:

  • Basal lamina

    • Composed of: type IV collagen, glycoproteins, and proteoglycan

  • Reticular lamina

    • Composed of delicate reticular fibrers

• Functions:

  • Forms a barrier between epithelium and connective tissue. Normally:

    • Lymphocytes pass through it during immune surveillance

    • Cancer cells pass through it during invasion

  • Serves as filter (e.g. renal glomuruli)

  • Plays a role in regeneration (epithelial, nervous and muscle cells use it as a scaffolding during regeneration)

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Lateral Surface specializations

A. Zonula occludens (Tight Junction):

• Formed by fusion of opposed cell membranes

• Extends completely around the apical cells to seal the underlying intercellular clefts from contact with outside environment

• May be leaky or tight

• Can be rapidly formed & disassembled e.g. in leukocyte migration across endothelium

B. Zonula Adharens (Intermediate Junction):

• Band like junction that serves in the attachment of adjacent epithelial cells.

• Often lies basal to zonula occludens, surround the perimeter below zonula occludens.

C. Desmosomes:

• Small discrete attachment

• Formed by juxtaposition of two disk-shaped plaques (desmolakins) in the cytoplasm of each adjacent cell.

• Tonofilaments (intermediate filaments) radiate away from the plaques

• Are common in lining membranes
  

D. Gap Junction (Nexus):

• Are subject to wear and tear
• Are considered spot welds that hold cells together

• Area of communication between adjacent cells.

• Allow very small particles and ions across a small intercellular gap within the junction. This allows conduction of electrical impulses

• Consists of a hexagonal lattice of tubular proteins, connexons which form hydrophilic channels connecting cytolplasm of adjacent cells.

Apical Surface Specialization

A. Microvilli:

• Are apical cell surface evaginations of cell membranes

• Core of each microvillus contains actin microfilaments.

• Is anchored to the terminal web in cytoplasm which itself is anchored to zonula adharens

• Function: increase the cell surface area available for absorption.

B. Stereocilia:

• Are elongated microvilli

• Found at the apices of cells lining the epididymis, ductus deferens and hair cells

C. Cilia:

• Are apical cell surface projections that contain microtubules

• Are inserted on centriole-like basal bodies present below the membrane surface

• Contain two central microtubules surrounded by a circle of nine peripheral microtubule doublets.

• The peripheral doublets are fused so that they share a common tubule wall and form 2 subunits, A & B.

• Adjacent doublets are linked to one another by Nexin links

• A pair of dynein arms is attached to each A subtubule.

• The arms bind to ATP and rearrange themselves so that a binding site for the B tubule on the tip of arm is exposed.

• B tubule interacts with the binding site, causing arm to snap back and movement to occur.

• Each cycle of a dynein arm slides adjacent doublets by 10 nm past each other.

• Function: cilia move back and forth to propel fluid and particles in one direction. Important in clearing mucus from the respiratory tract.

D. Flagella:

• Are longer than cilia but have the same microstructure

• Example: sperm