Cell Surface

Basement Membrane

The basement membrane is a sheetlike structure that underlies virtually all epithelia. It consists of the following:

• Basal lamina—composed of type IV collagen, glycoproteins (e.g., laminin), and proteoglycans (e.g., heparan sulfate) (Figure 1-1-9).

• Reticular lamina—composed of delicate reticular fibers.

Basal Lamina

Figure 1-1-9. Basal Lamina

Basal Lamina

Figure 1-1-9. Basal Lamina

Sealing Strands

Image copyright 1984 Lippincott Williams & Wilkins. Used with permission.

Figure 1-1-11. Freeze-Fracture Replica of a Tight Junction

Image copyright 1984 Lippincott Williams & Wilkins. Used with permission.

Figure 1-1-11. Freeze-Fracture Replica of a Tight Junction

Zonula adherens

A zonula adherens (adherent junction) often lies basal to the zonula occludens (Figure 1-1-10). It is a bandlike junction that serves in the attachment of adjacent epithelial cells.

Desmosome

The desmosome (macula adherens) is formed by the juxtaposition of two disk-shaped plaques contained within the cytoplasm of each adjacent cell (Figure 1-1-12).

• Intermediate filaments (tonofilaments) radiate away from the plaques (not seen in Figure I-1-11). These intermediate filaments are anchored by desmoplakins (plaques) that also bind to transmembrane linker proteins, lijakLng adjacent cells.

• Desmosomes are most common in lining membranes, are subject to wear and tear, and are considered spot welds that hold cells together.

Intermediate Density

Intermediate Density

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Figure 1-1-12. Desmosome

Copyright 2000 Gold Ständard Multimedia, Inc. All rights reserved.

Figure 1-1-12. Desmosome

Gap junction

The gap junction is an area of communication between adjacent cells that allows the passage of very small particles and Ions across a small intercellular gap within the junction (Figure 1-1-13).

The gap junction consists of a hexagonal lattice of tubular protein subunits called connexons, which form hydrophilic channels connecting the cytoplasm of adjacent cells (Figure 1-1-14). This permits the direct passage of ions and small molecules between cells to conduct electrical impulses.

Image copyright 1964 Lippincott Williams & WJIklns. Used with permission.

Figure 1-1-13. Freeze-Fracture Replica of a Gap (Communicating) Junction (CJ)

Image copyright 1964 Lippincott Williams & WJIklns. Used with permission.

Figure 1-1-13. Freeze-Fracture Replica of a Gap (Communicating) Junction (CJ)

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