FIGURE 5.23 • Atomic force microscopic (AFM) image of a gap junction. This image shows the extracellular surface of a plasma membrane preparation from the HeLa cell line. Multiple copies of the connexin-26 gene were incorporated into the HeLa cell genome to achieve overexpression of the connexin protein. Connexin-26 proteins self-assemble into functional gap junctions, and they were observed with AFM in two different buffer solutions. a. Gap junction containing individual connexons in a calcium-free buffer solution. 500,000. Inset shows a single connexon at higher magnification. Note the clear profiles of individual connexin molecules assembled into the connexon. The open profile of the channel is also visible. 2,000,000. b. The same preparation of connexons in a buffer containing Ca2 . 500,000. Inset: Note that the conformational change of the connexin molecules has caused the channel to close and has reduced the height of the connexon. 2,000,000. (Courtesy of Dr. Gina E. Sosinsky.) gallbladder epithelium. In active fluid transport, sodium ions are pumped out of the cytoplasm at the lateral plasma membrane by NaK-ATPase localized in that membrane. Anions then diffuse across the membrane to maintain electrical neutrality, and water diffuses from the cytoplasm into the intercellular space, driven by the osmotic gradient between the salt concentration in the intercellular space and the concentration in the cytoplasm. The intercellular space distends because of the accumulating fluid moving across the epithelium, but it can distend only to a limited degree because of junctional attachments in the apical and basal portions of the cell. Hydrostatic pressure gradually builds up in the intercellular space and drives an essentially isotonic fluid from the space into the underlying connective tissue. The occluding junction at the apical end of the inter-cellular space prevents fluid from moving in the opposite direction. As the action of the sodium pump depletes the cytoplasm of salt and water, these are replaced by diffusion across the apical plasma membrane, whose surface area is greatly increased by the presence of microvilli, thus allowing the continuous movement of fluid from the lumen to the connective tissue as long as the Na/K-ATPase is active.
The basal domain of epithelial cells is characterized by several features:  The basement membrane is a specialized structure lo cated next to the basal domain of epithelial cells and the underlying connective tissue stroma.
 Cell-to-extracellular matrix junctions anchor the cell to the extracellular matrix; they are represented by focal adhesions and hemidesmosomes.  Basal cell membrane infoldings increase the cell sur adjacent cells and extracellular matrix proteins.
The term basement membrane was originally given to an amorphous, dense layer of variable thickness at the basal surfaces of epithelia. Although a prominent structure referred to as basement membrane is observed with hematoxylin and eosin (H&E) stain in a few locations such as the trachea (Fig. 5.25) and, occasionally, the urinary bladder and ureters, basement membrane requires special staining to be seen in the light microscope. This requirement is caused, in part, by its thinness and by the effect of the eosin stain, which makes it indistinguishable from the immediately adjacent connective tissue. In the trachea, the structure that is often described as basement membrane includes not only the true basement membrane but also an additional layer of closely spaced and aligned collagen fibrils that belong to the connective tissue.