Individuals with the autosomal recessive condition known as familial hypomagnesemia, hypercalcuria, and nephrocalcinosis (FHHNC) have mutations in the gene that codes for claudin16. Claudin16 is located in the tight junction of the thick ascending portion of Henle's loop in the kidneys. It functions as a pathway for the paracellular reabsorption of Ca++ and Mg++ from the tubular fluid. In FHHNC, individuals lack functional copies of claudin16, resulting in reduced reabsorption of these divalent ions. This leads to hypomagnesemia, hypercalcuria, and nephrocalcinosis.

The apical membrane of epithelial cells has various specializations. Microvilli are small projections that increase the surface area of the apical membrane. They can be found in the epithelial cells of the small intestine and have a core structure primarily composed of actin. Stereocilia, on the other hand, are longer nonmotile membrane projections that also increase the surface area of the apical membrane. They can be up to 120 µm in length and are found in the epididymis of the male reproductive tract and the "hair cells" of the inner ear. Similar to microvilli, stereocilia contain actin filaments and accessory proteins.

Cilia are another feature of the apical membrane. They can be either motile or non-motile. Motile cilia have a characteristic arrangement of microtubules and are driven by the molecular motor dynein. These cilia are found in the epithelial cells lining the respiratory tract and play a role in mucociliary transport. Nonmotile cilia, on the other hand, serve as mechanoreceptors and are involved in embryological development and fluid flow sensing in the nephron of the kidneys. Only a single nonmotile cilium is present in the apical membrane of cells and lacks a motor protein.

The tight junction effectively separates the plasma membrane of an epithelial cell into two domains: the apical surface and the basolateral surface. The basolateral membrane of many epithelial cells is folded or invaginated to increase the membrane surface area. This is particularly seen in epithelial cells with high transport rates, allowing for the accommodation of a large number of membrane transporters such as Na+,K+-ATPase.
Mutationsinthegenethatcodesforclaudin16resultintheautosomalrecessiveconditionknowasfamilial hypomagnesemia, hypercalcuria, and nephrocalcinosis (FHHNC).Claudin16isfoundinthetightjunctionofthethickascendingportionofHenle’sloopinthekidneysandservesasaroutefortheparacellularreabsorptionofCa++ andMg++ fromthetubularfluid.IndividualswithFHHNClackfunctionalcopiesofclaudin16,andreabsorptionofthesedivalentionsisthusreduced,whichleadstohypomagnesemia,hypercalcuria,andnephrocalcinosis.
CHAPTER 2 Homeostasis: Volume and Composition of Body Fluid Compartments 29 • Fig. 2.9 Illustration of Apical Membrane Specializations of Epithelial Cells (Not Drawn to Scale). A, Microvilli 1 to 3 µm in length serve to increase the surface area of the apical membrane (e.g., those of the epithelial cells of the small intestine). B, Stereocilia can be up to 120 µm in length (e.g., those of the epididymis of the male reproductive tract). Both microvilli and stereocilia have a core structure composed primarily of actin, with a number of associated proteins. Both are nonmotile. (Redrawn from Pawlina, W. Histology: A Text and Atlas, with Correlated Cell and Molecular Biology. 7th ed. Philadelphia: Wolters Kluwer Health, 2016.) the base of the microvilli) and provides structural support for the microvilli. Another surface feature is stereocilia (see
Fig. 2.9B ). Stereocilia are long (up to 120 µm), nonmotile membrane projections that, like microvilli, increase the surface area of the apical membrane. They are found in the epididymis of the testis and in the “hair cells” of the inner ear. Their core also contains actin filaments and accessory proteins.
A third apical membrane feature is cilia (
Fig. 2.10 ). Cilia may be either motile (called secondary cilia) or non-motile (called primary cilia). The motile cilia contain a microtubule core arranged in a characteristic “9+2” pattern (nine pairs of microtubules around the circumference of the cilium, and one pair of microtubules in the center). Dynein is the molecular motor that drives the movement of the cilium. Motile cilia are characteristic features of the epithelial cells that line the respiratory tract. They pulsate in a synchronized manner and serve to transport mucus and inhaled particulates out of the lung, a process termed mucociliary transport (see
Chapter 26).
Nonmotile cilia serve as mechanoreceptors and are involved in determining left-right asymmetry of organs during embryological development, as well as sensing the flow rate of fluid in the nephron of the kidneys (see ). Only a single nonmotile cilium is found in the apical membrane of cells. Nonmotile cilia have a microtubule core (“9+0” arrangement) and lack a motor protein.
As noted previously, the tight junction effectively divides the plasma membrane of an epithelial cell into two domains: an apical surface and a basolateral surface. The basolateral membrane of many epithelial cells is folded or invaginated. This is especially so for epithelial cells that have high transport rates. These invaginations serve to increase the membrane surface area to accommodate the large number of membrane transporters (e.g., Na+,K+-ATPase) needed in the membrane.