All cells are constructed of the same basic elements and share the same basic metabolic and biosynthetic machinery. What distinguishes one cell type from another? Certainly, cells have different shapes and molecular structures. In addition, out of an extensive repertory of molecules that cells are capable of making, each cell type chooses which molecules to express, how to organize these molecules, and how to regulate them. It is this combination of choices that endows them with specific physiological functions. These specializations are the product of cell differentiation. Each of these cell types arises from a stem cell. Stem cells are mitotically active and can give rise to a spectrum of cellular lineages that can range from multiple to limited. Thus, they may be pluripotent, multipotent, oligopotent, or unipotent. Clearly, the zygote is the ultimate stem cell because through its divisions, it gives rise to every cell lineage present in the complete organism and is thus totipotent. Specific cell types arise from stem cells by activating a differentiation-specific program of gene expression. The interplay of environmental signals, temporal cues, and transcription factors that control the processes of cellular differentiation constitutes one of the great unraveling mysteries of modern biology.
Epithelial cells form a barrier between the internal and external milieu
How can an organism tightly regulate its internal fluid environment (i.e., internal milieu) without allowing this environment to come into direct and disastrous contact with the external world (i.e., external milieu)? The body has solved these problems by arranging a sheet of cells—an epithelium—between these two disparate environments. Because of their unique subcellular designs and intercellular relationships, epithelial cells form a dynamic barrier that can import or expel substances, sometimes against steep concentration gradients.
Two structural features of epithelia permit them to function as useful barriers (Fig. 2-23). First, epithelial cells connect to one another via tight junctions, which constrain the free diffusion of solutes and fluids around the epithelial cells, between the internal and external compartments. Second, the tight junctions define a boundary between an apical and a basolateral domain of the plasma membrane. Each of these two domains is endowed with distinct protein and lipid components, and each subserves a distinct function. Thus, the surface membranes of epithelial cells are polarized. Beginning on page 136, we discuss the mechanisms by which polarized epithelial cells exploit their unique geometry to transport salts and water from one compartment to the other. However, it is worth touching on a few of the cellular specializations that characterize polarized epithelia and permit them to perform their critical roles.
FIGURE 2-23 Epithelial cells. In an epithelial cell, the tight junction separates the cell membrane into apical and basolateral domains that have very different functional properties.
The apical membranes of the epithelial cells (see Fig. 2-23) face the lumen of a compartment that is often topologically continuous with the outside world. For example, in the stomach and intestine, apical membranes form the inner surface of the organs that come into contact with ingested matter. The apical membranes of many epithelial cells, including those lining kidney tubules, are endowed with a single nonmotile cilium. Known as the central cilium, this structure may sense the mechanical deformation associated with fluid flow. Mutations that disrupt individual components of the central cilium are associated with cystic disease of the kidney, in which the normal architecture of the kidney is replaced by a collection of large fluid-filled cysts.
The basolateral membranes of epithelial cells face the ECF compartment—which indirectly makes contact with the blood—and rest on a basement membrane. The basement membrane is composed of extracellular matrix proteins that the epithelial cells themselves secrete and include collagens, laminin, and proteoglycans. The basement membrane provides the epithelium with structural support and, most importantly, serves as an organizing foundation that helps the epithelial cells to establish their remarkable architecture.
Each epithelial cell is interconnected to its neighbors by a variety of junctional complexes (see Fig. 2-23). The lateral surfaces of epithelial cells participate in numerous types of cell-cell contacts, including tight junctions, adhering junctions, gap junctions, and desmosomes.
A tight junction (or zonula occludens) is a complex structure that impedes the passage of molecules and ions between the cells of the epithelial monolayer. This pathway between the cells is termed the paracellular pathway. The functional properties of tight junctions are related to their intriguing architecture (see Fig. 2-23). Viewed by transmission electron microscopy, tight junctions include regions of apparent fusion between the outer leaflets of the lipid bilayer membranes of neighboring epithelial cells. Freeze-fracture electron microscopy reveals that the tight junction comprises parallel strands of closely packed particles that represent the transmembrane proteins participating in the junction's formation. The degree of an epithelium's impermeability—or “tightness”—is roughly proportional to the number of these parallel strands. The claudins, a large family of proteins, are the principal structural elements of the tight junction. Interactions between the claudins present in the apposing membranes of neighboring cells form the permeability barrier (see p. 137).
Tight junctions play several roles. First, they are barriers in that they separate one compartment from another. In some epithelial cells, such as those of the renal thick ascending limb, the tight junctions form a boundary that blocks the flow of most ions and water between cells. In contrast, the tight junctions of the renal proximal tubule are leaky, permitting significant transepithelial movement of fluid and solutes.
Second, tight junctions can act as selective gates in that they permit certain solutes to flow more easily than others. Examples are the leaky tight junctions of tissues such as the proximal tubule. The permeability and selectivity of an epithelium's tight junctions are critical variables for determining that epithelium's transport characteristics (see pp. 136–137). Moreover, the permeability properties of the gate function of tight junctions can be modulated in response to various physiological stimuli. The inventory of claudins expressed by an epithelium in large measure determines the permeability properties of its tight junctions.
Third, tight junctions act as fences that separate the polarized surfaces of the epithelial plasma membrane into apical and basolateral domains. The presence of distinct populations of proteins and lipids in each plasma-membrane domain is absolutely essential for an epithelium to mediate transepithelial fluid and solute transport (see pp. 137–139).
An adhering junction (or zonula adherens) is a belt that encircles an entire epithelial cell just below the level of the tight junction. Epithelial cells need two pieces of information to build themselves into a coherent epithelium. First, the cells must know which end is up. The extracellular matrix (see above) provides this information by defining which side will be basolateral. Second, the cells must know that there are like neighbors with which to establish cell-cell contacts. Adhering junctions provide epithelial cells with clues about the nature and proximity of their neighbors. These cell-cell contacts are mediated by the extracellular domains of members of the cadherin family, transmembrane proteins discussed above. Epithelial cells will organize themselves into a properly polarized epithelium—with differentiated apical and basolateral plasma membranes—only if the cadherins of neighboring cells have come into close enough apposition to form an adhering junction. Formation of these junctions initiates the assembly of a subcortical cytoskeleton, in which anchor proteins (e.g., vinculin, catenins, α actinin) link the cytosolic domains of cadherins to a network of actin filaments that is associated with the cytosolic surfaces of the lateral membranes. Conversely, the disruption of adhering junctions can lead to a loss of epithelial organization. In epithelial tumors, for example, loss of expression of the adhering-junction cadherins tends to correlate with the tumor cell's loss of controlled growth and with its ability to metastasize—that is, to leave the epithelial monolayer and form a new tumor at a distant site in the body. N2-13
Role of Cell-Cell Adhesion Molecules in Development
Contributed by Michael Caplan
Formation of the first polarized epithelial cells in the developing mammalian embryo occurs when the adhesion proteins are synthesized and adhering junctions form. If these cell-cell interactions become disrupted during early embryogenesis, the embryonic cells separate from one another, preventing further development.
The channels that interconnect the cytosols of neighboring cells are called gap junctions (see pp. 158–159). They allow small molecules (less than ~1 kDa) to diffuse freely between cells. In some organs, epithelial cells are interconnected by an enormous number of gap junctions, which organize into paracrystalline hexagonal arrays. Because ions can flow through gap junctions, cells that communicate through gap junctions are electrically coupled. The permeability of gap junctions, and hence the extent to which the cytoplasmic compartments of neighboring cells are coupled, can be regulated in response to a variety of physiological stimuli.
A desmosome (or macula adherens) holds adjacent cells together tightly at a single, round spot. Desmosomes are easily recognized in thin-section electron micrographs by the characteristic dense plaques of intermediate filaments (see p. 23). The extracellular domains of desmoglein or desmocollin, which are transmembrane proteins in the cadherin family (see p. 17) mediate the interaction of adjacent cells. Anchor proteins link the cytosolic domains of the cadherins to intermediate filaments that radiate into the cytoplasm from the point of intercellular contact (see Fig. 2-23). These filaments interact with and organize the cytoplasmic intermediate filaments, thus coupling the structurally stabilizing elements of neighboring cells to one another. Epithelial cells are often coupled to adjacent cells by numerous desmosomes, especially in regions where the epithelium is subject to physical stress.
Epithelial cells are polarized
In many epithelia, the apical surface area is amplified by the presence of a brush border that is composed of hundreds of finger-like microvillar projections (see Fig. 2-23). In the case of the small intestine and the renal proximal tubule, the membrane covering each microvillus is richly endowed with enzymes that digest sugars and proteins as well as with transporters that carry the products of these digestions into the cells. The presence of a microvillar brush border can amplify the apical surface area of a polarized epithelial cell by as much as 20-fold, thus greatly enhancing its capacity to interact with, to modify, and to transport substances present in the luminal fluid.
The basolateral surface area of certain epithelial cells is amplified by the presence of lateral interdigitations and basal infoldings (see Fig. 2-23). Although they are not as elegantly constructed as microvilli, these structures can greatly increase the basolateral surface area. In epithelial cells that are involved in large volumes of transport—or in transport against steep gradients—amplifying the basolateral membrane can greatly increase the number of Na-K pumps that a single cell can place at its basolateral membrane.
Although the morphological differences between apical and basolateral membranes can be dramatic, the most important distinction between these surfaces is their protein composition. As noted above, the “fence” function of the tight junction separates completely different rosters of membrane proteins between the apical and basolateral membranes. For example, the Na-K pump is restricted to the basolateral membrane in almost all epithelial cells, and the membrane-bound enzymes that hydrolyze complex sugars and peptides are restricted to apical membranes in intestinal epithelial cells. The polarized distribution of transport proteins is absolutely necessary for the directed movement of solutes and water across epithelia. Furthermore, the restriction of certain enzymes to the apical domain limits their actions to the lumen of the epithelium and therefore offers the advantage of not wasting energy putting enzymes where they are not needed. The polarity of epithelial membrane proteins also plays a critical role in detecting antigens present in the external milieu and in transmitting signals between the external and internal compartments.
The maintenance of epithelial polarity involves complex intermolecular interactions. When tight junctions are disrupted, diffusion in the plane of the membrane leads to intermingling of apical and basolateral membrane components and thus a loss of polarity. The subcortical cytoskeleton beneath the basolateral surface may play a similar role in maintaining polarity by physically restraining a subset of membrane proteins at the basolateral surface.
However, such mechanisms for stabilizing the polarized distributions of membrane proteins do not explain how newly synthesized proteins come to be distributed at the appropriate plasma membrane. We give two examples of mechanisms that cells can use to direct membrane proteins to either the basolateral or apical membrane. The first example focuses on protein-protein interactions. As noted during our discussion of the secretory protein pathway, the sorting operation that separates apically from basolaterally directed proteins apparently occurs in the TGN (see p. 39). Some proteins destined for the basolateral membrane have special amino-acid motifs that act as sorting signals. Some of these motifs are similar to those that allow membrane proteins to participate in endocytosis. Members of the adaptin family recognize these motifs during the formation of clathrin-coated vesicles at the TGN and segregate the basolateral proteins into a vesicle destined for the basolateral membrane.
Another example of mechanisms that cells use to generate a polarized distribution of membrane proteins focuses on lipid-lipid interactions. In many epithelia, GPI-linked proteins are concentrated exclusively at the apical surface. It appears that the phospholipid components of GPI-linked proteins are unusual in that they cluster into complexes of fairly immobile gel-phase lipids during their passage through the Golgi apparatus. We saw above how lakes of phospholipids with different physical properties may segregate within a membrane (see p. 10). The “glycolipid rafts” of GPI-linked proteins incorporate into apically directed vesicles so that sorting can occur through lipid-lipid interactions in the plane of the membrane rather than through protein-protein interactions at the cytoplasmic surface of the Golgi membrane. From these two examples, it should be clear that a number of different mechanisms may contribute to protein sorting and the maintenance of epithelial polarity.