Occluding Junctions Form a Selective Permeability Barrier Across Epithelial Cell Sheets

All epithelia have at least one important function in common: they serve as selective permeability barriers, separating fluids on either side that have a different chemical composition. This function requires that the adjacent cells be sealed together by occluding junctions. Tight junctions have this barrier role in vertebrates, as we illustrate by considering the epithelium of the mammalian small intestine, or gut.

The epithelial cells lining the small intestine form a barrier that keeps the gut contents in the gut cavity, the lumen. At the same time, however, the cells must transport selected nutrients across the epithelium from the lumen into the extracellular fluid that permeates the connective tissue on the other side (see Figure 19-1). From there, these nutrients diffuse into small blood vessels to provide nourishment to the organism. This transcellular transport depends on two sets of membrane-bound membrane transport proteins. One set is confined to the apical surface of the epithelial cell (the surface facing the lumen) and actively transports selected molecules into the cell from the gut. The other set is confined to the basolateral (basal and lateral) surfaces of the cell, and it allows the same molecules to leave the cell by facilitated diffusion into the extracellular fluid on the other side of the epithelium. To maintain this directional transport, the apical set of transport proteins must not be allowed to migrate to the basolateral surface of the cell, and the basolateral set must not be allowed to migrate to the apical surface. Furthermore, the spaces between epithelial cells must be tightly sealed, so that the transported molecules cannot diffuse back into the gut lumen through these spaces (Figure 19-2).

Figure 19-2

The role of tight junctions in transcellular transport. Transport proteins are confined to different regions of the plasma membrane in epithelial cells of the small intestine. This segregation permits a vectorial transfer of nutrients across the epithelium (more...)

The tight junctions between epithelial cells are thought to have both of these roles. First, they function as barriers to the diffusion of some membrane proteins (and lipids) between apical and basolateral domains of the plasma membrane (see Figure 19-2). Mixing of such proteins and lipids occurs if tight junctions are disrupted, for example, by removing the extracellular Ca²⁺ that is required for tight junction integrity. Second, tight junctions seal neighboring cells together so that, if a low-molecular-weight tracer is added to one side of an epithelium, it will generally not pass beyond the tight junction (Figure 19-3). This seal is not absolute, however. Although all tight junctions are impermeable to macromolecules, their permeability to small molecules varies greatly in different epithelia. Tight junctions in the epithelium lining the small intestine, for example, are 10,000 times more permeable to inorganic ions, such as Na⁺, than the tight junctions in the epithelium lining the urinary bladder. These differences reflect differences in tight junction proteins that form the junctions.

Figure 19-3

The role of tight junctions in allowing epithelia to serve as barriers to solute diffusion. (A) The drawing shows how a small extracellular tracer molecule added on one side of an epithelium cannot traverse the tight junctions that seal adjacent cells (more...)

Epithelial cells can transiently alter their tight junctions to permit an increased flow of solutes and water through breaches in the junctional barriers. Such paracellular transport is especially important in the absorption of amino acids and monosaccharides from the lumen of the intestine, where their concentration can increase enough after a meal to drive passive transport in the desired direction.

When tight junctions are visualized by freeze-fracture electron microscopy, they seem to be composed of a branching network of sealing strands that completely encircles the apical end of each cell in the epithelial sheet (Figure 19-4A and B). In conventional electron micrographs, the outer leaflets of the two interacting plasma membranes are seen to be tightly apposed where sealing strands are present (Figure 19-4C). The ability of tight junctions to restrict the passage of ions through the spaces between cells is found to increase logarithmically with increasing numbers of strands in the network, suggesting that each strand acts as an independent barrier to ion flow.

Figure 19-4

The structure of a tight junction between epithelial cells of the small intestine. The junctions are shown (A) schematically, (B) in a freeze-fracture electron micrograph, and (C) in a conventional electron micrograph. Note that the cells are oriented (more...)

Each tight junction sealing strand is composed of a long row of transmembrane adhesion proteins embedded in each of the two interacting plasma membranes. The extracellular domains of these proteins join directly to one another to occlude the intercellular space (Figure 19-5). The major transmembrane proteins in a tight junction are the claudins, which are essential for tight junction formation and function and differ in different tight junctions. A specific claudin found in kidney epithelial cells, for example, is required for Mg²⁺ to be resorbed from the urine into the blood. A mutation in the gene encoding this claudin results in excessive loss of Mg²⁺ in the urine. A second major transmembrane protein in tight junctions is occludin, the function of which is uncertain. Claudins and occludins associate with intracellular peripheral membrane proteins called ZO proteins (a tight junction is also known as a zonula occludens), which anchor the strands to the actin cytoskeleton.

Figure 19-5

A current model of a tight junction. (A) This drawing shows how the sealing strands hold adjacent plasma membranes together. The strands are composed of transmembrane proteins that make contact across the intercellular space and create a seal. (B) This (more...)

In addition to claudins, occludins, and ZO proteins, several other proteins can be found associated with tight junctions. These include some that regulate epithelial cell polarity and others that help guide the delivery of components to the appropriate domain of the plasma membrane. Thus, the tight junction may serve as a regulatory center to help in coordinating multiple cell processes.

In invertebrates, septate junctions are the main occluding junction. More regular in structure than a tight junction, they likewise form a continuous band around each epithelial cell. But their morphology is distinct because the interacting plasma membranes are joined by proteins that are arranged in parallel rows with a regular periodicity (Figure 19-6). A protein called Discs-large, which is required for the formation of septate junctions in Drosophila, is structurally related to the ZO proteins found in vertebrate tight junctions. Mutant flies that are deficient in this protein not only lack septate junctions but also develop epithelial tumors. This observation suggests that the normal regulation of cell proliferation in epithelial tissues may depend, in part, on intracellular signals that emanate from occluding junctions.

Figure 19-6

A septate junction. A conventional electron micrograph of a septate junction between two epithelial cells in a mollusk. The interacting plasma membranes, seen in cross section, are connected by parallel rows of junctional proteins. The rows, which have (more...)

Anchoring Junctions Connect the Cytoskeleton of a Cell Either to the Cytoskeleton of Its Neighbors or to the Extracellular Matrix

The lipid bilayer is flimsy and cannot by itself transmit large forces from cell to cell or from cell to extracellular matrix. Anchoring junctions solve the problem by forming a strong membrane-spanning structure that is tethered inside the cell to the tension-bearing filaments of the cytoskeleton (Figure 19-7).

Figure 19-7

Anchoring junctions in an epithelium. This drawing illustrates, in a very general way, how anchoring junctions join cytoskeletal filaments from cell to cell and from cells to the extracellular matrix.

Anchoring junctions are widely distributed in animal tissues and are most abundant in tissues that are subjected to severe mechanical stress, such as heart, muscle, and epidermis. They are composed of two main classes of proteins (Figure 19-8). Intracellular anchor proteins form a distinct plaque on the cytoplasmic face of the plasma membrane and connect the junctional complex to either actin filaments or intermediate filaments. Transmembrane adhesion proteins have a cytoplasmic tail that binds to one or more intracellular anchor proteins and an extracellular domain that interacts with either the extracellular matrix or the extracellular domains of specific transmembrane adhesion proteins on another cell. In addition to anchor proteins and adhesion proteins, many anchoring junctions contain intracellular signaling proteins that enable the junctions to signal to the cell interior.

Figure 19-8

The construction of an anchoring junction from two classes of proteins. This drawing shows how intracellular anchor proteins and transmembrane adhesion proteins form anchoring junctions.

Anchoring junctions occur in two functionally different forms:

1.

Adherens junctions and desmosomes hold cells together and are formed by transmembrane adhesion proteins that belong to the cadherin family.

2.

Focal adhesions and hemidesmosomes bind cells to the extracellular matrix and are formed by transmembrane adhesion proteins of the integrin family.

On the intracellular side of the membrane, adherens junctions and focal adhesions serve as connection sites for actin filaments, while desmosomes and hemidesmosomes serve as connection sites for intermediate filaments (see Table 19-1, p. 1067).

Adherens Junctions Connect Bundles of Actin Filaments from Cell to Cell

Adherens junctions occur in various forms. In many nonepithelial tissues, they take the form of small punctate or streaklike attachments that indirectly connect the cortical actin filaments beneath the plasma membranes of two interacting cells. But the prototypical examples of adherens junctions occur in epithelia, where they often form a continuous adhesion belt (or zonula adherens) just below the tight junctions, encircling each of the interacting cells in the sheet. The adhesion belts are directly apposed in adjacent epithelial cells, with the interacting plasma membranes held together by the cadherins that serve here as transmembrane adhesion proteins.

Within each cell, a contractile bundle of actin filaments lies adjacent to the adhesion belt, oriented parallel to the plasma membrane. The actin is attached to this membrane through a set of intracellular anchor proteins, including catenins, vinculin, and α-actinin, which we consider later. The actin bundles are thus linked, via the cadherins and anchor proteins, into an extensive transcellular network (Figure 19-9). This network can contract with the help of myosin motor proteins (discussed in Chapter 16), and it is thought to help in mediating a fundamental process in animal morphogenesis—the folding of epithelial cell sheets into tubes and other related structures (Figure 19-10).

Figure 19-9

Adherens junctions. (A) Adherens junctions, in the form of adhesion belts, between epithelial cells in the small intestine. The beltlike junction encircles each of the interacting cells. Its most obvious feature is a contractile bundle of actin filaments (more...)

Figure 19-10

The folding of an epithelial sheet to form an epithelial tube. The oriented contraction of the bundles of actin filaments running along adhesion belts causes the epithelial cells to narrow at their apex and helps the epithelial sheet to roll up into a (more...)

The assembly of tight junctions between epithelial cells seems to require the prior formation of adherens junctions. Anti-cadherin antibodies that block the formation of adherens junctions, for example, also block the formation of tight junctions.

Desmosomes Connect Intermediate Filaments from Cell to Cell

Desmosomes are buttonlike points of intercellular contact that rivet cells together (Figure 19-11A). Inside the cell, they serve as anchoring sites for ropelike intermediate filaments, which form a structural framework of great tensile strength (Figure 19-11B). Through desmosomes, the intermediate filaments of adjacent cells are linked into a net that extends throughout the many cells of a tissue. The particular type of intermediate filaments attached to the desmosomes depends on the cell type: they are keratin filaments in most epithelial cells, for example, and desmin filaments in heart muscle cells.

Figure 19-11

Desmosomes. (A) An electron micrograph of three desmosomes between two epithelial cells in the intestine of a rat. (B) An electron micrograph of a single desmosome between two epidermal cells in a developing newt, showing clearly the attachment of intermediate (more...)

The general structure of a desmosome is illustrated in Figure 19-11C, and some of the proteins that form it are shown in Figure 19-11D. The junction has a dense cytoplasmic plaque composed of a complex of intracellular anchor proteins (plakoglobin and desmoplakin) that are responsible for connecting the cytoskeleton to the transmembrane adhesion proteins. These adhesion proteins (desmoglein and desmocollin), like those at an adherens junction, belong to the cadherin family. They interact through their extracellular domains to hold the adjacent plasma membranes together.

The importance of desmosome junctions is demonstrated by some forms of the potentially fatal skin disease pemphigus. Affected individuals make antibodies against one of their own desmosomal cadherin proteins. These antibodies bind to and disrupt the desmosomes that hold their skin epithelial cells (keratinocytes) together. This results in a severe blistering of the skin, with leakage of body fluids into the loosened epithelium.

Anchoring Junctions Formed by Integrins Bind Cells to the Extracellular Matrix: Focal Adhesions and Hemidesmosomes

Some anchoring junctions bind cells to the extracellular matrix rather than to other cells. The transmembrane adhesion proteins in these cell-matrix junctions are integrins—a large family of proteins distinct from the cadherins. Focal adhesions enable cells to get a hold on the extracellular matrix through integrins that link intracellularly to actin filaments. In this way, muscle cells, for example, attach to their tendons at the myotendinous junction. Likewise, when cultured fibroblasts migrate on an artificial substratum coated with extracellular matrix molecules, they also grip the substratum at focal adhesions, where bundles of actin filaments terminate. At all such adhesions, the extracellular domains of transmembrane integrin proteins bind to a protein component of the extracellular matrix, while their intracellular domains bind indirectly to bundles of actin filaments via the intracellular anchor proteins talin, α-actinin, filamin, and vinculin (Figure 19-12B).

Figure 19-12

Focal adhesions. (A) In these immunofluorescence micrographs, cells in culture have been labeled with antibodies against both actin (green) and the intracellular anchor protein vinculin (red). Note that vinculin is located at focal adhesions, which is (more...)

Hemidesmosomes, or half-desmosomes, resemble desmosomes morphologically and in connecting to intermediate filaments, and, like desmosomes, they act as rivets to distribute tensile or shearing forces through an epithelium. Instead of joining adjacent epithelial cells, however, hemidesmosomes connect the basal surface of an epithelial cell to the underlying basal lamina (Figure 19-13). The extracellular domains of the integrins that mediate the adhesion bind to a laminin protein (discussed later) in the basal lamina, while an intracellular domain binds via an anchor protein (plectin) to keratin intermediate filaments. Whereas the keratin filaments associated with desmosomes make lateral attachments to the desmosomal plaques (see Figure 19-11C and D), many keratin filaments associated with hemidesmosomes have their ends buried in the plaque (see Figure 19-13).

Figure 19-13

Desmosomes and hemidesmosomes. The distribution of desmosomes and hemidesmosomes in epithelial cells of the small intestine. The keratin intermediate filament networks of adjacent cells are indirectly connected to one another through desmosomes and to (more...)

Although the terminology for the various anchoring junctions can be confusing, the molecular principles (for vertebrates, at least) are relatively simple (Table 19-2). Integrins in the plasma membrane anchor a cell to extracellular matrix molecules; cadherin family members in the plasma membrane anchor it to the plasma membrane of an adjacent cell. In both cases, there is an intracellular coupling to cytoskeletal filaments, either actin filaments or intermediate filaments, depending on the types of intracellular anchor proteins involved.

Table 19-2

Anchoring Junctions.

Gap Junctions Allow Small Molecules to Pass Directly from Cell to Cell

With the exception of a few terminally differentiated cells such as skeletal muscle cells and blood cells, most cells in animal tissues are in communication with their neighbors via gap junctions. Each gap junction appears in conventional electron micrographs as a patch where the membranes of two adjacent cells are separated by a uniform narrow gap of about 2–4 nm. The gap is spanned by channel-forming proteins (connexins). The channels they form (connexons) allow inorganic ions and other small water-soluble molecules to pass directly from the cytoplasm of one cell to the cytoplasm of the other, thereby coupling the cells both electrically and metabolically. Dye-injection experiments suggest a maximal functional pore size for the connecting channels of about 1.5 nm, implying that coupled cells share their small molecules (such as inorganic ions, sugars, amino acids, nucleotides, vitamins, and the intracellular mediators cyclic AMP and inositol trisphosphate) but not their macromolecules (proteins, nucleic acids, and polysaccharides) (Figure 19-14). This cell coupling has important functional implications, many of which are only beginning to be understood.

Figure 19-14

Determining the size of a gap-junction channel. When fluorescent molecules of various sizes are injected into one of two cells coupled by gap junctions, molecules with a mass of less than about 1000 daltons can pass into the other cell, but larger molecules (more...)

Evidence that gap junctions mediate electrical and chemical coupling has come from many experiments. When, for example, connexin mRNA is injected into either frog oocytes or gap-junction-deficient cultured cells, channels with the properties expected of gap-junction channels can be demonstrated electrophysiologically where pairs of injected cells make contact.

The mRNA injection approach has been useful for identifying new gap-junction proteins. Genetic studies in the fruit fly Drosophila identified the gene shaking B, which, when mutated, resulted in flies that failed to jump in response to a visual stimulus. Although these flies had defective gap junctions, the sequence of the Shaking B protein did not resemble a connexin, and the function of the protein was unclear. An injection of the shaking B mRNA into frog oocytes, however, led to the formation of functional gap-junction channels, just like those formed by connexins. Shaking B thus became the first member of a new family of invertebrate gap-junction proteins called innexins. There are more than 15 innexin genes in Drosophila and 25 in the nematode C. elegans.

A Gap-Junction Connexon Is Made Up of Six Transmembrane Connexin Subunits

Connexins are four-pass transmembrane proteins, six of which assemble to form a channel, a connexon. When the connexons in the plasma membranes of two cells in contact are aligned, they form a continuous aqueous channel that connects the two cell interiors (Figure 19-15A). The connexons hold the interacting plasma membranes at a fixed distance apart—hence the gap.

Figure 19-15

Gap junctions. (A) A three-dimensional drawing showing the interacting plasma membranes of two adjacent cells connected by gap junctions. The apposed lipid bilayers (red) are penetrated by protein assemblies called connexons (green), each of which is (more...)

Gap junctions in different tissues can have different properties. The permeability of their individual channels can vary, reflecting differences in the connexins that form the junctions. In humans, for instance, there are 14 distinct connexins, each encoded by a separate gene and each having a distinctive, but sometimes overlapping, tissue distribution. Most cell types express more than one type of connexin, and two different connexin proteins can assemble into a heteromeric connexon, the properties of which differ from those of a homomeric connexon constructed from a single type of connexin. Moreover, adjacent cells expressing different connexins can form intercellular channels in which the two aligned half-channels are different (Figure 19-15B). Each gap junction can contain a cluster of a few to many thousands of connexons (Figure 19-16B).

Figure 19-16

Gap junctions as seen in the electron microscope. (A) Thin-section and (B) freeze-fracture electron micrographs of a large and a small gap junction between fibroblasts in culture. In (B), each gap junction is seen as a cluster of homogeneous intramembrane (more...)

Gap Junctions Have Diverse Functions

In tissues containing electrically excitable cells, coupling via gap junctions serves an obvious purpose. Some nerve cells, for example, are electrically coupled, allowing action potentials to spread rapidly from cell to cell, without the delay that occurs at chemical synapses. This is advantageous when speed and reliability are crucial, as in certain escape responses in fish and insects. Similarly, in vertebrates, electrical coupling through gap junctions synchronizes the contractions of both heart muscle cells and the smooth muscle cells responsible for the peristaltic movements of the intestine.

Gap junctions also occur in many tissues that do not contain electrically excitable cells. In principle, the sharing of small metabolites and ions provides a mechanism for coordinating the activities of individual cells in such tissues and for smoothing out random fluctuations in small molecule concentrations in different cells. In the liver, for example, the release of noradrenaline from sympathetic nerve endings in response to a fall in blood glucose levels stimulates hepatocytes to increase glycogen breakdown and release glucose into the blood. Not all the hepatocytes are innervated by sympathetic nerves, however. By means of the gap junctions that connect hepatocytes, the signal is transmitted from the innervated hepatocytes to the noninnervated ones. Thus, mice with a mutation in the major connexin gene expressed in the liver fail to mobilize glucose normally when blood glucose levels fall.

The normal development of ovarian follicles also depends on gap-junction-mediated communication—in this case, between the oocyte and the surrounding granulosa cells. A mutation in the gene that encodes the connexin that normally couples these two cell types causes infertility (Figure 19-17).

Figure 19-17

Gap junction coupling in the ovarian follicle. The oocyte is surrounded by a thick layer of extracellular matrix called the zona pellucida (discussed in Chapter 20). The surrounding granulosa cells are coupled to each other by gap junctions formed by (more...)

Cell coupling via gap junctions also seems to be important in embryogenesis. In early vertebrate embryos, beginning with the late eight-cell stage in mouse embryos, most cells are electrically coupled to one another. As specific groups of cells in the embryo develop their distinct identities and begin to differentiate, they commonly uncouple from surrounding tissue. As the neural plate folds up and pinches off to form the neural tube, for instance (see Figure 19-10), its cells uncouple from the overlying ectoderm. Meanwhile, the cells within each group remain coupled with one another and therefore tend to behave as a cooperative assembly, all following a similar developmental pathway in a coordinated fashion.

The Permeability of Gap Junctions Can Be Regulated

Like conventional ion channels (discussed in Chapter 11), individual gap-junction channels do not remain continuously open; instead, they flip between open and closed states. Moreover, the permeability of gap junctions is rapidly (within seconds) and reversibly reduced by experimental manipulations that decrease the cytosolic pH or increase the cytosolic concentration of free Ca²⁺ to very high levels. Thus, gap-junction channels are dynamic structures that can undergo a reversible conformational change that closes the channel in response to changes in the cell.

The purpose of the pH regulation of gap-junction permeability is unknown. In one case, however, the purpose of Ca²⁺ control seems clear. When a cell is damaged, its plasma membrane can become leaky. Ions present at high concentration in the extracellular fluid, such as Ca²⁺ and Na⁺, then move into the cell, and valuable metabolites leak out. If the cell were to remain coupled to its healthy neighbors, these too would suffer a dangerous disturbance of their internal chemistry. But the large influx of Ca²⁺ into the damaged cell causes its gap-junction channels to close immediately, effectively isolating the cell and preventing the damage from spreading to other cells.

Gap-junction communication can also be regulated by extracellular signals. The neurotransmitter dopamine, for example, reduces gap-junction communication between a class of neurons in the retina in response to an increase in light intensity (Figure 19-18). This reduction in gap-junction permeability helps the retina switch from using rod photoreceptors, which are good detectors of low light, to cone photoreceptors, which detect color and fine detail in bright light.

Figure 19-18

The regulation of gap-junction coupling by a neurotransmitter. (A) A neuron in a rabbit retina was injected with the dye Lucifer yellow, which passes readily through gap junctions and labels other neurons of the same type that are connected to the injected (more...)

Figure 19-19 summarizes the various types of junctions formed by vertebrate cells in an epithelium. In the most apical portion of the cell, the relative positions of the junctions are the same in nearly all vertebrate epithelia. The tight junction occupies the most apical position, followed by the adherens junction (adhesion belt) and then by a special parallel row of desmosomes; together these form a structure called a junctional complex. Gap junctions and additional desmosomes are less regularly organized.

Figure 19-19

A summary of the various cell junctions found in a vertebrate epithelial cell. The drawing is based on epithelial cells of the small intestine.

In Plants, Plasmodesmata Perform Many of the Same Functions as Gap Junctions

The tissues of a plant are organized on different principles from those of an animal. This is because plant cells are imprisoned within rigid cell walls composed of an extracellular matrix rich in cellulose and other polysacharides, as we discuss later. The cell walls of adjacent cells are firmly cemented to those of their neighbors, which eliminates the need for anchoring junctions to hold the cells in place. But a need for direct cell-cell communication remains. Thus, plant cells have only one class of intercellular junctions, plasmodesmata (singular, plasmodesma). Like gap junctions, they directly connect the cytoplasms of adjacent cells.

In plants, however, the cell wall between a typical pair of adjacent cells is at least 0.1 μm thick, and so a structure very different from a gap junction is required to mediate communication across it. Plasmodesmata solve the problem. With a few specialized exceptions, every living cell in a higher plant is connected to its living neighbors by these structures, which form fine cytoplasmic channels through the intervening cell walls. As shown in Figure 19-20A, the plasma membrane of one cell is continuous with that of its neighbor at each plasmodesma, and the cytoplasm of the two cells is connected by a roughly cylindrical channel with a diameter of 20–40 nm. Thus, the cells of a plant can be viewed as forming a syncytium, in which many cell nuclei share a common cytoplasm.

Figure 19-20

Plasmodesmata. (A) The cytoplasmic channels of plasmodesmata pierce the plant cell wall and connect all cells in a plant together. (B) Each plasmodesma is lined with plasma membrane that is common to two connected cells. It usually also contains a fine (more...)

Running through the center of the channel in most plasmodesmata is a narrower cylindrical structure, the desmotubule, which is continuous with elements of the smooth endoplasmic reticulum in each of the connected cells (Figures 19-20B and 19-21A and B). Between the outside of the desmotubule and the inner face of the cylindrical channel formed by plasma membrane is an annulus of cytosol through which small molecules can pass from cell to cell. As each new cell wall is assembled during the cytokinesis phase of cell division, plasmadesmata are created within it. They form around elements of smooth ER that become trapped across the developing cell plate (discussed in Chapter 18). They can also be inserted de novo through pre-existing cell walls, where they are commonly found in dense clusters called pit fields (Figure 19-21C). When no longer required, plasmadesmata can be readily removed.

Figure 19-21

Various views of plasmodesmata. (A) Electron micrograph of a longitudinal section of a plasmodesma from a water fern. The plasma membrane lines the pore and is continuous from one cell to the next. Endoplasmic reticulum and its association with the central (more...)

In spite of the radical difference in structure between plasmodesmata and gap junctions, they seem to function in remarkably similar ways. Evidence obtained by injecting tracer molecules of different sizes suggests that plasmo-desmata allow the passage of molecules with a molecular weight of less than about 800, which is similar to the molecular-weight cutoff for gap junctions. As with gap junctions, transport through plasmodesmata is regulated. Dye-injection experiments, for example, show that there can be barriers to the movement of even low-molecular-weight molecules between certain cells, or groups of cells, that are connected by apparently normal plasmodesmata; the mechanisms that restrict communication in these cases are not understood.

During plant development, groups of cells within the shoot and root meristems signal to one another in the process of defining their future fates (discussed in Chapter 21). Some gene regulatory proteins involved in this process of cell fate determination pass from cell to cell through plasmodesmata. They bind to components of the plasmodesmata and override the size exclusion mechanism that would otherwise prevent their passage. In some cases, the mRNA that encodes the protein can also pass through. Some plant viruses also exploit this route: infectious viral RNA, or even intact virus particles, can pass from cell to cell in this way. These viruses produce proteins that bind to components of the plasmodesmata to increase dramatically the effective pore size of the channel. As the functional components of plasmodesmata are unknown, it is unclear how endogenous or viral macromolecules regulate the transport properties of the channel to pass through it.

Summary

Many cells in tissues are linked to one another and to the extracellular matrix at specialized contact sites called cell junctions. Cell junctions fall into three functional classes: occluding junctions, anchoring junctions, and communicating junctions. Tight junctions are occluding junctions that are crucial in maintaining the concentration differences of small hydrophilic molecules across epithelial cell sheets. They do so in two ways. First, they seal the plasma membranes of adjacent cells together to create a continuous impermeable, or semipermeable, barrier to diffusion across the cell sheet. Second, they act as barriers in the lipid bilayer to restrict the diffusion of membrane transport proteins between the apical and the basolateral domains of the plasma membrane in each epithelial cell. Septate junctions serve as occluding junctions in invertebrate tissues.

The main types of anchoring junctions in vertebrate tissues are adherens junctions, desmosomes, focal adhesions, and hemidesmosomes. Adherens junctions and desmosomes connect cells together and are formed by cadherins, while focal adhesions and hemidesmosomes connect cells to the extracellular matrix and are formed by integrins. Adherens junctions and focal adhesions are connecting sites for bundles of actin filaments, whereas desmosomes and hemidesmosomes are connecting sites for intermediate filaments.

Gap junctions are communicating junctions composed of clusters of connexons that allow molecules smaller than about 1000 daltons to pass directly from the inside of one cell to the inside of the next. Cells connected by gap junctions share many of their inorganic ions and other small molecules and are therefore chemically and electrically coupled. Gap junctions are important in coordinating the activities of electrically active cells, and they have a coordinating role in other groups of cells as well. Plasmodesmata are the only intercellular junctions in plants. Although their structure is entirely different, and they can sometimes transport informational macromolecules, in general, they function like gap junctions.