Chapter 4: Cell Membrane and Cell Surface

I. Cell Membrane

II. Cell Junctions

III. Cell Adhesion

IV. Extracellular Matrix

http://www.cbi.pku.edu.cn/chinese/documents/chenjg/

I. Biomembranes: Their Structure, Chemistry and Functions

Learning objectives:

1. A brief history of studies on the structrure of the plasma membrane

2. Model of membrane structure: an experimental perspective

3. The chemical composition of membranes

4. Characteristics of biomembrane

5. An overview of the functions of biomembranes

1. 1. A brief history of studies on the structrure of the plasmic membrane

A. Conception: Plasma membrane(cell membrane), Intracellular membrane,

Biomembrane.

B. The history of study

Overton(1890s):

Lipid nature of PM;

J.D.Robertson(1959): The TEM showing:the trilaminar appearance of PM;

Unit membrane model;

S.J.Singer and G.Nicolson(1972):

fluid-mosaic model;

K.Simons et al(1997):

lipid rafts model;

Functional rafts in Cell membranes. Nature 387:569-572

2. Singer and Nicolson’s Model of membrane structure: The fluid-mosaic model is the “central dogma” of membrane biology.

A. The core lipid bilayer exists in a fluid state, capable of dynamic movement.

B. Membrane proteins form a mosaic of particles penetrating the lipid to varying degrees.

The Fluid Mosaic Model, proposed in 1972 by Singer and Nicolson, had two key features, both implied in its name.

3. The chemical composition of membranes

A. Membrane Lipids: The Fluid Part of the Model

Phospholipids:

Phosphoglyceride and sphingolipids

Glycolipids

Sterols ( is only found in animals)

Membrane lipids are amphipathic.

There are three major classes of lipids:

Figure 10-2. The parts of a phospholipid molecule. Phosphatidylcholine, represented schematically (A), in formula (B), as a space-filling model (C), and as a symbol (D). The kink due to the cis-double bond is exaggerated in these drawings for emphasis.

Figure 10-3. A lipid micelle and a lipid bilayer seen in cross-section. Lipid molecules form such structures spontaneously in water. The shape of the lipid molecule determines which of these structures is formed. Wedge-shaped lipid molecules (above) form micelles, whereas cylinder-shaped phospholipid molecules (below) form bilayers.

Figure 10-4. Liposomes. (A) An electron micrograph of unfixed, unstained phospholipid vesicles (liposomes) in water. The bilayer structure of the vesicles is readily apparent. (B) A drawing of a small spherical liposome seen in cross-section. Liposomes are commonly used as model membranes in experimental studies. (A, courtesy of Jean Lepault.)

Figure 10-5. A cross-sectional view of a synthetic lipid bilayer, called a black membrane. This planar bilayer is formed across a small hole in a partition separating two aqueous compartments. Black membranes are used to measure the permeability properties of synthetic membranes.

Figure 10-6. Phospholipid mobility. The types of movement possible for phospholipid molecules in a lipid bilayer.

Figure 10-7. Influence of cis-double bonds in hydrocarbon chains. The double bonds make it more difficult to pack the chains together and therefore make the lipid bilayer more difficult to freeze.

Figure 10-8. The structure of cholesterol. Cholesterol is represented by a formula in (A), by a schematic drawing in (B), and as a space-filling model in (C).

Figure 10-9. Cholesterol in a lipid bilayer. Schematic drawing of a cholesterol molecule interacting with two phospholipid molecules in one leaflet of a lipid bilayer.

Figure 10-10. Four major phospholipids in mammalian plasma membranes. Note that different head groups are represented by different symbols in this figure and the next. All of the lipid molecules shown are derived from glycerol except for sphingomyelin, which is derived from serine.

Figure 10-11. The asymmetrical distribution of phospholipids and glycolipids in the lipid bilayer of human red blood cells. The symbols used for the phospholipids are those introduced in Figure 10-10. In addition, glycolipids are drawn with hexagonal polar head groups (blue). Cholesterol (not shown) is thought to be distributed about equally in both monolayers.

Figure 10-12. Glycolipid molecules. Galactocerebroside (A) is called a neutral glycolipid because the sugar that forms its head group is uncharged. A ganglioside (B) always contains one or more negatively charged sialic acid residues (also called N-acetylneuraminic acid, or NANA), whose structure is shown in (C). Whereas in bacteria and plants almost all glycolipids are derived from glycerol, as are most phospholipids, in animal cells they are almost always produced from sphingosine, an amino alcohol derived from serine, as is the case for the phospholipid sphingomyelin. Gal = galactose; Glc = glucose, GalNAc = N-acetylgalactos-amine; these three sugars are uncharged.

Figure 10-13. Six ways in which membrane proteins associate with the lipid bilayer. Most trans-membrane proteins are thought to extend across the bilayer as a single a helix (1) or as multiple a helices (2); some of these "single-pass" and "multipass" proteins have a covalently attached fatty acid chain inserted in the cytoplasmic monolayer (1). Other membrane proteins are attached to the bilayer solely by a covalently attached lipid - either a fatty acid chain or prenyl group - in the cytoplasmic monolayer (3) or, less often, via an oligosaccharide, to a minor phospholipid, phosphatidylinositol, in the noncytoplasmic monolayer (4). Finally, many proteins are attached to the membrane only by noncovalent interactions with other membrane proteins (5) and (6). How the structure in (3) is formed is illustrated in Figure10-14.

Membrane proteins

Figure 10-14. The covalent attachment of either of two types of lipid groups can help localiz

e a water-soluble protein to a membrane after its synthesis in the cytosol. (A) A fatty acid chain (either myristic or palmitic acid) is attached via an amide linkage to an amino-terminal glycine. (B) A prenyl group (either farnesyl or a longer geranylgeranyl group - both related to cholesterol) is attached via a thioether linkage to a cysteine residue that is four residues from the carboxyl terminus. Following this prenylation, the terminal three amino acids are cleaved off and the new carboxyl terminus is methylated before insertion into the membrane. The structures of two lipid anchors are shown underneath: (C) a myristyl anchor (a 14-carbon saturated fatty acid chain), and (D) a farnesyl anchor (a 15-carbon unsaturated hydrocarbon chain).

Figure 10-15. A segment of a transmembrane polypeptide chain crossing the lipid bilayer as an a helix. Only the a-carbon backbone of the polypeptide chain is shown, with the hydrophobic amino acids in green and yellow. (J. Deisenhofer et al., Nature 318:618-624 and H. Michel et al., EMBO J. 5:1149-1158)

Figure 10-17. A typical single-pass transmembrane protein. Note that the polypeptide chain traverses the lipid bilayer as a right-handed a helix and that the oligosaccharide chains and disulfide bonds are all on the noncytosolic surface of the membrane. Disulfide bonds do not form between the sulfhydryl groups in the cytoplasmic domain of the protein because the reducing environment in the cytosol maintains these groups in their reduced (-SH) form.

Figure 10-18. A detergent micelle in water, shown in cross-section. Because they have both polar and nonpolar ends, detergent molecules are amphipathic.

Figure 10-19. Solubilizing membrane proteins with a mild detergent. The detergent disrupts the lipid bilayer and brings the proteins into solution as protein-lipid-detergent complexes. The phospholipids in the membrane are also solubilized by the detergent.

Figure 10-20. The structures of two commonly used detergents. Sodium dodecyl sulfate (SDS) is an anionic detergent, and Triton X-100 is a nonionic detergent. The hydrophobic portion of each detergent is shown in green, and the hydrophilic portion is shown in blue. Note that the bracketed portion of Triton X-100 is repeated about eight times.

Figure 10-21. The use of mild detergents for solubilizing, purifying, and reconstituting functional membrane protein systems. In this example functional Na+-K+ ATPase molecules are purified and incorporated into phospholipid vesicles. The Na+-K+ ATPase is an ion pump that is present in the plasma membrane of most animal cells; it uses the energy of ATP hydrolysis to pump Na+ out of the cell and K+ in, as discussed in Chapter 11.

Figure 10-22. A scanning electron micrograph of human red blood cells. The cells have a biconcave shape and lack nuclei. (Courtesy of Bernadette Chailley.)

Figure 10-24. SDS polyacrylamide-gel electrophoresis pattern of the proteins in the human red blood cell membrane. The gel in (A) is stained with Coomassie blue. The positions of some of the major proteins in the gel are indicated in the drawing in (B); glycophorin is shown in red to distinguish it from band 3. Other bands in the gel are omitted from the drawing. The large amount of carbohydrate in glycophorin molecules slows their migration so that they run almost as slowly as the much larger band 3 molecules. (A, courtesy of Ted Steck.)

Figure 10-25. Spectrin molecules from human red blood cells. The protein is shown schematically in (A) and in electron micrographs in (B). Each spectrin heterodimer consists of two antiparallel, loosely intertwined, flexible polypeptide chains called a and b these are attached noncovalently to each other at multiple points, including both ends. The phosphorylated "head" end, where two dimers associate to form a tetramer, is on the left. Both the a and b chains are composed largely of repeating domains 106 amino acids long. In (B) the spectrin molecules have been shadowed with platinum. (D.W. Speicher and V.T. Marchesi, Nature 311:177-180; B, D.M. Shotton et al., J. Mol. Biol. 131:303-329)

Figure 10-26. The spectrin-based cytoskeleton on the cytoplasmic side of the human red blood cell membrane. The structure is shown schematically in (A) and in an electron micrograph in (B). The arrangement shown in (A) has been deduced mainly from studies on the interactions of purified proteins in vitro. Spectrin dimers associate head-to-head to form tetramers that are linked together into a netlike meshwork by junctional complexes composed of short actin filaments (containing 13 actin monomers), tropomyosin, which probably determines the length of the actin filaments, band 4.1, and adducin. The cytoskeleton is linked to the membrane by the indirect binding of spectrin tetramers to some band 3 proteins via ankyrin molecules, as well as by the binding of band 4.1 proteins to both band 3 and glycophorin (not shown). The electron micrograph in (B) shows the cytoskeleton on the cytoplasmic side of a red blood cell membrane after fixation and negative staining. (B, courtesy of T. Byers and D. Branton, PNSA. 82:6153-6157)

Figure 10-31. The three-dimensional structure of a bacteriorhodopsin molecule. The polypeptide chain crosses the lipid bilayer as seven a helices. The location of the chromophore and the probable pathway taken by protons during the light-activated pumping cycle are shown. When activated by a photon, the chromophore is thought to pass an H+ to the side chain of aspartic acid 85. Subsequently, three other H+ transfers are thought to complete the cyclefrom aspartic acid 85 to the extra-cellular space, from aspartic acid 96 to the chromophore, and from the cytosol to aspartic acid 96. (R. Henderson et al. J. Mol. Biol.213:899-929)

Figure 10-32. The three-dimensional structure of a porin trimer of Rhodobacter capsulatus determined by x-ray crystallography. (A) Each monomer consists of a 16-stranded antiparallel b barrel that forms a transmembrane water-filled channel. (B) The monomers tightly associate to form trimers, which have three separate channels for the diffusion of small solutes through the bacterial outer membrane. A long loop of polypeptide chain (shown in red), which connects two b strands, protrudes into the lumen of each channel, narrowing it to a cross-section of 0.6 x 1 nm. (Adapted from M.S. Weiss et al., FEBS Lett.280: 379-382)

Figure 10-33. The three-dimensional structure of the photosynthetic reaction center of the bacterium Rhodopseudomonas viridis. The structure was determined by x-ray diffraction analysis of crystals of this transmembrane protein complex. The complex consists of four subunits, L, M, H, and a cytochrome. The L and M subunits form the core of the reaction center, and each contains five a helices that span the lipid bilayer. The locations of the various electron carrier coenzymes are shown in black. (Adapted from a drawing by J. Richardson based on data from J. Deisenhofer et al., Nature 318:618-624)

4. Characteristics of biomembraneA. Dynamic nature of biomembraneFluidity of membrane lipid. It give membranes the ability to fuse, form networks, and separate charge;

Motility of membrane protein.

The lateral diffusion of membrane lipids can demonstrated experimentally by a technique called Fluorescence Recovery After Photobleaching (FRAP).

Figure 10-34. Experiment demonstrating the mixing of plasma membrane proteins on mouse-human hybrid cells. The mouse and human proteins are initially confined to their own halves of the newly formed heterocaryon plasma membrane, but they intermix with time. The two antibodies used to visualize the proteins can be distinguished in a fluorescence microscope because fluorescein is green whereas rhodamine is red. (Based on observations of L.D. Frye and M. Edidin, J. Cell Sci. 7:319-335)

Figure 10-35. Antibody-induced patching and capping of a cell-surface protein on a white blood cell. The bivalent antibodies cross-link the protein molecules to which they bind. This causes them to cluster into large patches, which are actively swept to the tail end of the cell to form a "cap." The centrosome, which governs the head-tail polarity of the cell, is shown in orange.

Figure 10-37. Diagram of an epithelial cell showing how a plasma membrane protein is restricted to a particular domain of the membrane. Protein A (in the apical membrane) and protein B (in the basal and lateral membranes) can diffuse laterally in their own domains but are prevented from entering the other domain, at least partly by the specialized cell junction called a tight junction. Lipid molecules in the outer (noncytoplasmic) monolayer of the plasma membrane are likewise unable to diffuse between the two domains; lipids in the inner (cytoplasmic) monolayer, however, are able to do so.

Figure 10-38. Three domains in the plasma membrane of guinea pig sperm defined with monoclonal antibodies. A guinea pig sperm is shown schematically in (A), while each of the three pairs of micrographs shown in (B), (C), and (D) shows cell-surface immunofluorescence staining with a different monoclonal antibody (on the right) next to a phase-contrast micrograph (on the left) of the same cell. The antibody shown in (B) labels only the anterior head, that in (C) only the posterior head, whereas that in (D) labels only the tail. (Courtesy of Selena Carroll and Diana Myles.)

Figure 10-39. Four ways in which the lateral mobility of specific plasma membrane proteins can be restricted. The proteins can self-assemble into large aggregates (such as bacteriorhodopsin in the purple membrane of Halobacterium) (A); they can be tethered by interactions with assemblies of macromolecules outside (B) or inside (C) the cell; or they can interact with proteins on the surface of another cell (D).

Figure 10-41. Simplified diagram of the cell coat (glycocalyx). The cell coat is made up of the oligosaccharide side chains of glycolipids and integral membrane glycoproteins and the polysaccharide chains on integral membrane proteoglycans. In addition, adsorbed glycoproteins and adsorbed proteoglycans (not shown) contribute to the glycocalyx in many cells. Note that all of the carbohydrate is on the noncytoplasmic surface of the membrane.

cell coat

Figure 10-42. The protein-carbohydrate interaction that initiates the transient adhesion of neutrophils to endothelial cells at sites of inflammation. (A) The lectin domain of P-selectin binds to the specific oligosaccharide shown in (B), which is present on both cell-surface glycoprotein and glycolipid molecules. The lectin domain of the selectins is homologous to lectin domains found on many other carbohydrate-binding proteins in animals; because the binding to their specific sugar ligand requires extracellular Ca2+, they are called C-type lectins. A three-dimensional structure of one of these lectin domains, determined by x-ray crystallography, is shown in (C); its bound sugar is colored blue. Gal = galactose; GlcNAc = N-acetylglucosamine; Fuc = fucose; NANA = sialic acid.

5. An Overview of membrane functions

1. Define the boundaries of the cell and its organelles.

2. Serve as loci for specific functions.

3. provide for and regulate transport processes.

4. contain the receptors needed to detect external signals.

5. provide mechanisms for cell-to-cell contact, communication and adhesion

Learning Objectives:

1. Integrating Cells into Tissues

2. Cell junctons: Cell-cell adhension and communication;

3. Cell-Matrix adhension;

4. Extracellular matrix: Components and Functions;

5. Cell Walls

II. Cell junction, Cell adhension

Extracellular matrix

Table 19-1  A Functional Classification of Cell Junctions

1.  Occluding junctions (tight junctions)

2.  Anchoring junctions    a. actin filament attachment sites      i. cell-cell adherens junctions (e.g., adhesion belts)      ii. cell-matrix adherens junctions (e.g., focal contacts)      iii. septate junctions (invertebrates only)     b. intermediate filament attachment sites      i. cell-cell (desmosomes)      ii. cell-matrix (hemidesmosomes)

3.  Communicating junctions    a. gap junctions    b. chemical synapses    c. plasmodesmata (plants only)

Figure 19-1 Simplified drawing of a cross-section through part of the wall of the intestine. This long, tubelike organ is constructed from epithelial tissues (red), connective tissues (green), and muscle tissues (yellow). Each tissue is an organized assembly of cells held together by cell-cell adhesions, extracellular matrix, or both.

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 epithelial sheet from the gut lumen to the blood. In the example shown, glucose is actively transported into the cell by Na+-driven glucose symports at the apical surface, and it diffuses out of the cell by facilitated diffusion mediated by glucose carriers in the basolateral membrane. Tight junctions are thought to confine the transport proteins to their appropriate membrane domains by acting as diffusion barriers within the lipid bilayer of the plasma membrane; these junctions also block the backflow of glucose from the basal side of the epithelium into the gut lumen.

Tight junctions

Figure 19-3 Tight junctions allow cell sheets to serve as barriers to solut

e diffusion.    (A) Schematic drawing showing how a small extracellular tracer molecule added on one side of an epithelial cell sheet cannot traverse the tight junctions that seal adjacent cells together. (B) Electron micrographs of cells in an epithelium where a small, extracellular, electron-dense tracer molecule has been added to either the apical side (on the left) or the basolateral side (on the right); in both cases the tracer is stopped by the tight junction. (B, courtesy of Daniel Friend.)

Figure 19-4 Structure of a tight junction between epithelial cells of the small intestine.    The junctions are shown schematically in (A) and in freeze-fracture (B) and conventional (C) electron micrographs. Note that the cells are oriented with their apical ends down. In (B) the plane of the micrograph is parallel to the plane of the membrane, and the tight junction appears as a beltlike band of anastomosing sealing strands that encircle each cell in the sheet. The sealing strands are seen as ridges of intramembrane particles on the cytoplasmic fracture face of the membrane (the P face) or as complementary grooves on the external face of the membrane (the E face) (see Figure 19-5). In (C) the junction is seen as a series of focal connections between the outer leaflets of the two interacting plasma membranes, each connection corresponding to a sealing strand in cross-section. (B and C, from N.B. Gilula, in Cell Communication [R.P. Cox, ed.], pp. 1-29)

Figure 19-5 A current model of a tight junction.    It is postulated that the sealing strands that hold adjacent plasma membranes together are formed by continuous strands of transmembrane junctional proteins, which make contact across the intercellular space and create a seal. In this schematic the cytoplasmic half of one membrane has been peeled back by the artist to expose the protein strands. Two peripheral proteins associated with the cytoplasmic side of tight junctions have been characterized, but the putative transmembrane protein has not yet been identified. In freeze-fracture electron microscopy the tight-junction proteins would remain with the cytoplasmic (P face) half of the lipid bilayer to give the pattern of intramembrane particles seen in Figure 19-4B, instead of staying in the other half as shown here.

Figure 19-6 Anchoring junctions in an epithelial tissue. Highly schematized drawing of how such junctions join cytoskeletal filaments from cell to cell and from cell to extracellular matrix.

Anchoring junctions

Figure 19-7 Construction of an anchoring junction.    Highly schematized drawing showing the two classes of proteins that constitute such a junction: intracellular attachment proteins and transmembrane linker proteins.

Figure 19-8 Adhesion belts between epithelial cells in the small intestine. This beltlike anchoring junction encircles each of the interacting cells. Its most obvious feature is a contractile bundle of actin filaments running along the cytoplasmic surface of the junctional plasma membrane. The actin filaments are joined from cell to cell by transmembrane linker proteins (cadherins), whose extracellular domain binds to the extracellular domain of an identical cadherin molecule on the adjacent cell.

Adhesion belts

Figure 19-9 The folding of an epithelial sheet to form an epithelial tube.    It is thought that the oriented contraction of the bundle of actin filaments running along adhesion belts causes the epithelial cells to narrow at their apex and that this plays an important part in the rolling up of the epithelial sheet into a tube (although cellular rearrangements are also thought to play an important part). An example is the formation of the neural tube in early vertebrate development

Figure 19-11 A septate junction.    Electron micrograph of a septate junction between two epithelial cells of a mollusk. The interacting plasma membranes, seen in cross-section, are connected by parallel rows of junctional proteins. The rows, which have a regular periodicity, are seen as dense bars or septa. (From N.B. Gilula, in Cell Communication [R.P. Cox, ed.], pp. 1-29)

Septate junction

Figure 19-12 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 filaments. (C) A schematic drawing of a desmosome. On the cytoplasmic surface of each interacting plasma membrane is a dense plaque composed of a mixture of intracellular attachment proteins (including plakoglobin and desmoplakins). Each plaque is associated with a thick network of keratin filaments, which are attached to the surface of the plaque. Transmembrane linker proteins, which belong to the cadherin family of cell-cell adhesion molecules, bind to the plaques and interact through their extracellular domains to hold the adjacent membranes together by a Ca2+-dependent mechanism. (A, from N.B. Gilula, in Cell Communication, pp. 1-29; B, from D.E. Kelly, JCB. 28:51-59)

Desmosomes

Figure 19-13 The distribution of desmosomes and hemidesmosomes in epithelial cells of the small intestine.    The keratin filament networks of adjacent cells are indirectly connected to one another through desmosomes and to the basal lamina through hemidesmosomes.

Table 19-2  Anchoring Junctions

JunctionTransmembrane Linker Protein Extracellular Ligand

Intracellular Cytoskeletal Attachment

Some Intracellular Attachment Proteins

Adherens (cell-cell)

cadherin (E-cadherin) cadherin in neighboring cell

actin filaments

catenins, vinculin,  -actinin, plakoglobin

Desmosome cadherin (desmogleins & desmocollins)

cadherin in neighboring cell

intermediate filaments

desmoplakins, plakoglobin

Adherens (cell-matrix)

integrin extracellular matrix proteins

actin filaments

talin, vinculin,  -actinin

Hemidesmosome

integrin extracellular matrix (basal lamina) proteins

intermediate filaments

desmoplakinlike protein

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 smaller than about 1000 daltons can pass into the othe

r cell but larger molecules cannot.

Communicating junctions

gap-junction channel

Figure 19-15 A model of a gap junction.  The drawing shows the interacting plasma membranes of two adjacent cells. The apposed lipid bilayers (red) are penetrated by protein assemblies called connexons (green), each of which is thought to be formed by six identical protein subunits (called connexins). Two connexons join across the intercellular gap to form a continuous aqueous channel connecting the two cells.

Figure 19-16 Gap junctions as seen in the electron microscope.    Thin-section (A) and freeze-fracture (B) 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 particles associated exclusively with the cytoplasmic fracture face (P face) of the plasma membrane. (From N.B. Gilula, in Cell Communication [R.P. Cox, ed.], pp. 1-29)

Figure 11-33 Three classes of channel proteins.    The postulated relationship between the number of protein subunits and pore diameter. (Adapted from B. Hille, Ionic Channels of Excitable Membranes, 2nd ed. Sunderland, MA: Sinauer, 1992.)

Figure 19-17 A proposed model for how gap-junction channels may close in response to a rise in Ca2+ or a fall in pH in the cytosol.    A small rotation of each subunit closes the channel. The model is based on an image analysis of electron micrographs of rapidly frozen tissue in which the structure of gap junction channels in their presumed open state was compared with their structure in a Ca2+-induced closed state. It is possible that a similar mechanism operates in the opening and closing of the gated ion channels discussed in Chapter 11. (After P.N.T. Unwin and P.D. Ennis, Nature 307:609-613)

Figure 19-18 Summary of the various cell junctions found in animal cell epithelia.    This drawing is based on epithelial cells of the small intestine.

Figure 19-19 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 common to two connected cells. It usually also contains a fine tubular structure (20-40nm), the desmotubule, derived from smooth endoplasmic reticulum.

Figure 19-20 Plasmodesmata as seen in the electron microscope.    (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 desmotubule can be seen. (B) A similar plasmodesma in cross-section. (Courtesy of R. Overall.)

Figure 19-21 The simplest mechanism by which cells assemble to form a tissue.    The progeny of the founder cell are retained in the epithelial sheet by the basal lamina and by cell-cell adhesion mechanisms, including the formation of intercellular junctions.

There Are Two Basic Ways in Which Animal Cells Assemble into Tissues

III. Cell Adhension

Figure 19-22 An example of a more complex mechanism by which cells assemble to form a tissue.    Neural crest cells escape from the epithelium forming the upper surface of the neural tube and migrate away to form a variety of cell types and tissues throughout the embryo. Here they are shown assembling and differentiating to form two collections of nerve cells in the peripheral nervous system. Such a collection of nerve cells is called a ganglion. Other neural crest cells differentiate in the ganglion to become supporting (satellite) cells surrounding the neurons. Although it is not shown, the neural crest cells proliferate rapidly as they migrate.

Figure 19-23 Organ-specific adhesion of dissociated vertebrate embryo cells determined by a radioactive cell-binding assay.    The rate of cell adhesion can be measured by determining the number of radioactively labeled cells bound to the cell aggregates after various periods of time. The rate of adhesion is greater between cells of the same kind. In a commonly used modification of this assay, cells labeled with a fluorescent or radioactive marker are allowed to bind to a monolayer of unlabeled cells in culture.

Figure 19-24 Schematic drawing of a typical cadherin molecule.    The extracellular part of the protein is folded into five similar domains, three of which contain Ca2+-binding sites. The extracellular domain farthest from the membrane is thought to mediate cell-cell adhesion; the sequence His-Ala-Val in this domain seems to be involved, as peptides with this sequence inhibit cadherin-mediated adhesion. The cytoplasmic tail interacts with the actin cytoskeleton via a number of intracellular attachment proteins, including three catenin proteins. a-catenin is structurally related to vinculin. X represents uncharacterized attachment proteins involved in coupling cadherins to actin filaments.

Cadherin

Figure 19-25 Distribution of E- and N-cadherin in the developing nervous system.    Immuno-fluorescence micrographs of a cross-section of a chick embryo showing the developing neural tube labeled with antibodies against E-cadherin (A) and N-cadherin (B). Note that the overlying ectoderm cells express only E-cadherin, while the cells in the neural tube have lost E-cadherin and have acquired N-cadherin. (Courtesy of Kohei Hatta and Masatoshi Takeichi.)

Figure 10-42 The protein-carbohydrate interaction that initiates the transient adhesion of neutrophils to endothelial cells at sites of inflammation.    (A) The lectin domain of P-selectin binds to the specific oligosaccharide shown in (B), which is present on both cell-surface glycoprotein and glycolipid molecules. The lectin domain of the selectins is homologous to lectin domains found on many other carbohydrate-binding proteins in animals; because the binding to their specific sugar ligand requires extracellular Ca2+, they are called C-type lectins. A three-dimensional structure of one of these lectin domains, determined by x-ray crystallography, is shown in (C); its bound sugar is colored blue. Gal = galactose; GlcNAc = N-acetylglucosamine; Fuc = fucose; NANA = sialic acid.

Selectin

Figure 19-26 Three mechanisms by which cell-surface molecules can mediate cell-cell adhesion.    Although all of these mechanisms can operate in animals, the one that depends on an extracellular linker molecule seems to be least common.

Figure 19-27 Schematic drawing of four forms of NCAM.    The extracellular part of the polypeptide chain in each case is folded into five immunoglobulinlike domains (and one or two other domains called fibronectin type III repeats for reasons that will become clear later). Disulfide bonds (shown in red) connect the ends of each loop forming each Ig-like domain.

NCAM

Figure 19-28 A summary of the junctional and nonjunctional adhesive mechanisms used by animal cells in binding to one another and to the extracellular matrix. The junctional mechanisms are shown in epithelial cells, while the nonjunctional mechanisms are shown in nonepithelial cells. A junctional interaction is operationally defined as one that can be seen as a specialized region of contact by conventional and/or freeze-fracture electron microscopy. Note that the integrins and cadherins are involved in both nonjunctional and junctional cell-cell (cadherins) and cell-matrix (integrins) contacts. The cadherins generally mediate homophilic interactions, whereas the integrins mediate heterophilic interactions. Both the cadherins and integrins act as transmembrane linkers and depend on extracellular divalent cations to function; for this reason, most cell-cell and cell-matrix contacts are divalent-cation-dependent. The selectins and integrins can also act as heterophilic cell-cell adhesion molecules: the selectins bind to carbohydrate, while the cell-binding integrins bind to members of the immunoglobulin superfamily. The integrins and integral membrane proteoglycans that mediate nonjunctional adhesion to the extracellular matrix are discussed later.

Figure 19-29 Importance of the cytoskeleton in cell adhesion.   This drawing illustrates why cell-adhesion molecules must be linked to the cytoskeleton in order to mediate robust cell-cell or cell-matrix adhesion. In reality, many adhesion proteins would probably be pulled from the cell with bits of attached membrane, and the holes left in the membrane would immediately reseal.

IV. Cell Coat and The Extracellular Matrix

Figure 19-30 Cells surrounded by spaces filled with extracellular matrix.    The particular cells shown in this low-power electron micrograph are those in an embryonic chick limb bud. The cells have not yet acquired their specialized characteristics. (Courtesy of Cheryll Tickle.)

Figure 19-31 The connective tissue underlying an epithelial cell sheet.    It consists largely of extracellular matrix that is secreted by the fibroblasts.

Figure 19-32 Scanning electron micrograph of fibroblasts in connective tissue. The tissue is from the cornea of a rat. The extracellular matrix surroun-ding the fibroblasts is composed largely of collagen fibrils. The glycoproteins, glycosaminoglycans, and proteoglycans, which normally form a hydrated gel filling the interstices of the fibrous network, have been removed by enzyme and acid treatment. (T. Nishida et al. Invest. Ophthalmol. Vis. Sci. 29:1887-1890)

Figure 19-57 The comparative shapes and sizes of some of the major extracellular matrix macromolecules.    Protein is shown in green, glycosaminoglycan in red.

Figure 19-33 The repeating disaccharide sequence of a dermatan sulfate glycosaminoglycan (GAG) chain.    These chains are typically 70 to 200 sugar residues long. There is a high density of negative charges along the chain resulting from the presence of both carboxyl and sulfate groups.

Figure 19-35 The repeating disaccharide sequence in hyaluronan, a relatively simple GAG.    It consists of a single long chain of up to 25,000 sugar residues. Note the absence of sulfate groups.

Figure 19-36 The linkage between a GAG chain and its core protein in a proteoglycan molecule.    A specific link tetrasaccharide is first assembled on a serine residue. In most cases it is not clear how the serine residue is selected, but it seems to be a specific local conformation of the polypeptide chain, rather than a specific linear sequence of amino acids, that is recognized. The rest of the GAG chain, consisting mainly of a repeating disaccharide unit, is then synthesized, with one sugar residue being added at a time. In chondroitin sulfate the disaccharide is composed of D-glucuronic acid and N-acetyl-D-galactosamine; in heparan sulfate it is D-glucosamine (or ¬-iduronic acid) and N-acetyl-D-glucosamine; in keratan sulfate it is D-galactose and N-acetyl-D-glucosamine.

Figure 19-37 Examples of a large ( aggrecan ) and a small (decorin) proteoglycan found in the extracellular matrix.    They are compared to a typical secreted glycoprotein molecule (pancreatic ribonuclease B). All are drawn to scale. The core proteins of both aggrecan and decorin contain oligosaccharide chains as well as the GAG chains, but these are not shown. Aggrecan typically consists of about 100 chondroitin sulfate chains and about 30 keratan sulfate chains linked to a serine-rich core protein of almost 3000 amino acids. Decorin "decorates" the surface of collagen fibrils, hence its name.

Figure 19-38 An aggrecan aggregate from fetal bovine cartilage.  (A) Electron micrograph of an aggrecan aggregate shadowed with platinum. Many free aggrecan molecules are also seen. (B) Schematic drawing of the giant aggrecan aggregate shown in (A). It consists of about 100 aggrecan monomers (each like the one shown in Figure 19-37) noncovalently bound to a single hyaluronan chain through two link proteins that bind to both the core protein of the proteoglycan and to the hyaluronan chain, thereby stabilizing the aggregate; the link proteins are members of the hyaladherin family of hyaluronan-binding proteins discussed previously. The molecular weight of such a complex can be 108 or more, and it occupies a volume equivalent to that of a bacterium, which is about 2 x 10-12 cm. (A, courtesy of Lawrence Rosenberg.)

Figure 19-39 Electron micrograph of proteoglycans in the extracellular matrix of rat cartilage.    The tissue was rapidly frozen at -196°C and fixed and stained while still frozen (a process called freeze substitution) to prevent the GAG chains from collapsing. The proteoglycan molecules are seen to form a fine filamentous network in which a single striated collagen fibril is embedded. The more darkly stained parts of the proteoglycan molecules are the core proteins; the faintly stained threads are the GAG chains. (E.B. Hunziker et al., J. Cell Biol. 98:277-282.)

Figure 19-40 The structure of a typical collagen molecule.    (A) A model of part of a single collagen a chain in which each amino acid is represented by a sphere. The chain contains about 1000 amino acid residues and is arranged as a left-handed helix with three amino acid residues per turn and with glycine as every third residue. Therefore an a chain is composed of a series of triplet Gly-X-Y sequences in which X and Y can be any amino acid (although X is commonly proline and Y is commonly hydroxyproline). (B) A model of a part of a collagen molecule in which three alpha chains, each shown in a different color, are wrapped around one another to form a triple-stranded helical rod. Glycine is the only amino acid small enough to occupy the crowded interior of the triple helix. Only a short length of the molecule is shown; the entire molecule is 300 nm long. (From model by B.L. Trus.)

Collagen

Figure 19-41 Electron micrograph of fibroblasts surrounded by collagen fibrils in the connective tissue of embryonic chick skin.    The fibrils, which are organized into bundles that run approximately at right angles to one another, are produced by the fibroblasts. These cells contain abundant endoplasmic reticulum, where secreted proteins such as collagen are synthesized. (C. Ploetz et al. J. Struct. Biol. 106:73-81)

Table 19-4  Some Types of Collagen and Their PropertiesType Molecular Formula

Polymerized Form Tissue Distribution

FIBRIL-FORMING (FIBRILLAR)

I [  1(I)]2  2(I) fibril bone, skin, tendon, ligaments, cornea, internal organs (accounts for 90% of body collagen)

II [  1(II)]3 fibril cartilage, intervertebral disc, notochord, vitreous humor of the eye

III [  1(III)]3 fibril skin, blood vessels, internal organs

V [  1(V)]2  2(V) fibril (with type I)

as for type I

XI

 1(XI)  2(XI)  3(XI) fibril (with type II)

as for type II

FIBRIL-ASSOCIATED IX

 1(IX)  2(IX)  3(IX) with type II fibrils

lateral association

cartilage

XII

[  1(XII)]3 with some type I fi

brils

lateral association

tendon, ligaments, some other tissues

NETWORK-FORMING IV

[  1(IV)2  2(IV) sheetlike network

basal laminae

VII

[  1(VII)]3 anchoring fibrils

beneath stratified squamous epithelia

Note that types I, IV, V, and XI are each composed of 2 or 3 types of alpha chain, whereas types II, III, VII, and XII are composed of only 1 type of a chain each. Only 9 types of collagen are shown, but about 15 types of collagen and about 25 types of alpha chain have been defined so far.

Figure 19-42 Hydroxylysine and hydroxyproline residues.  These modified amino acids are common in collagen; they are formed by enzymes that act after the lysine and proline are incorporated into procollagen molecules

Figure 19-43 The intracellular and extracellular events involved in the formation of a collagen fibril.    Note that collagen fibrils are shown assembling in the extracellular space contained within a large infolding in the plasma membrane. As one example of how the collagen fibrils can form ordered arrays in the extracellular space, they are shown further assembling into large collagen fibers, which are visible in the light microscope.

Figure 19-44 How the staggered arrangement of collagen molecules gives rise to the striated appearance of a negatively stained fibril. (A) Since the negative stain fills only the space between the molecules, the stain in the gaps between the individual molecules in each row accounts for the dark staining bands. An electron micrograph of a portion of a negatively stained fibril is shown below (B). The staggered arrangement of the collagen molecules maximizes the tensile strength of the aggregate. (B, courtesy of Robert Horne.)

Figure 19-45 The covalent intramolecular and intermolecular cross-links formed between modified lysine side chains within a collagen fibril.    The cross-links are formed in several steps. First, certain lysine and hydroxylysine residues are deaminated by the extracellular enzyme lysyl oxidase to yield highly reactive aldehyde groups. The aldehydes then react spontaneously to form covalent bonds with each other or with other lysine or hydroxylysine residues. Most of the cross-links form between the short nonhelical segments at each end of the collagen molecules.

Figure 19-46 Electron micrograph of a cross-section of tadpole skin.    Note the plywoodlike arrangement of collagen fibrils, in which successive layers of fibrils are laid down nearly at right angles to each other. This arrangement is als

o found in mature bone and in the cornea. (Courtesy of Jerome Gross.)

Figure 19-47 Type IX collagen.    (A) Schematic drawing of type IX collagen molecules binding in a periodic pattern to the surface of a type-II-collagen-containing fibril. (B) Electron micrograph of a rotary-shadowed type-II-collagen-containing fibril in cartilage sheathed in type IX collagen molecules; an individual type IX collagen molecule is shown in (C). (B and C, from L. Vaughan et al., J. Cell Biol. 106:991-997.)

Figure 19-48 The shaping of the extracellular matrix by cells.    This micrograph shows a region between two pieces of embryonic chick heart (rich in fibroblasts as well as heart muscle cells) that has grown in culture on a collagen gel for four days. A dense tract of aligned collagen fibers has formed between the explants, presumably as a result of the fibroblasts in the explants tugging on the collagen. (D. Stopak and A.K. Harris, Dev. Biol. 90:383-398)

Figure 19-49 A network of elastic fibers.    These scanning electron micrographs show a low-power view of a segment of a dog's aorta (A) and a high-power view of the dense network of longitudinally oriented elastic fibers in the outer layer of the same blood vessel (B). All of the other components have been digested away with enzymes and formic acid. (K.S. Haas et al., Anat. Rec. 230:86-96.)

Elastin

Figure 19-50 Stretching a network of elastin molecules.    The molecules are joined together by covalent bonds (indicated in red) to generate a cross-linked network. In the model shown each elastin molecule in the network can expand and contract as a random coil, so that the entire assembly can stretch and recoil like a rubber band.

Figure 19-51 The structure of a fibronectin dimer.    As shown schematically in (A), the two polypeptide chains are similar but generally not identical. They are joined by two disulfide bonds near the carboxyl terminus. Each chain is almost 2500 amino acid residues long and is folded into five or six rodlike domains connected by flexible polypeptide segments. Individual domains are specialized for binding to a particular molecule or to a cell, as indicated for three of the domains. For simplicity, not all of the known binding sites are shown. (B) Electron micrographs of individual molecules shadowed with platinum; arrows mark the carboxyl termini. (C) The three-dimensional structure of a type III fibronectin repeat, as determined by nuclear magnetic resonance studies. It is the main type of repeating module in fibronectin and is also found in many other proteins. The Arg-Gly-Asp (RGD) sequence shown is part of the major cell-binding site (shown in blue in [A]) that we discuss in the text. (B,J. Engel et al., J. Mol. Biol. 150:97-120; C, A.L. Main et al., Cell 71:671-678)

Fibronectin

Figure 19-52 How type IV collagen molecules are thought to assemble into a multilayered network.    The model is based on electron micrographs of rotary-shadowed preparations of these molecules assembling in vitro. (Based on P.D. Yurchenco et al., J. Histochem. Cytochem. 34:93-102)

Figure 19-53 Three ways in which basal laminae (yellow lines) are organized.    They surround certain cells (such as muscle cells), underlie epithelial cell sheets, and are interposed between two cell sheets (as in the kidney glomerulus). Note that in the kidney glomerulus both cell sheets have gaps in them, so that the basal lamina serves as the permeability barrier determining which molecules will pass into the urine from the blood.

Figure 19-54 Scanning electron micrograph of a basal lamina in the cornea of a chick embryo.    Some of the epithelial cells (E) have been removed to expose the upper surface of the matlike basal lamina (BL). A network of collagen fibrils (C) in the underlying connective tissue interacts with the lower face of the lamina. (Courtesy of Robert Trelstad.)

Figure 19-55 The structure of laminin.    A schematic drawing of a laminin molecule is shown in (A), and electron micrographs of laminin molecules shadowed with platinum are shown in (B). This multidomain glycoprotein is composed of three polypeptides (A, B1, and B2) that are disulfide bond

ed into an asymmetric crosslike structure. Each of the polypeptide chains is more than 1500 amino acid residues long. Three types of Alpha chains, three types of B1 chains, and two types of B2 chain

s have been identified, which in principle can associate to form 18 different laminin isoforms. Several such isoforms have been found, each with a characteristic tissue distribution. There are also several isoforms of type IV collegen, each with a distinctive tissue distribution. Thus basal laminae are chemically diverse, which is not surprising in view of their functional diversity. (J. Engel et al., J. Mol. Biol. 150:97-120)

Figure 19-56 A current model of the molecular structure of a basal lamina. The basal lamina (A) is formed by specific interactions between the proteins type IV collagen, laminin, and entactin plus the proteoglycan perlecan (B). Arrows in (B) connect molecules that can bind directly to each other. (Based on P.D. Yurchenco and J.C. Schittny, FASEB J. 4:1577-1590.)

Figure 19-58 Regeneration experiments indicating the special character of the junctional basal lamina at a neuromuscular junction.    When the nerve, but not the muscle, is allowed to regenerate after both the nerve and muscle have been damaged (upper part), the junctional lamina directs the regenerating nerve to the original synaptic site. When the muscle, but not the nerve, is allowed to regenerate (lower part), the junctional lamina causes newly made acetylcholine receptors to accumulate at the original synaptic site. These experiments show that the junctional basal lamina controls the localization of other components of the synapseon both sides of the lamina.

Figure 19-59 Importance of cell-surface-receptor-bound protease.    In (A) human prostate cancer cells make and secrete the serine protease UPA, which binds to cell-surface UPA receptor proteins. In (B) the same cells have been transfected with DNA that encodes an excess of an inactive form of UPA, which binds to the UPA receptors but has no protease activity; by occupying most of the UPA receptors, the inactive UPA prevents the active protease from binding to the cell surface. Both types of cells secrete active UPA, grow rapidly, and produce tumors when injected into experimental animals. But the cells in (A) metastasize widely, whereas the cells in (B) do not. In order to metastasize, tumor cells have to crawl through basal laminae and other extracellular matrices on the way into and out of the bloodstream. This experiment therefore suggests that proteases must be cell-surface bound to mediate migration through the matrix.

Figure 19-60 The subunit structure of an integrin cell-surface matrix receptor.    Electron micrographs of isolated receptors suggest that the molecule has approximately the shape shown, with the globular head projecting more than 20 nm from the lipid bilayer. By binding to a matrix protein outside the cell and to the actin cytoskeleton inside the cell, the protein serves as a transmembrane linker. The alpha and beta chains are both glycosylated and are held together by noncovalent bonds.

Figure 19-29 Importance of the cytoskeleton in cell adhesion.    This drawing illustrates why cell-adhesion molecules must be linked to the cytoskeleton in order to mediate robust cell-cell or cell-matrix adhesion. In reality, many adhesion proteins would probably be pulled from the cell with bits of attached membrane, and the holes left in the membrane would immediately reseal.

Figure 19-61 Coalignment of extracellular fibronectin filaments and intracellular actin filament bundles.    The fibronectin is visualized in two rat fibroblasts in culture by the binding of rhodamine-coupled anti-fibronectin antibodies (A). The actin is visualized by the binding of fluorescein-coupled anti-actin antibodies (B). (R.O. Hynes and A.T. Destree, Cell 15:875-886)

Figure 19-62 How the extracellular matrix could propagate order from cell to cell within a tissue.    For simplicity, the figure represents a hypothetical scheme in which one cell influences the orientation of its neighboring cells. It is more likely, however, that the cells would mutually affect one another's orientation.

Figure 19-63 Cells can regulate the activity of their integrins. In (A) cell activation leads to a change in the extracellular binding site of the integrin so that it can now mediate cell adhesion. In (B) the tyrosine phosphorylation of the cytoplasmic tail of the integrins impairs their ability to bind to the actin cytoskeleton. As integrins must bind to the cytoskeleton to mediate robust cell-matrix adhesion, the phosphorylation causes the integrins to relax their grip on the extracellular matrix.

Cell wallsA. Plant cell walls provide protection against abrasion,

osmotic stress, and pathogens.

B. Microfibrils of cellulose form the fibrous component of the cell wall.

C. The matrix of cell wall contains hemicellulose, pectins, and hydroxyproline-rich,proline-rich, and glycine-rich structural proteins.

Figure 19-64 Plant cell walls.    (A) Electron micrograph of the root tip of a rush, showing the organized pattern of cells that results from an ordered sequence of cell divisions in cells with rigid cell walls. (B) Section of a typical cell wall separating two adjacent plant cells. The two dark transverse bands correspond to plasmodesmata that span the wall. (A, courtesy of Brian Gunning; B, courtesy of Jeremy Burgess.)

Figure 19-65 Scale model of a portion of a primary cell wall showing the two major polysaccharide networks.    The orthogonally arranged layers of cellulose microfibrils (green) are cross-linked into a network by H-bonded hemicellulose (red). This network is coextensive with a network of pectin polysaccharides (blue). The cellulose and hemicellulose network provides tensile strength, while the pectin network resists compression. Cellulose, hemicellulose, and pectin are typically present in roughly equal quantities in a primary cell wall. The middle lamella is pectin rich and cements adjacent cells together.

Figure 19-66 The orientation of cellulose microfibrils in the primary cell wall of an elongating carrot cell.    This electron micrograph of a shadowed replica from a rapidly frozen and deep-etched cell wall shows the largely parallel arrange-ments of cellulose microfibrils, oriented perpendicular to the axis of cell elongation. The microfibrils are cross-linked by, and interwoven with, a complex web of matrix molecules. (Brian Wells and Keith Roberts.)

Figure 19-68 The cortical array of microtubules in a plant cell.    (A) A grazing section of a root-tip cell from Timothy grass, showing a cortical array of microtubules lying just below the plasma membrane. These microtubules are oriented perpendicular to the long axis of the cell. (B) An isolated onion root-tip cell. (C) The same cell stained by immunofluorescence to show the transverse cortical array of microtubules. (A, courtesy of Brian Gunning; B and C, courtesy of K

im Goodbody.)

Figure 19-69 One model of how the orientation of newly deposited cellulose microfibrils might be determined by the orientation of cortical microtubules.    The large cellulose synthase complexes are integral membrane proteins that continuously synthesize cellulose microfibrils on the outer face of the plasma membrane. The distal ends of the stiff microfibrils become integrated into the texture of the wall, and their elongation at the proximal end pushes the synthase complex along in the plane of the membrane. Because the cortical array of microtubules is attached to the plasma membrane in a way that confines this complex to defined membrane channels, the microtubule orientation determines the axis along which the microfibrils are laid down.

作 业

• 简述细胞黏附分子、细胞外基质与肿瘤细胞迁移的关系。