Part III Organization of Cell Populations Chapter 11 Multicellular organisms such as the human body consist of various tissues such as epithelial tissues, bones and nerves, and organs such as heart and liver The key to the formation of these tissues and organs is adhesion between cells and that between cells and extracellular matrix. Besides building tissues and organs, cell adhesion plays a number of important roles including intercellular communication through cell adhesion and the reception of various extracellular signals. In this chapter, we first look at the mechanism of cell adhesion and the structure of extracellular matrix components the targets of the adhesion. Then, the roles that cell adhesion plays in the functions of various cells and in tissue architecture are discussed. 11 I. Cell-Cell Adhesion Many types of cell-cell adhesion have been reported (Fig. 11-1). Major types will also be discussed in the section on tissue architecture. Here, we describe cadherins common molecules that connect cells together and play a range of important roles. Discovery of Cadherin and its Characteristics It has long been known in embryology that if an embryo is cultured in a Ca 2+ free medium, germ cells dissociate from each other, and reagguregate with the addition of Ca 2+ to the medium,. To investigate this phenomenon, the molecules that stick germ cells together in the presence of Ca 2+ were studied, and cadherin was discovered as a protein located on the plasma membrane called cadherin was discovered. Several types of tissue-specific cadherin exist. As examples, E-cadherins are found in epithelial tissues, and N-cadherins are found in nerve tissues. These cadherins are bound only between the same types (e.g., E-cadherin and E-cadherin), and the presence of Ca 2+ is essential for the adhesion via Cadherins. CSLS / THE UNIVERSITY OF TOKYO 211
Chapter 11 Cadherin a transmembrane protein that penetrates the plasma membrane adheres to a cell that has the same type of cadherin using the part protruding from the cell (the extracellular domain). Extracellular signals (such as signals that indicate adherence to the same type cell) are conveyed to the cell using the part inside cells of cadherin protein (the intracellular domain). The intracellular domain of cadherin is attached to proteins that transduce extracellular signals into the cell. The intracellular domain is also bound with actin filaments, allowing the regulation of cytoskeletal construction by extracellular signals (Fig. 11-2). Figure 11-1 Cell adhesion model Cell adhesions are classified into the binding of a cell to another cell or to extracellular matrix material. Cadherins, the immunoglobulin superfamily and selectins participate cell-cell adhesion. Integrins are involved in the adhesion of a cell to extracellular matrix material. Figure 11-2 Adhesion model of cadherin Cadherins not only mediate cell-cell adhesion, but also influence gene expression and the establishment of cytoskeletal networks. The intracellular domain of cadherin is bound with catenins (signal transducer proteins) and actin filaments (cytoskeletal components). C SLS / THE UNIVERSITY OF TOKYO 21 2
Roles of Cadherins Cadherins within one group bind only to themselves. This characteristic plays an important role in tissue construction (discussed later), enabling cells of the same type bind together. Some other unique roles of cadherins are discussed below. One of these is cell sorting, a well-known phenomenon that occurs during the developmental process of animals (Fig. 11-3A). As an example, if two types of cell, which have different cadherins are mixed and cultured, those with the same cadherins stick together and form two separate groups. This characteristic of cadherins plays an important role in the developmental process phenomenon in which a single cell group is separated into several cell groups to form different tissues and organs. Another phenomenon involving cadherins is contact inhibition (Fig. 11-3B). As an example, if the number of cultured cells increases up to the point where they start to come into contact with each other, the cells stop dividing. Similarly, if migrating cells come into contact with each other, they temporarily stop moving and change direction to avoid one another. This phenomenon involves some types of cell adhesion molecule located on the plasma membrane, including cadherins. 11 (A) (B) Figure 11-3 Cell sorting and contact inhibition (A) A model of cell sorting. If two types of cell with different cadherins are mixed, cells with the same cadherins selectively adhere to each other, forming two separate cell groups. (B) A model of contact inhibition. If normal cells come into contact with each other during their locomotion, they recognize one another through the contact and stop moving. Then, these cells start to avoid each other. CSLS / THE UNIVERSITY OF TOKYO 213
Other Cell-Cell Adhesion In addition to cadherins, cell-cell adhesion involves the immunoglobulin superfamily and selectins. The immunoglobulin superfamily is a group of proteins that share structural features with immunoglobulins, and include many cell adhesion transmembrane proteins. Selectins, on the other hand, are transmembrane proteins that selectively adhere to particular carbohydrate chains located on the plasma membrane surface of other cells, and three types are known. They are found in the vascular system of vertebrates. II. Extracellular Matrix Extracellular matrix a substance secreted by cells is a main component of the human body. As an example, collagen fiber a main component of extracellular matrix material represents 25 to 30% of the total protein mass of our bodies. It serves to reinforce the body structure and plasma membranes so that we can withstand gravity and tension, and plays a role as an adhesive substance and a signaling molecule for cells. The material has many constituents such as fibrous components, glycosaminoglycans, proteoglycans and glycoproteins (Fig. 11-1). Figure 11-4 Collagen filaments Collagen proteins as a basic unit are bundled in a fibrous form and constitute collagen fibers. Three collagen proteins, after glycosylation to amino acids, are linked with disulfide bonds to form a triple-helical structure. Prolines of collagens are also modified with addition of the hydroxyl group and become hydroxyprolines. Hydrogen bonds are formed between the hydroxyl group of the hydroxyproline and other amino acids, thereby making the fibers strong and stable. The triple-helical structures are chemically bound to form thick collagen fibers. The bar in the photo is 50 nm long. CSLS / THE UNIVERSITY OF TOKYO 214
Fibrous Components Collagen fiber is a typical fibrous extracellular matrix component. Such fibers are distributed all over the human body, and are essential for the maintenance of the body s structure. As an example, collagen fiber is the main component of skin and bones. The unit protein that makes up this fiber consists of a repetition of three amino acids (glycine-proline-x, where X is any amino acid). Collagen proteins are polymerized to form thick collagen fibers (Fig. 11-4). Among the many types of collagen fiber that exist, one with a long, fibrous structure and another with a sheet structure (i.e., a two-dimensional network) are dominant. The former is mainly found in parts to which pressure is applied (such as bones and the dermis layer of skin), and plays a reinforcing role. The latter exists as a main component of basal lamina (located on the basal side of epithelial tissues), around muscle cells, in glomeruli (which filter urine in the kidneys), and plays an important role in each place. Glycosaminoglycans and Proteoglycans 11 Glycosaminoglycans are macromolecules consisting of a repetition of two saccharide types. They are also called mucopolysaccharides because they exhibit viscosity in an aqueous solution due to their polymer structure. Their molecular weight ranges from 100,000 to as much as 10,000,000. Many glycosaminoglycans form complexes known as proteoglycans with proteins (Fig. 11-5). Many proteoglycans, due to the addition of a sulfate group to their carbohydrates, have a negative charge. Since the aggregates of glycosaminoglycans hydrophilic molecules retain many water molecules, they play a role in expanding intercellular space and also as a shock absorber against external pressure. Their polymer structure also gives viscosity to extracellular matrix material. Glycoproteins Proteins that contain oligosaccharides attached to the amino acids located at particular sites of proteins are collectively known as Glycoproteins. Approximately 60% of proteins have oligosaccharides attached to them, and are therefore glycoproteins. As an example, most of the proteins and secretory proteins incorporated in the plasma membrane are glycoproteins. Many of the CSLS / THE UNIVERSITY OF TOKYO 215
Figure 11-5 Glycosaminoglycans and proteoglycans Many glycosaminoglycans, which consist of a repeating disaccharide unit, exist as proteoglycans by binding to Serines of the core protein. Among glycosaminoglycans, chondroitin sulfate and heparan sulfate attach to the amino acid via three sugars. glycoproteins secreted as extracellular matrix components act as cell adhesive molecules. These molecules have a great impact on cell division, movement and differentiation, through cell adhesion. Column Basal Lamina The basal lamina is a layer containing collagen fibers, glycosaminoglycans, proteoglycans, and glycoproteins (Column Fig. 11-1), and plays a number of important roles. The basal laminae located on the basal side of epithelial tissues contribute to tissue stabilization by adhering to epithelial cells. The basal laminae located around muscle cells contribute to plasma membrane reinforcement, and those located in glomeruli urine filtration structures play a role in filtering urine. During the developmental process of animals, molecules in basal laminae also have essential roles on cell migration, differentiation and growth. Column Figure 11-1 Electron micrograph of a basal lamina This photo shows the basal lamina located along the basal side of an epithelial cell. The epithelial cell is firmly attached to the basal lamina, below which thick collagen filaments can be observed. The bar in the photo is 200 nm long. CSLS / THE UNIVERSITY OF TOKYO 216
Chapter 11 III. Adhesion of Cells and Extracellular Matrix Many types of molecule are known to be involved in the adhesion between cells and extracellular matrix, of which integrins proteins located in the plasma membrane play the most important role. Integrins Integrins are transmembrane proteins that penetrate the plasma membrane and bind to extracellular matrix using their extracellular domain. They convey extracellular signals (e.g., a signal indicating binding with extracellular matrix material) into the cell using the intracellular domain. Like cadherins, the intracellular domain is bound with actin filaments (Fig. 11-6). Integrins function as dimers for the two distinct subunits of α and β. Since there are many types of α and β, many combinations of subunits forming a dimer are possible. The large number of combinations reflects the diversity of the tissues 11 where the dimers are located, the target molecules to be adhered, and the integrin functions. Figure 11-6 A model of adhesion between integrins and extracellular matrix Integrins not only serve as adhesive molecules that bind to extracellular matrix, but are also involved in conveying extracellular signals into the cell and regulating cytoskeleton assembly. The intracellular domain of an integrin is therefore bound with focal adhesion kinase (FAK) a signaling protein and actin filaments of the cytoskeleton. C S L S / T H E U N IV E R S IT Y OF T OK YO 217
Extracellular matrix components, to which integrins attach, have a common structure for adhering to integrins that consists of the three-amino-acid sequence (arginine-glycine-aspartic acid). Integrins recognize this sequence and selectively adhere to it. The adhesion between integrins and extracellular matrix serves to connect cells to extracellular matrix. The adhesion of epithelial cells and basal laminae is an example of this (see the Column on p.216). This adhesion plays the further role of conveying extracellular signals into the cell, a process in which particular extracellular matrix components serve as signaling molecules. The signals from extracellular matrix have a great impact on cell division, migration, differentiation and survival. Column Extracellular Matrix in Plants The main extracellular matrix components of plants is the cell wall that surrounds cells. Besides wrapping cells, cell walls provide plant tissues with structural support by solidly binding cells together. The main constituents of cell walls include cellulose, hemicellulose, pectin and lignin. Cellulose, hemicellulose and pectin are polysaccharides consisting of linear chains of sugars. Lignin is a molecule in which propyl benzene derivatives (the aromatic rings with hydroxyl and methoxyl groups) are intricately combined, and reinforces cell walls. The cell walls are permeable, and molecules with a molecular mass of less than 20,000 can penetrate by diffusion. Column Figure 11-2 Structure of a cell wall CSLS / THE UNIVERSITY OF TOKYO 218
IV. Intercellular Communication Through cell adhesion, cells communicate with others and receive signals from the outside. This section discusses the mechanisms and specific examples. Cell Adhesion and Intracellular Signaling Many cell adhesion molecules penetrate plasma membranes, including cadherins and integrins. They play an important role in transmiting extracellular signals, which are received by adhering to other cells and extracellular matrix components, into the cell. The intracellular domain of cell adhesion molecules is associated with a protein that transfers extracellular signals into the cell. As an example, the intracellular domain of a cadherin binds with a catenin (a protein that regulates gene expression), and that of an integrin binds with a focal adhesion kinase (FAK) (an enzyme that phosphorylates amino acids). The mechanism of extracellular signals being transferred into a cell through cell adhesion is discussed here, using an integrin as an example. If the extracellular domain of an integrin adheres to a particular extracellular matrix component, the signal is transduced to the intracellular domain of the integrin, thus activating the focal adhesion kinase attached to the intracellular domain (Fig. 11-6). As a result, a protein in the intracellular signaling system (a signaling factor) a target of FAK is phosphorylated, thus passing the extracellular signal on to the intracellular signaling system. Consequently, the signal is transmited into the nucleus to initiate the expression of genes. 11 Cell Recognition Cells recognize the type and character of other cells by adhering to them. As an example, through cell adhesion mediated by a cadherin, a cell recognizes whether the cell being adhered to is of the same type. A number of other mechanisms are also in place to identify the property of the cell being adhered to in further detail. One is the recognition of sugar chains located on the surface of the cell. Since sugar chains on the cell surface reflect a cell s type and changes in its characteristics (e.g., changes exhibited by a cell that has turned cancerous, or differences in blood type), the character of the cell can be identified by recognizing the structural changes of sugar chains. CSLS / THE UNIVERSITY OF TOKYO 219
Cell Adhesion and Locomotion Since most of the cells that form the tissues and organs of the human body are firmly adhered to each other or to extracellular matrix, they cannot move freely. However, some cells, such as white blood cells, actively circulate throughout the body performing various roles on biological defense. During early development stage in animals, significant locomotion of germ cells is also observed within the whole embryo, and this plays an important role in forming the body structure (see Chapter 10). Thus, cells inherently have migration activity. However, those that adhere to each other to form tissues and organs cannot move freely. Nevertheless, once the adhesion is unlocked and certain stimuli are applied, such cells initiate locomotive activity. Malignant cancer cells are an example of this; with their cell adhesion broken, they move freely around the body and multiply at their destinations, causing cancer metastasis. Cell locomotion is regulated by many extracellular substances; especially diffusible chemoattractants and various adhesive molecules in extracellular matrix material play important roles. When these substances are bound to receptors on the plasma membrane such as integrins and receptors for chemoattractants, the signal is transmitted into the cell. As a result, various changes necessary for cell locomotion such as the degradation and reconstruction of the cytoskeleton, the contraction motion of the cell and the activation of intracellular substance transport, are induced, thus initiating cell locomotion (Fig. 11-7). Cells may be engaged in directional movement. White blood cells moving toward an inflamed area (Fig. 11-8) and cells known as cellular slime molds moving toward their food bacteria are examples of this. In such cases, cells detect the concentration of chemoattractants diffused from the target and move toward it. This phenomenon is called chemotaxis, and the chemoattractants that cause it are called chemotactic factors. When this cellular behavior occurs, the chemoattractant receptors located on the plasma membrane play an important role. Cells detect subtle changes in the concentration gradient of chemoattractants using their receptors, and the intracellular signaling pathway are activated. Cell protrusions are formed on the high-concentration side, and the formation of protrusions is suppressed on the opposite side. As a result, cellular locomotion toward the area of high chemoattractant concentration occurs (Fig. 11-9A). Another example of directional cellular movement is the phenomenon observed when neuron protrusions extend toward sensory cells and muscle cells. Although nerves, sensory organs and muscle tissues form independently during the developmental process, they are subsequently connected by a functional network of nerve fibers. For this purpose, it is necessary for the tip of the nerve fiber (called growth cone) to extend toward the target CSLS / THE UNIVERSITY OF TOKYO 220
(a) (c) (b) (d) Figure 11-7 A model showing the steps of cell locomotion (a) A static cell attached to matrix material. (b) To initiate movement, the cell breaks the adhesion on the side of the direction of movement (right) and forms a protrusion in that direction. (c) The protrusion adheres to the matrix material, and the adhesion on the other side (left) is detached. (d) Lastly, the posterior part of the cell is contracted to push the cytoplasm forward, allowing the forward movement of the cell. Figure 11-8 Directional movement of a cell A chemotactism model of a white blood cell. White blood cells travel in blood vessels in a rolling motion while loosely adhering to the vascular endothelial cells. When a cell detects a chemoattractant (a chemotactic factor) released from an inflamed site during circulation, its adhesion to the endothelial cells becomes stronger. The cell then slips through the endothelial cells out of the vessel and migrates toward the area of higher chemoattractant concentration. 11 (A) (B) Figure 11-9 Concentration gradient of chemoattractants and chemotaxis (A) A model of a cell being attracted by and moving toward chemoattractants. (1) The cell is exposed to the concentration gradient of the chemoattractants. (2) The chemoattractants bind to the receptors located on the plasma membrane on the high-concentration side at a high frequency. (3) As a result, cell protrusions are formed on the high-concentration side, while, the formation of protrusions is suppressed and cell contraction is induced on the opposite side. Through these steps, the cell moves toward the high-concentration area. (B) An extension model of neuron protrusions (growth cones). The extension direction of the growth cones moving toward the target is regulated by attractants or repulsion substances, such as diffusible chemicals and extracellular matrix components. Arrows indicate the extension direction of the cell protrusions. CSLS / THE UNIVERSITY OF TOKYO 221
cell and form synapse (Column Fig. 11-3) with it. For the nerve fiber to reach the target, a mechanism that accurately leads the extending tip is necessary. This mechanism is made possible by the receptors on the plasma membrane and the substances (ligands) that specifically bind to the receptors. Receptors bind to ligands may either extend protrusions toward an area with a high concentration of ligands or change the direction of the protrusions in a way that avoids the ligands (Fig. 11-9B). The extension direction of nerve fiber tips is regulated through these types of receptor. Plasmodesma of Plant Cells Unlike animal cells, plant cells are surrounded by a cell wall, meaning that they cannot adhere to each other in the same manner as animal cells. However, plant cells still need to communicate with each other, and for this purpose they have a structure known as plasmodesma (Fig. 11-10). In a plasmodesma, the plasma membranes of adjacent cells are merged, forming tube-like structure with a diameter of 20 100 nm through which the cytoplasm and endoplasmic reticula are shared by the cells. Substances with a molecular weight of 800 or less can freely pass through the plasmodesmata by diffusion. However, a far greater molecules (such as certain proteins, RNA and viruses) are also known to pass through the plasmodesma. Special mechanisms may be involved in this phenomenon. Plasmodesmata are similar to gap junctions in animal cells, in that cytoplasm is shared by two cells. The exchange of various substances through plasmodesmata allows intercellular communication. Figure 11-10 Plasmodesma of plant cells In plants, whose cells are surrounded by a cell wall, plasmodesmata are formed between adjacent cells, allowing them to share their cytoplasm. Endoplasmic reticula also pass through them between the cells. CSLS / THE UNIVERSITY OF TOKYO 222
Column Nerves and Synapses An information network to respond to stimuli and to take action based on the consideration of various factors is formed in the human body. This is the nervous system, and is centered on the brain and the spinal cord. Junctions called synapses connect the neurons that constitute the system with other neurons and cells such as sensory cells, muscle cells and secretory cells. Synapses are specialized cell adhesion junctions that are necessary to efficiently transmit the excitation of sensory cells and neurons to other neurons and effectors (muscle cells and secretory cells). Generally, cell excitation means changes in membrane potential. In other words, excitatory transmission by neurons means the transmission of changes in membrane potential to other cells via synapses. The two types of synapses are electrical synapses and chemical synapses (Column Fig. 11-3). Electrical synapses link two cells through gap junctions, which directly transmit changes in membrane potential between the cells. The transmission rate is therefore fast. On the other hand, chemical synapses indirectly transmit changes in membrane potential using chemical transmitters. For this reason, the transmission rate is relatively slow (although it still takes only milliseconds). In chemical synapses, a neuron on the excitement-transmitting side releases neurotransmitters such as adrenalin and acetylcholine, to the receiving cell. When these neurotransmitters bind to the receptors on the plasma membrane of the excitementreceiving cell, ion channels located on this plasma membrane open, causing changes in membrane potential of the receiving cell and allowing the transmission of the excitation. 11 Column Figure 11-3 Electrical and chemical synapses The excitatory transmission manner differs between electrical and chemical synapses. Electrical synapses transmit excitation directly through gap junctions, while chemical synapses transmit signals indirectly using chemical transmitters. CSLS / THE UNIVERSITY OF TOKYO 223
Column Junctional Complexes Junctional complexes join epithelial cells together and consist of a number of adhesion types (Column Fig. 11-4). First, tight junctions tightly join the cells together. This type of junction encompasses the epithelial cells and forms a close association between them; the resulting gap is impermeable even to ions. Second, desmosomes are structures that physically and firmly connect epithelial cells. In this type of adhesion, the cells are firmly connected through the linking of cell adhesion molecules, which are in the same group of cadherins, to intracellular intermediate filaments. Third, there is a special cell adhesion mechanism called a gap junction, which is composed of two ion channels linked in series. Gap junctions serve as cytoplasm connection paths, through which ions and other small molecules freely move to the adjacent cell. Signaling molecules such as Ca 2+ and camp can easily travel through gap junctions to mediate intercellular communication. Other important cell adhesion mechanisms in epithelial tissues is hemidesmosome for binding the basal side of epithelial cells to basal laminae. Hemidesmosome is the connection of integrins and extracellular matrix. Epithelial cells are firmly connected to basal lamina by linking cell adhesion molecules to intermediate filaments, resembling desmosomes, Column Figure 11-4 Junctional complexes that connect epithelial cells Epithelial cells are connected by adhesion structures called junctional complexes, which consist of tight junctions, desmosomes and gap junctions on the lateral side. On the basal side, cells are adhered to basal laminae by hemidesmosomes. CSLS / THE UNIVERSITY OF TOKYO 224
V. Tissue Architecture Epithelial tissues represent fundamental tissues in animals bodies. As examples, many tissues and organs, including exocrine glands (e.g., salivary glands and the pancreas), the skin, the liver, the kidneys, the lungs and the gastrointestinal tract mainly consist of epithelial tissues. In this section, the formation of epithelial tissues key players in tissue architecture and the roles cell adhesion plays in epithelial tissues are discussed. The direction of cells (cell polarity), which is one of the characteristics of epithelial cells, is also covered. Epithelial Tissues Epithelial tissues are sheet or tubular tissues consisting of cells connected by unique cell adhesion structures called junctional complexes (Column Fig. 11-4). Epithelial cells have apical, lateral and basal sides, and are bound on their lateral sides by junctional complexes. The basal side adheres to a basal lamina. One of the characteristics of epithelial tissues is their role in creating an impermeable wall separating two environments (the body s external and internal environments). Another characteristic is their active involvement in the transport of materials between the two environments. The roles played by epithelial tissues are closely associated with the junctional complexes formed between epithelial cells as well as with cell polarity. (B) 11 (A) Figure 11-11 Cell polarity of epithelial cells (A) A model of the cell polarity of epithelial cells. Epithelial cells have apical, basal and lateral sides. Organelles and the cytoskeleton are arranged in particular directions in association with the directional intracellular trafficking one of the roles of epithelial cells. (B) Cell polarity is observed in many cases other than epithelial cells. indicates the direction of polarity. CSLS / THE UNIVERSITY OF TOKYO 225
Cell Polarity Epithelial cells have cell polarity; the structure and functions differ between the apical side and the basal side (Fig. 11-11A). The nucleus, endoplasmic reticula, Golgi bodies and the cytoskeleton are arranged in certain directions in accordance with cell polarity. The reason epithelial cells have polarity is that they come into contact with and mediate between two different environments, and one of their important roles as a mediator is material transport between the two. The transport of nutrients from the outside to the inside of the body (through epithelial cells in the small intestine) and the transport of secretory materials from the inside to the outside of the body (through secretory cells in the pancreas and salivary glands) are examples of these functions. To fulfill these roles, epithelial cells have obtained polarity. There are many other examples of cell polarity besides epithelial cells (Fig. 11-11B), including amphibian eggs (which have animal and vegetal poles), asymmetric cell division, migrating cells (which have anterior and posterior directionality), and neurons (which have directionality in axons and dendrites). This cellular directionality is formed in accordance with the functions of each cell. CSLS / THE UNIVERSITY OF TOKYO 226
Summary Chapter 11 Multicellular organisms consist of cells and extracellular matrix (secretory substances). Extracellular matrix consists of many types of components and acts as a support for the body structure, a signaling molecule and an adhesive molecule for cells. Cells adhere to other cells as well as to extracellular matrix around them. Cell adhesion plays an important role in forming tissues and organs as well as in intercellular communication and obtaining extracellular information. The main molecules involved in cell adhesion are transmembrane proteins located on the plasma membrane, and are classified into several groups. Cell adhesion molecules convey extracellular signals, obtained through cell-cell adhesion and cell-extracellular matrix adhesion, into the cell. The signals conveyed into the cell via cell adhesion molecules influence various cellular functions (e.g., cell migration, growth and differentiation, and intracellular material transport) through the intracellular signal transduction system. Epithelial tissues are the main tissues formed by cell adhesion, and many parts of animal bodies consist of these tissues. The epithelial cells that make up epithelial tissues have directionality (cell polarity), and the apical and basal sides face the outside and inside of the body, respectively. Epithelial tissues serve as a wall separating two environments as well as a medium of transport between the environments. Plant cells have extracellular matrix consisting of cellulose and other substances, and intercellular communication is made through plasmodesmata. 11 CSLS / THE UNIVERSITY OF TOKYO 227
Problems [1] If two types of cell expressing different types of cadherin are dissociated and then reassembled and cultured, what arrangement might they take? [2] Explain the functions of tight junctions in normal tissues. [4] If a tobacco leaf is treated with enzymes, hemicellulase and cellulase, to degrade its cell walls, protoplasts are produced. How will these protoplasts be shaped in a solution with appropriate osmotic pressure? Also, explain why endogenous cellulase is necessary for plants to grow. [3] Which of the following substances can pass through both gap junctions and plasmodesmata? Amino acids, ATP, Ca 2+, phospholipids in the plasma membrane, endoplasmic reticula (Answers on p.258) CSLS / THE UNIVERSITY OF TOKYO 228