細(xì)胞質(zhì)膜與細(xì)胞表面講義_cell_membrane_and_cell_surfa-資料下載頁

　　

【正文】 interacting plasma membranes, seen in crosssection, are connected by parallel rows of junctional proteins. The rows, which have a regular periodicity, are seen as dense bars or septa. (From . Gilula, in Cell Communication [. Cox, ed.], pp. 129) Septate junction Figure 1912 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 posed of a mixture of intracellular attachment proteins (including plakoglobin and desmoplakins). Each plaque is associated with a thick work of keratin filaments, which are attached to the surface of the plaque. Transmembrane linker proteins, which belong to the cadherin family of cellcell 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 . Gilula, in Cell Communication, pp. 129。 B, from . Kelly, JCB. 28:5159) Desmosomes Figure 1913 The distribution of desmosomes and hemidesmosomes in epithelial cells of the small intestine. The keratin filament works of adjacent cells are indirectly connected to one another through desmosomes and to the basal lamina through hemidesmosomes. Table 192 Anchoring Junctions Junction Transmembrane Linker Protein Extracellular Ligand Intracellular Cytoskeletal Attachment Some Intracellular Attachment Proteins Adherens (cellcell) cadherin (Ecadherin) cadherin in neighboring cell actin filaments catenins, vinculin, actinin, plakoglobin Desmosome cadherin (desmogleins amp。 desmocollins) cadherin in neighboring cell intermediate filaments desmoplakins, plakoglobin Adherens (cellmatrix) integrin extracellular matrix proteins actin filaments talin, vinculin, actinin Hemidesmosome integrin extracellular matrix (basal lamina) proteins intermediate filaments desmoplakinlike protein Figure 1914 Determining the size of a gapjunction 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 other cell but larger molecules cannot. Communicating junctions gapjunction channel Figure 1915 A model of a gap junction. The drawing shows the interacting plasma membranes of two adjacent cells. The apposed lipid bilayers (red) are perated 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 1916 Gap junctions as seen in the electron microscope. Thinsection (A) and freezefracture (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 . Gilula, in Cell Communication [. Cox, ed.], pp. 129) Figure 1133 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 1917 A proposed model for how gapjunction 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 pared 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 . Unwin and . Ennis, Nature 307:609613) Figure 1918 Summary of the various cell junctions found in animal cell epithelia. This drawing is based on epithelial cells of the small intestine. Figure 1919 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 mon to two connected cells. It usually also contains a fine tubular structure (2040nm), the desmotubule, derived from smooth endoplasmic reticulum. Figure 1920 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 crosssection. (Courtesy of R. Overall.) Figure 1921 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 cellcell adhesion mechanisms, including the formation of intercellular junctions. There Are Two Basic Ways in Which Animal Cells Assemble into Tissues III. Cell Adhension Figure 1922 An example of a more plex 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 n