Chapter all: Cell Junctions

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1 Dec 2016
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Cell Junctions and the Extracellular Matrix
Direct and extracellular matrix interactions are social interactions between
cells and hold cells together. These interactions make multicellular life
possible - shape, strength, architecture of a life form.
Extracellular Matrix - a complex network of proteins and polysaccharide
chains that cells secrete.
Cell Junctions/Extracellular Matrix have two broad categories of tissues that
are found in all animals (see below figure):
Connective tissues - e.g. bone or tendon, are formed from an extracellular
matrix produced by cells in the matrix. The matrix that bears most of the
mechanical stress the tissue is subjected to, not the individual cells. Direct
attachments between cells are rare, but cells are attached to the matrix. Cell
matrix junctions link the cytoskeleton to the matrix, allowing the cells to move
through the matrix and monitor changes in its mechanical properties.
Epithelial tissues - e.g., the lining of the gut or the epidermal covering of the
skin, cells are bound together as sheets called epithelia.
basal lamina/basement membrane - thin mat of cellular matrix within
epithelium.
cell-cell junction - direct connections in the epithelium.
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Anchoring junctions types of linkages between adjacent cellular
cytoskeletons:
Four major types of connections: Adherens Junctions,
Desmosomes, Actin-linked cell-matrix junctions,
hemidesmosomes
Other connection types: tight junctions, gap junctions
Anchoring junction types depends on transmembrane adhesion proteins
that span the plasma membrane, with one end linking to the cytoskeleton
inside the cell and the other end linking to other structures outside it. There
are two protein superfamilies, corresponding to two kinds of external
attachments:
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Cadherin superfamily chiefly mediate attachment of cell to cell
Integrin superfamily chiefly mediate attachment of cells to matrix.
CELLCELL JUNCTIONS
Cadherins Form a Diverse Family of Adhesion Molecules
Cadherins are present in all multicellular animals whose genomes have been
analyzed.
They are also present in the choanoflagellates (unicellular organisms
or multicellular colonies), thought to represent the group of protists
from which all animals evolved.
Lacking Cadherins: Fungi and plants, bacteria and archaea.
Cadherins names for reliance on Ca2+ ions. Removing Ca will cause
adhesions to break up (thus Ca2+ mediated).
Classical and nonclassical cahejrins make up the Cadherin Superfamily
Classical cadherins are closely related in sequence throughout their
extracellular and intracellular domains. 3 first cahedrin types to be discovered:
E- epithelial cells, N - nerve cells, P - placenta and epidermis. All are
also found on other tissue and are closely related in sequence.
There are a large number of nonclassical cadherins distantly related in
sequence.
More than 50 expressed in the brain alone
Proteins with adhesive function: protocadherins, desmocollins,
desmogleins
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Document Summary

Cell junctions and the extracellular matrix: direct and extracellular matrix interactions are social interactions between cells and hold cells together. The matrix that bears most of the mechanical stress the tissue is subjected to, not the individual cells. Direct attachments between cells are rare, but cells are attached to the matrix. There are two protein superfamilies, corresponding to two kinds of external attachments: cadherin superfamily chiefly mediate attachment of cell to cell. Integrin superfamily chiefly mediate attachment of cells to matrix. Removing ca will cause adhesions to break up (thus ca2+ mediated): classical and nonclassical cahejrins make up the cadherin superfamily, classical cadherins are closely related in sequence throughout their extracellular and intracellular domains. 3 first cahedrin types to be discovered: e- epithelial cells, n - nerve cells, p - placenta and epidermis. Individual cadehrins bond weakly, but there are many of them velcro principle .

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