BIOB10H3 Chapter Notes - Chapter 7: Globulin, Gap Junction, Desmosome

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18 Apr 2012
The outer layer of skin (the epidermis) is a type of epithelial tissue.
the epidermis consists largely of closely packed cells attached to one another and to an underlying
noncellular layer by specialized contacts
These contacts provide a mechanism for cells to adhere to and communicate with one another.
the deeper layer of the skin (the dermis) is a type of connective tissue.
Extracellular Matrix:
Many types of animal cells are surrounded by an extracellular matrix (ECM)an organized
network of extracellular materials that is present beyond the immediate vicinity of the plasma
The ECM is more than an inert packing material or a nonspecific glue that holds cells together; it
often plays a key regulatory role in determining the shape and activities of the cell.
One of the best defined extracellular matrices is the basement membrane (or basal lamina), a
continuous sheet 50 to 200 nm thick that (1) surrounds nerve fibers, muscles, and fat cells, (2)
underlies the basal surface of epithelial tissues, such as the epidermis of the skin
ECM is mostly composed of macromolecules.
Most proteins of the extracellular space are typically extended, fibrous species. These proteins are
secreted into the extracellular space where they are capable of self-assembling into an
three-dimensional network.
the proteins of the ECM serve as trail markers, scaffolds, girders, wire, and glue
An ECM protein
Collagens comprise a family of fibrous glycoproteins that are present only in extracellular
Collagens are found throughout the animal kingdom and are noted for their high tensile strength,
that is, their resistance to pulling forces.
It is estimated that a collagen fiber 1 mm in diameter is capable of suspending a weight of 10 kg
(22 lb) without breaking.
Collagen is the single most abundant protein in the human body.
Collagen is produced primarily by fibroblasts, the cells found in various types of connective
tissues, and also by smooth muscle cells and epithelial cells
To date, 27 distinct types of human collagen have been identified.
All collagen molecules are trimers consisting of three polypeptide chains, called alpha chains
Along at least part of their length, the three polypeptide chains of a collagen molecule are wound
around each other to form a unique, rodlike triple helix
The alpha chains of collagen molecules contain large amounts of proline
A number of the collagens, including types I, II, and III, are described as fibrillar collagens
because they assemble into rigid, cable-like fibrils, which in turn become packaged into thicker
fibers that are typically large enough to be seen in the light microscope
Among the various components of the ECM, collagen molecules provide the insoluble framework
that determines many of the mechanical properties of the matrix
Not all collagens form fibrils
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An ECM protein
polysaccharide complex
it consists of a core protein molecule to which chains of glycosaminoglycans (GAGs) are
covalently attached
Each glycosaminoglycan chain is composed of a repeating disaccharide; that is, it has the
structure -A-B-A-B-A-, where A and B represent two different sugars
GAGs are highly acidic due to the presence of both sulfate and carboxyl groups attached to the
sugar ring
proteoglycans bind huge numbers of cations, which in turn bind large numbers of water
molecules. As a result, proteoglycans form a porous, hydrated gel that fills the extracellular space
like packing material
An ECM protein:
consists of a linear array of distinct ―building blocks‖ that gives each polypeptide a modular
Each fibronectin polypeptide is constructed from a sequence of approximately 30 independently
folding Fn modules
In fibronectin, the 30 or so structural modules combine to form five or six larger functional
domains, illustrated by the colored cylinders
An ECM protein
Laminins are a family of extracellular glycoproteins that consist of three different polypeptide chains
linked by disulfide bonds and organized into a molecule resembling a cross with three short arms and one
long arm
At least 15 different laminins have been identified.
extracellular laminins can greatly influence a cell’s potential for migration, growth, and differentiation.
Properties of ECM:
a static structure.
the ECM can exhibit dynamic properties, both in space and over time.
the components of an ECM are subject to continual degradation and reconstruction
The degradation of extracellular materials, along with cell-surface proteins, is accomplished largely by a
family of zinc-containing enzymes called matrix metalloproteinases (MMPs) that are either secreted into
the extracellular space or anchored to the plasma membrane
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