Essential Cell Biology 5th edition

Epithelial Sheets and Cell Junctions
701
aggrecan-hyaluronan aggregate
1 µm
core protein
link protein
hyaluronan
molecule
chondroitin sulfate
keratan
sulfate
aggrecan
(A)
(B)
1 µm
Figure 20–17 Proteoglycans and GAGs can form large aggregates. (A) Electron micrograph of an aggrecan–
hyaluronan aggregate from cartilage, spread out on a flat surface. (B) Schematic drawing of the giant aggregate
illustrated in (A), showing how it is built up from aggrecan subunits bristling with numerous GAG chains—chondroitin
sulfate (long blue lines) and keratan sulfate (short blue lines)—attached to a core protein (light green). These
subunits then aggregate via link proteins (green) to the GAG hyaluronan (blue). The mass of such a complex can be
10 8 daltons or more, and it occupies a volume equivalent to that of a bacterium, which is about 2 × 10 –12 cm 3 .
(A, from J.A. Buckwalter, P.J. Roughley, and L.C. Rosenberg, Microscopy Research & Technique 28:398–408, 1994.
With permission from John Wiley & Sons.)
Even at very low concentrations, GAGs form hydrophilic gels: their multiple
negative charges attract a cloud of cations, such as Na + , that are
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osmotically active, causing large amounts of water to be sucked into the
matrix. This gives rise to a swelling pressure, which is balanced by tension
in the collagen fibers interwoven with the GAGs. When the matrix
is rich in collagen and large quantities of GAGs are trapped in the mesh,
both the swelling pressure and the counterbalancing tension are enormous.
Such a matrix is tough, resilient, and resistant to compression. The
cartilage matrix that lines the knee joint, for example, has this character:
it can support pressures of hundreds of kilograms per square centimeter.
Proteoglycans perform many sophisticated functions in addition to providing
hydrated space around cells. They can form gels of varying pore
size and charge density that act as filters to regulate the passage of molecules
through the extracellular medium. They can bind secreted growth
factors and other proteins that serve as extracellular signals for cells.
They can block, encourage, or guide cell migration through the matrix.
In all these ways, the matrix components influence the behavior of cells,
often the same cells that make the matrix—a reciprocal interaction that
has important effects on cell differentiation and the arrangement of cells
in a tissue. Much remains to be learned about how cells weave the tapestry
of matrix molecules and how the chemical messages they deposit in
this intricate biochemical fabric are organized and act.
QUESTION 20–3
Proteoglycans are characterized by
the abundance of negative charges
on their sugar chains. How would
the properties of these molecules
differ if the negative charges were
not as abundant?
EPITHELIAL SHEETS AND CELL JUNCTIONS
There are more than 200 visibly different cell types in the body of a
vertebrate. The majority of these are organized into epithelia (singular
epithelium)—multicellular sheets in which adjacent cells are joined