Chapter 4
Tissue: The Living Fabric
127
4
Ground Substance
Ground substance
is the unstructured material that fills the
space between the cells and contains the fibers. It is composed of
interstitial
(
tissue
)
fluid, cell adhesion proteins
, and
proteoglycans
(pro
0
te-o-gli
9
kanz). Cell adhesion proteins (
fibronectin
,
laminin
,
and others) serve mainly as a connective tissue glue that allows
connective tissue cells to attach to matrix elements. Te pro-
teoglycans consist of a protein core to which
glycosaminoglycans
(GAGs) (gli
0
kos-ah-me
0
no-gli
9
kanz) are attached. Te strand-
like GAGs, most importantly
chondroitin sulfate
and
hyaluronic
acid
(hi
0
ah-lu-ron
9
ik), are large, negatively charged polysaccha-
rides that stick out from the core protein like the fibers of a
bottle brush. Te proteoglycans tend to form huge aggregates in
which the GAGs intertwine and trap water, forming a substance
that varies from a fluid to a viscous gel. Te higher the GAG
content, the more viscous the ground substance.
Te ground substance consists of large amounts of fluid
and functions as a molecular sieve, or medium, through which
nutrients and other dissolved substances can diffuse between
the blood capillaries and the cells. Te fibers embedded in the
ground substance make it less pliable and hinder diffusion
somewhat.
Connective Tissue Fibers
Te fibers of connective tissue provide support. Tree types
of fibers are found in connective tissue matrix: collagen, elas-
tic, and reticular fibers. Of these, collagen fibers are by far the
strongest and most abundant.
Collagen fibers
are constructed primarily of the fibrous
protein
collagen
. Collagen molecules are secreted into the ex-
tracellular space, where they assemble spontaneously into cross-
linked fibrils, which in turn are bundled together into the thick
collagen fibers seen with a microscope. Because their fibrils
cross-link, collagen fibers are extremely tough and provide high
tensile strength (that is, the ability to resist being pulled apart)
to the matrix. Indeed, stress tests show that collagen fibers are
stronger than steel fibers of the same size!
Elastic fibers
are long, thin fibers that form branching net-
works in the extracellular matrix. Tese fibers contain a rubber-
like protein,
elastin
, that allows them to stretch and recoil like
rubber bands. Connective tissue can stretch only so much be-
fore its thick, ropelike collagen fibers become taut. Ten, when
the tension lets up, elastic fibers snap the connective tissue back
to its normal length and shape. Elastic fibers are found where
greater elasticity is needed, for example, in the skin, lungs, and
blood vessel walls.
Reticular fibers
are short, fine, collagenous fibers with a
slightly different chemistry and form. Tey are continuous with
collagen fibers, and they branch extensively, forming delicate
networks (
reticul
5
network) that surround small blood ves-
sels and support the soF tissue of organs. Tey are particularly
abundant where connective tissue abuts other tissue types, for
example, in the basement membrane of epithelial tissues, and
around capillaries, where they form fuzzy “nets” that allow
more “give” than the larger collagen fibers.
Connective Tissue
Indicate common characteristics of connective tissue, and
list and describe its structural elements.
Connective tissue
is the most abundant and widely distributed
of the primary tissues, but its amount in particular organs varies.
±or example, skin consists primarily of connective tissue, while
the brain contains very little.
Tere are four main classes of connective tissue and several
subclasses (
Table 4.1
on p. 129). Te main classes are (1)
con-
nective tissue proper
(which includes fat and the fibrous tissue of
ligaments), (2)
cartilage
, (3)
bone
, and (4)
blood
.
Connective tissue does much more than just
connect
body
parts. Its major functions include (1)
binding and supporting
,
(2)
protecting
, (3)
insulating
, (4)
storing
reserve fuel, and (5)
transporting
substances within the body. ±or example, bone and
cartilage support and protect body organs by providing the hard
underpinnings of the skeleton. ±at insulates and protects body
organs and provides a fuel reserve. Blood transports substances
inside the body.
Common Characteristics
of Connective Tissue
Connective tissues share three characteristics that set them
apart from other primary tissues:
Common origin.
All connective tissues arise from
mesen-
chyme
(an embryonic tissue).
Degrees of vascularity.
Connective tissues run the gamut of
vascularity. Cartilage is avascular. Dense connective tissue is
poorly vascularized, and the other types of connective tissue
have a rich supply of blood vessels.
Extracellular matrix.
All other primary tissues are composed
mainly of cells, but connective tissues are largely nonliving
extracellular matrix
(ma
9
triks; “womb”), which separates,
oFen widely, the living cells of the tissue. Because of its
matrix, connective tissue can bear weight, withstand great
tension, and endure abuses, such as physical trauma and
abrasion that no other tissue can tolerate.
Structural Elements of Connective Tissue
Connective tissues have three main elements:
ground substance
,
fibers
, and
cells
(²able 4.1). ²ogether ground substance and fi-
bers make up the extracellular matrix. (Note that some authors
use the term
matrix
to indicate the ground substance only.)
Te composition and arrangement of these three elements
vary tremendously. Te result is an amazing diversity of con-
nective tissues, each adapted to perform a specific function in
the body. ±or example, the matrix can be delicate and fragile to
form a soF “packing” around an organ, or it can form “ropes”
(tendons and ligaments) of incredible strength. Nonetheless,
connective tissues have a common structural plan, and we use
areolar connective tissue
(ah-re
9
o-lar) as our
prototype
, or model
(
Figure 4.7
and ±igure 4.8a). All other subclasses are simply
variants of this plan.
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