Tissue: The Living Fabric
is the unstructured material that ﬁlls the
space between the cells and contains the ﬁbers. It is composed of
ﬂuid, cell adhesion proteins
kanz). Cell adhesion proteins (
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
kanz) are attached. Te strand-
like GAGs, most importantly
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
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.
are constructed primarily of the ﬁbrous
. 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!
are long, thin ﬁbers that form branching net-
works in the extracellular matrix. Tese ﬁbers contain a rubber-
, 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.
are short, ﬁne, collagenous ﬁbers with a
slightly diﬀerent chemistry and form. Tey are continuous with
collagen ﬁbers, and they branch extensively, forming delicate
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.
Indicate common characteristics of connective tissue, and
list and describe its structural elements.
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
on p. 129). Te main classes are (1)
nective tissue proper
(which includes fat and the ﬁbrous tissue of
, and (4)
Connective tissue does much more than just
parts. Its major functions include (1)
binding and supporting
reserve fuel, and (5)
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.
of Connective Tissue
Connective tissues share three characteristics that set them
apart from other primary tissues:
All connective tissues arise from
(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.
All other primary tissues are composed
mainly of cells, but connective tissues are largely nonliving
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:
(²able 4.1). ²ogether ground substance and ﬁ-
bers make up the extracellular matrix. (Note that some authors
use the term
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
o-lar) as our
, or model
and ±igure 4.8a). All other subclasses are simply
variants of this plan.