58
UNIT 2
Basic Tissues
LEUKOCYTES
,
white blood cells, are considered the transient
cells of connective tissue. They migrate from the blood vessels
into connective tissue by the process of
diapedesis
. This process
increases greatly during various infl ammatory conditions. After
entering connective tissue, leukocytes, with the exception of
lymphocytes, do not return to the blood. The following leuko-
cytes are commonly found in connective tissue: (1)
Lymphocytes
:
These cells have a round or bean-shaped nucleus and are often
located in the subepithelial connective tissue. (2)
Neutrophils
(
polymorphs
): Each cell has a multilobed nucleus and functions
in the defense against infection. (3)
Eosinophils
: Each cell has a
bilobed nucleus and reddish granules in the cytoplasm (Figs. 4-2
and 4-3). They have antiparasitic activity and moderate the aller-
gic reaction function. (4)
Basophils
: These cells are not easy to
± nd in normal tissues. Their primary function is similar to that of
mast cells. A detailed account of the structure and the function of
leukocytes is given in Chapter 8, “Blood and Hemopoiesis.”
ADIPOCYTES (FAT CELLS)
arise from undifferentiated mes-
enchymal cells of connective tissue. They gradually accumu-
late cytoplasmic fat, which results in a signi± cant fl
attening of
the nucleus in the periphery of the cell.
Adipocytes
are found
throughout the body, particularly in loose connective
tissue (Figs.
4-2 and 4-18). Their function is to store energy in the form of
triglycerides and to synthesize hormones such as
leptin
.
Connective Tissue Fibers
Three types of ±
bers are found in connective tissue:
colla-
gen
,
elastic
, and
reticular
. The amount and type of ± bers that
dominate a connective tissue are a refl
ection of the structural
support needed to serve the function of that particular tissue.
These three ±
bers all consist of proteins that form elongated
structures, which, although produced primarily by ± broblasts,
may be produced by other cell types in certain locations. For
example, collagen and elastic ±
bers can be produced by smooth
muscle cells in large arteries and chondrocytes in cartilages.
COLLAGEN FIBERS
are the most common and widespread
±
bers in connective tissue and are composed primarily of type
I collagen. The collagen molecule (
tropocollagen
) is a product
of the ±
broblast. Each collagen molecule is 300 nm in length
and consists of three polypeptide amino acid chains (
alpha
chains
) wrapped in a right-handed triple helix. The molecules
are arranged head to tail in overlapping parallel, longitudinal
rows with a gap between the molecules within each row to
form a
collagen f
bril
. The parallel array of ±
brils forms cross-
links to one another to form the collagen ±
ber. Collagen ± bers
stain readily with acidic and some basic dyes. When stained
with H&E and viewed with the light microscope, they appear
as pink, wavy ±
bers of different sizes (Fig. 4-13). When stained
with osmium tetroxide for EM study, the ±
bers have a transverse
banded pattern (light–dark) that repeats every 68 μm along the
±
ber. The banded pattern is a refl ection of the arrangement
of collagen molecules within the ±
brils of the collagen ± ber
(Figs. 4-5 to 4-7).
ELASTIC FIBERS
stain glassy red with H&E but are best
demonstrated with a stain speci± cally for elastic ±
bers, such
as
aldehyde fuchsin. Elastic ± bers have a very resilient nature
(stretch and recoil), which is important in areas like the lungs,
aorta, and skin. They are composed of two proteins,
elastin
and
f brillin
, and do not have a banding pattern. These ± bers are pri-
marily produced by the ± broblasts but can also be produced by
smooth muscle cells and chondrocytes (Figs. 4-8 and 4-9).
RETICULAR FIBERS
are small-diameter ±
bers that can only
be adequately visualized with silver stains; they are called
argy-
rophilic f
bers
because they appear black after exposure to sil-
ver salts (Figs. 4-10 and 4-11). They are produced by modi±
ed
± broblasts (
reticular cells
) and are composed of type III colla-
gen. These small, dark-staining ±
bers form a supportive, mesh-
like framework for organs that are composed mostly of cells
(such as the liver, spleen, pancreas, lymphatic tissue, etc.).
Ground Substance o± Connective Tissue
Ground substance
is a clear, viscous substance with a high
water content, but with very little morphologic structure.
When stained with basic dyes (periodic acid-Schiff [PAS]), it
appears amorphous, and with H&E, it appears as a clear space.
Its major component is
glycosaminoglycans (GAGs)
, which
are long, unbranched chains of
polysaccharides
with repeating
disaccharide units. Most GAGs are covalently bonded to a large
central protein to form larger molecules called
proteoglycans
.
Both GAGs and proteoglycans have negative charges and attract
water. This semifl
uid gel allows the diffusion of water-soluble
molecules but inhibits movement of large macromolecules and
bacteria. This water-attracting ability of ground substance gives
us our extracellular body fl
uids.
Types o± Connective Tissues
CONNECTIVE TISSUE PROPER
Dense Connective Tissue
can be divided into
dense irregu-
lar connective tissue
and
dense regular connective tissue
.
Dense
irregular connective tissue
consists of few connective tissue cells
and many connective tissue ±
bers, the majority being type I col-
lagen ±
bers, interlaced with a few elastic and reticular ±
bers.
These ± bers are arranged in bundles without a de± nite orien-
tation. The dermis of the skin and capsules of many organs
are typical examples of dense irregular connective tissue (Figs.
4-13 and 4-14).
Dense regular connective tissue
also consists
of fewer cells and more ±
bers, with a predominance of type I
collagen ±
bers like the dense irregular connective tissue. Here,
the ±
bers are arranged into a de±
nite linear pattern. Fibroblasts
are arranged linearly in the same orientation. Tendons and liga-
ments are the most common examples of dense regular connec-
tive tissue (Fig. 4-15).
Loose Connective Tissue
,
also called
areolar connective
tissue
, is characterized by abundant ground substance, with
numerous connective tissue cells and fewer ±
bers (more cells
and fewer ±
bers) compared to dense
connective
tissue. It is
richly vascularized, fl
exible, and not highly resistant to stress.
It provides protection, suspension, and support for the tissue.
The lamina propria of the digestive tract and the mesentery are
good examples of loose connective tissue (Figs. 4-16 and 4-17).
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