130
UNIT 1
Organization of the Body
4
to the tissue. A glistening oil droplet (almost pure triglyceride)
occupies most of a fat cell’s volume and displaces the nucleus
to one side (Figure 4.8b). Mature adipocytes are among the
largest cells in the body. As they take up or release fat, they
become plumper or more wrinkled, respectively.
Adipose tissue is richly vascularized, indicating its high met-
abolic activity. Without the fat stores in our adipose tissue, we
could not live for more than a few days without eating. Adipose
tissue is certainly abundant: It constitutes 18% of an average
person’s body weight, and a chubby person’s body can be 50%
fat without being considered morbidly obese.
Adipose tissue may develop almost anywhere areolar tissue
is plentiful, but it usually accumulates in subcutaneous tissue,
where it acts as a shock absorber, as insulation, and as an energy
storage site. Because fat is a poor conductor of heat, it helps pre-
vent heat loss from the body. Other sites where fat accumulates
include surrounding the kidneys, behind the eyeballs, and at ge-
netically determined fat depots such as the abdomen and hips.
Te abundant fat beneath the skin serves the general nutrient
needs of the entire body, and smaller depots of fat serve the local
nutrient needs of highly active organs. Such depots occur around
the hard-working heart and around lymph nodes (where cells of
the immune system are furiously fighting infection), within some
muscles, and as individual fat cells in the bone marrow, where
new blood cells are produced at a rapid rate. Many of these local
depots are highly enriched in special lipids.
Te adipose tissue just described is sometimes called
white
fat
, or
white adipose tissue
, to distinguish it from
brown fat
, or
brown adipose tissue
. White fat stores nutrients (mainly for
other cells), but brown fat contains abundant mitochondria,
which use the lipid fuels to heat the bloodstream to warm the
body (rather than to produce A±P molecules). Te richly vas-
cular brown fat occurs mainly on the back of babies who (as
yet) lack the ability to produce body heat by shivering. Scant
deposits occur in adults, mostly above the collar bones, on the
neck and abdomen, and around the spine.
Reticular Connective Tissue
Reticular connective tissue
re-
sembles areolar connective tissue, but the only fibers in its matrix
are reticular fibers, which form a delicate network along which
fibroblasts called
reticular cells
(Figure 4.8c) are scattered. Al-
though reticular
fibers
are widely distributed in the body, re-
ticular tissue is limited to certain sites. It forms a labyrinth-like
stroma
(literally, “bed” or “mattress”), or internal framework,
that can support many free blood cells (mostly lymphocytes) in
lymph nodes, the spleen, and bone marrow.
Connective Tissue Proper—Dense Connective
Tissues
Te three varieties of dense connective tissue are dense regular,
dense irregular, and elastic. Since all three have fibers as their
prominent element, dense connective tissues are o²en called
fibrous connective tissues
.
Dense Regular Connective Tissue
Dense regular connective
tissue
contains closely packed bundles of collagen fibers run-
ning in the same direction, parallel to the direction of pull (Fig-
ure 4.8d). Tis arrangement results in white, flexible structures
Connective Tissue Proper—Loose Connective
Tissues
Connective tissue proper
has two subclasses:
loose connec-
tive tissues
(areolar, adipose, and reticular) and
dense connec-
tive tissues
(dense regular, dense irregular, and elastic). Except
for bone, cartilage, and blood, all mature connective tissues are
connective tissue proper.
Areolar Connective Tissue
Te functions of
areolar connec-
tive tissue
(Figure 4.8a) include:
Supporting and binding other tissues (the job of the fibers)
Holding body fluids (the ground substance’s role)
Defending against infection (via the activity of white blood
cells and macrophages)
Storing nutrients as fat (in fat cells)
Fibroblasts
, flat branching cells that appear spindle shaped
in profile, predominate, but numerous macrophages are also
seen and present a formidable barrier to invading microorgan-
isms. Fat cells appear singly or in clusters, and occasional mast
cells are identified easily by the large, darkly stained cytoplasmic
granules that o²en obscure their nuclei. Other cell types are
scattered throughout.
Te most obvious structural feature of this tissue is the
loose arrangement of its fibers. Te rest of the matrix, occupied
by ground substance, appears to be empty space when viewed
through the microscope, and in fact, the Latin term
areola
means
“a small open space.” Because of its loose nature, areolar connec-
tive tissue provides a reservoir of water and salts for surrounding
body tissues, always holding approximately as much fluid as there
is in the entire bloodstream. Essentially all body cells obtain their
nutrients from and release their wastes into this “tissue fluid.”
Te high content of hyaluronic acid makes its ground sub-
stance viscous, like molasses, which may hinder the movement
of cells through it. Some white blood cells, which protect the
body from disease-causing microorganisms, secrete the enzyme
hyaluronidase to liquefy the ground substance and ease their
passage. (Unhappily, some harmful bacteria have the same abil-
ity.) When a body region is inflamed, the areolar tissue in the
area soaks up excess fluids like a sponge, and the affected area
swells and becomes puffy, a condition called
edema
(ĕ-de
9
mah).
Areolar connective tissue is the most widely distributed con-
nective tissue in the body, and it serves as a universal packing
material between other tissues. It binds body parts together while
allowing them to move freely over one another; wraps small
blood vessels and nerves; surrounds glands; and forms the subcu-
taneous tissue, which cushions and attaches the skin to underly-
ing structures. It is the connective tissue that most epithelia rest
on and is present in all mucous membranes as the
lamina propria
.
(Mucous membranes line body cavities open to the exterior.)
Adipose (Fat) Tissue
Adipose tissue
(ad
9
ĭ-pōs) is similar
to areolar tissue in structure and function, but its nutrient-
storing ability is much greater. Consequently,
adipocytes
(ad
9
ĭ-po-sītz), commonly called
adipose
or
fat cells
, account
for 90% of this tissue’s mass. Te matrix is scanty and the cells
are packed closely together, giving a chicken-wire appearance
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