1 12 Unit 1 Organization of the Body
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(b) Merocrine gland
(c) Apocrine gland
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Figure 4.6 Modes of secretion in exocrine glands. (a) In holocrine glands, the
entire secretory cell ruptures, releasing secretions and dead cell fragments• (b) Merocrine glands secrete their products by exocytosis• (c) In apocrine glands, the apex of
each secretory cell pinches off and releases its secretions.
Holocrine (h6'-luh-krin) glands accumulate their
products within them until the secretory cells rupture. (They are replaced by the division of underlying
cells.) Since holocrine gland secretions include the
synthesized product plus dead cell fragments (holos
= all), you could say that their cells "die for their
cause." Sebaceous (oil) glands of the skin are the only
true example of holocrine glands (Figure 4.6a).
Apocrine (a'-puh-krin) glands also accumulate
their products, but in this case, accumulation occurs
only at the cell apex (just beneath its free surface).
Eventually, the apex of the cell pinches off (apo =
from, off) and the secretion is released. The cell repairs
its damage and repeats the process again and again.
The mammary glands and some sweat glands release
their secretions by this mechanism (Figure 4.6c).
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Connective tissue does much more than connect
body parts; it has many forms and many functions. Its
chief subclasses are connective tissue proper, carti-
lage, bone, and blood. Its major functions include
binding, support, protection, insulation, and, as blood,
transportation of substances within the body. For
example, cordlike connective tissue structures con-
nect muscle to bone (tendons) and bones to bones (ligaments), and fine, resilient connective tissue invades
soft organs and supports and binds their cells together.
Bone and cartilage support and protect body organs
by providing hard "underpinnings"; fat cushions,
insulates, and protects body organs as well as providing reserve energy fuel.
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Common Characteristics of
Connective Tissue
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Connective Tissue
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Despite their multiple and varied functions in the body,
connective tissues have certain common properties
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that set them apart from other primary tissues:
Connective tissue is found everywhere in the body. It
is the most abundant and widely distributed of the
primary tissues, but its amount in particular organs
varies greatly. For example, bone and skin are made
up primarily of connective tissue, whereas the brain
contains very little.
1. Common origin. All connective tissues arise from
mesenchyme, an embryonic tissue derived from the
mesoderm germ layer, and hence have a common kin-
ship (Figure 4.7).
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Chapter 4 Tissue: The Living Fabric 113
i,
l'
Mesenchyme
Common
embryonic
origin:
Cellular
descendants:
Fibroblast
Fibrocyte
Chondroblast
Osteoblast
Hemocytoblast
Chondroeyte
Osteocyte
Blood cells*
(and.macrophages)
Class of
connective
tissue
resulting:
Subclasses:
Connective tissue
proper
1. Loose connective
tissue
Types: Areolar
Adipose
Reticular
Osseous (bone)
Cartilage
1. Compact
bone
1. Hyaline
cartilage
2. Fibrocartilage
3. Elastic
cartilage
2. Spongy
(cancellous)
bone
Blood
*Blood cell formation
and differentiation are
quite complex.
Details are provided
in Chapter 18.
2. Dense connective
tissue
Types: Regular
Irregular
Elastic
Figure 4.7 Major classes of connective tissue. All of these classes arise from the
same common embryonic cell type (mesenchyme).
have a rich vascular supply, connective tissues run
the entire gamut of vascularity. Cartilage is avascular;
dense connective tissue is poorly vascularized; and
the other types have a rich supply of blood vessels.
matrix. (However, you should be aware that some
authors use the term matrix to indicate the ground
substance only.) The properties of the cells and the
composition and arrangement of extracellular matrix
elements vary tremendously, giving rise to an amazing
diversity of connective tissues, each uniquely adapted
to perform its specific function in the body. For exam-
3. Matrix. Whereas all other primary tissues are com-
ple, the matrix can be delicate and fragile to form a
2. Degrees of vascularity. Unlike epithelium, which
is avascular, and muscle and nervous tissue, which
posed mainly of cells, connective tissues are com-
posed largely of nonliving extracellular matrix, which
separates, often widely, the living cells of the tissue.
Because of this matrix, connective tissue is able to
bear weight, withstand great tension, and endure
abuses, such as physical trauma and abrasion, that no
other tissue could withstand.
Structural Elements of
Connective Tissue
In any type of connective tissue, three elements must
be considered: ground substance, fibers, and ceils. The
ground substance and fibers make up the extracellular
soft "packing" around an organ, or it can form "ropes"
(tendons and ligaments) of incredible strength.
Ground Substance
Ground substance is an amorphous (unstructured)
material that fills the space between the cells and contains the fibers. It is composed of interstitial fluid,
glycoproteins, and glycosaminoglycans (glY'-k6-suhm6"-n6-glÿ-kanz") (GAGs), a diverse group of large,
negatively charged polysaccharides. The long, strandlike GAGs coil, intertwine, and trap water, forming a
substance that varies from a fluid to a semistiff hydrated
gel. One type of GAG, hyaluronic (hy'-yul-yoo-rah'-
nik) acid, is found in virtually all connective tissues,
f,
1 14 Unit 1 Organization of the Body
"sieve," or medium, through which nutrients and other
tissue proper: fibroblast; (2) cartilage: chondroblast
(kon'-dr6-blast"); (3) bone: osteoblast (ah'-stÿ-6-blast");
and (4) blood: hemocytoblast (h6"-m6-sV-t6-blast).
Once the matrix has been synthesized, the blast
dissolved substances can diffuse between the blood
capillaries and the cells. The fibers in the matrix impede
diffusion somewhat and make the ground substance
less pliable.
by the suffix cyte (see Figure 4.7); this mode is responsible for maintaining the matrix in a healthy state.
However, if the matrix is injured, the mature cells can
and its relative amount helps determine the viscosity
and permeability of the ground substance.
The ground substance functions as a molecular
cells assume their less active, mature mode, indicated
easily revert to their more active state to make repairs
Fibers
Three types of fibers are found in the matrix of connective tissue: collagen, elastic, and reticular fibers.
Of these, collagen is by far the most important and
and regenerate the matrix. (Note that the hemocytoblast, the stem cell of bone marrow, always remains
actively mitotic.)
Additionally, connective tissue proper, especially
the loose connective tissue type called areolar, is
abundant.
"home" to an assortment of other cell types, such as
Collagen fibers are constructed primarily of the
fibrous protein collagen. Collagen molecules are
secreted into the extracellular space, where they
assemble spontaneously into fibers. Collagen fibers
are extremely tough and provide high tensile strength
(that is, the ability to resist longitudinal stress) to the
matrix. When fresh, they have a glistening white
appearance; they are therefore also called white fibers.
Elastic fibers are formed largely from another
fibrous protein, elastin. Elastin has a randomly coiled
structure that allows it to stretch and recoil like a rubber band. The presence of elastin in the matrix gives
it a rubbery, or resilient, quality. Collagen fibers, always
found in the same tissue, stretch a bit and then "lock"
in full extension, which limits the extent of stretch
and prevents the tissue from tearing. Elastic fibers then
snap the connective tissue back to its normal length
when the tension lets up. Elastic fibers are found where
greater elasticity is needed, for example, in the skin,
lungs, and blood vessel walls. Since flesh elastic fibers
appear yellow, they are sometimes called yellow fibers.ÿ
Reticular fibers are believed to be fine collagenous fibers (with a slightly different chemistry) and are
continuous with collagen fibers. They branch extensively, forming a netlike reticulum in the matrix. They
fat cells and cells that migrate into the connective tissue matrix from the bloodstream. The latter include
white blood cells (neutrophils, eosinophils, lymphocytes) and other cell types that act in the inflammatory
and immune responses that protect the body, such as
mast ceils, macrophages, and plasma cells.
Although all of these accessory cell types are
described in later chapters, the macrophages are so
significant to overall body defense that they deserve
a brief mention here. Macrophages (ma'-kr6-fÿ"-juz)
are large, irregularly shaped cells that avidly phagocytize both foreign matter that has managed to invade
the body and dying or dead tissue cells. They are also
central actors in the immune system, as you will see
in Chapter 22. In connective tissues, they may be fixed
(attached to the connective tissue fibers) or they may
migrate freely through the matrix. However, macrophages are not limited to connective tissue. In fact,
their body distribution is so broad and their numbers
so vast that they are often referred to collectively as
the macrophage system.
Macrophages are peppered throughout loose connective tissue, bone marrow, lymphatic tissue, the
spleen, and the mesentery that suspends the abdominal viscera. Those in certain sites are given specific
and other tissue types, for example, in the basement
names; they are called histiocytes (his'-tÿ-6-sits) in
loose connective tissue, Kupffer (koop'-fer) cells in
the liver, and microglial (mi"-kr6'-glÿ-ul) cells in the
brain. Although all these cells are phagocytes, some
membrane of epithelial tissues.
have selective appetites. For example, the macro-
construct a fine mesh around small blood vessels,
support the soft tissue of organs, and are particularly
abundant at the junction between connective tissue
Cells
phages of the spleen function primarily to engulf aging
red blood cells; but they will not turn down other
"delicacies" that come their way.
Each major class of connective tissue has a funda-
mental cell type that exists in immature and mature
forms (see Figure 4.7). The undifferentiated cells,
indicated by the suffix blast (literally, "bud," or "sprout,"
but meaning "forming"), are actively mitotic cells that
secrete both the ground substance and the fibers characteristic of their particular matrix. The primary blast
cell types by connective tissue class are (1) connective
Types of Connective Tissue
As noted, all classes of connective tissue consist of
living cells surrounded by a matrix. Their major differences reflect cell type, fiber type, and proportion
of the matrix contributed by fibers. Collectively, these
three factors determine not only major connective tis-
Chapter 4 Tissue: The Living Fabric ] 15
sue classes, but also their subclasses and types. The
connective tissue classes described in this section are
illustrated in Figure 4.8. Additionally, since the mature
connective tissues arise from a common embryonic
tissue, it seems appropriate to describe this here as
well.
Embryonic Connective Tissue: Mesenchyme
Mesenchyme (meh'-zin-kim), or mesenchymal tissue,
is embryonic connective tissue and represents the first
definitive tissue formed from the mesoderm germ layer.
It arises during the early weeks of development and
eventually differentiates (specializes) into all other
connective tissues. Mesenchyme is composed of star-
shaped mesenchymal cells and a fluid ground substance containing fine fibrils (Figure 4.8a).
Mucous connective tissue is a temporary tissue,
derived from mesenchyme and similar to it, that
appears in the fetus in very limited amounts. Whar-
ton's jelly, which supports the umbilical cord, is the
best representative of this scant embryonic tissue.
Connective Tissue Proper
Connective tissue proper has two subclasses: the loose
connective tissues (areolar, adipose, and reticular) and
dense connective tissues (dense regular, dense irreg-
ular, and elastic). Except for bone, cartilage, and blood,
all mature connective tissues belong to this class.
Areolar Connective Tissue. Areolar (uh-rO'-uh-ler)
connective tissue has a semifluid ground substance
formed primarily of hyaluronic acid in which all three
fiber types are loosely dispersed (Figure 4.8b). Fibroblasts, flat, branching cells that appear spindle-shaped
in profile, are the predominant cell type of this tissue.
Numerous macrophages are also seen, but other cell
types are scattered throughout.
Fat cells appear singly or in small clusters. Mast
cells are identified easily by the large, darkly stained
cytoplasmic granules that often obscure their nucleus.
Mast cell granules contain (1) heparin (heh'-puh-rin),
an anticoagulant that is released into the capillaries
and helps prevent blood clotting, and (2) histamine
(his'-tuh-mÿn), which is released during inflammatory reactions and makes the capillaries leaky. (The
inflammatory process is discussed in Chapter 22.)
Perhaps the most obvious structural feature of this
tissue is the loose arrangement of its fibers, which
account for only small portions of matrix. The rest of
the matrix, occupied by fluid ground substance, appears
to be empty space when viewed through the microscope; in fact, the Latin term areola means "a small
open space." Because of its loose and fluid nature,
areolar connective tissue provides a reservoir of water
and salts for surrounding body tissues. If extracellular
fluids accumulate in excess, the affected areas swell
and become puffy, a condition called edema.
Areolar connective tissue is soft and pliable and
serves as a kind of universal packing material between
other tissues. The most widely distributed connective
tissue in the body, it separates muscles, allowing them
to move freely over one another; wraps small blood
vessels and nerves; surrounds glands; and forms the
subcutaneous tissue, which attaches the skin to
underlying structures. It is present in all mucous
membranes as the lamina propria.
Adipose (Fat) Tissue. Adipose (a'-dih-p6s) tissue is
basically an areolar connective tissue in which the
adipocytes (a'-dih-p6-sits), commonly called fat cells,
have accumulated in large numbers. A glistening oil
droplet (almost pure neutral fat), which occupies most
of a fat cell's volume, compresses the nucleus and dis-
places it to one side; only a thin rim of surrounding
cytoplasm is seen. Since the oil-containing region looks
empty, and the thin cytoplasm containing the bulging
nucleus looks like a ring with a seal, fat cells have
been called "signet ring" cells (Figure 4.8c). Mature
adipocytes are among the largest cells in the body and
are fully specialized cells that are incapable of cell
division. As they take up or release fat, they become
more plump or more wrinkled looking, respectively.
Compared to other connective tissues, adipose
tissue is very cellular; adipose cells account for
approximately 90% of the tissue mass and are packed
closely together, giving a chicken wire appearance to
the tissue. Very little matrix is seen, except for that
separating the adipose cells into lobules (cell clusters)
and permitting the passage of blood vessels and nerves
to the cells. Adipose tissue is richly vascularized,
indicating its high metabolic activity, and it has many
functions; most importantly, it acts as a storehouse of
nutrients. Without stored fat, we could not live for
more than a few days without eating.
Adipose tissue may develop almost anywhere
areolar tissue is plentiful, but it usually accumulates
in subcutaneous tissue, where it acts as a shock absorber
and as insulation. Since fat is a poor conductor of heat,
it helps prevent heat loss from the body. Other sites
of fat accumulation include genetically determined fat
depots such as the abdomen and hips, the bone marrow, around the kidneys, and behind the eyeballs.
Some nutritionists believe that obesity in later life
results from overfeeding during infancy and
childhood. Since unused nutrients are converted to
fat for storage, excessive food intake may encourage
differentiation of excessive numbers of fat cells, which
are capable of storing large amounts of fat throughout
r
122 Unit 1 Organization of the Body
life. Fat cells may even release chemicals into the blood
that make you hungry. Obese people have millions of
these little "gluttons" screaming for food. Notice,
however, that these theories are still controversial? •
Reticular Connective Tissue. Reticular connective tissue consists of a delicate network of interwoven retic-
ular fibers associated with primitive reticular cells,
which resemble mesenchymal cells (Figure 4.8d).
Although reticular fibers are widely distributed in the
body, reticular tissue is limited to certain sites. It forms
the stroma, or internal supporting framework, of lymph
nodes, the spleen, bone marrow, and the liver. Some
of the reticular cells are fibroblast-like; others differentiate into phagocytic macrophages.
Dense Regular Connective Tissue. Dense regular con-
nective tissue is one variety of the dense connective
tissues, all of which have fibers as their predominant
element. For this reason, the dense connective tissues
are often referred to as dense fibrous connective tissues.
Dense regular connective tissue contains regu-
larly arranged bundles of closely packed collagen fibers
running in the same direction. This results in a white,
flexible tissue with great resistance to pulling forces.
It is found in areas where tension is always exerted in
a single direction. Crowded between the collagen fibers
are rows of fibroblasts that continue to form fibers and
scant ground substance. As seen in Figure 4.8e, col-
lagen fibers are slightly wavy. This allows the tissue
to stretch somewhat, but once the fibers are straightened out, there is no further "give" to this tissue.
With its enormous tensile strength, dense regular
connective tissue forms the tendons, cords that attach
muscles to bones, and flat, sheetlike tendons called
aponeuroses (a"-p6-noo-r6'-s6s), which attach muscles to other muscles or to bones. Dense regular con-
nective tissue also forms the ligaments that bind bones
together at joints. Ligaments contain more elastic fibers
than do tendons and thus are slightly more stretchy.
Dense Irregular Connective Tissue. Dense irregular
connective tissue has the same structural elements as
the regular variety, but the collagen fibers are interwoven and arranged irregularly, that is, they run in
more than one plane (Figure 4.8f). This type of tissue
usually forms sheets in body areas where tension is
exerted from many different directions. It is found in
the skin as the dermis, and it forms the fibrous capsules of some organs (testes, lymph nodes, and liver)
and the fibrous coverings of bones, cartilages, and
nerves. It is also the basis of most fasciae (fash'-e-ah),
glistening white sheets that surround the muscles.
Elastic Connective Tissue. The vocal cords and some
ligaments, such as the ligamenta flava (lih-guh-men'tuh flÿ'-vuh) connecting adjacent vertebrae, are com-
posed almost entirely of elastin fibers. These struc-
tt
tures combine strength with elasticity. They yield easily to a pulling force (or pressure) and then recoil to
their original length as soon as the tension is released.
This dense, fibrous tissue is called elastic connective
tissue to distinguish it from the dense varieties in which
collagen fibers predominate (Figure 4.8g).
Cartilage
Cartilage (kar'-tih-lij) has qualities intermediate
between dense connective tissue and bone; it is tough
and yet flexible, providing a resilient rigidity to the
structures it supports (see the box on p. 128). Cartilage
is avascular and devoid of nerve fibers. Its ground
substance consists of large amounts of the GAG chondroitin sulfate, as well as hyaluronic acid bound to
proteins. The ground substance is heavily invested
with firmly bound collagen fibers and, in some cases,
reticular or elastic fibers. As a result, the matrix is
usually quite firm.
Chondroblasts produce the matrix and are the
predominant cell type in cartilage. Their mature forms,
chondrocytes, are found singly or in small groups
within cavities called lacunae (]uh-koo'-n0). The rigid
nature of the cartilage matrix prevents the cells from
becoming widely separated. The surfaces of most cartilage structures are surrounded by a well-vascular-
ized dense irregular connective tissue membrane called
a perichondrium (payr"-ih-kon'-dr0-um) (peri =
around; chondro = cartilage), from which the nutrients
diffuse through the matrix to the chondrocytes. This
mode of nutrient delivery limits cartilage thickness.
Cartilages heal slowly when injured--a phenomenon excruciatingly familiar to those experiencing sports injuries. During later life, cartilages tend to
calcify or even ossify (become bony). In such cases,
the chondrocytes are poorly nourished and die. •
There are three varieties of cartilage: hyaline cartilage, fibrocartilage, and elastic cartilage.
Hyaline Cartilage. Hyaline (hV-uh-lin) cartilage, or
gristle, is very resistant to wear and tear. Although it
contains large amounts of collagen fibers, they are not
apparent and the matrix appears amorphous and glassy
white (Figure 4.8h).
The most widely distributed cartilage type in the
body, hyaline cartilage provides firm support with some
pliability. It covers the ends of long bones as articular cartilage, providing springy pads that absorb
compression stresses at joints. Hyaline cartilage also
supports the tip of the nose, connects the ribs to the
sternum, and forms most of the larynx and supporting
cartilages of the trachea and bronchial tubes. Most of
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Chapter 4 Tissue: The Living Fabric 123
the embryonic skeleton is formed of hyaline cartilage
before bone is formed. Hyaline cartilage persists during childhood as the epiphyseal (eh-pih"-fih-sÿ'-ul)
plates, actively growing regions near the end of long
bones that provide for continued growth in length.
Fibrocartilage. The coarse collagenic fibers of fibrocartilage are arranged in thin, roughly parallel bundles that give the matrix a grainy fibrous appearance.
soluble protein molecules that only become visible
during blood clotting. Still, we must recognize that
blood is quite atypical as connective tissues go. Blood
acts as the transport vehicle for the cardiovascular
system, carrying nutrients, wastes, respiratory gases,
and many other substances throughout the body. Blood
is considered in detail in Chapter 18.
The chrondrocytes are seen squeezed between the col-
lagen bundles (Figure 4.8i). Fibrocartilage looks quite
similar to dense regular connective tissue. Because it
is compressible and resists tension well, fibrocartilage
is found where strong support and the ability to withstand heavy pressure are required. For example, the
intervertebral discs, which provide resilient cushions
between the bony vertebrae, and the spongy cartilages
of the knee are fibrocartilage structures.
Elastic Cartilage. Histologically, elastic cartilage
resembles hyaline cartilage (Figure 4.8j). However,
elastic cartilage contains more elastin fibers than other
cartilage varieties, which gives this tissue a yellow
color in the fresh state. It is found where strength and
exceptional ability to stretch are needed. Elastic cartilage forms the "skeletons" of the auditory tubes, the
external ear, and the epiglottis. The epiglottis is the
flap that covers the opening to the respiratory passageway when we swallow, preventing food or fluids
from entering the lungs.
Bone (Osseous Tissue)
Because of its hardness, bone, or osseous (ah'-sÿ-us)
tissue, has an exceptional ability to support and protect softer tissues. Bones of the skeleton also provide
cavities for fat storage and synthesis of blood cells.
The matrix of bone is similar to that of cartilage, but
it is harder and more rigid because bone matrix has
far more collagen fibers and deposits of inorganic calcium salts (bone salts).
Osteoblasts produce the organic portion of the
matrix; then, bone salts are deposited on and between
the fibers. Mature bone cells, or osteocytes, reside in
the lacunae within the matrix they have made (Figure
4.8k). Unlike cartilage, the next firmest connective tissue, bone is very well supplied by blood vessels, which
invade the bone tissue. We will consider the structure
and metabolism of bone further in Chapter 6.
Muscle Tissue
Muscle tissues are highly cellular, well-vascularized
tissues that are responsible for most types of body
movement. Among the most important characteristics
of muscle cells are their elongated shape, which
enhances their shortening (contraction) function, and
their possession of specialized myof!laments, composed of the contractile proteins actin and myosin (mi'6-sin). There are three types of muscle tissue: skeletal,
cardiac, and smooth.
Skeletal muscle is packaged by connective tissue
sheets into organs called skeletal muscles that are
attached to the bones of the skeleton; these muscles
form the flesh of the body. As the muscles contract,
they pull on bones or skin, causing gross body movements or facial expressions. Skeletal muscle cells are
long, cylindrical cells that contain many nuclei. Their
obvious banded, or striated, appearance reflects the
alignment of their myofibrils (Figure 4.9a).
Cardiac muscle makes up the walls of the heart;
it is found nowhere else in the body. Its contractions
propel blood through the blood vessels to all parts of
the body. Like skeletal muscle ceils, cardiac muscle
cells are striated. However, they differ structurally in
that (1) they are uninucleate cells and (2) they are
branching cells that fit together tightly at unique junctions called intercalated (in-ter'-kuh-lÿ"-tid) discs
(Figure 4.9b).
Smooth muscle is so named because no externally
visible striations can be seen. Individual smooth muscle cells are spindle-shaped and contain one centrally
located nucleus (Figure 4.9c). Smooth muscle occurs
in the walls of hollow organs (digestive and urinary
tract organs, uterus, and blood vessels). It generally
acts to propel substances through the organ by alternately contracting and relaxing.
Since skeletal muscle contraction is under our
conscious control, skeletal muscle is often called vol-
Blood
Blood or vascular tissue is considered a connective
tissue because it has living cells, called formed elements or blood cells, surrounded by a fluid matrix
called plasma (Figure 4.81). The "fibers" of blood are
untary muscle, while the other two types are called
involuntary muscle. Skeletal and smooth muscle are
described in detail in Chapter 9; cardiac muscle is
discussed in Chapter 19.