The purpose of this section of the unit is to help you become familiar
with the different types of animal tissues. A tissue is an integrated group
of cells, usually similar in both structure and function, that are bound
together by intercellular material. Animal tissues are divided into four
categories: epithelial, connective, muscle, and nerve. It should be emphasized
that the classification is based on vertebrate animals, especially human
beings, and that its application to other animals is often not appropriate.
The classification of animal tissues can be summarized as follows:
Connective tissue proper
Loose connective tissue
Dense connective tissue
of slides has been set up for you here on the course
website to help you visualize some different tissue types. Use the following notes and pages 856-859 of your text as
guidelines to the slides.
A. Epithelial Tissue
Epithelial tissues form the covering or lining of all the various body
surfaces, both external and internal, in animals. Epithelial tissues
function in protection and absorption, forming sheets covering a surface; in secretion,
forming glands; and in excretion. Some tissues do several of these. The
of the intestine
for instance, functions in absorption, and secretes mucus for protection.
The cells of epithelial tissues are tightly packed together, with
very little intercellular material or spaces. The cells are tightly
bound to one another by specialized junctions (Fig. 6.32, p. 121).
Epithelial cells are customarily divided into three principal types:
flattened or squamous, cube-shaped or cuboidal
(when viewed in a section perpendicular to the tissue surface), and
columnar (see Fig. 40.5, p 856). Epithelial tissue
may be only one cell thick, in which case it is called simple
epithelium, or it may be two or more cells thick and called
stratified epithelium. (It is the cells of the outermost
layer that determine the name of stratified epithelia.) Thus we can
recognize simple squamous epithelium, simple columnar epithelium,
stratified squamous epithelium, stratified cuboidal epithelium, etc.
Epithelium, regardless of type, is usually separated from the underlying
tissue by an extracellular basement membrane. The
basement membrane is not penetrated by blood vessels, so all epithelia
are dependent upon diffusion of material from underlying tissues.
Slide 1: Cells from the lining of the human cheek, stained. Are these
squamous, cuboidal, or columnar?
Slide 2: Squamous epithelial cells. The cells in the upper half of the
slide are shown in surface view. Note how the irregularly shaped cells
with their prominent nuclei fit tightly together forming a protective sheet.
Slide 3: Stratified squamous epithelium, x.s. This section of frog skin
shows several layers of epithelial cells, the basement membrane, and the
underlying dermal layer. The pink layer across the middle of the slide
is the extracellular basement membrane which separates the epithelium from
the dermis. The outermost layer of epithelium consists of squamous cells
so this epithelium is classified as stratified squamous.
Slide 4: Duodenum, x.s., low power. The fingerlike projections are called
villi. Between the bases of the villi there are small tubular glands composed
of epithelial cells; these are the intestinal glands that secrete mucus.
Slide 5: Intestine, x.s., high power. The surface
of the digestive tract throughout is covered by a single layer
of epithelial cells. The cells lining the small intestine are
an excellent example of simple, columnar epithelium. The outer
surfaces of these cells are covered with a brush border (not visible
here) consisting of countless cylindrical projections of the cell
membrane, the microvilli, which extend into the lumen of the intestine
(see Fig. 41.15, p. 889).
Slide 6: Ciliated epithelial tissue. Much of the human respiratory tract
is lined by ciliated columnar epithelial tissue as seen here. What is the
function of this tissue?
B. Connective Tissue
Connective tissue, with its many varieties, is the most widespread and abundant
tissue in the body. It was given its name because its chief function is to
connect the other three tissues of the body (epithelial, muscular, and nervous)
together. The primary function is to provide structural and metabolic support
for other tissues and organs throughout the body. It surrounds cells, carries
blood vessels, encases internal organs, sheathes muscles, wraps bones, encloses
joints, composes the blood, forms the immune system, and forms the supporting
framework of all organs. Structures made of connective tissue differ widely.
Delicate tissue-paper webs, strong tough cords, rigid bones, liquid blood--all
are made of connective tissue. Microscopically, connective tissue consists
of cells and intercellular material (a matrix of ground substance embedded
in which are a variety of fibers). The ground substance contains long unbranched
polysaccharide chains to which tissue fluid binds. The fibers are primarily
collagen, elastin, and structural glycoproteins. One of the major differences
between connective and epithelial tissues is the proportion of cells to intercellular
material. Cells predominate in epithelial tissues, with very little intercellular
material; in connective tissues just the reverse is true, there being a large
amount of intercellular material and comparatively few cells. The ground
substance or matrix, which is produced by the cells and extruded, may be
or very hard.
Connective Tissue with Liquid Matrices
Slide 7: Human blood. Blood and lymph are good examples of connective
tissues with liquid matrices. In blood, the red blood cells and white blood
cells are floating in the plasma, which is the liquid matrix. The fibers
are made of the soluble protein fibrinogen (not visible); when clotting
occurs, fibrinogen is converted into fibrin fibers which make up the clot.
Connective Tissue with Semisolid Matrices
Slide 8: Loose fibrous connective tissue (often called areolar tissue)
is very widespread throughout the body, functioning to bind together the
individual cells of muscles and nerves, to bind organs together and hold
them in place in a loose and movable but strong manner, etc. The matrix
includes some large tough collagen fibers and some thinner elastic elastin
fibers; both types of fibers are produced by the cells. In loose connective
tissue, the fibers are not packed tightly together and they are oriented
in many different directions permitting movement in all directions.
Slide 9: Tendon, l.s. Tendons, which connect muscles to bones, and ligaments,
which connect bones to other bones, are composed of dense fibrous connective
tissue. This tissue differs from loose fibrous connective tissue in that
its elastin and collagen fibers are tightly packed together and are all
oriented in the same direction, thus increasing strength in that direction.
These fibers are wavy in shape, and closely packed in bundles, giving the
tissue a characteristic wavy appearance. Between the bundles can be seen
the cells arranged in rows.
Slide 10: Adipose or fat tissue is a modified type of connective tissue;
its cells are involved in the storage and metabolism of fat. Fat is constantly
being deposited or removed from the cells. The tissue is well supplied
with blood capillaries. The stored fat droplet occupies a large part of
each cell so that only a small margin of cytoplasm and the nucleus can
be noted just inside the plasma membrane. This gives the cells a so-called "signet
ring" shape. Note how small the blood capillaries (C) are compared
to the size of the adipose cells. Fat tissue is not only important in nutrient
storage, but also in protection of other tissues, insulation, and padding.
Connective Tissue with Solid Matrices
Slide 11: Hyaline cartilage. Cartilage has a firm rubbery matrix consisting
of many tightly packed collagen fibers and glycoproteins. Cartilage cells
are located in small spaces (lacunae) scattered throughout the matrix.
Cartilage varies in its texture, color, and elasticity. It is found in
your body in such places as the nose, larynx, trachea, ear, intervertebral
discs, and many parts of the skeleton, particularly the immature skeleton,
which is initially mostly cartilage. No blood vessels penetrate this tissue;
materials move in and out by diffusion through ground substance.
Slide 12: Compact bone, x.s., which is a specialized
form of connective tissue in which the extracellular components
are mineralized, producing a hard, relatively rigid matrix. The
matrix contains numerous collagen fibers which are impregnated
with calcium carbonate and calcium phosphate salts. Compact bone
is composed of numerous structural units called Haversian systems
that run lengthwise through the bone. Each Haversian system is
seen as a nearly round area, the central core of which is the
Haversian canal. Bone is living tissue; the blood vessels, lymph
vessels, and nerves that supply the tissue run through the Haversian
canal. Around the canal is the matrix, laid down in concentric
layers called lamellae. The lamellae are perforated by small holes
(lacunae) where the bone cells are located. The matrix is hard
and calcified. (See Fig. 40.5, p. 857.)
or Contractile Tissue
of muscle tissue have greater capacity for contraction than do other cells,
although most cytoplasm possesses this property to some extent.
are usually quite elongate. Muscles are responsible for most movement in animals.
Three principal types of muscle are recognized: striated muscle, which is responsible
for most voluntary movement; smooth muscle, which is involved in most involuntary
movements of internal organs; and cardiac muscle, the tissue of which the heart
Slide 13: Skeletal muscle, x.s. Note that each muscle cell, or fiber,
is long and cylindrical, and that each contains many nuclei. Each fiber
is crossed by numerous alternating light and dark bands, or striations,
and for this reason skeletal muscle is designated as striated muscle. The
fibers are usually bound together into bundles by connective tissue. Skeletal
muscle, the "meat" of the body, is attached to the skeleton and
is under voluntary control. Compare fibers of striated muscle with those
of the next two kinds of muscle tissue.
Slide 14: Smooth muscle fibers. Note the absence of striations. Each
fiber is spindle-shaped and elongate, pointed at each end, and contains
a single centrally located nucleus. They are much shorter than skeletal
Slide 15: Section through a sheet of smooth muscle. Groups of smooth
muscle fibers interlace to form sheets of muscle rather than bundles. The
fibers are arranged parallel to one another with the thickest part of one
cell lying against the thin parts of adjacent cells. A network of connective
tissue (stained blue) supports the groups of smooth muscle cells. The uninucleate
cells are spindle shaped.
Slide 16: Cardiac muscle, found only in the heart. The cells of cardiac
muscle branch and interdigitate, forming a complex three-dimensional network.
The tiny brown cells in the spaces between the fibers are red blood cells;
cardiac muscle is well supplied with blood. Note that cardiac muscle has
striations but the striations are not as apparent as those of striated
D. Nerve Tissue
All protoplasm possesses the property of irritability to some extent, but
nerve tissue (sometimes called conductile tissue) is highly specialized
capacity. The nerve cells, or neurons, are easily stimulated and can transmit
impulses very rapidly.
Slide 17: Neuron. Each neuron consists of a central cell body containing
the nucleus and other organelles, and long thin extensions called fibers
that conduct information and communicate with other cells. In humans, an
individual neuron may be a meter long or longer. Neurons are thus well
suited to conduct messages over long distances in the body.