AP Biology
ANIMAL STRUCTURE AND FUNCTION : Tissue
Life is characterized by hierarchical levels of organization. Animals are multicellular organisms
with their specialized cells grouped into tissues. In most animals, combinations of various tissues
make up functional units called organs, and groups of organs that work together form organ systems.
Tissues are groups of cell with a common structure and function.
· Different types of tissues have different structures that are especially suited to their
functions. A tissue may be held together by a sticky
extra cellular matrix that coats
the cells or weaves them together in a fabric of fibers.
Tissues are classified into four main categories:
1. Epithelial tissue
Function:
Occurring in sheets of tightly packed cells, epithelial tissue covers the outside of the body
and lines organs and cavities within the body The cells of a epithelium are closely joined
and in many epithelia, the cells are riveted together by tight
junctions. The epithelium
functions as a barrier protecting against mechanical injury, invasive microorganisms, and
fluid loss. The free surface of the epithelium is exposed to air or fluid, and the cells at the
base of the barrier are attached to a basement membrane, a dense mat of
extra cellular matrix.
Structure:
Epithelia are classified by the number of cell layers and the shape of the cells on the free surface.
*A simple epithelium has a single layer of cells,
*A stratified epithelium has multiple tiers of cells.
*The shapes of cells may be:
Cuboidal (like dice)
Columnar (like bricks on end)
Squamous (flat like floor tiles).
Examples:
Some epithelia, called glandular epithelia, absorb or secrete chemical solutions.
For example, glandular epithelia lining tubules in the thyroid gland secrete a hormone
that regulates fuel consumption. The glandular epithelia that line the lumen of the digestive
and respiratory tracts form a mucous membrane that secretes a slimy solution called mucus
that lubricates the surface and keeps it moist. The free epithelial surfaces of some mucous
membranes have beating cilia that move the film of mucus along the surface. In the respiratory
tubes, this traps dust and particles.
2. Connective tissue
Function:
Connective tissue functions mainly to bind and support other tissues.
Connective tissues
have a sparse population of cells scattered through an extracellular matrix.
The matrix
generally consists of a web of fibers embedding in a uniform foundation that may be liquid,
jellylike, or solid. In most cases, the connective tissue cells secrete the matrix.
There are three kinds of connective tissue fibers:
Collagenous (nonelastic,
do not tear)
Collagenous fibers are made of collagen. Collagenous fibers are nonelastic
and do not tear easily when pulled lengthwise.
Elastic (rubbery)
Elastic fibers are long threads of
elastin. Elastin fiber provides
a rubbery
quality
Reticular (connects tissues)
Reticular fibers are very thin and branched. Composed of collagen and
continuous with collagenous fibers, they form a tightly woven fabric that
joins connective
tissue to adjacent tissues
The major types of connective tissues in vertebrates are:
A. Loose connective tissue:
Loose connective tissue
binds epithelia to underlying tissues and functions as
packing materials, holding organs in place. Loose
connective tissue has all three fiber types.
B. Adipose tissue: Adipose tissue is a specialized form
of loose connective tissues that store fat in adipose cells
distributed
throughout the matrix. Adipose tissue pads
and insulates the body and stores fuel as fat molecules.
Each adipose cell contains a large fat droplet that swells
when fat is stored and shrinks when the body uses fat as
fuel.
C. Fibrous connective tissue:
Fibrous connective tissue
is dense, due to its large number of collagenous fibers.
The fibers are organized into parallel bundles, an arrangement
that maximizes nonelastic strength.
This type of connective
tissue forms tendons, attaching muscles to bones, and
ligaments,
joining bones
to bones at joints.
D. Cartilage: Cartilage has an abundance of collagenous fibers 
embedded in a rubbery matrix made of a substance called
chondroitin sulfate, a protein-carbohydrate complex.
Chondrocytes secrete collagen and chondroitin sulfate.
The
composite of collagenous fibers and chondroitin sulfate
makes cartilage a strong yet somewhat
flexible support
material. The skeleton of a shark is made of cartilage and
the embryonic skeletons of many vertebrates
are cartilaginous.
We retain cartilage as flexible supports in certain locations,
such as the nose, ears, and vertebral disks.
E. Bone: The skeleton supporting most vertebrates is made 
of bone, a mineralized connective tissue. Osteoblasts
deposit a matrix of collagen. Then, calcium, magnesium,
and phosphate ions combine and harden within the matrix
into the mineral hydroxyapatite. The combination of hard
mineral and flexible collagen makes bone harder than cartilage
without being brittle. The microscopic structure of hard mammalian
bones consists of repeating units called osteons. Each osteon has
concentric layers of mineralized matrix deposited around a central
canal containing blood vessels and nerves that service the bone.
F. Blood: Blood functions differently from other connective
tissues, but it does have an extensive
extra cellular matrix. The
matrix is a liquid called plasma, consisting of water, salts, and
a variety of dissolved proteins. Suspended in the plasma are
erythrocytes (red blood cells), leukocytes (white blood cells) and
cell fragments called platelets. Red cells carry oxygen.
White cells
function in defense against viruses, bacteria, and other invaders.
Platelets aid in blood clotting. 
3. Nervous tissue: Nervous tissue senses stimuli and transmits
signals from one part of the animal to another. The functional
unit of nervous tissue is the neuron, or nerve cell. It consists of
a cell body and two or more extensions, called dendrites and
axons. Dendrites transmit nerve impulses from their tips toward
the rest of the neuron. Axons transmit impulses toward another
neuron or toward an effector, such as a muscle
cell
4. Muscle tissue: Muscle tissue is composed of long cells called
muscle fibers that are capable of contracting when
stimulated by
nerve impulses. Arranged in parallel within the cytoplasm of muscle
fibers are large numbers of myofibrils made of the
contractile proteins
actin and myosin. Muscle is the most abundant tissue in most animals,
and muscle contraction accounts for most of the
energy-consuming
cellular work in active animals.
There are three types of muscle tissue
in the vertebrate body:
A. Skeletal muscle: Attached to bones by tendons, skeletal muscle
is responsible for voluntary movements. Skeletal muscle is also
called striated muscle because the overlapping filaments give the
cells a striped (striated) appearance under the microscope.
B. Cardiac muscle:
Cardiac muscle forms the contractile
wall of the heart. It is striated like cardiac muscle, but
cardiac cells are branched. The ends of the cells are
joined by intercalated disks, which relay signals from
cell to cell during a heartbeat.
C. smooth muscle:
Smooth muscle, which lacks
striations, is found in the walls of the digestive tract,
urinary bladder, arteries, and other internal organs.
The cells are spindle-shaped. They contract more
slowly than skeletal muscles but can remain contracted
longer. Controlled by different kinds of nerves than those
controlling skeletal muscles, smooth muscles are responsible
for involuntary body activities. These include churning of the
stomach and constriction of arteries.