Animal Tissues
Tissues
Tissues are groups of specialized cells that work
together for a particular function. In humans,
combinations of different types of tissues make up
organs, and groups of organs work together to
form organ systems.
Four Types of Tissue
1. Epithelial
2. Connective
3. Nervous
4. Muscle
Epithelial Tissue
Epithelial tissues cover body surfaces. Skin and the lining
of organs are epithelial tissues. Epithelial tissues play roles
in absorption, secretion, and protection against foreign
substances.
Simple Columnar Epithelium
The cells of simple columnar epithelium (arrows) are taller, as their
name suggests. This tissue is usually associated with secretion or
absorption and is most often found lining the digestive tract. The section
shown here is from an amphibian.
Simple Squamous Epithelium
SS
Simple squamous (SS) tissue is composed of flat, scale-like cells. It
lines the walls of blood vessels, pulmonary alveoli (shown here), and
the lining of the heart, lung, and peritoneal cavities.
Simple Cuboidal Epithelium
This tissue is composed of a single layer of boxy cells
(arrows). It lines the walls of kidney tubules. In describing
epithelium, the term “simple” means one cell-layer thick.
Ciliated Epithelium
BM
Some epithelial membranes are made up of cells with cilia, tiny
projections that beat in unison to move mucus along the surface.
Ciliated epithelia in the trachea, for example, sweep debris out of the
respiratory tract. This tissue lines the larger respiratory passageways. It
is often ciliated (arrows).
Muscle Tissue
Muscle is a contractile tissue. There are
three types of muscle: skeletal, cardiac and
smooth.
Skeletal Muscle
Skeletal muscle is distinguishable by its striations
and its long, unbranched, multinucleate cells or
fibers. The striations are due to the arrangement of
myosin and actin filaments, along the length of the
fiber. Skeletal muscle is attached to bone and is
responsible for voluntary movement. When
signaled to contract, all the fibers in a particular
bundle contract. The number and size of the
bundles involved determines the strength of the
contraction.
Skeletal Muscle
Nucleus
Striations are visible. Each fiber runs the entire length of
the muscle. Nuclei can be found around the cell periphery.
Cardiac Muscle
Cardiac muscle makes up the wall of the heart and
is also striated. The cells are branched and usually
contain only one nucleus. Cardiac muscle fibers
are composed of a number of cardiac cells.
Adjacent muscle cells are held together by
intercalated disk. At each disk, the cell membranes
of cardiac muscle cells are intertwined and held
together by gap junctions. Because of these
connections, impulses spread quickly through the
muscular walls of each chamber.
Cardiac Muscle
Striations are evident as well as intercalated disks,
the light bands between cells in each fiber.
Nervous Tissue
Nervous tissue, which occurs throughout the body,
receives and transmits stimuli. It converts a
stimulus, whether chemical or physical in nature,
into an electrical impulse that is conducted by
neurons. Neurons, also called nerve cells, are the
functional unit of nervous tissues.
Neuron
Axon
Cell Body
Dendrites
A neuron consists of a cell body, dendrites, and axons.
Dendrites carry impulses to the cell body, whereas axons
transmit impulses toward another cell body or an effector,
a muscle or organ that responds to the impulse.
Axons
Axons are usually much longer than dendrites, sometimes reaching a
meter in length. Many axons are enclosed in an insulating, lipid layer
called a myelin sheath. This sheath is produced by Schwann cells, a
type of supporting cell for the nervous system. The sheath not only
protects the axon but also allows impulses to move more quickly along
the axon. Axons arranged in ropelike bundles wrapped in connective
tissue make up nerves. Nerves carry sensory impulses to the brain and
transmit motor responses from the brain to effector organs. Nerves that
make up the peripheral nervous system bring impulses from the entire
body to the spinal cord, which then transmits them to the brain. The
spinal cord and brain are the central nervous system.
Connective Tissue
Connective tissue differs from other tissues
in that it contains large amounts of
intercellular matrix. Connective tissues
function to bind other tissues together,
provide support, provide nourishment, store
wastes, or repair damaged tissues.
Loose Connective Tissue
Loose connective tissue has few fibers (collagen protein
fibers), a number of cell types, and a large amount of
matrix. It functions to bind epithelia to underlying tissues.
Blood
Blood is considered a connective tissue because it
has an extracellular matrix called plasma. This
liquid matrix is made up of water, salts, nutrients
and an assortment of dissolved proteins.
Suspended in the matrix are two types of cells,
erythrocytes (red blood cells) and leukocytes
(white blood cells), as well as specialized cell
fragments called platelets that function in clotting
blood.
Erythrocytes
RBCs
Red blood cells (RBCs) in humans are flattened disks because the cells lack a
nucleus. The pigment hemoglobin, which gives blood its red color, binds to
oxygen. Red blood cells are responsible for transporting oxygen to all the cells
of the body.
Leukocytes
White blood cells function mostly in fighting diseases.
Some of them move through the walls of blood vessels and
enter body tissues to engulf bacteria. There are five types
of white blood cells: neutrophils, lymphocytes, monocytes,
eosinophils, and basophils.