Tissue is a group of cells that have similar structure and
that function together as a unit. A nonliving material, called the
intercellular matrix, fills the spaces between the cells. This may be abundant in
some tissues and minimal in others. The intercellular matrix may contain
special substances such as salts and fibers that are unique to a specific
tissue and gives that tissue distinctive characteristics. There are four main
tissue types in the body: epithelial, connective, muscle, and nervous. Each is
designed for specific functions. Use the hyperlinks below to branch into a
tissue type and learn more about the topic.
Epithelial
tissues are widespread throughout the body. They form the covering of all body
surfaces, line body cavities and hollow organs, and are the major tissue in
glands. They perform a variety of functions that include protection, secretion,
absorption, excretion, filtration, diffusion, and sensory reception.
The
cells in epithelial tissue are tightly packed together with very little
intercellular matrix. Because the tissues form coverings and linings, the cells
have one free surface that is not in contact with other cells. Opposite the
free surface, the cells are attached to underlying connective tissue by a
non-cellular basement membrane. This membrane is a mixture of carbohydrates and proteins secreted by the
epithelial and connective tissue cells.
Epithelial
cells may be squamous, cuboidal, or columnar in shape and may be arranged in
single or multiple layers.
Simple
cuboidal epithelium is found in glandular tissue and in the kidney tubules.
Simple columnar epithelium lines the stomach and intestines. Pseudostratified
columnar epithelium lines portions of the respiratory tract and some of the
tubes of the male reproductive tract. Transitional epithelium can be distended
or stretched. Glandular epithelium is specialized to produce and secrete substances.
Connective
tissues bind structures together, form a framework and support for organs and
the body as a whole, store fat, transport substances, protect against disease,
and help repair tissue damage. They occur throughout the body. Connective
tissues are characterized by an abundance of intercellular matrix with
relatively few cells. Connective tissue cells are able to reproduce but not as
rapidly as epithelial cells. Most connective tissues have a good blood supply
but some do not.
Numerous cell types are found in connective tissue. Three of the
most common are the fibroblast, macrophage, and mast
cell. The types of connective tissue include loose connective tissue,
adipose tissue, dense fibrous connective tissue, elastic connective tissue,
cartilage, osseous tissue (bone), and blood.
Muscle
tissue is composed of cells that have the special ability to shorten or
contract in order to produce movement of the body parts. The tissue is highly
cellular and is well supplied with blood vessels. The cells are long and
slender so they are sometimes called muscle fibers, and these are usually
arranged in bundles or layers that are surrounded by connective tissue. Actin
and myosin are contractile proteins in muscle tissue.
Muscle tissue can be categorized into skeletal muscle
tissue, smooth muscle tissue, and cardiac muscle tissue.
Skeletal
muscle fibers are cylindrical, multinucleated, striated, and under voluntary
control. Smooth muscle cells are spindle shaped, have a single, centrally
located nucleus, and lack striations. They are called involuntary muscles.
Cardiac muscle has branching fibers, one nucleus per cell, striations, and
intercalated disks. Its contraction is not under voluntary control.
Nervous tissue is found in the brain, spinal cord, and nerves. It
is responsible for coordinating and controlling many body activities. It
stimulates muscle contraction, creates an awareness of the environment, and
plays a major role in emotions, memory, and reasoning. To do all these things,
cells in nervous tissue need to be able to communicate with each other by way
of electrical nerve impulses.
The cells in nervous tissue that generate and conduct impulses are
called neurons or nerve cells. These
cells have three principal parts: the dendrites, the cell body, and one
axon. The main part of the
cell, the part that carries on the general functions, is the cell body.
Dendrites are extensions, or processes, of the cytoplasm that carry impulses to
the cell body. An extension or process called an axon carries impulses away from
the cell body. Nervous tissue also
includes cells that do not transmit impulses, but instead support the
activities of the neurons. These are the glial cells (neuroglial cells),
together termed the neuroglia. Supporting, or glia,
cells bind neurons together and insulate the neurons. Some are phagocytic and
protect against bacterial invasion, while others provide nutrients by binding
blood vessels to the neurons.
EXTRACELLULAR
JUNCTIONS
Tight
Junctions
Tight junctions, or zonula occludens, are the closely associated
areas of two cells whose membranes join together forming a
virtual impermeable barrier to fluid. It is a type of junctional complex only present in
vertebrates. The corresponding junctions that occur in invertebrates are
septate junctions.
Tight junctions are composed of a branching network of sealing
strands, each strand acting independently from the others. Therefore, the
efficiency of the junction in preventing ion passage increases exponentially
with the number of strands Each strand is formed from a row of transmembrane
proteins embedded in both plasma membranes, with extracellular domains joining
one another directly. Although more proteins are present, the major types are
the claudins and the occludins. These asociate with
different peripheral membrane proteins located on the intracellular side of
plasma membrane which anchor the strands to the actin cytoskeleton. Thus, tight junctions
join together the cytoskeletons of adjacent cells.
Gap Junction
A gap junction or nexus is a junction between certain animal cell-types that allows
different molecules and ions, mostly small
intracellular signaling molecules (intracellular mediators), to pass freely
between cells. The junction connects the cytoplasm of cells. One gap
junction is composed of two connexons (or hemichannels) which
connect across the intercellular
space. They are analogous to the plasmodesmata that join plant cells
Adhesion Junction
Adhesions
are bands of scar-like tissue that form between two surfaces inside the
body. Inflammation, surgery, or
injury can cause tissues to bond to other tissue or organs, much like the
process of forming scar tissue.
Glands
Glands
of the body are classified as either exocrine or endocrine types. Exocrine
glands are glands that retain ducts to body surfaces. During development,
endocrine glands lose their contacts to embryological surfaces(ducts) and
become isolated as small blocks of tissues. Endocrine glands are
therefore referred to as "ductless" glands.
Endocrine
and exocrine glands secrete various products. These include hormones,
enzymes, metabolites, and other molecules. In exocrine glands, products
of these cells collect in the duct of the gland and flow toward the surface to
which the duct is in contact. Since endocrine glands lack ducts, the
product is released across the cell membrane into interstitial spaces around the
cells. Diffusion of the product into capillaries follows.
Exocrine
Glands
Most
glands of the body are exocrine types with ducts connecting to anatomical
surfaces. Contrast your salivary glands that open into the oral cavity
with sweat glands that deposit their product on the body surface. Both
types of glands are buried in deeper tissues but their products appear on a
superficial surface. Connecting the glands to the surfaces are ducts!
Endocrine Gland
Endocrine glands
produce hormones into the blood or lymph systems. These glands include the thyroid, parathyroid, hypothalamus, pineal,
pituitary, adrenal, islets of Langerhans in the pancreas, and the gonads
(testes and ovaries). The effects of these hormones may affect one organ or
tissue, or the entire body.
Endocrine glands release hormones (chemical messengers) into the bloodstream to be transported to various organs and tissues throughout the body. For instance, the pancreas secretes insulin, which allows the body to regulate levels of sugar in the blood. The thyroid gets instructions from the pituitary to secrete hormones which determine the pace of chemical activity in the body (the more hormone in the bloodstream, the faster the chemical activity; the less hormone, the slower the activity).
Membranes
Mucous
Membranes
Mucosa is moist
tissue that lines particular organs and body cavities throughout the body,
including your nose, mouth, lungs, and gastrointestinal tract. Glands along the
mucosa secrete mucus (a thick fluid).
Serous Membrane
Produce a secretion called
serous fluid. Acts to protect and lubricate surfaces.
Synovial Membrane
A
continuous sheet of connective tissue lining the capsule; its cells produce
synovial fluid that lubricates the joint and prevents the two cartilage caps on
the bones from rubbing together.
Meninges
The
connective tissue covering on the brain and spinal cord, within the dorsal
cavity, are called meninges. They provide protection for these vital
structures. 
Cutaneous
Membrane
Cutaneous
membranes of the skin cover the surface of the body. It consists of stratified
squamous epithelium and the underlying connective tissues. Cutaneous membranes
are thick, relatively waterproof, and dry.
