Tissues
“Man is an intelligence in servitude to his organs.”
- Aldous Huxley
Tissue Level of Organization
• The cells of the body vary widely in terms of
function and design.
• About 200 different types of cells in the body.
• Collections of
specialized cells
that perform
certain functions
are called
tissues.
Epithelial Tissue
• Cells that cover internal or external surfaces
form epithelial tissue.
• Characteristics of epithelial tissue include:
▫ Cells are closely bound together.
▫ An apical surface that is exposed to the
environment or an internal passage.
▫ A basement membrane that attaches to
underlying tissues.
▫ Avascular, meaning an absence of blood vessels.
▫ Continuous replacement or regeneration of lost
and damaged surface cells.
• Epithelia protects exposed
and internal surfaces from
damage by physical abrasion,
dehydration, infection, and
by chemicals.
• Any substance that enters or
leaves the body must do so
through a layer of epithelium.
• The nerve cells that make up
the five senses are located
within epithelium.
• Glands; organs that produce secretions, are
made of epithelial cells.
▫ Exocrine glands
discharge secretions
to surfaces exposed to
the outside, such as
the lining of the
digestive tract or skin.
▫ Endocrine glands
secrete directly into
tissue fluid or blood.
Types of Epithelium
• Epithelial cells are classified by shape and
whether they form layers.
• Simple epithelium consists of a thin, single
layer of cells covering the basement membrane.
• Stratified epithelium has several layers of
cells above the basement membrane, creating a
greater degree of protection.
▫ Found in areas commonly exposed to mechanical
and chemical stress, such as the mouth.
• Simple squamous epithelium is found in
internal surfaces where absorption takes place,
or there is little friction due to surrounding fluid.
▫ Examples: Lining of the heart, blood vessels,
kidney tubules, air sacs of lungs.
• Simple cuboidal epithelium occurs in areas
where increased amounts of secretion and
absorption occur.
• The cells are larger and provide more room more
needed organelles.
▫ Examples: Pancreas, salivary glands, urine
production areas of kidneys.
• Simple columnar epithelium provides more
protection than cuboidal in areas of absorption
and secretion.
• Held together with tight junctions; waterproof
attachments made with membrane proteins.
▫ Examples: Stomach lining, intestinal tract.
• Stratified squamous epithelium is a thick
protective lining found in areas of high
mechanical stress.
▫ Held together by desmosomes, which are
cytoskeleton attachments at the cell membrane.
 Causes the cells to peel off in sheets instead of
individually..
▫ Examples: Lining of the mouth, skin, esophagus,
and anus.
• Pseudostratified ciliated columnar
epithelium is a mixture of columnar and
cuboidal cells that appears layered, but all cells
actually do touch the basement membrane.
▫ Cilia are present to move mucus.
▫ Examples: Lining of the nasal cavity, trachea, and
bronchi.
• Transitional epithelium can stretch and
recoil, so its appearance changes.
▫ Examples: Urinary bladder.
Connective Tissue
• Connective tissues are distributed throughout
the body, never exposed to the outside
environmental and have a diverse set of
functions and properties.
▫
▫
▫
▫
Support and protect other tissues.
Transport materials throughout the body.
Storage of energy reserves as fat.
Defense against microorganisms.
• Connective tissue is made of a mixture of
different types of cells and protein fibers.
Reticular
fibers
Macrophage
Mast cell
Elastic
fibers
Macrophage
Collagen
fibers
Blood Vessel
Ground
substance
Fibroblast
• Fibroblasts produce the protein fibers and the
ground substance that fills the empty spaces
in connective tissue.
• Macrophages will phagocytize or “eat”
damaged cells, bacteria, or viruses.
• Mast cells contain vesicles that release
defensive chemicals during an injury or
infection.
• Collagen fibers are long, straight, and flexible.
• Elastic fibers are branched and wavy, giving
them the ability to stretch and return back to
their original shape.
• Reticular fibers are thinner and form a
branching, interwoven network.
Loose Connective Tissues
• Loose connective tissues are the “packing
materials” of the body, filling in spaces to
provide cushioning and support.
• Areolar tissue is elastic and has an extensive
blood supply.
▫ Has enough collagen and elastin fibers to be
strong, and enough ground substance to move
independently.
▫ Example: Between the skin and muscle.
• Adipose tissue is a collection of fat cells, also
called adipocytes.
▫ Provides padding, shock absorption, and thermal
insulation.
▫ Example: Breasts, bottom layer of skin.
• Reticular tissue is a network of protein fibers
that holds other cells in place, such as white and
red blood cells.
▫ Example: Spleen, liver, bone marrow, lymph
nodes.
Dense Connective Tissues
• Dense connective tissues consist mostly of
collagen fibers, with fewer cells and less ground
matter.
• Dense regular connective tissue contain
collagen fibers and fibroblasts that stabilize
connections between body structures.
▫ Example: Tendons connecting bone and muscle.
Cartilage
• Cartilage is a firm gel containing protein fibers
and a type of cell called chondrocytes.
▫ Chondrocytes are found within small pockets
called lacunae.
▫ No blood supply.
• Hyaline cartilage is the most common,
providing stiff and flexible support.
▫ Matrix is made of densely packed collagen fiber.
▫ Example: Respiratory tract, connecting ribs to
sternum, and covering the ends of bones.
• Elastic cartilage is extremely resilient; able to
endure distortions while still maintaining its
original shape.
▫ Contains many more elastic fibers than hyaline
cartilage.
▫ Example: External ear.
• Fibrocartilage has very little ground
substance, with densely packed collagen fibers.
▫ Tough, durable, and resist compression.
▫ Example: Spinal cord, knee joint, between bones
of pelvis.
• Bone has a matrix mostly made of calcium
compounds and collagen fibers, making it the
strongest tissue.
▫ Like cartilage, the cells reside in pits called
lacunae.
▫ Canals provide a blood supply.
Fluid Connective Tissues
• Blood and lymph are the only two types of fluid
connective tissues.
▫ Transport oxygen, carbon dioxide, nutrients, and
wastes throughout the body.
Tissue Membranes
• At the tissue level, a membrane is a physical
barrier made of an epithelium supported by
connective tissue.
• Mucous membranes line tracts that must be
kept moist at all times, such as the esophagus,
intestines, and trachea.
▫ Example: The small intestines has a combination
of columnar epithelium and areolar tissue.
• Serous membranes line internal body cavity
divisions.
▫ Coated with serous fluid to prevent friction.
▫ Example: The lungs are lined by simple squamous
epithelium and areolar tissue.
• Cutaneous membranes are thick, dry, and
relatively waterproof.
▫ Example: Skin, made of stratified squamous
epithelium above areolar and dense connective
tissue.
• Synovial membranes protect the ends of
bones at joints.
▫ Lined with synovial fluid to prevent friction.
▫ Example: The bones of the knee joint are lined by
mostly areolar tissue with an incomplete
epithelium.
Muscle Tissue
• Muscle tissue is made of arrangements of
protein filaments that can contract and generate
force.
• Skeletal muscle tissue is made of large,
multinucleated cells that can be up to a foot
long.
▫ The repeating groups of muscle filaments create a
series of bands called striations.
• Cardiac muscle tissue is smaller than skeletal
muscle and the cells only contain a single
nucleus.
▫ Cells are connected at specialized attachment sites
called intercalated discs.
▫ Each disc contains a gap junction that allows the
movement of small molecules and ions between
cells.
• Smooth muscle tissue is made of cells smaller
and more slender than skeletal muscle.
▫ Striations are not present.
▫ Found in layers around various hollow organs that
must contract.
Nervous Tissue
• Nervous tissue is
specialized for the
conduction of
electrical impulses.
▫ Combination of
supporting cells
called neuroglia
and long, branched
neurons.
▫ Mostly found in the
brain and spinal
cord.