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TISSUES
Tissues are groups of cells that work together to perform similar functions. Tissues characteristics
will determine the function of the organ. There are four types of tissue: epithelial, connective,
muscle, nervous. Each type performs specific functions that maintain homeostasis. Histology is the
study of tissue.
EPITHELIAL TISSUE
Location: Lines and covers body’s surface or inner cavities & forms glands in the body
Functions: protection, secretion, absorption, filtration, propulsion, excretion
Characteristics:
1. Cells are tightly packed together forming a
continuous sheet. Adjacent cells are bound together
by lateral contacts – tight junctions or desmosomes.
2. Cells have polarity --- cell regions near the apical
surface differ from those near the basal surface in
both structure and function.
 Apical surface – free or unattached surface
that lines external surfaces or inner cavity of
organs; can be slick, smooth or modified with
cilia or microvilli. Microvilli are fingerlike
extensions of plasma membrane that increase the exposed surface area for absorption
and secretion. The microvilli are so dense that the cell has a fuzzy appearance called a
brush border.
 Basal surface – the lower attached surface of the epithelium. The basal surface is
attached to the basement membrane which is composed of noncellular material secreted
by both the epithelial cells and the underlying connective tissue. The basement
membrane reinforces the epithelial sheet and resists stretching and tearing. The
basement membrane is actually composed of 2 layers – basal lamina (superior) &
reticular lamina (inferior). The basal lamina is a noncellular, adhesive sheet composed of
glycoproteins and collagen fibers – acts as a selective filter that determines which
molecules diffuse form the underlying connective tissue. The reticular lamina is a network
of collagen fibers.
3. Innervated - contain nerves.
4. Avascular – Epithelium has no blood vessels instead gets nutrient by diffusion from underlying
connective tissue.
5. Regenerates rapidly – stimulated by loss of
apical-basal polarity and lateral contacts or
nutrient availability.
Classification:
Epithelial are classified by their cellular arrangement
(simple or stratified) and shape. Stratified epithelium
is named for cells at the apical surface.
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TYPES OF EPITHELIAL TISSUE:
A. SIMPLE SQUAMOUS
 Two other locations – endothelium (lining of lymphatic vessels, blood vessels and heart) and
mesothelium (serous membranes in ventral body cavity)
B. SIMPLE CUBOIDAL
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C. SIMPLE COLUMNAR
D. PSEUDOSTRATIFIED COLUMNAR
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E. STRATIFIED SQUAMOUS
F. STRATIFIED CUBOIDAL – rare; found in some sweat and mammary glands. Typically two cell
layers thick.
G. STRATIFIED COLUMNAR – rare; small amounts in pharynx, male urethra and lining of some
glandular ducts.
H. TRANSITIONAL EPITHELIUM
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I. GLANDULAR EPITHELIUM – involved in production and secretion of aqueous fluid into ducts or
into bloodstream.
 Endocrine glands – secretions released directly into blood or lymph and travel to specific
target organs.
 Exocrine glands – secretions
released through ducts onto
body surfaces or into body
cavities. Examples include
mucous, sweat and oil. Mucous
and goblet cells line the
intestines and respiratory tracts
and produce mucus.
CONNECTIVE TISSUE
Location: most abundant tissue – underlies epithelial & surrounds nerves and blood vessels
Functions: support, attachment, insulating, protection, storage, transport
Characteristics:
1. Composed of scattered cells & nonliving extracellular material called matrix. Matrix fills space
between cells and is composed of ground substance and fibers. Composition and arrangement
of cells and matrix vary.
 Ground substance serves as a medium through which solutes diffuse between capillaries
and cells. Components include interstitial fluid, cell adhesion proteins and proteoglycans
(trap water in various amounts).
 Fibers provide support. Collagen fibers are strong flexible to resist stress. Elastic fibers
provide strength and stretching capabilities. Reticular fibers are thin branched collagen fibers
that form networks that offer more “give”.
 Cells – ‘Blast’ cells are immature forms that secrete ground substances and fibers; fibroblast,
chondroblast, osteoblast and hematopoietic stem cells in bone marrow. ‘Cyte’ cells are
mature forms that maintain the matrix; chondrocytes and osteocytes. Fat cells store
nutrients. White blood cells respond to injury, mast cells initiate local inflammatory response
by releasing heparin (anticoagulant) and histamine (promotes inflammation). Macrophages
engulf dead cells.
2. Vascularity varies – most have a good
blood supply
3. Common Embryonic Tissue –
mesenchyme cells
Classification:
Connective tissues are classified based on
cells, fibers and consistency of matrix.
 Types: connective tissue proper,
cartilage, bone and blood.
 Connective tissue proper is divided
into two subclasses: loose connective
tissue and dense connective tissue.
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TYPES OF CONNECTIVE TISSUE:
A. AREOLAR TISSUE (loose) – most widely distributed; serves as a reservoir of water and salts
for surrounding tissues. Supports and bind other tissues – called tissue fluid.
B. ADIPOSE TISSUE (loose)
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C. RETICULAR (loose)
D. DENSE REGULAR
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E. DENSE IRREGULAR
F. DENSE ELASTIC
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G. CARTILAGE – Cells are avascular; get nutrients from membrane surrounding it. Lacks nerve
fibers and is composed of 80% water. 3 types: hyaline, elastic and fibrocartilage
HYALINE CARTILAGE
FIBROCARTILAGE
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ELASTIC CARTILAGE
H. BLOOD – HEMATOPOIETIC TISSUE
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I. OSSEOUS TISSUE - BONE
MUSCLE TISSUE
Muscle tissues consist of fibers (cells) that are modified for contraction and thus provide motion,
maintenance of posture and heat production. 3 types – skeletal, smooth and cardiac muscle
SKELETAL MUSCLE
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SMOOTH MUSCLE
CARDIAC MUSCLE
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NERVOUS TISSUE
Nerve tissues are made up of cells called neurons and neuroglia. Neurons receive impulse from
stimuli, convert stimuli into impulse & conduct impulse to other neurons, muscles or glands. Neurons
do NOT regenerate. Neuroglia cells protect and support neurons and are capable of regenerating.
TISSUE REPAIR
Tissue repair is determined by the type of tissue damaged and the severity of the tissue damage.
 Regeneration - replacement of destroyed tissue and original function is restored.
 Fibrosis - dense connective tissue replaces destroy tissue and the original function is lost.
Regenerate extremely well – epithelial tissue, bone, areolar, dense irregular, hematopoietic
Moderate regenerating capacity – smooth muscle, dense regular
Virtually no functional regenerative capacity – cardiac muscle, nervous tissue
Events of tissue repair –
1. Inflammation occurs – inflammatory chemicals
released; local blood vessels become permeable
to allow WBC, fluid, clotting proteins and other
plasma proteins to seep into injured area; clotting
occurs.
2. The clot is replaced by granulation tissue.
Fibroblast produce collagen fibers to bridge gap;
macrophages phagocytize dead cells and debris;
surface epithelial cells multiply and migrate over
granulation tissue.
3. Regeneration and fibrosis effect permanent repair. Area mature and contract and epithelium
thicken; fully regenerated epithelium with underlying scar tissue form.
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MEMBRANES
Cutaneous membranes (SKIN) – a dry membrane with a superficial epidermis composed of
stratified squamous epithelium and underlying dermis is dense irregular connective tissue. The main
function is protection.
Mucous membranes (mucosa) – composed of
varies types of epithelium resting on areolar
connective tissue called lamina propria.
Lines body cavities open to the
exterior and organs of the respiratory,
digestive, urinary and reproductive
tracts. The mucosa is adapted for
secretion of mucus.
Serous Membranes (serosa) – moist
membrane found in ventral body cavities. The walls of ventral cavities and outer surface of visceral
organs are covered by a thin, double-layered membrane. Serosa is composed of simple squamous
epithelium with underlying areolar connective tissue. The main function is lubrication and
cushioning.
Serous membranes occur in pairs. The
parietal layer lines a specific portion of the
wall of the ventral body cavity. The visceral
layer covers the outside of the organs in the
cavity.
The serous space between the layers is
filled with lubricating fluid called serous fluid. The fluid is secreted by both layers and
allows organs to slide without friction.
The membranes lining the abdominal cavity and organs are called peritoneum. The
membranes lining the thoracic cavity and the lungs are called pleura. The membranes
lining pericardial cavity and covers the heart is called pericardium.
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