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Tissues
Dr. Nabil Khouri
The Tissue Level of
Organization
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Learning Objectives
• Identify the four major tissue types and describe their functions.
• Describe the relationship between form and function for each tissue type.
• Discuss the types and functions of epithelial tissues.
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• Tissues are:
• Collections of specialized cells and cell products organized to perform a limited number of functions
• The four tissue types are:
• Epithelial
• Connective
• Muscular
• Nervous
Tissues of the Body: An Introduction
Tissues and tissue types
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4
Embryonic Tissues
1. Embryo begins as a single cell
• divides into many cells that form layers (strata)
2. Three primary germ layers
A. Ectoderm (outer) gives rise to: epidermis & nervous system
B. Endoderm (inner): mucous membranes: GI tract and respiratory linings; digestive glands.
C. Mesoderm (middle) forms mesenchyme (gelatinuous tissue) and then give rise to muscle, bone, and blood
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Tissues = groups of cells that are similar in structure and function.
Epithelium
Coverings
Linings of surfaces
Connective
Support
Bone, ligaments, fat
Muscle
Movement
Nervous
Control
Brain, nerves, spinal cord
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Function of Epithelial Tissue
Protection
Skin protects from sunlight & bacteria & physical damage.
Absorption
Lining of small intestine, absorbing nutrients into blood
Filtration
Lining of Kidney tubules filtering wastes from blood plasma
Secretion
Different glands produce perspiration, oil, digestive enzymes and mucus
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Characteristics of Epithelial Tissue
Form continuous sheets (fit like tiles)
Apical Surface
All epithelial cells have a top surface that borders an open space – known as a lumen
Basement Membrane
Underside of all epithelial cells which anchors them to connective tissue
Avascularity (a = without)
Lacks blood vessels
Nourished by connective tissue
Regenerate & repair quickly
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• Includes epithelium & glands
• Glands are secretory epithelium lining organs
• The epithelium :
• Is avascular
• Forms a protective barrier that regulates permeability
• Cells may show polarity
Epithelial tissue
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• Cells attach via cell adhesion molecules (CAM)
• Cells are attach at specialized cell junctions
• Tight junctions
• Desmosomes
• Gap junctions
Maintaining the integrity of epithelium
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Intercellular connections
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• Basal lamina attaches to underlying surface
• Lamina lucida
• Lamina densa
• Germinative cells replace short-lived epithelial cells
Structure of typical epithelium
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The Polarity of Epithelial Cells
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Classification of epithelia
Cell Shape
Squamous – flattened like fish scales
Cuboidal - cubes
Columnar - columns
Cell Layers (NUMBER)
Simple (one layer)
Stratified (many layers)
It is then named for the type of cell at the apical surface.
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Simple Squamous Epithelium
Mesothelia or Endothelia
Structure Single Layer of
flattened cells
Function Absorption, secretion
permeability and filtration
Reduce frection
Not effective
protection – single layer of cells.
Location Walls of capillaries, air sacs in
lungs
Form serous membranes in body cavity
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Stratified Squamous Epithelium
Structure
Many layers (usually cubodial/columnar at bottom and squamous at top)
Function
Physical Protection
Keratin (protein) is accumulated in older cells near the surface – waterproofs and toughens skin.
Location
Skin (keratinized), mouth & throat
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Simple Cuboidal Epithelia
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Simple Cuboidal Epithelium
Structure
Single layer of cube shaped cells
Function
Secretion and transportation in glands, filtration in kidneys
Location
Glands and ducts (pancreas & salivary), kidney tubules, covers ovaries
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Stratified Cuboidal Epithelia
Stratified Cubodial (layers of
cubodial only)
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Columnar Epithelia
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Simple Columnar Epithelium
Structure
Elongated layer of cells with nuclei at same level
Function
Absorption, Protection & Secretion
When open to body cavities called mucous membranes
Special Features
Microvilli, bumpy extension of apical surface, increase surface area and absorption rate.
Goblet cells, single cell glands, produce protective mucus.
Location
Linings of entire digestive tract
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Columnar Epithelia
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Stratified columnar epithelium
• Is a rare type of epithelial tissue composed of column shaped cells arranged in multiple layers.
• Stratified columnar epithelia are found in the ocular conjunctiva of the eye, in parts of the pharynx and anus, the female's uterus, the male urethra and vas deferens. Also found in Lobar ducts in salivary glands.
• The cells function in secretion and protection.
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Pseudostratified ciliated Columnar Epithelia
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Pseudostratified Epithelium
Structure
Irregularly shaped cells with nuclei at different levels – appear stratified, but It is not.
All cells reach basement membrane
Function
Absorption and Secretion
Goblet cells produce mucus
Cilia (larger than microvilli) sweep mucus
Location
Respiratory Linings & Reproductive tract
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Transitional Epithelium
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Transitional Epithelium
Structure
Many layers
Very specialized – cells at base are cuboidal or columnar, at surface will vary.
Change between stratified & simple as tissue is stretched out.
Function
Allows stretching (change size)
Location
Urinary bladder, ureters & urethra
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• Unicellular
• Individual secretory cells
• Multicellular
• Organs containing glandular epithelium
• Classified according to structure
Glands
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Glands
One or more cells that make and secrete a product.
Secretion = protein in aqueous solution: hormones, acids, oils.
Endocrine glands
No duct, release secretion into blood vessels
Often hormones
Thyroid, adrenal and pituitary glands
Exocrine glands
Contain ducts, empty onto epithelial surface
Sweat, Oil glands, Salivary glands, Mammary glands.
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a. Sweet Glands
There are 2 types of sweat glands:
•Eccrine sweat glands, all over body
except lips and part of external genitalia;
•Apocrine sweat glands, only in axilla, areola, nipple
of mammary gland, and circumanal region and
the external genitalia. The ceruminous glands of
ear and glands of Moll of eyelid are also apocrine.
•Both the Eccrine and the Apocrine sweat
glands are innervated by the sympathetic nervous
system.
•Eccrine glands respond differently
to heat and nervous state.
•The apocrine glands respond
to emotional and sensory stimuli but not heat.
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•Are simple coiled glands that regulate body
temperature.
•The secretory segment is deep in
the dermis or upper hypodermis.
•Its duct leads to surface.
•In the secretory region there are clear cells that
produce the watery
component of sweat and dark cells that
produce a protein secretion.
•Duct cells form the walls from the secretory
portion to the area near the surface where the
epidermal cells form the wall.
•The duct is stratified cuboidal epithelium .
Eccrine Sweat Glands
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Eccrine Sweat Glands
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b. Sebaceous Glands or Oil Glands
Found in the skin of mammals and these glands
secrete sebum.
Sebum is made of fat (lipids) and the debris of
dead fat-producing cells.
•These glands exist in humans throughout the
skin except in the palms of the hands and soles of
the feet.
•Sebum acts to protect and waterproof hair and
skin, and keep them from becoming dry, brittle,
and cracked. It can also inhibit the growth of
microorganisms on skin.
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Sebaceous Glands or Oil Glands
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•Are involved in skin problems such as acne and
keratosis pilaris. A blocked sebaceous gland can
result in a sebaceous cyst.
•Earwax, also known by the medical term cerumen, is
a yellowish, waxy substance secreted in the ear canal
of humans and many other mammals. It plays a vital
role in the human ear canal, assisting in cleaning and
lubrication, and also provides some protection from
bacteria, fungus, and insects
c. Ceruminous glands
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Are the organs that, in the female mammal, produce milk for the
sustenance of the young. These Exocrine glands are enlarged and
modified sweat glands and are the characteristic of mammals
which gave the class its name.
D. Mammary glands
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•The human mammary glands are modified sweat glands and are
developed from two sources --the parenchyma (alveoli and ductules )
from the surface ectoderm ,the fibrofatty stroma from the underlying
endoderm.
•At birth mammary glands of both sexes remain in infantile form .This
condition persists throughout life in normal male.
•First change is seen at puberty in females ,in the form of deposition of
fat and increase in size and attain hemispherical outlines. With the start
of reproductive cycle after puberty ,glandular tissue show changes with
the alteration in concentration of oestrogen and progesterone in each
cycle.
•Ultimately during pregnancy final maturation of the glands takes place
and they are ready for milk secretion under the influence of oestrogen
,progesterone ,prolactin and probably hCG..
•Some milk is secreted into the ducts as early as 5 months but the
amount is less compared to large amount secreted at child birth.Milk is
secreted within an hour of child birth and first formed milk is
called colostrum which is yellowish in colour and rich in protein and
antibodies and provide immunity to the baby.Normal milk production
starts 2-3 days after child birth.
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• Merocrine (product released through exocytosis)
• Apocrine (involves the loss of both product and cytoplasm)
• Holocrine (destroys the cell)
Glandular secretions can be:
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Apocrine Sweat Glands
•The secretory product from the gland.
•The duct has a narrow lumen.
•Apocrine
secretions contain protein, carbohydrate, ammonia and l
ipid.
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Merocrine gland
• Cells that secrete products via the merocrine method form membrane-bound secretory vesicles internal to the cell.
• These are moved to the apical surface where the vesicles coalesce with the membrane on the apical surface to release the product. Most glands release their products in way.
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Holocrine Glands
• The third type of secretory release, Holocrine, involves death of the cell. The secretory cell is released and as it breaks apart, the contents of the cell become the secretory product. This mode of secretion results in the most complex secretory product. Some sweat glands located in the axillae, pubic areas, and around the areoli of the breasts release their products in this manner. Sebaceous glands also are of this type.
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Mechanisms of Glandular Secretion
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A Structural Classification of Exocrine Glands