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Epithelial cells are tightly packed together.
• There cohesion is due to the adhesion junctions.
• Cells are polarized
Epithelium is derived from: ectoblast endoblast, mesoblast.
• Epithelial tissue is divided in:
1. Covering epithelium
2. Glandular epithelium
1. Covering epithelium:
-It is innervated & NON-vascularized.
- Has a great capacity for regeneration (stem cell)
-It is selectively permeable.
- It covers:
•Surface of the body : Epidermis (Ex: Skin)
•Open cavities: Mucosal epithelium (Ex: Digestive system ,uro-genital tract, airways)
•Closed cavities : Mesothelium (ex :Heart, pericardial, pleural, peritoneal) and
Endothelium (ex: vessels)
-Characteristics of Epithelial cells:
•Cohesion: cells are tightly packed by adhesion molecules and junctions (tight and
Intermediate junctions)
•Morphology: Squamous /Cuboidal /Columnar
• Presence of keratin (intermediate filaments): keratin helps in preventing the
dehydration of cells.
•Basal pole :rests on the basal lamina
and contains hemi-desmosomes
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•Lateral pole :Contains proteins that are needed for the adhesion of cells ,and contain
anchoring and adhering junctions also,
tight junction (near to apical pole)
•Apical pole :in contact with the lumen .IT'S highly specialized ,and contains most
proteins.
May have micro-villi or cilia.
-Classification of Covering epithelium:
Uni-stratified | Simple Epithelium:
Consist of one layer of cells.
Simple Squamous:
• Cell is flattened
Selectively permeable
• Function: filtration, diffusion, osmosis.
• Types:
~Endothelium covers the blood & lymphatic vessels, there cells are poorly attached by
tight junction. Its role is phagocytosis.
~Mesothelium that covers the closed serous cavities (Perichonral, pleural,
peritoneal) ,Its role is phagocytosis.
Simple cuboidal:
• Cuboidal cells and spherical nucleus (on the same level).
• Tightly packed together.
• Function: secretion & absorption.
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• Examples:
~Surface of ovaries
~Excretory duct (salivary glands, pancreas..)
Simple columnar:
• Nucleus are on the same level & on: lower third part
• Function: absorption & secretion.
• Examples: stomach
• Types:
~without specializations
~Ciliated: have cilia that helps in movement (fallopian tubes).
~Having Microvilli that helps in absorption (intestine).
Pseudo-stratified columnar Epithelium:
• Consist of one layer (single)
• Cells (basal cells) are cylindrical with different sizes & there nucleus are in various
heights.
• Function: absorption & secretion & transport of mucus
• It contains goblet cells (uni-cellular gland)
Stratified Epithelium:
Made up of several layers.
Basal / Germinative layer is the deepest layer.
Superficial layer is the apical layer.
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Function:
resistant to friction (like epidermis)
protects the underlying connective tissue.
It is classified according to the shape of the superficial
cells:
Squamous:
• Most abundant (mouth)
• Apical layer is made up of flat squamous cells, & degenerate everytime.
• Basal layer is made up of cuboidal and columnar
•Keratinized: in skin, it prevents dehydration.
Cuboidal:
• Less abundant
• Layers are of cuboidal cells
• Example: secretory duct of sweat glands
Columnar:
• Rare
• Irregular polyhedral cells in basal layer.
• Superficial cylindrical cells.
• Role: protection & secretion.
• Example: urethra
Transitional / urothelium
• Found in urinary bladder.
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• Bladder is empty —> cells
are round and bulky.
• Bladder is full of urine —> cells
will be flat and Squamous .
2. Glandular Epithelium:
-Secretory cell: any cell that exports out of its cytoplasm specific molecules.
- Glandular cells: are epithelial cells that store their secretion in vesicles, then they
release them.
-Glands are tightly packed and surrounded by vessels (vascularized) & rest on basal
lamina.
-In a specific zone, epithelial cells increase in number toward the connective tissue.
-it divided into exocrine and endocrine glands.
exocrine gland:
-Secretes there products into the environment or cavity.
-We can classify them according to:
1- number of cells:•Uni-cellular goblet cells
,Opened cell and have no duct(Example: intestine, stomach, respiratory tract..)
•Multi-cellular
2-Characteristics of excretory canal:
•Simple :Contain ONE excretory duct
Types:
~Simple exocrine gland: have one
secretory duct + one secretory portion.
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~ Branched or ramified simple exocrine
gland: one secretory duct + many
secretory portions.
•Compound :Contain many excretory ducts, each has several secretory portions.
3-Secretory portion:
Acinus gland: Spherical (or vial like)
secretory portion. Its wall made of
glandular cells (pyramidal in shape), and the Lumen is too small.
•Tubular gland: Tubular secretory
portion. Its wall made up of cuboidal or columnar cells ,and It can be coiled
Alveolar gland: It look like acinus gland but its lumen is larger .Its secretory portion is
in the form of bag .
4-Type of secreted products:
•Serous gland: Its secretion is protein
(enzymes) with fluid consistency.
•Mucus gland: Secretes mucus rich in
GAGs & proteoglycans.
Mixed gland: Secrete sero-mucosa
(both protein & mucus).
•endocrine gland:
-They secrete molecules into blood called hormones.
-Cells that secrete there products can be: steroids ,thyroid, peptide, amino acid
derivatives.
-Classification according to mode
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of action:
•Intracellular receptors :Present inside the cell, on the nucleus. For liposoluble
hormones . They can cross the Plasma membrane.
•Cell-Surface receptors :present on cell membrane. For water soluble hormones.
Amphicrine Gland:
-Homotypic Gland: Single type of endocrine & exocrine cells. Cells can play the role of
both glands at the
same time (Example: Liver)
-Heterotypic Gland: Where 2 types of cells are present in the same parenchyma.
(Example: pancreas, testes)

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