Epithelial Tissue
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Epithelial Tissue
Tissue is the collection of cells specifying the same certain functions
Tissue Classification:
| 1) Epithelial tissue → lined the surface or body cavities, and secretory in function |
| 2) Connective tissue → support & protection |
| 3) Muscular tissue → For movement |
| 4) Nervous tissue → For transmission of nervous impulses |
| • Glands are the modification of epithelial cells. • And its cells (parenchyma) are supported by connective tissues (Stroma). |
Basically, in development aspect:
1) Nervous tissue
1) Nervous tissue
→ From ectoderm
2) Muscular & connective tissue
2) Muscular & connective tissue
→ mesoderm
3) Epithelial tissue
3) Epithelial tissue
→ From all three germ layers
Examples:
| Lining epithelium related to the external outer environment like 1) Epidermis of skin 2) Corneal epithelium & conjunctiva of eye |
| 3) Lining epithelium of lower part of the anal canal 4) Lining epithelium of urethral opening in penis, vagina |
| 5) The epithelial lining of the mouth cavity, nose cavity, paranasal sinuses These all develop from ectoderm. |
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Lining epithelium of
• Respiratory tract, GIT Tract,
• Glands of the digestive tract (Liver, Pancreas, Gallbladder)
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| → develop from endoderm |
| Lining epithelium of • Heart (Endocardium-simple squamous), • Blood vessels, Pleura, Pericardium |
| → develop from mesoderm |
Basic structures of epithelial cells
• Surface epithelial cells
• Basement membrane
• Lamina propria (loose connective tissue)
• Basement membrane
• Lamina propria (loose connective tissue)
Epithelial tissue:
They are the collection of closely aggregated cells with very little intercellular substance covering the external surface and internal body tube and cavity.
• Tightly packed
• Avascular
They are the collection of closely aggregated cells with very little intercellular substance covering the external surface and internal body tube and cavity.
• Tightly packed
• Avascular
Characteristics of Epithelial Tissue
| • There is no direct blood supply to the epithelial cells. It gets nutrition from underlying blood vessels (in lamina propria) by diffusion. |
| • Cells are closely packed to each other • Cells rest on basement membrane |
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• Origin from all 3 germ layers
• Maximum cellular substance and minimum intracellular substance (opposite of connective tissue) |
Classification of Epithelial Tissue:
1. Covering Epithelium
2. Glandular Epithelium
Glandular Epithelium
• Exocrine
• Endocrine
Covering Epithelium
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1. Single-layer
• Simple
(Squamous, Cuboidal, Columnar)
• Pseudostratified Columnar
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2. Multi-layer
• Stratified
(Squamous, Cuboidal, Columnar) • Transitional
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Covering Epithelium Lined
1. External Surface (Epidermis)
2. Internal closed cavity
• Pericardium
• Pleural
• Peritoneum
3. All body tube which communicate externally
• Respiratory Tube
• GIT
• Urogenital
Naming of Epithelium
A/c to Body Location
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Neuroepithelium
For Receptor
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Olfactory Epithelium
Nasal cavity receptor for smell
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Endothelium
Lining of Blood & Lymphatic vessels
Simple Squamous
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Endocardium
Lining of the inner part of the heart
Simple Squamous
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Mesothelium
Lining of serous cavities
Simple Squamous
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Respiratory Epithelium
Lining of most parts of the Respiratory Tract
Pseudo-stratified Squamous
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| Nasal Cavity has two epithelium • In upper 1/3 rd of roof
Olfactory Epithelium
• Remaining
Respiratory Epithelium
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Functions of Epithelial Tissue
| 1. Covering and lining the surface (By skin) |
| 2. Absorption (From endothelial lining of intestine) |
| 3. Secretion (From gland) |
| 4. Lubrication (From serous cavities i.e Pleural, Peritoneal & Pericardium) |
| 5. Prevent water loss (By skin) |
| 6. Prevent reabsorption (By urinary bladder) |
Distribution of Covering Epithelium
Basic knowledge:
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Almost every air-filled cavities/ Air pathway
→ lined by pseudo-stratified ciliated columnar epithelium
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| Example (Nasal cavity, Trachea, Larynx, Bronchus, Middle ear cavity, Auditory tube) |
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Exception
• Oropharynx - Both air & food pathway → lined by Non-Keratinized Stratified Squamous Epithelium
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• Respiratory Part of lung (Lung alveoli)
→ lined simple squamous
→ because gases needed to be exchanged between pulmonary blood vessels and lung alveoli.
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| Area related to the outer external surface, which need extra-large protection |
| like Mouth cavity, Pharynx, Lower part of anal canal and vagina → lined by non-keratinized stratified squamous epithelium |
| Skin → by keratinized stratified squamous epithelium |
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• Dry membrane → Keratinized
• Mucous related wet membrane → Non keratinized
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For absorption
→ simple columnar epithelium
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Maximum part of GIT Tract (from the Lower part of esophagus up to the level of the rectum)
→ lined by simple columnar epithelium
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• For Diffusion, Secretory
→ Simple Squamous
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• For Absorption
→ simple columnar
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• Urine related
→ Transitional epithelium
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• Air related
→ Pseudo-stratified ciliated columnar epithelium
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• Protection
→ Keratinized/Non-keratinized stratified squamous epithelium
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• Ciliated
→ in the respiratory tract for movement of mucous
→ & in uterine tube for sperm mobility |
Simple Squamous Epithelium
• Flat scale-like cells
• Nucleus in the center of the cell
• Flat scale-like cells
• Nucleus in the center of the cell
Distribution:
• Lining of lung alveoli
• Lining of descending limb of loops of Henle & Bowmans capsule.
• Lining of descending limb of loops of Henle & Bowmans capsule.
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A/c to the location
Simple squamous epithelium has different names • Endocardium
The lining of the heart (Endocardium) (Innermost layer of the heart) |
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• Endothelium
The lining of blood vessels & lymph
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• Mesothelium
Lining of serous cavities: Pericardium, pleura, peritoneum
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Heart Layer
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• Epicardium
Form by Visceral layer of serous pericardium
Simple Squamous Epithelium
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• Myocardium
By Cardiac Muscle
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• Endocardium
Form by Simple Squamous Epithelium
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Functions:
• Protection
• Gaseous exchange in lung alveoli
• Facilitates the movement of the viscera (By mesothelium)
• Secretion of biologically active molecules.
• Protection
• Gaseous exchange in lung alveoli
• Facilitates the movement of the viscera (By mesothelium)
• Secretion of biologically active molecules.
Simple Squamous Epithelium lined blood vessels because:
• It is composed of a single layer of flattened epithelial cells that helps to reach in a narrow space &
• Cells transfer O2, CO2, substrates & metabolites from blood to tissues & from tissues to blood.
• Cells make a smooth surface that prevents blood coagulation in the vessels.
• It is composed of a single layer of flattened epithelial cells that helps to reach in a narrow space &
• Cells transfer O2, CO2, substrates & metabolites from blood to tissues & from tissues to blood.
• Cells make a smooth surface that prevents blood coagulation in the vessels.
Lung alveoli are lined with simple squamous epithelium
• In the human body, where there is an exchange of nutrients or gases or other substances,
• In the human body, where there is an exchange of nutrients or gases or other substances,
→ lined by simple squamous epithelium developmentally.
• Simple squamous epithelium are typically specialized as the lining of vessels & cavities & regulate substances that can enter underlying tissue from the vessels or cavities.
• In lung alveoli, gaseous exchange takes place.
• Simple squamous epithelium are typically specialized as the lining of vessels & cavities & regulate substances that can enter underlying tissue from the vessels or cavities.
• In lung alveoli, gaseous exchange takes place.
So, they are lined by simple squamous epithelium
Fallopian tube (uterine tube) is lined Ciliated simple columnar epithelium.
• The cilia causes movement of the viscous fluid film that covers the epithelial surface
• This favors the transport of the ovulated secondary oocyte into the tube.
• By the ciliary activity of the epithelium, the oocyte enters the infundibulum & moves to the ampulla.
• The secretion from secretory cells provide nutrition & protective functions for the oocyte & the sperm
• The cilia causes movement of the viscous fluid film that covers the epithelial surface
• This favors the transport of the ovulated secondary oocyte into the tube.
• By the ciliary activity of the epithelium, the oocyte enters the infundibulum & moves to the ampulla.
• The secretion from secretory cells provide nutrition & protective functions for the oocyte & the sperm
Pseudostratified ciliated columnar epithelium:
• Modified form of simple ciliated columnar epithelium.
• Cells are of different heights as their nuclei are placed at different levels, giving a false impression of many layers of cells.
• Modified form of simple ciliated columnar epithelium.
• Cells are of different heights as their nuclei are placed at different levels, giving a false impression of many layers of cells.
Distribution:
1. Lining of mostly air-filled space like nasal cavity, lungs, trachea, bronchi, bronchiole.
2. Middle ear cavity, auditory tube
1. Lining of mostly air-filled space like nasal cavity, lungs, trachea, bronchi, bronchiole.
2. Middle ear cavity, auditory tube
Functions:
• Protection
• Secretion
• Transport of particles out of the air passages.
• Lubrication
• Protection
• Secretion
• Transport of particles out of the air passages.
• Lubrication
Significance of pseudo-stratified ciliated columnar epithelium in the respiratory tract
• Because the propelling action of the cilia removes the foreign particles towards the outside to prevent lung infection
• Because the propelling action of the cilia removes the foreign particles towards the outside to prevent lung infection
Stratified squamous Epithelium:
Characteristics
• Cells closer to the underlying connective tissue are usually cuboidal or low columnar
• Most superficial cells are flattened (squamous type)
• Cells closer to the underlying connective tissue are usually cuboidal or low columnar
• Most superficial cells are flattened (squamous type)
It is of two types:
1. Non-keratinized squamous Epithelium
2. Keratinized squamous Epithelium
1. Non-keratinized squamous Epithelium
2. Keratinized squamous Epithelium
Non-keratinized stratified squamous Epithelium (Wet Membrane)
• Present around the moist surface
• Present to withstand the increased wear & tear that occurs in the moist cavities.
• The epithelia provide protection against micro-organisms, but not covered by keratin (as water loss usually does not occur in the moist membrane)
• Found in oral cavity, tongue, oropharynx, laryngopharynx, upper esophagus, vagina, lower part of the anal canal, cornea, etc.
| • In the oral cavity, there is mastication of food so, the oral cavity is lined by non-keratinized squamous epithelium to prevent friction. |
| • Esophagus is lined by this epithelium as the broken food particles pass through the esophagus towards the stomach. |
| • Vocal cord is lined by non-keratinized stratified squamous epithelium because the food & particles that accidentally enter the larynx could damage the vocal cord. |
Keratinized stratified squamous Epithelium (Dry Membrane)
• Superficial layer is covered by keratin.
• Keratin layer → are non-living, contain no nuclei.
- Found in Epidermis of skin
The skin is lined by keratinized squamous epithelium to-
• Provides mechanical protection to underlying tissue.
• Provides mechanical protection to underlying tissue.
• Prevents loss of water & salt.
• Pigment (Melanin) of epidermis protects tissues against the harmful effects of light.
• Pigment (Melanin) of epidermis protects tissues against the harmful effects of light.
Difference between keratinized & Non-keratinized stratified squamous epithelium
| Keratinized stratified squamous epithelium | Non-keratinized stratified squamous epithelium |
| Superficial cells are not living | All cells are living |
| Superficial cells have no nuclei | All cells contain nuclei |
| Surface is dry | Surface is moist |
| Keratin is found | Absent keratin |
Transitional Epithelium:
Features
• Comprises 4-6 layers of cells arranged in 3 zones.
Features
• Comprises 4-6 layers of cells arranged in 3 zones.
| • Upper layer → contains umbrella-shaped cells (dome-shaped) |
| • Middle layer → contains cuboidal a polyhedral cells |
| • Basal cells → are cuboidal/columnar |
• Tight junction present between the cells (like in blood-brain barrier) to prevent reabsorption of urine.
Distribution:
→ Found in urinary pathway
• Renal pelvis & calyces
• Mucous membrane (Epithelium) of the ureter & urinary bladder
• Parts of urethra
• Renal pelvis & calyces
• Mucous membrane (Epithelium) of the ureter & urinary bladder
• Parts of urethra
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Opening of both male & female urethra
Lined by non-keratinized stratified squamous epithelium
As related to the external environment (extra-protection)
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Function:
• Prevent reabsorption of urine
• Allows free distension a contraction
• Prevent reabsorption of urine
• Allows free distension a contraction
Difference between stratified squamous & transitional epithelium
| Stratified squamous epithelium | Transitional epithelium |
| Numerous superimposed cell layers | Only 4-6 layers of cells |
| Superficial cells are squamous type | Superficial cells are of umbrella (dome-shaped) |
| Don't allow distension & contraction | Allow free distension & contraction |
| Main function is to prevent wear & tear | Main function is to prevent urine absorption |
| Found in moist cavities like oral cavity, tongue, lower pharynx, vagina | Found in urinary tract |
| Stratified (multi-layer): • They are named after the cell type present in the superficial layer (whatever be the cell beneath them) |
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