Epithelial Tissue

Epithelial Tissue
Tissue is the collection of cells specifying 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
    → From ectoderm
2) Muscular & connective tissue
    → mesoderm
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.

Lining epithelium of
• Respiratory tract, GIT Tract,
• Glands of the digestive tract (Liver, Pancreas, Gallbladder)
→ 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)
 
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
 
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
• 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
1. Single-layer
Simple
 (Squamous, Cuboidal, Columnar)
 Pseudostratified Columnar
2. Multi-layer
Stratified
 (Squamous, Cuboidal, Columnar)
Transitional
 
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
Neuroepithelium
For Receptor
Olfactory Epithelium
Nasal cavity receptor for smell
Endothelium
Lining of Blood & Lymphatic vessels
Simple Squamous
Endocardium
Lining of the inner part of the heart
Simple Squamous
Mesothelium
Lining of serous cavities
Simple Squamous
Respiratory Epithelium
Lining of most parts of the Respiratory Tract
Pseudo-stratified Squamous
 
Nasal Cavity has two epithelium
In upper 1/3 rd of roof
Olfactory Epithelium
Remaining 
Respiratory Epithelium 
 
 
 
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: 
Almost every air-filled cavities/ Air pathway
→ lined by pseudo-stratified ciliated columnar epithelium
Example
(Nasal cavity, Trachea, Larynx, Bronchus, Middle ear cavity, Auditory tube)
Exception
• Oropharynx - Both air & food pathway
→ lined by Non-Keratinized Stratified Squamous Epithelium
• Respiratory Part of lung (Lung alveoli)
→ lined simple squamous
because gases needed to be exchanged between pulmonary blood vessels and lung alveoli.


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
• Dry membrane → Keratinized
• Mucous related wet membrane → Non keratinized
 
 
For absorption
→ simple columnar epithelium
Maximum part of GIT Tract (from the Lower part of esophagus up to the level of the rectum
lined by simple columnar epithelium
 
 
• For Diffusion, Secretory
→ Simple Squamous
• For Absorption
→ simple columnar
• Urine related
→ Transitional epithelium
• Air related
→ Pseudo-stratified ciliated columnar epithelium
• Protection
→ Keratinized/Non-keratinized stratified squamous epithelium
• 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
 
Distribution: 
• Lining of lung alveoli
• Lining of descending limb of loops of Henle & Bowmans capsule.
A/c to the location
Simple squamous epithelium has different names
Endocardium
The lining of the heart (Endocardium)
(Innermost layer of the heart)
• Endothelium
The lining of blood vessels & lymph
• Mesothelium
Lining of serous cavities:
Pericardium, pleura, peritoneum
 
Heart Layer
• Epicardium
Form by Visceral layer of serous pericardium
Simple Squamous Epithelium
• Myocardium
By Cardiac Muscle
• Endocardium
Form by Simple Squamous Epithelium
 
Functions: 
• 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.
 
Lung alveoli are lined with simple squamous epithelium
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.
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
 
 
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.
 
Distribution: 
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
 
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
 
 
Stratified squamous Epithelium: 
Characteristics
• 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

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.
• Prevents loss of water & salt.
• 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.
• 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
Opening of both male & female urethra 
Lined by non-keratinized stratified squamous epithelium
As related to the external environment (extra-protection)
 
Function: 
• 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)