Surface Modification of Cell Membrane & Junctional Complex

Surface Modification of Cell Membrane:
 
Surface modification of cell membrane: 
  • Microvilli
  • Stereocilia
  • Cilia
  • Flagella
 
Microvilli:
• Microvilli are the finger-like projections of the surface epithelium.
• Found in small intestine & Proximal convoluted tubules (termed as brush border)
• Functions: 
- Increase surface area for increasing the absorption.
 (Characteristic feature of the lining epithelium of small intestine)


Stereocilia:
• They are the long, nonmotile processes like microvilli found in the apical surface of cells.
• Found in Epididymis & Hair cells of internal ear
 
Cilia:
• Elongated motile(movable) hair-like structures on the surface of the epithelial cells.
• Found in respiratory tract and genital tract.
• Like lining epithelium of respiratory tract → is Pseudo-stratified ciliated columnar epithelium
• Characteristic feature of Respiratory Tract

Function of cilia: -
• Helps in the movements of fluid, mucus from the respiratory tract in the upward direction toward the pharynx
• Helps to movements of spermatozoa & ovum through the uterine tube (fallopian tube) & spermatozoa through the male genital tract.
• Helps to remove dust particles from the respiratory tract.
 
Cilia have the rhythmic to & fro motion that helps to make airways clear of dirt & mucus.
 
Flagella:
• Only found in spermatozoa
• Helps in the mobility of spermatozoon within the female reproductive tract.


Difference between Vili and Microvilli
Villi Microvilli
• Villi are the finger-like or leaf-like projection of lamina propria(loose connective tissue beneath epithelium) covered by surface epithelium • Microvilli are short finger-like projections of the cell membrane of the surface epithelium.
• It is composed of
- loose connective tissue
- lymphatic channels
- Capillary
- Diffused lymphoid tissue
• It is composed of cytoplasm of surface epithelium

But the function of both villi and microvilli is to increase the surface area for increasing the absorption.
 
 
Difference between Cilia and Stereocilia:
Cilia Stereocilia
• Soft-movable Hair-like structure • Quite rigid like non-movable processes
• Present in the respiratory tract, Genital tract • Present in epididymis and hair cells of the internal ear.
• Cilia of the respiratory tract help to sweep the fluid from the respiratory tract & protect by eliminating bacteria & foreign particles.
• Cilia helps in the movement of spermatozoa through the uterine tube and the male genital tract.
• Stereocilia of Hair cells help in signal transduction of auditory impulse.
 
 
- Remember that respiratory tract surface epithelium is covered with mucous.

 
Transport in cell:
Exocytosis: 
The process by which macro-molecules are exported by cells to outside extracellular space.
• Intracellular vesicles contain materials (like neurotransmitters, enzymes, hormone) move towards the cell membrane, where vesicles fuse with the cell membrane, then the contents of vesicles are released outside.
Examples: -
- Release of neurotransmitter in the synapse
- Export of proteins from cells to blood.
- Export of chylomicron by enterocytes to lymphatics
 
 
Endocytosis: 
The process by which micro-molecules are engulfed (or taken in) by cells either in a solid form or in liquid.
 Two types:
1) Phagocytosis (cell eating):
It is the process by which bacteria, dead tissue, are engulfed by cells. The material makes contact with the cell membrane, which then invaginates inside the cell.
It is the process of ingestion/eating of substances visible under the microscope.
2) Pinocytosis (cell drinking process): 
It is the process of ingestion of substances not visible under the microscope.
It is quite the same as that of phagocytosis.
The only difference is that substances ingested are in solution and not visible under the microscope.
 
Membrane trafficking: 
The process by which proteins and other macromolecules are transported throughout the cell by the process of endocytosis and exocytosis is called
membrane trafficking.
During endocytosis, a portion of the cell membrane is used to form endocytotic vesicles by which macromolecules are transported inside the cell.
During exocytosis, secretory vesicles formed by the Golgi apparatus fused with the cell membrane and release their product to extracellular space.

• Vesicles enclose macromolecules for its transport.