17. Specialised cell membrane structures

Question 1

Specialised Cell membrane structures? Microvilli? Stereo cilia? Cilia and Flagella?

Answer 1

MICROVILLI

• These projections maybe short/ long finger like extensions /folds
• They range in number from a few to many
• Most are temporary, reflecting cytoplasmic movements and the activity of actin filaments.
• In absorptive cells (epithelium of small intestine/ kidney/ gall bladder) the apical surface = 100’s of more permanent microvilli.
• The complex of microvilli and glycocalyx is easily seen in the light microscope and is called brush/ striated border.
• W/ in each microvillus are bundles of actin filaments - cross linked to each other and the to the surrounding plasma membrane by other proteins.
• The microfilaments stabilizes the microvillus/ causes it to contract slightly –> helps maintain optimal conditions for absorption across its plasma lemma (cell membrane).

STEREOCILIA

• Long apical processes of cells/ absorptive epithelia (lining the epididymis/ ductus deferens)
• These structures are branched, much longer and less motile than microvilli
• Like microvilli - stereocilia increases cell’s surface area
• Facilitates movement of molecules into/out of the cell

CILIA AND FLAGELLA = UNDULIPODIA

• They are elongated/ highly motile structures on the surface of some epithelial cells
• Much longer/ wider than a typical microvillus
• Each cilium is bounded by a cell membrane, has a axoneme, central pair of microtubules, surrounded by 9 peripheral micro tubular pairs.
• The motion:

1) MOTILE:
* MC are found in the lungs/respiratory tract/middle ear
* These cilia have a rhythmic waving/beating motion
* Keep the airways clear of mucus/ dirt = breathe easily and they also help propel (drive) sperm.
* Structure: 9 outer microtubule doublets + a central pair of microtubules

2) NON-MOTILE (PRIMARY CILIA)
* Act as a sensory antenna for the cell - receiving signals from other cells/fluids nearby
* Structure: Single appendages microtubules on the apical surface of cells + do not have central pair of microtubules

• Cilia are cylindrical motile structures on the surface of some epithelial cells.
• Flagella are structurally similar to cilia but are longer - only found in sperm cells.