Introduction
Specialized cell membrane structures are adaptations or modifications in the cell membrane that enable cells to perform specific functions essential to their role in the body. These structures often involve changes in shape, composition, or the presence of specialized proteins or lipid arrangements to enhance cellular communication, transport, adhesion, or protection. Key examples include:
Microvilli:
These are finger-like extensions from the cell surface that increase the surface area for absorption, cell movement and phagocytosis.
They are found in the intestines, kidney and gall bladder.
Microvilli are mostly temporary, reflecting cytoplasmic movement.
Actin filaments crosslink to form the protrusion.
Microvilli that are densely packed form a brush border. Brush borders are found in the small intestine and proximal tubules of the kidney.
When microvilli are viewed under a light microscope they look like a wave of long projections.
Microvilli are much shorter than cilia.
The epithelial cells lining the proximal convoluted tubule of the kidney are highly specialised for absorption and have long invaginations of the basal cell membrane and many microvilli.
Stereocilia:
These are long, irregular microvilli with no internal structure.
Stereocilia are not motile, because they lack an internal structure.
They do not have a basal body.
Like microvilli, they increase the surface area for absorption.
Cilia and Flagella:
Cilia and flagella are hair-like extensions consisting of microtubules in a specific 9+2 arrangement.
Flagella are 100–200 micrometers long, while cilia are shorter.
Cilia:
Cilia move substances along the cell surface.
They are found in the respiratory tract, where they move mucus.
Cilia can be seen as a wave of long projections when viewed under a light microscope.
When cilia are viewed under a transmission electron microscope, their internal structure of microtubule doublets can be seen.
Flagella:
Flagella provide cell motility.
In humans, flagella are found only on sperm cells.
Basal Lamina and Basal Infoldings:
Epithelial cells have a basal pole, which is the region of the cell that is in contact with the underlying connective tissue.
The basal pole is attached to a basal lamina by hemidesmosomes, which are junctions that contain integrin proteins. Integrins are transmembrane linker proteins that bind to extracellular matrix components.
The basal lamina is a thin, specialised sheet of extracellular matrix that underlies epithelial cells. It is part of the basement membrane, which consists of the basal lamina and the underlying reticular lamina.