Specialized Cell Membrane Structures and Functions
Specialized Cell Membrane Structures
Specialized membranes are morphological or structural distinctions of the cell’s plasma membrane that enable specific functions. While present in various cell types, they are particularly common in polarized cells. These cells exhibit distinct apical, basal, and lateral poles, with the lateral walls connecting to neighboring cells. Lateral membranes differ and are classified based on their location:
Apical Membrane Specializations
- Microvilli: Cylindrical cytoplasmic extensions involved in absorption, increasing the absorptive surface area in organs requiring extensive exchange. Microvilli can be arranged in an ordered (uniform shape and size) or disordered (variable shape, size, and direction) manner.
- Stereocilia: Long cytoplasmic extensions resembling large microvilli, found in bundles on the cell surface. They are present in the male genital tract, fallopian tubes, and inner ear, guiding the evacuation of substances.
- Pits: Invaginations in epithelial cells resting on connective tissue.
Basal Membrane Specializations
- Hemidesmosomes: Differentiations of the basement membrane that ensure adherence of the epithelial sheet to the basal lamina. They are classified based on configuration (zonula, macula, fascia) and intercellular space thickness (occludens, adherens, gap).
- Zonula: A continuous band encircling the entire cell surface.
- Macula: Oval-shaped differentiations in the lateral membranes.
- Fascia: Polygon-shaped unions larger than maculae.
- Occludens: No intercellular space.
- Adherens: Wide intercellular space (250 Å).
- Gap: Smaller intercellular space (20 Å).
Lateral Membrane Specializations
1. Interdigitations
In areas of high mechanical stress, the lateral membranes of adjacent cells form interdigitating invaginations and evaginations, providing reserve plasma membrane for expansion (e.g., in the bladder).
2. Tight Junctions (Zonula Occludens)
Areas of close connection that seal the intercellular space.
3. Intermediate Junctions (Zonula Adherens)
Zonular junctions encircling the cell surface, located below tight junctions, providing adhesion between neighboring cells.
4. Septate Junctions (Desmosomes)
Characteristic of invertebrates, these junctions enable the exchange of molecules. Also called gap junctions.
5. Gap Junctions (Nexus)
Fascia-type junctions with intercellular intramembranous particles that align with those in the adjacent cell. Each particle, called a connexon, has a diameter of 60-65 Å and is composed of connexin protein subunits, forming a central channel with a diameter of 20 Å.
6. Desmosomes (Macula Adherens)
The most complex and differentiated intercellular junctions, typically macular in shape. They have extracellular and intracellular components:
- Extracellular: Denser cell coat at the desmosome level with an intermediate dense line.
- Intracellular: Thickening of the inner leaflet of the cytoplasmic membrane, forming a laminar structure attached to the inner membrane and actin microfilaments. Composed of cadherins associated with calcium ions. The intracellular domain of cadherin, bound to calcium, forms the intermediate dense line.
Desmosomes hold cell surfaces together and are found in all epithelia, mesothelium, and endothelium. They are abundant in tissues under stress, such as cardiac muscle, skin epithelium, and the cervix.
In some cases, these specializations are associated with each other, forming junctional complexes or strips of closure. From the apical pole to the cell’s center, they are typically arranged as follows: tight junction, intermediate junction or gap junction, and desmosome.