Epithelial Cell Junctions and Specializations
Apical Specializations
Microvilli
Microvilli are cylindrical, membrane-bound cytoplasmic extensions that increase the absorptive surface of cells without increasing cell volume. They are abundant in organs involved in substance exchange. A collection of microvilli is called a brush border. Microvilli are classified into two types based on their arrangement: orderly and disorderly (or accidental). They have a diameter of 0.1μm and a length of 0.6 to 0.8μm, observable with electron microscopy. Microvilli have two distinct zones: a peripheral zone (200-300 Å wide) lacking a central structure, and a core composed of 10-50 actin microfilaments bundled together. These filaments extend into the cytoplasm and reach a terminal network of filaments near the membrane. Within the core, actin microfilaments are cross-linked by proteins called fimbrin and villin. In the presence of calcium, villin bundles the actin microfilaments, increasing their resistance.
Stereocilia
Stereocilia are long cytoplasmic extensions, similar to microvilli but longer, found in the male genital tract, fallopian tubes, and inner ear. Their primary function is to facilitate the movement of secretions. The core of stereocilia consists of actin microfilaments linked by fimbrin molecules. Unlike microvilli, there is no contractile terminal network; their movement is passive, driven by fluid flow.
Basal Specializations
Basal Pits
Epithelial cells rest on a connective tissue basement membrane. Basal pits are invaginations of the basement membrane into the basal surface of the cell. These invaginations increase the surface area for exchange and cellular transport. The appearance of basal pits varies depending on the epithelium type and transport needs. The presence of mitochondria within the invaginations suggests ATP-dependent transport processes.
Hemidesmosomes
Hemidesmosomes are membrane specializations that anchor epithelial cells to the basal lamina. They are oval-shaped, approximately 2000Å in diameter and 300Å thick. Hemidesmosomes have intracellular and extracellular components. Intracellular components include a thickening of the inner membrane leaflet (cytoplasmic plaque) and microfibrils of the basal lamina (500Å). The extracellular components interact with the basal lamina, securing the cell’s attachment.
Lateral Specializations
Interdigitations
Interdigitations are formed by the interlocking of complementary folds and protrusions of the lateral membranes of adjacent cells. This arrangement strengthens cellular cohesion, increases the surface area for exchange between neighboring cells, and facilitates intercellular communication.
Intermediate Junctions (Zonula Adherens)
Zonula adherens are belt-like junctions that encircle the entire cell, providing strong adhesion between neighboring cells. They are composed of proteins, including E-cadherin, which spans the membrane with intracellular and extracellular domains. Intracellularly, E-cadherin connects to other proteins that bind to actin microfilaments. Zonula adherens promote cell adhesion, communication, and maintain epithelial rigidity.
Tight Junctions (Zonula Occludens)
Tight junctions are areas of close membrane contact that seal the intercellular space, creating a diffusion barrier and regulating the passage of molecules between cells. They form a continuous band around the apical portion of the cell. The protein ocludin, with extracellular and intracellular domains, is a key component of tight junctions. Intracellularly, ocludin binds to a calcium-dependent protein (E-cadherin), which in turn binds to ZO-1 and ZO-2 proteins. Tight junctions are crucial for maintaining cell polarity and regulating paracellular transport.
Septate Junctions (Septated Desmosomes)
Septate junctions are found in invertebrates and are characterized by ladder-like structures bridging the intercellular space.
Gap Junctions
Gap junctions are fascia-type junctions formed by the alignment of connexons in adjacent cell membranes. Connexons are composed of six connexin subunits arranged around a central channel (20Å diameter) that allows the passage of small molecules and ions between cells. Mutations in connexin genes can lead to hereditary neuropathies characterized by muscle atrophy and loss of reflexes.
Desmosomes (Macula Adherens)
Desmosomes are complex intercellular junctions that provide strong adhesion between cells. They are typically oval-shaped (4000-5000Å diameter) with an intercellular space of 240-500Å. Desmosomes have both extracellular and intracellular components. Extracellular components include a dense cell coat and an intermediate dense line. Intracellular components include a thickening of the inner membrane leaflet (cytoplasmic plaque) and a laminar structure attached to the inner leaflet and actin microfilaments. Desmosomes are abundant in tissues subjected to mechanical stress, such as cardiac muscle and skin epithelium. Several of these junctions can be associated together, forming junctional complexes that contribute to cell adhesion, communication, and barrier function.