Keratin is not a microfilament. It belongs to a different category of cell structural fibers called intermediate filaments. This is a common point of confusion because cells contain three types of protein-based scaffolding, and they’re easy to mix up. Keratin filaments are about 10 nm in diameter, while microfilaments are only about 7 nm and are made from an entirely different protein: actin.
Three Types of Cytoskeletal Filaments
Every cell maintains its shape and internal organization using a network of protein fibers collectively called the cytoskeleton. These fibers come in three distinct sizes and compositions, each with different jobs.
- Microfilaments (about 7 nm diameter): Made of actin protein. These are the thinnest fibers and are highly dynamic, constantly assembling and disassembling. They drive cell movement, help cells change shape, and power muscle contraction.
- Intermediate filaments (8 to 10 nm diameter): Keratin belongs here, along with other proteins like vimentin and neurofilaments. These are rope-like and primarily mechanical, providing structural strength and resisting physical stress.
- Microtubules (about 25 nm diameter): Made of tubulin protein. These are the thickest fibers and serve as tracks for transporting cargo within the cell, while also forming the scaffolding that pulls chromosomes apart during cell division.
Intermediate filaments got their name precisely because their diameter falls between microfilaments and microtubules. As a class, they are less dynamic than the other two types, acting more like permanent structural cables than rapidly shifting networks.
How Keratin Filaments Are Built
Keratin filaments are assembled from two families of keratin proteins: type I (acidic) and type II (neutral or basic). There are 28 known type I keratins and 26 type II keratins. The two types are strictly interdependent for assembly. A type I monomer pairs with a type II monomer in parallel to form a coiled-coil dimer, somewhat like two strands of rope twisted around each other. Two dimers then come together in an antiparallel, staggered arrangement to form a tetramer, which is the main building block available in living cells. These tetramers interact along their sides and end-to-end to build the final 10 nm filament.
This assembly process is fundamentally different from how microfilaments form. Actin monomers are small, globe-shaped proteins that stack head-to-tail like beads on a string. Three monomers cluster together as a starting nucleus, then additional monomers add to both ends of the growing chain, with one end (the “plus end”) growing five to ten times faster than the other. Actin filaments constantly polymerize and depolymerize depending on the cell’s needs, giving them a dynamic, treadmill-like quality. Keratin filaments, by contrast, tend to mature into stable bundles once they’re in place.
Where Keratin Works in the Body
Keratin is found specifically in epithelial cells, the cells that form your skin, line your organs, and cover body surfaces. Every epithelial cell produces at least one type I and one type II keratin, which pair up to form the cell’s internal scaffolding. This is distinct from other intermediate filament proteins: vimentin appears in connective tissue cells and white blood cells, desmin is specific to muscle cells, and neurofilaments are found in neurons.
Inside epithelial cells, keratin filaments form a network that radiates through the cytoplasm, surrounding the nucleus in a cage-like structure and extending outward to anchor at specialized cell junctions called desmosomes and hemidesmosomes. Desmosomes connect neighboring cells to each other, while hemidesmosomes attach cells to the underlying tissue layer. By anchoring to these junctions, keratin networks link individual cells into a continuous, mechanically resilient sheet of tissue. Once filaments attach to these junctions, their normal cycle of assembly and disassembly slows down considerably, creating a stable framework that maintains mechanical integrity.
Different Jobs: Structure vs. Movement
The functional difference between keratin and actin microfilaments is significant. Keratin filaments exist primarily to absorb and distribute mechanical stress. Epithelia are constantly exposed to stretching, compression, and friction. The cross-linked keratin network acts as an internal shock absorber, maintaining cell and tissue integrity under those forces.
Actin microfilaments, on the other hand, are the engine behind cell movement. Cycles of actin polymerization and depolymerization at the cell’s leading edge push the membrane forward, forming sheet-like protrusions called lamellipodia that allow cells to crawl. This treadmilling behavior is essential for wound healing, immune cell migration, and embryonic development. Interestingly, research on skin cells (keratinocytes) has shown that the keratin network and actin network interact during cell spreading. The keratin scaffold influences the direction of actin-driven protrusions, helping to guide polarized cell migration.
The Full Intermediate Filament Family
Keratin is part of a larger family of more than 50 intermediate filament proteins, classified into six types. Types I and II are the acidic and basic keratins. Type III includes vimentin and desmin. Type IV covers neurofilament proteins (designated light, medium, and heavy based on their molecular weight). Type V consists of nuclear lamins, which line the inside of the nuclear envelope in most cells. Type VI is nestin, expressed in neural stem cells during early development. Intermediate filaments commonly work alongside microtubules, providing strength and support to the more fragile tubulin-based structures.
What Happens When Keratin Goes Wrong
Because keratin filaments are the mechanical backbone of epithelial tissues, mutations in keratin genes lead to diseases characterized by tissue fragility. The best-studied example is epidermolysis bullosa simplex, a condition in which the skin blisters in response to minor friction or trauma. The defective keratin filaments can’t absorb normal mechanical stress, so cells rupture and the skin layers separate. Other keratin-related disorders include pachyonychia congenita (painful thickening of the nails and skin on the feet), monilethrix (beaded, fragile hair that breaks easily), and several forms of ichthyosis involving scaly, thickened skin. These diseases illustrate exactly why keratin is classified separately from microfilaments: its job is structural durability, and when it fails, tissues that endure physical wear simply fall apart.