Keratinocytes
Keratinocytes — the predominant cell type of the epidermis, responsible for producing keratin, the protein that gives skin its strength and protective barrier. They form in the deepest epidermal layer and gradually move upward, flattening and dying to become the corneocytes of the surface. This continuous journey drives skin renewal and barrier maintenance.
Key points
- Keratinocytes make up roughly 90 percent of the cells in the epidermis.
- They produce keratin, a tough structural protein essential to the skin's barrier function.
- As keratinocytes migrate from the basal layer to the surface, they differentiate and eventually form flattened dead corneocytes.
- This upward migration and shedding, known as desquamation, underpins the skin's natural renewal cycle.
The journey in stages
| Stage | Where it sits | What the cell is doing |
|---|---|---|
| Basal keratinocyte | Stratum basale, on the basement membrane | Dividing; the main source of new epidermal cells |
| Spinous keratinocyte | Stratum spinosum | Building keratin bundles and desmosomes; assembling lamellar bodies |
| Granular keratinocyte | Stratum granulosum | Filling with keratohyalin granules; secreting lamellar body lipids |
| Transitional cell | Granular–corneal boundary | Losing its nucleus and organelles; cornified envelope cross-linked |
| Corneocyte | Stratum corneum | Flat, dead, keratin-filled; set in lipid, holding water via NMF |
| Shed cell | Skin surface | Corneodesmosomes cut by enzymes; released invisibly, a few layers a day |
What the cell makes on the way up
A keratinocyte spends its life manufacturing structural protein. Basal cells make keratins 5 and 14; once committed to differentiation they switch to keratins 1 and 10, which bundle into dense filaments anchored to desmosomes. In the granular layer, keratohyalin granules release filaggrin, which compacts those bundles into the flat mass of a corneocyte and is later broken down into natural moisturising factor.
The second product is lipid. From the spinous layer onward the cell assembles lamellar bodies filled with glucosylceramides, cholesterol and phospholipids, then extrudes them at the granular boundary, where enzymes convert them into the ceramides and fatty acids that stack into ordered sheets. A keratinocyte therefore builds both the bricks and the mortar, and a fault in either arm leaves a barrier that loses water more easily.
Not only a structural cell
Keratinocytes are also immune-signalling cells, which short definitions usually omit. They carry pattern-recognition receptors for bacterial, fungal and viral components, and secrete antimicrobial peptides such as beta-defensins and cathelicidin into the surface lipids. On injury they release cytokines and chemokines, including pre-formed interleukin-1, so a keratinocyte is often the first cell to raise the alarm.
They also talk to their neighbours. They accept melanin from melanocytes and hold it as a cap shielding the nucleus, they signal to the Langerhans cells threaded between them, and under ultraviolet B they convert 7-dehydrocholesterol into the precursor of vitamin D. Damage beyond repair pushes them into programmed cell death — the apoptotic cells that make sunburnt skin peel.
What sets the pace
The familiar 28-day skin cycle is a convenient approximation rather than a measured constant. Estimates of how long a cell takes to travel from the basal layer to shedding vary with the method used, the body site and the person's age, and in adults the journey is commonly longer than a month. Transit also tends to slow with age.
Several things speed division up. Barrier disruption — from harsh surfactants, low humidity or repeated friction — triggers a repair response in which basal cells divide faster and the stratum corneum thickens, which is how a callus forms. Ultraviolet exposure has a broadly similar effect. In psoriasis the cycle is drastically shortened, a condition for a clinician to diagnose and manage.
Frequently asked
What do keratinocytes do?
Keratinocytes produce keratin and form the bulk of the epidermis, building and maintaining the skin's protective barrier as they migrate toward the surface.
Where are keratinocytes formed?
They originate in the stratum basale, the deepest layer of the epidermis, and gradually move upward as they mature.
Are keratinocytes and corneocytes the same cell?
They are the same cell at different stages. A corneocyte is a keratinocyte that has finished differentiating, having lost its nucleus and organelles to become a flat, keratin-filled shell.
Can a cream reach the basal layer where keratinocytes divide?
Most cosmetic ingredients stay within the stratum corneum, and only small, suitably lipophilic molecules cross into the living epidermis. Claims that a product acts directly on dividing basal cells should be treated with caution.
Do keratinocytes ever run out?
No. The basal layer holds long-lived stem cells, with a further reservoir in the hair follicle, and these keep dividing throughout life. Output slows with age rather than stopping.
Why does skin thicken where it is rubbed?
Repeated pressure or friction increases basal cell division and the number of corneocyte layers, producing a callus. It is a protective adaptation that usually reverses once the friction stops.
Related topics
This is a foundational entry in the SYNC Skin Encyclopedia and is expanded over time. Educational information only — not medical advice.
