Keratinocytes are the most abundant cell type, making up approximately 90% of the cells in the epidermis, the outermost layer of the skin. They play a role in forming the skin’s protective barrier, shielding the body from environmental damage such as heat, UV radiation, and water loss, and protecting against bacteria, fungi, parasites, and viruses.
The Keratinocyte’s Journey to the Surface
A keratinocyte’s journey from its formation to the skin’s surface typically takes about 28 to 40 days. This process begins in the stratum basale, the deepest layer of the epidermis, where new keratinocytes are continuously produced through cell division.
From the stratum basale, these newly formed cells begin their upward migration through several distinct layers of the epidermis. They first move into the stratum spinosum, where they start to produce keratin, a fibrous protein that provides strength to the skin. In this layer, cells also become more polyhedral in shape and are held together by specialized junctions called desmosomes.
The cells then progress to the stratum granulosum, where they flatten and undergo further changes, including the degradation of their internal organelles and nucleus. During this stage, they release lipids into the spaces between cells, which are important for forming the skin’s water barrier.
Finally, these cells reach the stratum corneum, the outermost layer of the epidermis. Here, they are known as corneocytes, which are essentially dead, flattened keratinocytes filled with keratin. These tough, anucleated cells form a protective layer that eventually sheds through a process called desquamation as new cells arrive from below.
Factors Affecting Keratinocyte Turnover
Several factors influence keratinocyte movement from the basal layer to the skin surface. Age is a factor, with the turnover process slowing down in older individuals. For example, in humans, the complete cycle from formation to shedding can take 40 to 56 days.
The rate of keratinocyte turnover can also vary depending on the specific body area. Areas like the palms and soles of the feet have a faster turnover due to increased friction and wear, while areas such as the eyelids may experience a slower rate.
Certain health conditions can alter keratinocyte turnover. Psoriasis, for instance, is a condition characterized by an accelerated turnover, where cells reach the surface in just a few days instead of weeks, leading to thickened, scaly patches of skin. Conversely, some diseases or nutritional deficiencies can slow this process, impairing the skin’s ability to renew itself.
Environmental factors influence skin cell turnover. Chronic sun exposure can damage keratinocytes and affect their normal life cycle, while extreme climates, either very dry or very humid, can impact the skin barrier and, indirectly, the rate of cell replacement. External influences such as regular exfoliation can manually remove dead skin cells, encouraging the underlying keratinocytes to differentiate and move upwards more quickly. Additionally, certain skincare ingredients, like retinoids, stimulate keratinocyte proliferation and differentiation, thereby accelerating the skin’s renewal process.