How does cellular renewal influence skin ageing?

Cellular renewal refers to the process by which skin cells continuously regenerate to maintain its integrity and protective function. This mechanism relies on the proliferation and differentiation of keratinocytes, the principal cells of the epidermis. These cells originate in the basal layer, at the junction between the dermis and the epidermis, then progressively migrate towards the surface of the skin while undergoing a series of morphological and biochemical transformations. As they ascend, they lose their nucleus and become corneocytes, forming the stratum corneum, the outermost layer of the skin. This complete cycle, from the basal keratinocyte to the desquamated corneocyte, lasts on average 28 days in young adults.

With age, this renewal cycle tends to lengthen, leading to an accumulation of dead cells on the surface and a duller complexion.

A study examined the duration of cellular transit within the epidermis, that is, the time required for cells to pass from one layer to another: from the basal layer to the Malpighian layer, then to the granular layer and finally to the stratum corneum. The experiment involved volunteers aged between 18 and 80 years. The results revealed that with advancing age, the epidermal cell migration time was prolonged by approximately ten days compared with younger subjects, reflecting a marked slowdown in skin renewal. The values obtained for each age group are presented in the table below, with asterisks indicating statistically significant differences.

The skin ageing is accompanied by a progressive weakening of the skin’s regenerative capabilities. This decline is largely due to the reduced efficiency of epidermal and follicular stem cells, which are responsible for the continuous renewal of the epidermis. Although their number only declines moderately with age, their capacity for division and differentiation decreases markedly, notably due to an altered microenvironment: the extracellular matrix loses its cohesion, cell signalling is disrupted and inflammatory signals intensify. These disturbances reduce the stem cells’ ability to maintain regular renewal, resulting in skin that is less resilient and slower to heal.

At the molecular level, the slowing of cell turnover is exacerbated by the accumulation of senescent cells. These cells, unable to divide, secrete a range of pro-inflammatory mediators known as SASP (Senescence-Associated Secretory Phenotype), which impair the function of neighbouring cells. At the same time, phenomena such as telomere shortening, oxidative stress and the deregulation of proliferation-related genes, such as p53, HES1, KLF6 or COL17A1, contribute to the slowdown in cell renewal. These internal changes create a vicious cycle in which the skin renews ever more slowly, while its structural integrity deteriorates.

Extrinsic factors, such as the UV rays and pollution, amplify these intrinsic ageing effects. They cause damage to stem cell DNA, disrupt their microenvironment and hasten their functional exhaustion. The skin thus ages visibly, with the appearance of wrinkles, a duller complexion and a loss of elasticity, and functionally, with a weakened skin barrier and reduced repair capacity.

The slowdown in cellular renewal is not merely a feature of ageing: it is also one of its principal driving forces.

To offset the natural slowdown of cellular renewal, the exfoliation remains one of the most effective approaches today. By removing dead skin cells from the surface of the skin, mechanical scrubs or chemical exfoliants stimulate keratinocyte proliferation and promote renewal of the stratum corneum. However, caution is advised: while exfoliation is an important part of a skincare routine, it should only be performed once or twice a week to avoid weakening the skin.

Meanwhile, research is currently exploring new avenues to reactivate the biological mechanisms of cellular renewal. Among these, exosomes, tiny extracellular vesicles released by cells, are attracting growing interest. These structures play a role in intercellular communication and could, according to some studies in vitro, stimulate keratinocyte proliferation and promote skin regeneration. However, these observations remain, for the time being, confined to laboratory models and their efficacy in vivo on human skin has yet to be demonstrated. Further scientific work is therefore still required.

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