In this study, the researchers first confirmed that the skin produces serotonin. In mice, as in humans (skin samples taken during abdominoplasties), they found the full set of machinery needed for serotonin metabolism: synthesis enzymes, transporters, and receptors are present in several skin cell types. Serotonin was found more specifically in the epidermis and around hair follicles, and its concentration was measured using a precise analytical chemistry technique (HPLC).

To identify its role in the skin, the researchers used genetically modified mice unable to produce peripheral serotonin (Tph1⁻/⁻ knockout mice). Comparing their skin to that of normal mice, they observed: thinner skin, with reduced epidermal and dermal thickness; a less effective skin barrier, with greater water loss through the skin (measured by "TEWL," transepidermal water loss); a decrease in key proteins involved in the differentiation of the skin's outer layers -filaggrin, loricrin, keratins - proteins essential for forming healthy, protective skin; less collagen in the dermis, suggesting a weakened supportive structure; and finally, fewer blood vessels and fewer proliferating cells.

In its absence, the skin shows clear changes in its organization: the epidermis and dermis are altered, and the dermal extracellular matrix is remodeled. At the same time, barrier function is disrupted, suggesting that the skin loses part of its ability to protect itself and maintain its balance.

The significance of this discovery extends beyond basic biology. The authors note that abnormalities in the serotonergic system could play a role in certain inflammatory skin diseases, such as atopic dermatitis (eczema), or in other situations where the skin barrier is compromised, such as during skin aging. The authors also raise the possibility that certain drugs that affect serotonin (such as some antidepressants, SSRIs -selective serotonin reuptake inhibitors) could potentially have an impact on the skin.

By opening up this avenue, the study invites us to think of the skin as a chemically highly active organ, capable of engaging with signaling pathways once thought to be reserved for other functions. Furthermore, this research, which shows that serotonin is not solely a brain molecule, could eventually open new avenues for better understanding, and perhaps better treating, certain skin diseases.