Plant-Derived Bioactive Compounds in Skin Cell Biology: Mechanisms of Barrier Repair, Photoaging Protection, and Microbiome Modulation

Natural ingredient-based cosmeceuticals represent an expanding area at the interface of cosmetics; these include polyphenols, phenolic acids, flavonoids, carotenoids, fatty acids, polysaccharides, and essential oil constituents. They have attracted increasing attention because of their antioxidant, anti-inflammatory, antimicrobial, photoprotective, and skin barrier-supporting properties. These compounds may contribute to the prevention of premature skin aging by reducing oxidative stress, modulating inflammatory signaling, inhibiting extracellular matrix degradation, and supporting epidermal homeostasis. In parallel, the emerging concept of microbiome-friendly skincare has created new opportunities for natural ingredients that act as prebiotics, postbiotics, or microbial balance supporting agents. Before clinical translation, cell-based experimental systems employing keratinocytes, dermal fibroblasts, melanocytes, and reconstructed skin models can be used to validate the relevant mechanisms of action at the cellular level. However, the translation of natural bioactivities into effective cosmetic products remains limited by poor chemical stability, low skin penetration, variability in extract composition, formulation incompatibility, and insufficient clinical validation. Advanced formulation technologies, including liposomes, nanoemulsions, solid lipid nanoparticles, hydrogels, and biopolymer-based delivery systems, may improve the stability, bioavailability, and controlled release of natural actives. This review summarizes major classes of plant-derived cosmeceutical ingredients, their mechanisms of action in skin aging, photoaging, barrier repair, and microbiome modulation, and discusses extraction, characterization, formulation, safety, and regulatory considerations. Future development should prioritize standardized phytochemical profiling, mechanism-based efficacy testing, microbiome-aware formulation design, sustainable sourcing, and well-controlled human studies.