First Assessment of Hippophae rhamnoides Seed Extract’s Retinoid-Like Activity and Evidence for Additive/Synergistic Epidermal Reinforcement with Harungana madagascariensis

Background: Skin ageing manifests as epidermal thinning, impaired barrier function, and dermal matrix degradation. Retinol and its derivatives effectively counteract these changes but can elicit irritation. Among plant-derived alternatives, a Harungana madagascariensis extract (HME) shows retinol-like effects. However, a comprehensive anti-ageing strategy must also reinforce the epidermis. During screenings for natural compounds, we identified a sea buckthorn (Hippophae rhamnoides L.) seed extract (BSE). Herein, we characterise HME and BSE effects on skin explants, benchmarking them against retinol. Materials and Methods: Skin explants from six female donors (26 - 47 years) were UV-irradiated and topically treated with creams containing 0.1% HME, 0.1%/0.5% BSE, HME-BSE combinations, or retinol benchmark. Treatments spanned 10 days with applications on days 2, 3, 5, 7, and 9. CRABP-II, Ki67 (day 3), COL-I, STRA6, and TJP1 (day 10) were assessed by immunofluorescence. Results: HME enhanced dermal COL-I/CRABP-II and epidermal CRABP-II/Ki67. BSE upregulated dermal CRABP-II (+43%), COL-I (+28% - 56%), epidermal CRABP-II (+52% - 58%), and Ki67 (+59% - 84%), mimicking retinol/HME. HME-BSE combinations showed CRABP-II antagonism yet superior epidermal responses for STRA6 (+39% - 49% vs. no individual effect) and TJP1 (+91% - 140% vs. +72% HME; +27% BSE 0.1%), suggesting additive/synergistic effects. Conclusion: BSE demonstrates retinol-like anti-ageing potential. HME-BSE synergy, possibly via CRABP-II-independent mechanisms, should reinforce epidermal proliferation (STRA6) and the barrier function (TJP1), complementing the extracts’ dermal effects. These findings position HME, BSE, and their combination as promising natural anti-ageing actives warranting clinical validation.