Green Histopathology: A Global Systematic Review of Natural Staining Agents as Sustainable Alternatives to Synthetic Dyes (2003 to 2025)

Despite increasing publications on the potential of natural dyes in histology and laboratory sciences, there is limited comparative synthesis of available literature, which hampers the development of consolidated guidance for practice, regulation and innovation. This study presents a global systematic review of natural histological staining agents reported between 2003 and 2025, evaluating their sources, staining performance, methodological approaches, and sustainability attributes. Following PRISMA 2020 guidelines, peer-reviewed studies assessing plant- and biologically derived dyes for routine and specialized histological applications were analyzed. The findings show that while early investigations reported inconsistent or poor staining outcomes for some extracts, growing evidence supports the eosin-like potential of several natural dyes, particularly Curcuma longa, Hibiscus sabdariffa, Lawsonia inermis, Zingiber officinale, betacyanin-rich plants, and Tectona grandis. Moreover, staining effectiveness was strongly influenced by extraction methods, pH adjustment, mordant use, and staining duration. Despite demonstrated benefits in safety, biodegradability, and cost, challenges related to standardization, reproducibility, and nuclear staining remain. Overall, the review highlights the promise of natural dyes as complementary, sustainable options in histopathology and underscores the need for further optimization and large-scale validation. These findings support the cautious integration of optimized natural dyes as sustainable adjuncts in histopathology rather than direct one-to-one replacements for synthetic stains.