Prolonged high-density culture with restricted medium renewal induces epithelial-like plasticity in apical papilla stem cells

Stem cells from the apical papilla (SCAPs) are dental mesenchymal stem cells known for their high proliferative and multilineage potential. However, prolonged in vitro expansion may alter their phenotype due to microenvironmental stress. This exploratory study aimed to investigate whether prolonged high-density culture with restricted medium renewal is associated with morphological, protein-level, and transcriptional changes indicative of partial mesenchymal-epithelial transition (MET) in SCAPs. Donor-derived SCAPs were cultured under standard or prolonged high-density conditions. Morphological assessment was conducted using phase-contrast microscopy, protein expression was evaluated through immunofluorescence, and gene expression was analyzed using a Human epithelial-mesenchymal transition (EMT) reverse-transcription quantitative polymerase chain reaction (RT-qPCR) array. Prolonged culture resulted in a transition from spindle-shaped mesenchymal cells to compact epithelial-like clusters. This morphological change was accompanied by the upregulation of epithelial-associated genes, such as cadherin 1 (CDH1) and keratin 14 (KRT14), alongside the downregulation of extracellular matrix (ECM) and adhesion-related genes, including collagen type I alpha 2 chain (COL1A2), collagen type V alpha 2 chain (COL5A2), fibronectin 1 (FN1), and several integrins. Key regulators associated with EMT, including snail family transcriptional repressor 1 (SNAI1), snail family transcriptional repressor 3 (SNAI3), zinc finger E-box-binding homeobox 1 (ZEB1), notch receptor 1 (NOTCH1), and epidermal growth factor receptor (EGFR), were downregulated. In contrast, bone morphogenetic proteins 2 (BMP2) and 7 (BMP7), transforming growth factor beta 2 (TGFB2), matrix metalloproteinases 2 (MMP2) and 3 (MMP3), and secreted phosphoprotein 1 (SPP1) were upregulated. Immunofluorescence further demonstrated the expression of enamel-associated proteins, including amelogenin and ameloblastin, in long-term cultures. In conclusion, prolonged high-density culture with limited medium renewal was associated with coordinated epithelial-like phenotypic and transcriptional changes in SCAPs, suggesting the potential for partial mesenchymal-epithelial plasticity and emphasizing the sensitivity of dental stem cells to the culture microenvironment during in vitro expansion.