The human umbilical cord–mesenchymal stem cell secretome regulates hair growth and cycle transition by promoting methylthioadenosine synthesis via the PI3K/AKT/mTOR pathway

• SCT promotes telogen-to-anagen transition, hair thickening, and elongation in mice, and increases human hair density and diameter in a clinical trial. • Mechanistically, SCT regulates dermal papilla cells and hair matrix cells via cysteine and methionine metabolism, activating the PI3K/AKT/mTOR pathway to stimulate methylthioadenosine synthesis. • SCT is a cell-free therapy with no observed adverse reactions on scalp or hair, offering a promising alternative for hair loss prevention. • This study provides a scientific basis for developing SCT-based hair care products and regenerative treatments.