San Francisco startup Muse Bio began paying women to donate menstrual fluid as it builds a biobank of menstrual‑derived stem cells, aiming first at cosmetics and later at regenerative‑medicine applications. The company offers modest per‑cycle payments and packages donations as a low‑barrier source of mesenchymal stem cells shed from the uterine lining. Founders argue menstrual stem cells are a rich, non‑invasive source with clinical potential; critics note ethical, consent, and commercialization questions around compensating biological donors. Muse’s move reflects growing commercial interest in unconventional cell sources and the early‑stage market for menstrual‑derived therapies and biobanking.