The present invention provides for compositions and methods for modulating hematopoetic stem cell populations by using HCS modulators, which are agents that either increase HSC numbers or decrease HSC numbers as desired by a particular indication. For example, HSC modulators found to increase HSC numbers include prostaglandin E2 (PGE2) and agents that stimulate the PGE2 pathway. Conversely, HSC modulators that prevent PGE2 synthesis decrease HSC numbers. HCS modulators may be used in vitro, in vivo, or ex vivo.