A 32-year-old woman is undergoing controlled ovarian hyperstimulation as part of an in vitro fertilization (IVF) protocol. Beginning on day 2 of her menstrual cycle, she receives daily exogenous follicle-stimulating hormone (FSH). The goal is to recruit multiple follicles capable of maturing and ovulating. At which stage of follicular development do ovarian follicles first express sufficient FSH receptors and become dependent on FSH for continued growth?