Perifollicular stromal cells sustain folliculogenesis via MDK-NCL signaling in mice and humans.

Oocyte development requires coordinated metabolic and signaling support from granulosa and theca cells. By performing integrated single-cell RNA sequencing and spatial transcriptomic analyses of murine and human ovaries, we discovered a functionally specialized stromal subtype essential for folliculogenesis. These stromal cells (SCs) with glutamyl aminopeptidase (ENPEP) function, designated perifollicular SCs based on their circumferential follicle localization, exhibit two hallmark features: (1) dynamic proliferation synchronized with follicular maturation from primary to secondary to antral stages, and (2) secretion of midkine (MDK), which activates nucleolin (NCL) receptor signaling to drive granulosa cell (GC) expansion. Furthermore, analyses of ovarian aging revealed the concurrent depletion of perifollicular SCs and the attenuation of MDK-NCL signaling between perifollicular SCs and GCs. The unique spatial confinement and regulatory capacity of perifollicular SCs endow them with the potential to become important components of the follicular functional unit, providing new theoretical support for understanding the molecular regulatory mechanisms of ovarian aging from the perspective of the follicular microenvironment.