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Female mammals are born with a lifetime's supply of oocytes individually enveloped in flattened epithelial cells to form primordial follicles. It is not clear how sufficient primordial follicles are maintained to sustain the reproductive lifespan, while providing an adequate supply of mature oocytes for ovulation. Locally produced growth factors are thought to be critical regulators of early follicle growth, but knowledge of their identity and source remains incomplete. Here, we have used a simple approach of spatial analysis of structures in histological tissue sections to identify likely sources of such regulatory molecules, narrowing the field for future screening for candidate growth factors or antagonists. We have quantified the relative spatial positions of primordial (resting) follicles and growing follicles in mice on days 4, 8, and 12 after birth, and calculated interfollicular distances. Follicles were significantly less likely to have started growing if they had 1 or more primordial follicles close by (within 10 m), predicting that primordial follicles inhibit each other. This approach allows us to hypothesize that primordial follicles produce a diffusible inhibitor that prevents neighboring primordial follicles from growing. Such an approach has wide applicability within many branches of developmental and cell biology for studying spatial signaling within tissues and cells.diffusing inhibitor ͉ follicle growth ͉ signal gradient

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