Regulation of follicular development. The modelled signalling network. In brief, FSH- and LH-signalling enables IGF signalling in several ways (arrow A1), and enhances the production of androgens (A2), as well as the production and activity of aromatase (A3), the enzyme that catalyzes androgens into estradiol. IGF signalling is necessary for the gonadotropin-dependent expression of aromatase (A4), enhances the production of FSH and LH receptors (A5, A6), and reduces the expression of estrogen receptors (A7). Estrogen signalling enhances the production of aromatase (A8), as well as the production of the receptors for FSH (A9), LH (A10), and estradiol (A11). FSH receptor expression is observed also in the absence of FSH, LH, and estrogen signalling, and we therefore introduce a further regulation-independent component ϑ, which may represent testosterone-dependent signalling (A12). Blue arrows indicate exchange with the blood, black arrows indicate chemical reactions (binding or catalysis), green arrows indicate activating impacts and red arrows indicate inhibitory impacts. All components also decay, but for greater clarity decay reactions have not been included in the scheme. For a detailed description of the network interactions along with the evidence see the main text.