Estrogeny to hormony steroidowe pełniące fundamentalną rolę w regulacji procesów rozrodczych u samic, działając na wszystkich poziomach osi podwzgórze-przysadka-jajnik (HPG) w sposób endokrynny, parakrynny i autokrynny, a także są włączone w regulację procesów metabolicznych. Wyróżniamy trzy główne estrogeny: estron (E1), estradiol (E2) i estriol (E3). Najbardziej aktywny biologicznie jest E2 produkowany przede wszystkim przez pęcherzyki jajnikowe, choć niewielkie ilości E2 są syntetyzowane w nadnerczach i mózgu ptaków. Biosynteza estrogenów rozpoczyna się już na początku rozwoju embrionalnego. Estradiol jest wykrywany w niezróżnicowanych gonadach już w 3-4 dniu embriogenezy (Bruggeman i wsp., 2002).

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