Folikülogenezis ve Moleküler Mekanizması
Özet
Ovarian foliküller, ovaryumun temel fonksiyonel birimidir ve bir oosit ile onu çevreleyen granüloza ve teka hücrelerinden meydana gelir. Folikülogenezis süreci; primordiyal, gelişen (primer ve sekonder) ve olgun (graaf) folikül olmak üzere üç temel morfolojik evrede incelenir. Primordiyal germ hücreleri, embriyonik gelişim sırasında fötal gonadlara göç ederek oogoniumlara ve ardından mayoz bölünmenin profaz evresinde duraklayan primer oositlere farklanır. Puberteyle birlikte gonadotropinlerin ve lokal büyüme faktörlerinin etkisiyle aktive olan foliküller, tek katlı yassı hücrelerden çok katlı kübik granüloza hücrelerine dönüşerek primer ve sekonder folikül evrelerine ilerler. Bu süreçte zona pellüsida şekillenir ve granüloza hücreleri arasında sıvı dolu boşlukların birleşmesiyle antrum adı verilen yapı oluşur. Folikül gelişiminin erken evreleri büyük oranda gonadotropinden bağımsız otokrin ve parakrin faktörlerce (AMH, GDF-9, BMP ailesi, PTEN, Foxo3a) denetlenirken, ileri evrelerde ve dominant folikül seçimi ile ovulasyon aşamalarında FSH ve LH hormonları kritik roller üstlenir. Her siklusta çok sayıda folikül gelişmeye başlasa da, sadece bir tanesi tam anlamıyla olgunlaşarak ovulasyona ulaşır, diğerleri ise atreziye uğrar.
Ovarian follicles are the primary functional units of the ovary, consisting of an oocyte surrounded by granulosa and theca cells. The process of folliculogenesis is examined in three basic morphological stages: primordial, developing (primary and secondary), and mature (graafian) follicles. During embryonic development, primordial germ cells migrate to the fetal gonads, differentiating into oogonia and subsequently into primary oocytes that arrest in the prophase of the first meiotic division. With puberty, under the influence of gonadotropins and local growth factors, activated follicles transition from single-layered flat cells to multi-layered cubic granulosa cells, progressing into primary and secondary follicle stages. During this process, the zona pellucida develops, and fluid-filled spaces between granulosa cells coalesce to form a cavity called the antrum. While the early stages of follicle development are largely regulated by gonadotropin-independent autocrine and paracrine factors (AMH, GDF-9, BMP family, PTEN, Foxo3a), FSH and LH hormones play critical roles in the advanced stages, dominant follicle selection, and ovulation. Although multiple follicles initiate development in each cycle, only one reaches full maturity and undergoes ovulation, while the remaining follicles undergo atresia.
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