In mammals, members of the transforming growth factor-beta (TGF-β) superfamily are known to have key roles in the regulation of follicular growth and development. The aim of the study was to evaluate the expression of TGF-β superfamily growth factors, their receptors, downstream SMAD signalling molecules and TGF-β/bone morphogenetic protein (BMP) antagonists during early human folliculogenesis. Human pre-antral follicles were enzymatically isolated from surplus ovarian tissue obtained from women having ovarian cortical tissue frozen for fertility preservation. A total of 348 human pre-antral follicles, ranging from 40 to 200 µm in diameter, were isolated from ovarian tissue obtained from 15 women, aged 24-34 years. Isolated pre-antral follicles were grouped according to diameter in five size-matched populations spanning the primordial, primary and secondary stage follicles and analysed by whole-genome microarray analysis. Selected proteins/genes were analysed by immunocytochemistry and quantitative RT-PCR. TGF-β superfamily genes with overall highest mRNA expressions levels included growth differentiation factors 9 (GDF9), BMP15, BMP6, BMP-receptor-2 (BMPR2), anti-Müllerian hormone receptor 2 (AMHR2), TGFβR3, inhibin-α (INHA) and intracellular SMAD3 and SMAD4. Moreover, genes which were differentially expressed from the primordial to the late secondary stage follicles included GDF9, BMP15, AMH, INHBB, TGFβR3, SMAD4 and antagonists Follistatin (FST) and GREM1. Collectively, these data indicate that the active TGF-β superfamily pathways in early human folliculogenesis consist of primarily GDF9 combined with possible synergistic effects of BMP15 through the BMPR2 and intracellular activation of SMAD3 and SMAD4, and that AMH and INHBB are engaged in intrafollicular events from the onset of follicular growth. Moreover, the presence of multiple TGF-β/BMP antagonists imply that certain growth factors are subjected to local regulation on different levels that address another important level of intraovarian regulation of follicle development in humans.
Keywords: follicle development; gene expression; growth factors.