Bone Morphogenetic Protein 15 May Promote Follicle Selection in the Hen

In the hen, optimal ovulation rate depends on selection of a single follicle into the pre-ovulatory hierarchy. Follicle selection is associated with increased oocyte growth and changes in gene expression in granulosa cells surrounding the oocyte, in preparation for ovulation. This study investigated the expression, function and regulation of bone morphogenetic protein-15 (BMP15) during follicle development in the hen. BMP15 mRNA expression was analyzed in the ooplasm and granulosa cells of 3mm follicles and was confirmed to be primarily in the ooplasm. BMP15 was detected by immunoblotting in 6 and 8mm follicles near the time of follicle selection. Expression of mRNA for BMP15 receptors (BMPR1B and BMPR2) in granulosa cells increased with follicle size, indicating that BMP15 may play an important role around follicle selection. The function of BMP15 was examined by culturing granulosa cells from 3-5mm and 6-8mm follicles with recombinant human BMP15 (rhBMP15). BMP15 increased expression of follicle stimulating hormone receptor (FSHR) mRNA and decreased anti-Müllerian hormone (AMH) mRNA and occludin (OCLN), factors associated with follicle maturation and growth in the hen. Hormonal regulation of BMP15 was assessed by whole follicle culture with estradiol (E2) which increased BMP15 mRNA expression. The distinct expression pattern of BMP15 and its receptors, coupled with the effects of BMP15 to increase FSHR mRNA and decrease AMH mRNA and OCLN mRNA and protein expression suggest that the oocyte may have a role in follicle selection in the chicken.