Follicle Selection And Growth In The Domestic Hen Ovary

Abstract

Ovarian follicle selection and development in domestic hens is highly regulated. A single small follicle (approximately 6-8 mm in diameter) is selected daily and enters the pre-ovulatory hierarchy, continuing to grow until ovulation occurs. We investigated the mechanism(s) involved in follicle selection and subsequent growth before ovulation. In the first study, we found bone morphogenetic protein 15 (BMP15) mRNA primarily in the oocyte, and BMP15 protein was detected in 6 and 8 mm follicles around the time of follicle selection. In addition, BMP15 altered mRNA expression of genes associated with follicle selection. Granulosa cell culture with BMP15 increased mRNA levels of follicle stimulating hormone receptor (FSHR) and decreased both anti-Müllerian hormone (AMH) and occludin (OCLN). OCLN protein, a tight junction protein, also decreased, suggesting that this oocyte factor may play a pivotal role in follicle selection into the pre-ovulatory hierarchy. Liver-derived yolk material enters follicles via capillaries in the theca layer, ultimately reaching oocytes via paracellular spaces between granulosa cells. Yolk is incorporated into the oocyte by receptor-mediated endocytosis. OCLN may regulate access of yolk material to the oocyte surface. We determined expression and regulation of OCLN in small follicles. OCLN mRNA expression decreased with increasing follicle size. The regulation of OCLN transcripts in laying hens was investigated by granulosa cell culture. BMP15, epidermal growth factor (EGF) and activin B decreased OCLN expression, whereas activin A increased OCLN mRNA, suggesting local factors may regulate yolk access to the oocyte. Broiler breeder hens have aberrant follicle development that is modulated by limiting feed intake. Restricted diets produce more ordered follicle development and yolk uptake, but the causative factors are unknown. We examined metabolic-related factors in the liver and global gene expression to identify altered transcripts that contribute to abnormal follicular growth. The GH/IGF1-axis may be overstimulated in hens on an ad libitum diet and feed intake alters genes involved in lipid metabolism. These studies offer new insights into specific intra-ovarian and extra-ovarian factors mediating follicle selection and growth in the hen.