The domestic chicken: Causes and consequences of an egg a day.

The domestic laying chicken has been intensely selected to be a persistent ovulator. That is, the tendency for broodiness has been nearly eliminated and, given the appropriate lighting and nutrition, many strains of laying hens produce an egg on almost every day. The regulatory mechanisms involved in coordination of neuroendocrine and ovarian events have been well studied and described. In spite of this, there has been little attention focused on the oocyte itself. Recent findings in mammals have indicated that the oocyte produces several oocyte-specific factors, including growth differentiation factor 9 (GDF9) and bone morphogenetic factor 15 (BMP15), which influence the surrounding cells and follicular development. Our studies indicate that GDF9 is present in the hen oocyte and influences granulosa cell proliferation. Additionally, Bmp15 mRNA is most abundant in oocytes of small follicles and stimulates an increase in follicle stimulating hormone (FSH) receptor mRNA in granulosa cells. BMP15 also enhances yolk uptake in growing follicles by decreasing tight junctions between granulosa cells. These studies indicate that the oocyte likely contributes to follicle development. Commercial laying hens also spontaneously develop ovarian cancer at a high rate, and susceptibility to this disease has been associated with ovulatory events in women. Studies have shown that ovulation, or events associated with ovulation, increase the prevalence of ovarian cancer in hens. Inhibition of ovulation in hens through a hormonal strategy mimicking oral contraceptives results in a decrease of ovarian cancer incidence. Recent studies in women have suggested that some ovarian tumors may arise from the distal oviduct. Gene expression profiles in very early stage tumors from hens show a high expression of oviduct-related genes, supporting the possibility of oviduct origin for some ovarian tumors. Genetic selection for high productivity in commercial laying hens has generated an efficient and valuable food source as well as an important animal model for human ovarian cancer.