The avian ovary and follicle development: some comparative and practical insights

This review focuses on recent research related to the avian ovary and, in particular, cellular mechanisms mediating embryonic growth of the ovary, postembryonic follicular development, and the process of follicular selection. Attention is drawn to a variety of fundamental, yet unresolved, aspects of ovarian function, several of which are uniquely related to avian reproductive strategies. Some notable differences in ovarian function between birds and the most closely related extant group, the crocodilians, are illustrated. Moreover, because the sauropsid (reptilian and avian) and synapsid (mammalian) lineages diverged shortly after the emergence of amniotes some 340 million years ago, consideration is given to both derived and convergent characteristics of ovarian function in avian species versus those in eutherian mammals. Two examples of divergent characteristics are the differences in sex-determining mechanisms between birds and mammals and the requirement for progesterone synthesized within the follicular granulosa as a requirement for ovulation in virtually all species of birds, compared to the site of synthesis and role for estradiol in mammals. As an example of a convergent process, both birds and select mammals (e.g., humans, cattle, and horses) typically ovulate a single egg per reproductive cycle, yet the processes associated with follicular selection occur via unique mechanisms. Finally, reference is made to several practical outcomes from studies of the avian ovary, including applications within the poultry industry and use of the domestic hen ovary as a model for human ovarian cancer.