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

Abstract

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.

Reproductive management of poultry

“…The testes develop due to gene dosing with increased expression of the Z-linked transcription factor gene, doublesex and mab-3-related transcription factor 1 (DMRT1). 6,7 Anti-M€ ullerian hormone (AMH) is synthesized and secreted by the embryonic testis with greater expression in the embryonic testes than the ovaries. 8e10 AMH directs the regression of the paired M€ ullerian ducts.…”

“…11 The avian oviduct is derived from the embryonic M€ ullerian duct; the former term encompassing the entire reproductive tract from infundibulum to the cloaca. 7 Regression of the right oviduct is induced by AMH. 10 Parenthetically, AMH also plays an important role in development of tubules in the testes.…”

Expression of FOXL2 and RSPO1 in Hen Ovarian Follicles and Implication of Exogenous Leptin in Modulating Their mRNA Expression in In Vitro Cultured Granulosa Cells

“…A few signaling pathways have been implicated in the process of follicle recruitment in domestic laying hens [1,[5][6][7]; however, the exact underlying molecular mechanisms are still unclear. It has been reported that surrounding somatic cells have a well-coordinated interplay with the oocytes, and this interaction is believed to promote the process of follicle recruitment from the available pool [3].…”

“…Apparently, this process seems to be complex and highly coordinated, involving a number of divergent biological effects on the maturation of oocytes and differentiation and proliferation of granulosa and theca cells within the ovarian follicles [8]. In addition, the follicle-stimulating hormone (FSH) has been implicated in facilitating the recruitment of follicles [6,7,[9][10][11][12]. Similarly, follicle stimulating hormone receptors (FSHR) are reportedly expressed on the granulosa layer of developing follicles, and mRNA expression of these receptors (FSHR) changes with progressing follicular maturation [3,6,13].…”

Functional Morphology of Thecal Glands in the Ovary of Japanese Quails (<b><i>Coturnix japonica</i></b>)

“…Small follicles also produce a significant amount of ovarian oestrogens [Armstrong, 1984]. Cells expressing the enzyme aromatase, which synthesise oestrogen from androgen precursors, are localised to the theca externa of larger follicles, whereas pregnenolone, progesterone, and androgen precursors are almost exclusively produced by the theca interna [Johnson, 1986[Johnson, , 2014. On the other hand, the granulosa cells of the growing previtellogenic and vitellogenic follicles synthesise progesterone de novo, i.e., from cholesterol and pregnenolone.…”