Ovulation in the animal kingdom: A review with an emphasis on the role of contractile processes

Abstract

INTRODUCTIONIn most sexually reproducing animals, oocytes develop within an organized tissue sac, the follicle; oocytes and follicle cells are assembled together in the unovulated follicle, which is solid in all but therian mammals. Before fertilization, the oocyte generally abandons the follicle and moves to another site for subsequent spawning or further development. The process of escape from the follicle is termed ovulation; we shall also use this term to refer to the escape of the oocyte from its surrounding tissue in those metazoans that lack an organized follicle. In general, ovulation is at least a two-part phenomenon involving enzymatic degradation and rupture of the follicle followed by the actual movement of the oocyte out of the follicle. These steps were proposed by Schochet [1916] for mammalian ovulation, and by Rugh El9351 for frogs, and seem to be generally applicable to those phyla in which no intrafollicular antral space is present at the time of ovulation.We have reviewed information on the mechanism of oocyte extrusion in animals throughout the phylogenetic series, paying particular attention to the role of contractility in the process. Contraction of a follicle implies the presence of contractile cells in or around the follicle. A variety of contractile systems has been described in follicles from diverse animal types, and we feel it is now worthwhile to review the question of contractile mechanisms in ovulation from an evolutionary point of view. For each group we discuss, we will examine, to the extent information is available, 1) evidence that contractile cells are present near the developing oocyte, 2) experimental evidence that contractile cells are involved in ovulation, and 3) factors that may regulate contraction of these cells. It will become apparent that information on the mechanics of ovulation in other than eutherian mammals is scarce. Current information makes it clear that cells that are contractile or potentially capable of

Structural and immunohistochemical aspects of the postovulatory follicle in Japanese quail

“…Morphological and physiological results (for review see Schroeder and Talbot, 1985;Jones, 1987) as well as our observations supported the prevailing hypothesis that cells of the theca externa might also have a role in ovulation. Yoshimura et al (1983) showed that the theca externa contracts intermittently in preovulatory follicles, suggesting a vitellodynamic function .…”

Localization of smooth‐muscle markers in the ovaries of some ectothermic vertebrates

“…Larsen et al (1977) stated that the inner ovarian epithelium is "a sheet of typical smooth-muscle cells." In unpublished data, Larsen reported a similar layer in other frog and salamander species (Schroeder and Talbot, 1985).…”

Identification of ovarian gene expression patterns during vitellogenesis in Atlantic cod (Gadus morhua)

“…These results suggest that smooth muscle genes are upregulated in the late vitellogenic follicle wall and may function in ovulatory competency for follicular rupture during ovulation. Although the presence and function of smooth muscle during ovulation has been controversial for decades [18], some evidence has existed for its role in teleost follicles [30,64], and mammalian theca cells can contract during follicular rupture [35]. While ovulation in fish has been described as an inflammatory-like reaction [8,42], other processes such as smooth muscle contractions may cooperate with these factors, and further research is needed to investigate the role of follicular smooth muscle.…”