Tokmakov AA , Stefanov VE , Sato KI .

             gonadotropin hormone-releasing hormone activity
             ovulation
             luteinizing hormone signaling pathway involved in ovarian follicle development

	FIGURE 1. In vivo and in vitro ovulation of frog oocytes. In vivo, favorable environmental factors initiate a hormonal cascade via the hypothalamic-pituitary-gonadal axis, to cause oocyte ovulation resulting in egg spawning. In vitro, oocyte ovulation can be induced in the ovarian fragments or isolated ovarian follicles by pituitary extracts, or luteinizing hormone (LH), or human Chorionic Gonadotropin (hCG), a functional analog of LH, or multiple steroid hormones.
	FIGURE 2. Major events of ovulation in the mammalian ovarian follicle (see details in the text).
	FIGURE 3. Signaling pathways of Xenopus oocyte maturation. Molecular components of CSF and MPF are colored green and orange, correspondingly. The components of MPF auto-amplification loops mediated by polo-like kinase (Plx1) and Greatwall kinase (Gwl) are presented in light blue, and the factors involved in the initiation of egg maturation are shown in gray. Detailed explanations are provided in the text (see section “Maturation”).
	FIGURE 4. Timing of maturation and follicle rupture during Xenopus oocyte ovulation. Progression of meiotic maturation in the developed in vitro ovulation model (Tokmakov et al., 2019) was judged by appearance of a white spot (WS) on the animal hemisphere of oocytes, reflecting germinal vesicle breakdown (GVBD), and occurrence of follicular rapture (FR) was defined by the loss of the follicle layer (FL). (A) Theoretically, two alternative scenarios, GVBD-FR and FR-GVBD, can occur during ovulation, as indicated in the figure. To distinguish between them, the four morphologically different types of cells were counted during hormone-induced ovulation. They included oocytes without WS surrounded by FL (WS-FL+), oocytes with WS and FL (WS + FL+), oocytes without WS and FL (WS–FL–), and oocytes with WS and without FL (WS + FL). The WS-FL + phenotype characterizes original oocyte population, and the WS + FL– phenotype corresponds to ovulated mature eggs. (B) Experimentally observed relative frequencies of the two intermediate phenotypes, WS + FL + and WS–FL–, are very close, indicating that GVBD and FR occur almost simultaneously.

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