Figure 8.

Hypothetical model for IGF-I regulation of kitlga expression and its signaling mechanisms in the zebrafish follicle cells. Kitlga from the follicle cells may promote oocyte growth while suppressing maturation during vitellogenic stage, and its high expression level may likely be maintained by growth factors such as IGF-I via the PI3K-Akt pathway. When the follicle approaches the preovulatory stage, the expression of kitlga reduces and its response to IGF-I diminishes, which is probably due to a decreased signaling of the stimulatory PI3K-Akt pathway and a relatively constant or slightly increased MEK-ERK pathway. Paracrine signals from the oocyte such as EGF family may contribute to the shifting of the relative signaling intensity of the 2 pathways.