Figure 4.

Working model for maternal deposition of cortisol into zebrafish oocytes. In oocyte development, cortisol (blue) is incorporated during vitellogenesis, along with vitellogenin (yellow). Therefore, excess cortisol may be deposited in the embryos in response to maternal stress and this is represented in the bottom panel of the figure; the cortisol content shown is based on the highest level observed in the breeding study (approx. 35 pg per embryo on day 3; figure 2c). Given that zebrafish are asynchronous breeders, oocytes are at multiple stages in the ovarian follicles and, therefore, pre-vitellogenic oocytes will also be exposed to cortisol excess during maternal stress. We propose these pre-vitellogenic oocytes will upregulate 11βHSD2 levels (by 4 h; figure 3) in the theca/granulosa cells surrounding the oocyte (top panel) in response to cortisol excess. This will result in the breakdown of cortisol to cortisone, thereby preventing excess maternal cortisol from being incorporated into the oocytes during subsequent vitellogenesis. This is reflected in the basal embryo cortisol levels (approx. 4 pg per embryo; figure 2c) seen in the cortisol group on days more than 4 days of breeding.