Microscopic detection of IgY‐Fc binding signal in the inner layers of ovarian follicular tissue in quail

In avian species, it has been assumed that an Fc receptor in the ovarian follicles mediates immunoglobulin Y (IgY) transport into the yolk. However, no such receptor responsible for IgY has been identified to date. To examine potential IgY binding activity in the entire ovarian follicle, whole‐mount sections of quail ovarian follicle were incubated with the Fc fragment of chicken IgY (cIgY). Whole‐mount frozen sections of the second largest ovarian follicle were prepared, and then the sections were incubated with digoxigenin‐labeled Fc or Fab fragments of cIgY. Microscopic observation revealed that incubation with the cIgY‐Fc fragment produced a binding signal in the inner layer of the ovarian follicular tissues, most likely in the granulosa cell layer. However, no such signal was detected when the sections were incubated with cIgY‐Fab. Coincubation of the ovarian sections with Alexa488‐labeled cIgY‐Fc and antiserum raised against ZP1, an envelope protein specifically localized in the perivitelline layer, demonstrated that the source of the Fc binding signals partly coincided with the perivitelline layer. In conclusion, our data suggest that potential IgY binding substances interacting with the Fc domain are present in the inner layers of ovarian follicular tissues, most likely in the granulosa cell layer and/or in the perivitelline layer.