Apical surface ring formation in Cyprinus carpio scale epidermis

Apical Rings (ARs) are novel circular structures located at the apical surface of fish scale epidermal cells. Different stages in the life cycle of ARs were established using time-lapse video microscopy of Koi scale epidermis in situ. The organization of the F-actin cytoskeleton corresponding to the stages of ring formation in live tissue was determined from phalloidin staining of scale epidermis. ARs form from localized swollen regions of microridges that subsequently elongate laterally, progressing into complete circles with well-defined inner and outer borders that encircle unperturbed microridges. The ARs close in a centripetal movement to form a circular structure, the terminal disc, and subsequently an underlying vesicle, the terminal body. During this latter stage, a rapid rearrangement of the actin cytoskeleton occurs resulting in the emergence of a newly identified corkscrew-like structure, termed the Helical Core. Furthermore, fluid-phase markers are incorporated into the terminal bodies, identifying these vesicles as macropinosomes. The findings are discussed with respect to F-actin organization and macropinocytosis at the epidermal surface of fish.