# Development of branchial ionocytes in embryonic and larval stages of cloudy catshark, Scyliorhinus torazame

## TL;DR

This study examines how ionocytes, which help regulate body fluids, develop in the cloudy catshark during its embryonic and larval stages.

## Contribution

The study reveals the unique development and asymmetric distribution of follicular ionocytes in the gills of cloudy catshark embryos.

## Key findings

- NKA- and V-ATPase-rich ionocytes appear as single cells in gill filaments from stage 31.
- Follicular structures of NKA-immunoreactive cells develop in the inter-filament region of gill septa from stage 32.
- Follicular ionocytes are distributed asymmetrically on the rostral side of the gill septum, aiding efficient fluid homeostasis.

## Abstract

In teleost fish, branchial ionocytes are important sites for osmoregulation and acid-base regulation by maintaining ionic balance in the body fluid. During the early developmental stages before the formation of the gills, teleost ionocytes are localized in the yolk-sac membrane and body skin. By comparing with teleost fish, much less is known about ionocytes in developing embryos of elasmobranch fish. The present study investigated the development of ionocytes in the embryo and larva of cloudy catshark, Scyliorhinus torazame. We first observed ionocyte distribution by immunohistochemical staining with anti-Na+/K+-ATPase (NKA) and anti-vacuolar-type H+-ATPase (V-ATPase) antibodies. The NKA- and V-ATPase-rich ionocytes appeared as single cells in the gill filaments from stage 31, the stage of pre-hatching, while the ionocytes on the body skin and yolk-sac membrane were also observed. From stage 32, in addition to single ionocytes on the gill filaments, some outstanding follicular structures of NKA-immunoreactive cells were developed to fill the inter-filament region of the gill septa. The follicular ionocytes possess NKA in the basolateral membrane and Na+/H+ exchanger 3 in the apical membrane, indicating that they are involved in acid-base regulation like single NKA-rich ionocytes. Three-dimensional analysis and whole-mount immunohistochemistry revealed that the distribution of follicular ionocytes was limited to the rostral side of gill septum. The rostral sides of gill septum might be exposed to faster water flow than caudal side because the gills of sharks gently curved backward. This dissymmetric distribution of follicular ionocytes is considered to facilitate efficient body-fluid homeostasis of catshark embryo.

The online version contains supplementary material available at 10.1007/s00441-024-03897-4.