Morphological and functional characterization of a novel Na^sup +^/K^sup +^-ATPase-immunoreactive, follicle-like structure on the gill septum of Japanese banded houndshark, Triakis scyllium

In teleost fishes, it is well-established that the gill serves as an important ionoregulatory organ in addition to its primary function of respiratory gas exchange. In elasmobranch fish, however, the ionoregulatory function of the gills is still poorly understood. Although mitochondria-rich (MR) cells have also been found in elasmobranch fish, these cells are considered to function primarily in acid-base regulation. In this study, we found a novel aggregate structure made up of cells with basolaterally-expressed Na^sup +^/K^sup +^-ATPase (NKA), in addition to NKA-immunoreactive MR cells that have already been described in the gill filament and lamella. The cell aggregates, named follicularly-arranged NKA-rich cells (follicular NRCs), were found exclusively in the epithelial lining of the venous web in the cavernous region of the filament and the inter-filamental space of the gill septum. The follicular NRCs form a single-layered follicular structure with a large lumen leading to the external environment. The follicular NRCs were characterized by: (i) well-developed microvilli on the apical membrane, (ii) less prominent infoldings of the basolateral membrane and (iii) typical junction structures including deep tight junction between cells. In addition, large numbers of vesicles were observed in the cytoplasm and some of them were fused to the lateral membrane. The follicular NRCs expressed Na^sup +^/H^sup +^ exchanger 3 and Ca^sup 2+^ transporter 1. The follicular NRCs thus have the characteristics of absorptive ionoregulatory cells and this suggests that the elasmobranch gill probably contributes more importantly to body fluid homeostasis than previously thought.[PUBLICATION ABSTRACT]