Development of multicellular complexes of chloride cells in the yolk-sac membrane of tilapia ( Oreochromis mossambicus ) embryos and larvae in seawater

Abstract

Morphological changes in the chloride cells (CCs) in the yolk-sac membrane of euryhaline tilapia (Oreochromis mossambicus) embryos and larvae were examined in relation to environmental salinity. Half of a brood of embryos spawned in fresh water (FW) were transferred directly to seawater (SW) 1 day before hatching; the other half was maintained in FW. The embryos and larvae in both FW and SW contained a rich population of CCs in the yolk-sac membrane; the CCs were visualized by whole-mount immunocytochemistry with an antiserum specific for Na+,K+-ATPase. The sectional areas of CCs increased markedly following SW transfer, whereas they remained small in the embryos and larvae maintained in FW. Scanning electron microscopy showed that the apical opening of CCs was enlarged in the fish transferred to SW. Transmission electron microscopy revealed enhanced cellular activity in SW, as evidenced by well-developed mitochondria and tubular systems. The CCs in SW frequently formed a multicellular complex, consisting of a main CC and one or two accessory cells. Accessory cells interdigitated with the main cells and extended their cytoplasmic processes to the apex of the main cell. The three-dimensional arrangement of the cells participating in the complex was identified by confocal laser scanning microscopy. Such complexes were rarely observed in FW fish. The activated CCs in the yolk-sac membrane in the SW fish probably function as ion-extruding sites during embryonic and larval stages until gill CCs become functional.

Chloride turnover and ion-transporting activities of yolk-sac preparations (yolk balls) separated from Mozambique tilapia embryos and incubated in freshwater and seawater

“…In yolk balls incubated in seawater, where Cl -permeability is greatly increased, it is inevitable that excess Cl -be extruded in order to cope with diffusional Cl -influx and maintain the Cl -concentration in the physiological range. It would be reasonable to attribute active Cl -secretion to MRCs, which are present in the yolk-sac membrane of tilapia embryos and larvae (Ayson et al, 1994;Shiraishi et al, 1997) as well as in that of yolk balls .…”

“…On the basis of their morphology and responses to environmental salinity changes, branchial and extrabranchial MRCs in Mozambique tilapia have largely been classified into freshwater and seawater types (Ayson et al, 1994;Shiraishi et al, 1997;Hiroi et al, 1999;Uchida et al, 2000). The freshwater-type cells that are expected to absorb ions in hypoosmotic environments are small and located individually.…”

“…The embryos are able to survive direct transfer from freshwater to seawater and vice versa, even though the developing gills are not functional (Ayson et al, 1994). Previous studies have demonstrated that MRCs located in the yolk-sac membrane are the extrabranchial site of ion exchange during the late embryonic stages of tilapia (Ayson et al, 1994(Ayson et al, , 1995Shiraishi et al, 1997). MRCs in the yolk-sac membrane became larger and frequently formed multicellular complexes when embryos were transferred from freshwater to seawater, whereas small MRCs existed individually in freshwater (Shiraishi et al, 1997).…”

“…Previous studies have demonstrated that MRCs located in the yolk-sac membrane are the extrabranchial site of ion exchange during the late embryonic stages of tilapia (Ayson et al, 1994(Ayson et al, , 1995Shiraishi et al, 1997). MRCs in the yolk-sac membrane became larger and frequently formed multicellular complexes when embryos were transferred from freshwater to seawater, whereas small MRCs existed individually in freshwater (Shiraishi et al, 1997). Such cellular complexes, consisting of well-developed MRCs and accessory cells, are considered to be characteristic of seawater-type MRCs with ion-secreting functions .…”

Physiological study of larval fishes: challenges and opportunities

“…Such techniques have been employed, for example, to reveal that chloride cells of the yolk-sac membrane of embryos and larvae of the Mozambique tilapia, Oreochromis mossambicus, differ in size and density between those adapted to freshwater and those adapted to seawater (Ayson et al, 1994). Additional differences in the fine structure of these cells has been studied with both scanning and transmission electron microscopy (Shiraishi et al, 1997). The ion secreting and absorbing functions of the chloride cells can be detected with the vibrating probe technique (Foskett and Scheffey, 1982) and the chloride test, in which Cl -reacts with Ag + resulting in photosensitive AgCl (Kaneko and Shiraishi, 2001).…”