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Early ionocytes have been studied in the European sea bass (Dicentrarchus labrax) embryos. Structural and functional aspects were analyzed and compared with those observed in the same conditions (38 ppt) in post hatching stages. Immunolocalization of Na(+) /K(+) -ATPase (NKA) in embryos revealed the presence of ionocytes on the yolk sac membrane from a stage 12 pair of somites (S), and an original cluster around the first gill slits from stage 14S. Histological investigations suggested that from these cells, close to the future gill chambers, originate the ionocytes observed on gill arches and gill filaments after hatching. Triple immunocytochemical staining, including NKA, various Na(+) /K(+) /2Cl⁻ cotransporters (NKCCs) and the chloride channel "cystic fibrosis transmembrane regulator" (CFTR), point to the occurrence of immature and mature ionocytes in early and late embryonic stages at different sites. These observations were completed with transmission electronic microscopy. The degree of functionality of ionocytes is discussed according to these results. Yolk sac membrane ionocytes and enteric ionocytes seem to have an early role in embryonic osmoregulation, whereas gill slits tegumentary ionocytes are presumed to be fully efficient after hatching., (© 2011 The Authors. Journal compilation © 2011 Japanese Society of Developmental Biologists.)

Because of the permeability of the chorion, sea bass embryos are exposed to seawater before hatching and hence require precocious osmoregulatory processes. Several studies of other species have demonstrated the existence of ion-transporting cells located on the yolk sac membrane of embryos. In these cells, called ionocytes, ion movements are controlled by a pool of transmembrane proteins. Among them, the Na(+)/K(+)-ATPase, an abundant driving enzyme, has been used to reveal the presence or absence of ionocytes. We have immunostained the Na(+)/K(+)-ATPase in sea-bass embryos and shown the presence of the first ionocytes on the yolk sac membrane at stage 12 somites and the occurrence of ionocytes at other sites before hatching. Ionocytes located on the first gill slits have been identified at stage 14 somites. Primitive enteric ionocytes have also been detected at stage 14 somites in the mid and posterior gut. The presence of these cells might be related to the early opening of the gut to perivitelline fluids, both anteriorly by the gill slits and posteriorly by the anus. The role of embryonic ionocytes in osmoregulation before hatching is discussed.

The European sea bass Dicentrarchus labrax is a marine teleost important in Mediterranean aquaculture. The development of the entire digestive tract of D. labrax, including the pharynx, was investigated from early embryonic development to day 5 post hatching (dph), when the mouth opens. The digestive tract is initialized at stage 12 somites independently from two distinct infoldings of the endodermal sheet. In the pharyngeal region, the anterior infolding forms the pharynx and the first gill slits at stage 25 somites. The other three gill arches and slits are formed between 1 and 5 dph. Posteriorly, in the gut tube region, a posterior infolding forms the foregut, midgut and hindgut. The anus opens before hatching, at stage 28 somites. Associated organs (liver, pancreas and gall bladder) are all discernable from 3 dph. Some aspects of the development of the two independent initial infoldings seem original compared with data in the literature. These results are discussed and compared with embryonic and post-embryonic development patterns in other teleosts.