Location and morphology of chloride cells were studied in the sea bass ( Dicentrarchus labrax) from hatching to the juvenile stage to determine the development of the adult osmoregulatory function as seen in adult fish. During the studied developmental sequence changes were observed in the location, number, size and structure of these cells, that were studied by microscopy (light, scanning electron, transmission electron and confocal) and immunocytochemistry. Chloride cells were found on the tegument and on the gills. They were present on the tegument already at hatching, before the development of the gills. Their density as well as their association in multicellular complexes decreased during the postembryonic development. In old larvae and in juveniles, cutaneous chloride cells were associated with the fins, the developing scales and the lateral line. Gills developed gradually during the prelarval stage and the gill arches were present at mouth opening. At that time chloride cells were already numerous on the gill arches. In older larvae, during the progressive development of the gill filaments, chloride cells were numerous on these structures and formed multicellular complexes. Several stages in the differentiation of these cells were studied, including the development of the tubulovesicular system at the end of the prelarval stage, as well as the stratification appearance of the cytoplasm that was concomitant with the considerable development of the tubular system and its association with the endoplasmic reticulum during the larval period. The involvement of different epithelia in the osmoregulatory process during the postembryonic development of this species, as well as the role of chloride cells during successive developmental stages, is discussed.

Branchial chloride cells (CC) were studied in sea bass (Dicentrarchus labrax) maintained in seawater (SW: 35 per thousand) or gradually adapted to and subsequently maintained in fresh water (0.2 per thousand) or doubly concentrated seawater (DSW: 70 per thousand). Changes were observed in the location, number, and structure of CCs, that were discriminated by light, scanning, and transmission electron microscopy, as well as by immunofluorescence on the basis of their high Na(+)/K(+)-ATPase antigen content. The number of CCs increased in both fresh water and doubly concentrated seawater compared to control fish maintained in SW. In both experimental conditions, these cells were found on the gill filament (as in control fish) and even on the lamellae, especially in hypersaline conditions. Structural changes concerned the shapes and sizes of CCs and their apical outcrops and particularly the structures of their functional complexes (mitochondria, tubular system, and endoplasmic reticulum), which developed significantly in DSW adapted fish. The changes in the expression of the Na(+)/K(+)-ATPase were evaluated by assessing the enzyme's density at the ultrastructural level following immunogold labeling. This parameter was significantly higher in doubly concentrated seawater. The adaptative significance of the quantitative and morphofunctional changes in branchial chloride cells is discussed in relation to the original osmoregulatory strategy of this marine euryhaline teleost.

European sea bass (Dicentrarchus labrax) are euryhaline fish that tolerate wide salinity fluctuations owing to several morphofunctional adaptations. Among the osmoregulatory sites (tegument, branchial chambers, digestive tract, urinary system), little is known about the kidney and the urinary bladder. The present study describes the ontogeny of the urinary system (kidney and urinary bladder) and focuses on the progressive expression of the Na(+)/K(+)-ATPase in the cells of these ion-transporting epithelia. A structural approach has shown that two pronephric urinary tubules are already present at hatching while the urinary bladder starts to differentiate. The glomus, an ultrafiltration site, occurs at day 5 (D5). The opisthonephros differentiates at D19/25 from the pronephric collecting tubules, then it rapidly grows longer and becomes folded. Na(+)/K(+)-ATPase immunolocalization and transmission electron microscopy show that ionocyte-like cells line the urinary tubules and the dorsal wall of the urinary bladder from D2/D5 on. Tubule ionocytes present a basolateral-localized fluorescence. Ionocytes of the collecting ducts and of the dorsal wall of the bladder present a fluorescence distributed in the whole cytoplasm. Fluorescence becomes stronger in later stages, suggesting a progressively increasing functionality of the urinary system in active ion transports. This observation is closely correlated with the ontogeny of osmoregulatory abilities. In juvenile and preadult fish kept in seawater, osmolality measurements demonstrate that urine is isotonic to blood. At low salinity, urine is hypotonic to blood in both stages. The capacity to produce hypotonic urine increases during ontogeny, a fact that suggests an increasing involvement of the urinary system in osmoregulation. The occurrence and the progressive functionality of the urinary system during the ontogeny, along with those of other osmoregulatory sites, are major adaptations allowing the sea bass to live in habitats of variable salinity such as lagoons and estuaries.

The ontogeny of the antennal glands was studied during the embryonic and post-embryonic development of Astacus leptodactylus. The future glands arising from undifferentiated columnar cells were detectable at the metanauplius stage EI 150 microm (EI: eye index; approximately 440 microm at hatching). The tubule and labyrinth differentiated in embryos at EI 190 microm, and the bladder and coelomosac at EI 250 microm. At EI 350 microm, the tubule lengthened and divided into proximal and distal sub-regions. In later stages, the gland retained the same morpho-anatomy but the differentiation and size of each part increased. The cells of the coelomosac displayed the cytological features of podocytes in late embryonic development at EI 440 microm. Only small apical microvilli and a few mitochondria were observable in the labyrinth cells at EI 250 microm; by EI 440 microm, these cells presented well-shaped apical microvilli, formed bodies, basal infoldings and mitochondria. In the cells of the tubules and bladder, mitochondria and basal infoldings occurred at EI 440 microm and EI 250 microm, respectively. The differentiation of the tubules and bladder cells suggested that they were involved in active transport at EI 440 microm. Following hatching, the differentiation of the cells and the size of the glands increased. The ontogeny of the antennal glands thus starts in early embryos, the specific cellular functional features being differentiated in the various parts of the glands by EI 440 microm. The antennal glands are probably functional just before hatching, i.e., before the juveniles are confronted with the low osmolality of freshwater.

The olfactory organ of the European sea bass (Dicentrarchus labrax) in adults and during development has been studied by light microscopy and by transmission and scanning electron microscopy. This organ includes two cavities, each extended by an accessory sac and opening to the outside through two nostrils. It contains a rosette consisting of about forty lamellae. The olfactory epithelium is characterized by the presence of two types of receptor cells, ciliated or with microvilli, and numerous ciliated nonsensory cells. Rod cells, essentially found in the altered epithelia of farmed bass, and rodlet cells are also observed. The olfactory organ forms very early in the developmental process. Two olfactory pits holding both types of sensory receptors appear 24 h before hatching. The ciliated nonsensory cells only appear at the end of the endotrophic period, shortly before the mouth opens. Although it is rather unspectacular during the larval stage, the development of the olfactory organ is characterized at the start of the juvenile stage by three important events: the formation of the nostrils, the hollowing of the accessory sacs, and the development of the rosette. This is created by raising the floor of the cavity and forming successive folds, which are the lamellae where the sensory epithelium is found.