Résumé
The ontogeny of osmoregulation was examined in the branchial cavity of embryonic and early post-embryonic stages of the crayfish Astacus leptodactylus maintained in freshwater, at the sub-cellular level through the detection of the sodium-potassium adenosine triphosphatase (Na(+),K(+)-ATPase). The embryonic rate of development was calculated according to the eye index (EI) which was 430-450 microm at hatching. The distribution of the enzyme was identified by immunofluorescence microscopy using a monoclonal antibody IgGalpha5 raised against the avian alpha-subunit of the Na(+),K(+)-ATPase. Immunoreactivity staining, indicating the presence of Na(+), K(+)-ATPase appeared in the gills of late embryos (EI>/=400 microm), i.e. a few days before hatching time, and steadily increased throughout the late embryonic and early post-embryonic development. The appearance of the enzyme correlates with the ability to osmoregulate which also occurs late in the embryonic development at EI 410-420 microm and with tissue differentiation within the gill filaments. These observations indicate that the physiological shift from osmoconforming embryos to hyper-regulating late embryos and post-hatching stages in freshwater must originate partly from the differentiation in the gill epithelia of ionocytes which are the site of ion pumping, as suggested by the location of Na(+),K(+)-ATPase. Only the gills were immunostained and a lack of specific staining was noted in the lamina and the branchiostegites. Therefore, osmoregulation through Na(+)active uptake is likely achieved in embryos at the gill level; all the newly formed gills in embryos function in ion regulation; other parts of the branchial chamber such as the branchiostegites and lamina do not appear to be involved in osmoregulation.