Morphometric of blastomeres in Salmo salar

For Salmo salar, there is a lack of information on the morphology of the first blastomeres formed during embryonic development and which could be used as a diagnostic tool for the first stages of development. The purpose of this investigation, therefore, was to characterize morphometrically the first blastomeres of S. salar. From a pool of eggs incubated at 7.5°C, 100 microphotographs of blastodiscs were extracted and analyzed at different incubation periods: 12, 14, 16, 20 or 24 h. Blastodiscs were characterized morphologically after 16, 20 or 24 h incubation, and classified into symmetric or asymmetric groups according to their morphology. The ratio of length (L) versus width (W) of each blastomere was determined, to establish its symmetry. In addition, 20 microphotographs of blastodiscs of normal appearance were analysed morphologically (control blastodisc: CB) for comparison (20 or 24 h). Results show that the first cleavage ends after 16 h of development. Seven categories were established during blastomere characterization: 47% normal (G1); 27% with dispersed margins (G2); 10% unequal (G3); 9% 'pie-shaped' (G4); 3% amorphous (G5); 2% three equal blastomeres and one different one (G6); and 2% with eccentric cleavage (G7). Although the incidence of abnormal cleavage in S. salar is uncertain, there is a potential for some asymmetries to be corrected during embryogenesis to generate viable individuals. More studies are necessary to correlate these abnormal cleavage patterns with indicators of quality in the later stages of embryogenesis in this species, to establish a quality assessment tool for gametes and/or embryos in salmonid species.