Distribution of Glycogen in the Developing Salmon (Salmo Salar L.)

Abstract

1. Quantitative estimations and histological methods have been used to determine the presence and distribution of glycogen in fertilized salmon eggs and subsequent stages of development. 2. A check upon the occurrence of glycogen in each sample was obtained by the use of amylase and the presence of glucose as a result of this technique confirmed, in certain large samples, by the formation of phenylglucosazone crystals. 3. Results of estimations agree with the general distribution of glycogen as shown in histological sections. They differ from those of Hayes & Hollett (1940) who, using water extraction, found no glycogen in stages corresponding to stages I and II and only recorded it as doubtfully present in the yolk-sac. 4. In stages I and II glycogen is concentrated in the blastoderm and perivitelline space. Later the main sources are the muscles, liver and yolk-sac envelope. 5. Glycogen is present in embryonic muscle tissue when fibrils are being laid down (stage III). Subsequently it occurs in both sarcoplasm and muscle fibres. 6. Eight days before hatching (stage V) there is strong staining reaction for glycogen in liver cells, and it is present in all later stages. This glycogen is not obtained from engulfed food or from direct absorption of yolk by liver cells. 7. There is histological evidence that yolk is taken up by the yolk-sac blood vessels after absorption by the yolk-sac cells and dermis. This absorption is accompanied by the appearance of glycogen in these cells and in yolk lying adjacent to them. 8. An increase in amounts of glycogen in both embryo and yolk-sac coincides with a rapid absorption of fat which takes place about 20 days after hatching. 9. The presence of glycogen in blastoderm cells, before the gastrula stage, is similar to the condition in developing Aves and Amphibia. This is true, also, for its appearance early in the formation of muscle tissue. Other similarities are the presence of glycogen in the perivitelline fluid of salmon and Amphibia and the manner of its distribution in the liver of early and late stages of the salmon and chick. 10. There is extra-embryonic glycogen present in the meroblastic eggs of both the salmon and chick. It is concentrated in the embryonic shield of the latter which corresponds, in development, to the glycogen-carrying envelope of the salmon yolk-sac. Accompanying these deposits in both cases is a well-developed circulatory system to assist in the absorption of yolk. 11. The presence of glycogen in the yolk-sac cells of the salmon refutes a suggestion that the embryo receives glucose only by direct diffusion from the yolk. 12. The liver cells and yolk-sac cells overlap in the function of laying down glycogen during development. The latter, therefore, do not form a ‘transitory’ but rather function as a ‘supplementary’ liver, connected with the liver by a venous system until the cells rejoin the true embryo after complete yolk exhaustion.