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Lindahl, K. 1980. The gonadotropic cell in parr, precocious parr male and smolt of the Atlantic salmon, Salmo salar. An immunocytological, light‐ and electron microscopical study. (Department of Zoology, University of Stockholm, Sweden.) — Acta zool. (Stockh.) 61(2): 117–125. Only one GTH cell type was identified in the pars distalis of the parr, smolt and precocious parr male. On a fine structural basis four phases with transitional forms of the GTH cell were recognized. In parr and smolt the GTH cells have numerous secretory granules and their cytology substantiate that they are in a storing phase with a possibly low release rate. On the contrary, in the precocious parr male all phases of the GTH cell are present. The great number of active cells and frequent degranulation and vacuolization indicate a high activity and release level.

LINDAHL, K., H. LUNDQVIST, and M. RYDEVIK. 1983. Plasma thyroxine levels and thyroid gland histology in Baltic salmon (Salmo salar L.) during smoltification. Can. J. Zool. 61: [1954][1955][1956][1957][1958]. Plasma thyroxine (T4) levels in Salmo salar L. showed a clearly defined surge which coincided with a rapid rise in the ambient water temperature and with the period of natural seaward migration. It is sugggested that the elevation of T4 levels has a major role in the completion rather than the onset of the smoltification in Baltic salmon parr. The body coloration (i.e., smolt or parr appearance) could not be correlated to the levels of plasma T4 and silvering does not at all times reflect the degree of smoltification. A relationship between size (length and weight) and plasma levels of T4 was not detected for smolting Baltic salmon parr. Increased circulating levels of T4 is not reflected by an overall morphological change of the thyroid cells during smoltification.LINDAHL, K., H. LUNDQVIST et M. RYDEVIK. 1983. Plasma thyroxine levels and thyroid gland histology in Baltic salmon (Salmo salar L.) during smoltification. Can. J. Zool. 61: [1954][1955][1956][1957][1958]. Les concentrations plasmatiques de thyroxine (T4) subissent une augmentation appreciable au moment de l'augmentation de la temperature de l'eau et de la periode naturelle de migration vers la mer chez le saumon de la Baltique. I1 semble que cette augmentation joue un r61e important au moment oh le tacon achkve de se transformer en saumoneau plutat qu'au debut de la transformation. La coloration du corps, c.-a.-d. l'aspect du tacon ou du saumoneau, n'est pas en correlation avec la concentration de T4 du plasma et l'apparition des reflets argent n'indique pas toujours le degre du developpement en saumoneau. I1 ne semble pas y avoir non plus de relation entre la taille du poisson (longueur et masse) et les concentrations de T4 du plasma chez le tacon en transformation. L'augmentation des concentrations de T4 en circulation ne s'acccompagne pas de changements morphologiques importants des cellules thyroi'diennes durant la transformation en saumoneau.[Traduit par le journal]

In March (sunrise at 0530, sunset at 1800, water temperature 0.04 °C, and the fish not feeding) plasma T3 showed no diel variation and plasma T4 varied only slightly. In May (sunrise at 0200, sunset at 2120, water temperature 9.0 °C, and the fish feeding) plasma T4 showed a pronounced diel variation with its peak value appearing just before sunset and its nadir occurring at 0800. The plasma T3 level varied slightly with its peak value at 0400. The constant or slightly varying diel levels of plasma T3 suggests an efficient regulatory mechanism that maintains the plasma T3 concentration relatively stable throughout the 24 h. It is concluded that the diel patterns of T4 and T3 are dependent on factors related to the season. The influence of photoperiod and feeding on the diel pattern of T4 is discussed.