Endocrinology of the head-kidney tissues in teleost fish

Abstract

This is a study of the structure and functions of the endocrine tissues in the head-kidney of the teleost fish, the homologous tissues to the mammalian adrenal cortex (=adrenocortical tissue) and adrenal medulla (=chromaffin tissue). The study is divided into three main sections: 1.The first section comprises a study of the general morphology, at the anatomical and histological level, of the different types of head-kidney which occur in teleost fish. The range of types is illustrated by studies on twenty-four species, some of which have been previously investigated, and including in particular sixteen marine species from the Red Sea coast of Saudi Arabia collected by the author. 2. The second section of this thesis comprises a detailed study of the morphology of the head-kidney of one particular species, Phoxinus phoxinus (Linnaeus), and a two year study of seasonal variations in the activity of its adrenocortical and chromaffin tissue. Samples of fish were collected from a population in the Walton Reservoir, Scotland, at monthly intervals. The activity of the adrenocortical tissue was assessed by measuring nuclear diameter of the adrenocortical cells, a criterion already widely used for this purpose. The activity of the chromaffin cells was similarly assessed, though the methodology is less well established in this case. The effects were compared of electro-fishing followed by anaesthesis and immersion in Bouin's fixative while still under electronarcosis. Both proved to be relatively stress-free methods. 3. The third section of this thesis comprises a study of the fine structure of the endocrine tissues of the head-kidney of Phoxinus phoxinus and Salmo qairdnerii. Electron micrographs were prepared using fish caught under stress-free conditions in the Walton Reservoir, and from aquarium-maintained fish. The adrenocortical cells in both species are characterised by having a great many conspicuous mitochondria with tubule-vesicular internal structure. The nucleus is circular in section, and centrally situated in the cell. There is an extensive smooth endoplasmic reticulum and numerous ribosomes, Microvilli occur on cell surfaces in contact with veins. There is a wide range in the structure of the adrenocortical cells of individual fish; mitochondria range from small, elongated structure, with dark matrix to large, circular structures in which the internal structures eventually breaks down. The cytoplasm as a whole tends to be pale in cells with small dense mitochondria, and dense in cells with large, paler mitochondria. Pale highly vacuolated cytoplasm is associated with cells in which the mitochondria are breaking down; the vacuoles are probably associated with cells in which the mitochondria are breaking down; the vacuoles are probably associated with the degenerating mitochondria, In Phoxinus maintained in aquaria for twenty-four hours before killing, the proportion of adrenocortical cells with small mitochondria with dark matrices, -. as compared to fish caught by stress-free methods and fixed immediately. In Salmo which had been maintained in aquaria for longer periods, the proportion oft- cells showing mitochondrial degeneration and cytoplasmic vacuolation is higher. It is concluded that small, dark matrix mitochondria are typical of early stages of adrenal activity; dense cytoplasm and an increased number of large, circular mitochondria are typical of maximum activity; and mitochondrial degeneration and vacuolation of the cytoplasm is typical of exhaustion. Chromaffin cells, not hitherto described in teleost fish, are of the type found in other vertebrates, with many chromaffin vesicles containing varying amounts of granular inclusion. Synaptic contacts occur commonly, apparently all of cholinergic type. In Salmo the chromaffin and adrenocortical cells lie separately, but in Phoxinus the adrenocortical cells form a sheath round the posterior cardinal veins and their main tributaries, and the chromaffin cells lie beyond them, against the haemopoietic tissue. These chromaffin cells communicate with the vein by elongated projections running amongst the adrenocortical cells.