Studies on the ultrastructure of the pituitary of a non parasitic lamprey, Lampetra planeri, throughout the larval, metamorphosing and adult stages

Abstract

A comparative study of the fine structure of the pituitary of a non-parasitic lamprey, Lampetra planeri has been made during its larval, metamorphosing and adult stages. The pro-adenohypophysis (rostral pars distalis) is the first region to differentiate cytologically and already in the large ammocoete contains well developed basophils. These cells increase in number and in the density of their granulation during the pre-metamorphic phase, but the granulation tends to become sparser in the course of metamorphosis, finally increasing once again to reach a maximum in the adult stage. The diameter of the granules shows an increase from larval to adult stages. In the meso-adenohypophysis (proximal pars distalis) cellular differentiation accompanied by an overall growth of the lobe only begins at metamorphosis and is due mainly to an increase in cytoplasmic volumes. Basophils first appear in considerable numbers only during metamorphosis and at this stage are distinguished from those of the pro-adenohypophysis by their prominent cytoplasmic whorls of endoplasmic reticulum and the distinctly smaller size and relative sparsity of their secretory granules. Both lobes also contain chromophobes, some small and undifferentiated and others, ’active' or granular types with a few secretory granules, smaller than those of the basophils. These chromophobes are only numerous in the meso-adenohypophysis in metamorphic and pre-metamorphic stages. Stellate cells with either light or dark cytoplasm are present throughout the life cycle, although the latter are most conspicuous in the adult where they show evidence of phagocytotic activity. The meta-adenohypophysis (pars intermedia) shows overall growth and an increase in the numbers of granulated cells in the pre-metamorphic stages. These cells are only sparsely granulated, with conspicuous whorls of endoplasmic reticulum, but the granules increase in size from larval to adult stages. In the neurohypophysis the pre-metamorphic, metamorphic and adult stages show evidence of increased activity involving the increased depth of the fibre layer, the greater degree of neurosecretory granulation, the appearance of Herring bodies, the wider distribution of neurosecretory endings and indications of activity at the boundary between the neural lobe and the meta-adenohypophysis. Measurements of neurosecretory granules suggest the presence of four size classes of which the largest (Class IV) are absent in the larval stage. Comparisons have been made between the cytology of the pituitary of L. planeri and that of comparable stages of parasitic lampreys, especially the closely related river lamprey, L. fluviatills. The possible functional implications of the present studies have been considered against the background of recent findings based on the application of immunocytochemical techniques to the pituitaries of other species.