Quarterly Journal of Microscopical Science, Vol s3-97, 393-409, Copyright © 1956 by Company of Biologists

Some Cytological Characteristics of Thyroidal Function in the Endostyle of the Ammocoete Larva

¹ Department of Zoology, The University, Nottingham

An account is given of the distribution of thyroidal function in the epithelium of the endostyle of the ammocoete larva, based on the localization of bound iodine as revealed by autoradiographs; and the cytological characteristics of the various types of cells are analysed in relation to this. The binding of iodine is shown to be correlated with the production of a PAS-positive ‘thyroidal prosecretion’ which, in fully active cells, tends to become transformed into a more granule-like phase. These thyroidal granules appear to have some degree of permanence and to enlarge by the accumulation of material; the precise significance of this is not clear, but it might involve the layingdown either of secretory reserves or of the by-products of thyroidal metabolism. The granules differ from the initial phase of the prosecretion in developing a brown pigmentation, a positive ferric-ferricyanide response, and a yellowish fluorescence, together with some affinity for orange G. These properties, together with the PASpositive response of both phases of the prosecretion, give considerable support to the view that the secretory process involved is homologous with that which produces thyroid colloid in higher forms, although the histochemical reactions concerned are not sufficiently specific to be decisive. Thyroidal function is widely spread in the epithelium, except in the glandular tracts (type 1 cells), but it varies in its degree of expression; the pattern of this, and its relationship to the alimentary activity of the endostyle, can conveniently be described in terms of the embryological concept of ‘gradient fields’. The present results are discussed in relation to the fate of certain regions of the epithelium at metamorphosis.