The elaboration and release of the colloid of the thyroid

Abstract

The development of the thyroid of the salamander Ambystoma opacum was studied on serial sections of thyroids in various stages before and after metamorphosis.

The proportion of colloid and epithelium was found by weighing separately wax models of the colloid and epithelium. During the larval period the colloid increases more rapidly than does the epithelium. From 13 per cent., shortly after hatching, it increases to 45 per cent. of the total thyroid mass just before metamorphosis. The larval period is not a period of colloid release, but of colloid elaboration and storage. At the beginning of metamorphosis, the colloid percentage drops suddenly below 30 per cent. This drop is due partly to the sudden and excessive release and disappearance of the colloid from the follicles, and partly to an excessive increase of the epithelial mass, owing to the swelling of the individual cells. With increasing age the thyroid again enters upon a stage of colloid accumulation. In animals 4.5 years of age, 56 per cent. of the thyroid mass is colloid. This relative increase is not due entirely to an absolute colloid increase, but also to an absolute decrease of the epithelium.

The colloid was studied on sections stained, according to Kraus'method, with polychrome methylene blue and acid fuchsin.

Colloid is elaborated long before follicles are formed, at a time when the cells still contain yolk and are devoid of granules, a structure frequently considered to be prerequisite to colloid elaboration. In thyroids of larvæ fed exclusively thymus gland, the cells remain permanently primitive, consisting almost entirely of nucleus, possess very small amounts of plasma and are devoid of granules. Yet these cells elaborate large quantities of colloid (55 per cent. of the total thyroid mass).