X-ray microanalysis on the thyroid follicle of the hagfish, Eptatretus burgeri and lamprey, Lampetra japonica.

Intracellular dense bodies, cytoplasmic matrices, and luminal colloids in the thyroid follicle of cyclostomes, hagfish and lamprey were examined to identify the distribution of iodine using an energy dispersive X-ray microanalyzer attached to a scanning transmission electron microscope. High level of iodine was detected only in the large dense body of the hagfishes, while it was too slight in quantity to detect by this method in the cytoplasmic matrix as well as in the luminal colloid. In the adult lamprey thyroid, an appreciable amount of iodine was detected in a few large dense bodies. In mice and rats, it is very hard to detect iodine in the luminal colloid, intracellular colloid droplet, and in the lysosomal dense granule by this method, though these structures have been well known to contain a fairly large amount of iodine which is combined with thyroglobulin. These facts mean that intracellular dense bodies in the thyroid follicular epithelium of the cytoclostome, especially of the hagfish have an extremely larger amount of iodine. These bodies are considered to be secondary lysosomes or residual bodies containing reabsorbed colloid materials which are highly condensed.