Thyroid capsule changes during the development of thyroid hyperplasia in the rat.

Young adult male Fischer rats were fed 0.25% thouracil in a low-iodine diet to produce hyperplasia of the thyroid gland. The capsule of the thyroid gland increased in thickness from approximately one cell in controls to a substantial multilayered structure. Increase in capsule thickness was noted by 3 days. The cell population of the capsule was largely fibroblasts, but during a period within the interval from 14 to 28 days, the capsule tended to be exceptionally thick and contained many mononuclear leukocytes. At later times the capsule was not quite as thick and the leukocytes largely disapproved. Capillaries developed in the capsule probably by sprouting. The capsule growth was so extensive that certain neighboring tissues were often incorporated into the capsule, including arteries, veins, nerves, striated muscle, and lymph nodes. There was some regional specificity in the development of capsular hyperplasia. Connective tissue increased around the thyroid and parathyroid glands but not between them. Connective tissue in partitions with the thyroid gland also increased in thickness, although the extent of accumulation of cells and intercellular matrix was much less than in the capsule.