Lipolysis and Blood Capillary Enlargement in Adipose Tissue Pads on Thyroids of Rats Fed Thiouracil

In the young adult male Fischer rat there are mixed brown and white adipose tissue pads on the surface of the thyroid gland at the upper and lower poles of the lobes. When the thyroid gland was stimulated by feeding the rats 0.25% thiouracil in a low iodine diet, these adipose tissue pads became much less prominent. Examination of histological sections revealed that there were marked morphological changes in these pads. After feeding the animals the thiouracil-containing diet, thyroids and the adipose pads were fixed by perfusion with a solution of 2% formaldehyde, 2.5% glutaraldehyde, and 3% dextran in cacodylate buffer and postfixed in 1% osmium tetroxide in buffer, and whole lobes were embedded in Epon. The lobes were sectioned serially starting at one pole, and sections were usually examined every 0.3 mm. Adipose tissue was found only within 1.5 mm from each pole. Although thyroid tissue showed changes within 1 or 2 days, no obvious morphological changes were seen in the adipose tissue pads until approximately 7–14 days. At this time, there was obvious enlargement of blood capillaries, primarily in areas of brown adipose tissue. By 20 days, there was extensive lipolysis, and in some areas brown adipocytes were no longer recognized, but there was further enlargement of the capillaries. A thick thyroid capsule developed between the follicles and the adipose tissue, but did not surround the latter. Polymorphonuclear leukocytes were present along the interior of many capillary walls in the stimulated adipose tissue, and in some places these leukocytes were present in adjacent tissue. In general, this is in contrast to the capsule at 20 days, in which polymorphonuclear leukocytes were not observed but in which there was an abundance of lymphocytes and monocytes. The changes in the adipose tissue were reversible. Within 8 days of removal of the thiouracil diet and its replacement by a high iodine diet, there was an extensive reaccumulation of large fat droplets and a return of blood capillary size toward normal.