Some observations on the fine structure of thyroids of hibernating and aroused bats

The fine structure of thyroids of hibernating and aroused bats, Myotis lucifugus, was observed with the electron microscope. (1) In the hibernating bat for 5 months the thyroid follicular epithelial cell is markedly attenuated and the rough endoplasmic reticulum and Golgi apparatus are very poor in development, while the mitochondria are larger and distributed throughout the cytoplasma. Intracellular colloid droplets are difficult to find. Homogeneously dense small granules suggesting primary lysosomes are numerous. Half an hour to 2 hours after injection of 100 μc of 125I, very few silver grains are recognized in a follicle lumen electron microscopic autoradiographically. (2) Half an hour to 2 hours after the intraperitoneal injection of 1 unit of thyroid stimulating hormone (TSH) into hibernating bats, several colloid droplets, many small smooth-surfaced vesicles and a few coated vesicles appear in the apical cytoplasm of the thyroid follicle epithelial cell. The small vesicles are gathered around the large droplet, and some of them are fused with it. The rough endoplasmic reticulum does not show any marked reaction. These facts suggest the possibility that the luminal colloid is reabsorbed in the form of large droplets as well as in the form of small vesicles. Dense granules (lysosomes) gathered near the colloid droplets and some of the colloid droplets themselves become heterogeneously dense. Half an hour to 2 hours after injection of 100 μc of 125I simultaneously with TSH into hibernating bats, silver grains which are markedly increased in number, as compared with those of the non-treated hibernating animal, are recognized in the follicle lumen electron microscopic autoradiographically. (3) At 2–3 hours after injection of TSH into the hibernating bat, the appearance of crystals in over half of the colloid droplets is characteristic. Each crystal consists of aggregates of numerous needle-like filaments, about 65 A in diameter, running parallel with one another. Center to center distances between these filaments cut in longitudinal section are about 150 A. The filaments are arranged regularly along two or three axes. These crystals are considered to be altered thyroglobulin. (4) The follicular epithelial cells of aroused bats (24° C, laboratory room in winter) from the hibernation show heterogeneity in their fine structure. Some cells seem active, and the others look inactive. In 15-day aroused bats, though most cells are flattened and the rough endoplasmic reticulum is sparse, a few cells show a fairly well developed rough endoplasmic reticulum with somewhat dilated cisternae. Large colloid droplets occur sometimes in a few cells. Lysosomal dense granules, reduced in number as compared with those of hibernating animals, are considered to originate from the Golgi apparatus. In 30 and 40-day aroused bats, the rough endoplasmic reticulum of most cells becomes more enlarged, and some follicular cells are quite similar to those of non-hibernating bats obtained in the field. In 60-day aroused bats, all the follicular epithelial cells show quite the same feature as the active thyroid cell of non-hibernating mammals.