Time course of peroxisome biogenesis during adaptation to mild hyperthyroidism in rat liver: a morphometric/stereologic study by electron microscopy.

The subcellular reaction pattern of the peroxisome compartment in rat liver cells has been studied after thyroid hormone administration for 1, 3, 5, 8, and 15 days (20 micrograms of 3, 3', 5'-triiodothyronine (T3) per 100 gm. of body weight per day). Mild hyperthyroidism caused by T3 resulted in the enlargement of the peroxisome volume fraction. This was achieved exclusively by an increase in organelle number (31 x 10(9) +/- 1.5 x 10(9) per 1 cu. cm. per cu. cm. of cytoplasm in controls), since the average organelle volume (0.27 +/- 0.02 cu. micrometer in controls) was simultaneously reduced. The remodeling of the peroxisome compartment was, for the most part, completed on day 3 and consisted of a first preliminary doubling of peroxisome numbers after 1 day, followed by a second doubling on day 3. Each doubling was accompanied by a one-third reduction in the average organelle volume. Peroxisome biogenesis due to T3 resulted in (1) the formation of microperoxisomes, consisting of small profiles, lacking the core-structure and (2) an increase of peroxisomes arranged in clusters. Cluster types with two to three profiles appeared from the beginning of biogenesis, whereas clusters with four or more profiles were mostly observed at the end. The latter was taken as an indication of a late phase of adaptation of the peroxisome compartment to hyperthyroidism, following a rapid early phase, mainly expressed on the organellar level and completed for the most on day 3. The parallel response of peroxisomes and mitochondria, as seen by the simultaneous enlargement of the fractional volume of both compartments, is taken as an indication of a close functional relationship of both organelle systems under stimulated cellular respiration.