Abstract
Large mitochondria that appeared in osteoclasts during the early stage of parathyroid hormone (PTH) treatment were examined histochemically and morphologically in order to elucidate how their appearance is related to ruffled border (RB) form or cytochrome c oxidase activity. Three types of osteoclasts including large mitochondria, that were characterized by closely packed cristae, appeared 1-2 hr after PTH treatment. The first type (PTH-1) showed a poorlydeveloped RB, and many large mitochondria concentrated on the basal membrane side. The second type (PTH-2) showed a well-developed RB, but the terminal infoldings did not expand. Many large mitochondria were scattered freely in PTH-2. The third type (PTH-3) showed a well-developed RB, and many expanded terminal infoldings and vacuoles could be seen in the RB. Large mitochondria in PTH-3 were few in number. Cytochrome c oxidase activities both in the control and in PTH-3 osteoclasts were significantly higher than that in the PTH-1 osteoclasts. Over 90% of large mitochondria showed high enzymatic activity. Only 0.7% of control osteoclasts had a cross-sectional mitochondrial area of over 0.867 μm2 (large mitochondria), whereas the area of over 0.867 μm2 was 4.7% of PTH-1, 9% of PTH-2, and 2.3% of PTH-3 respectively. The present results suggest that large mitochondria demonstrate high ATP production, and that a close relationship exists between the location and number of large mitochondria and RB form during the early period when osteoclastic bone resorption is promoted by PTH.
