Ultrastructural changes of osteoclasts on hen medullary bone during the egg-laying cycle

1. The ultrastructure of osteoclasts on hen medullary bone during the egg-laying cycle was observed by electron microscopy. 2. At 0 to 6 h after oviposition, osteoclasts lacking ruffled borders were attached to the bone via the clear zone and appeared to have ceased bone resorption. Small vacuoles were scattered throughout this cytoplasm. 3. At 9 to 21 h after oviposition, most of the osteoclasts had ruffled borders and appeared to be resorbing bone. The ruffled borders at 15 h were well developed, whereas at 9, 18 and 21 h they were poorly developed or showed similarities in structure to the clear zone. The small vacuoles were concentrated under the ruffled borders at 9 h and then decreased. 4. These results demonstrate cyclic changes in osteoclasts during the egg-laying cycle, indicate that ruffled borders form at the beginning of bone resorption and suggest that they are derived from the clear zone and the small vacuoles. The results also indicate that the ruffled borders fuse into the clear zone and disappear at the completion of bone resorption.