Osteoclasts
Osteoclasts are mobile cells and originate from hematopoietic stem cells of the bone marrow.
Their immediate precursor cells accumulate as monocytes at sites of boneresorption and fuse into osteoclasts as required. Osteoclasts are multinucleated giant cells, have a well developed Golgi apparatus, little endoplasmic reticulum, but are rich in mitochondria and lysosome enzymes. Osteoclasts use proteolytic enzymes to degrade the intercellular substance of bone tissue and absorb the degradation products.
Differentiation and activation of osteoclasts are controlled by hormones (parathormones, estrogens, thyroid hormones) and cytokines.
Osteoclasts also possess estrogens receptors via which the estrogens inhibit the recruitment of osteoclasts. An osteoclast can break down the same amount of bone that 100 osteoblasts build up during this time. Although both cell types act in the opposite direction, they work in the context of the remodelling processes in the bone tissue - hormone controlled and coordinated with each other.