osteoclast

From the Greek 'osteo' bone & 'clast' broken Cell involved in bone resorption, derived from fusion of cells of the monocyte-macrophage cell line. High expression of tartrate-resistant acid phosphatase (TRAP) & cathepsin K. Morphology: 1) large cells 2) multinuclear 3) cytoplasm with foamy appearance due to vesicles & vacuoles 4) osteoclasts lie in small cavity called Howship's lacuna Function: 1) remodeling of bone in conjunction with osteoblasts 2) osteoclast forms specialized membrane 'ruffled border' at site of active bone resorption 3) 'ruffled border' facilitates removal of bone matrix 4) hydroxyapatite is absorbed by endocytosis into vesicles of osteoclast & released into the blood through a 'secretory domain' 5) once ativated osteoclases move to areas of microfracture by chemotaxis 6) attachment to bone is facilitated by osteoclast integrin receptors & bone matrix proteins, including osteopontin 7) osteoclasts release H⁺ via vacuolar ATPase & hydrolytic enzymes, cathepsins & matrix metalloprotease Formation of osteoclasts: 1) requires RANKL & M-CSF expressed on neighboring stromal cells & osteoblasts 2) M-CSF acts on c-fms on monocyte/macrophage precursor 3) RANKL activated NF-kappa-B 4) osteoclast differentiation is inhibited by osteoprotegerin which binds RANKL Additional regulation by: 1) parathyroid hormone 2) calcitonin 3) IL6

References

Wikepedia, The Free Encyclopedia http://en.wikipedia.org/wiki/osteoclast