osteoclast

Introduction

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:

• large cells
• multinuclear
• cytoplasm with foamy appearance due to vesicles & vacuoles
• osteoclasts lie in small cavity called Howship's lacuna

Function:

• remodeling of bone in conjunction with osteoblasts
• osteoclast forms specialized membrane 'ruffled border' at site of active bone resorption
• 'ruffled border' facilitates removal of bone matrix
• hydroxyapatite is absorbed by endocytosis into vesicles of osteoclast & released into the blood through a 'secretory domain'
• once ativated osteoclases move to areas of microfracture by chemotaxis
• attachment to bone is facilitated by osteoclast integrin receptors & bone matrix proteins, including osteopontin
• osteoclasts release H+ via vacuolar ATPase & hydrolytic enzymes, cathepsins & matrix metalloprotease

Formation of osteoclasts:

• requires RANKL & M-CSF expressed on neighboring stromal cells & osteoblasts
• M-CSF acts on c-fms on monocyte/macrophage precursor
• RANKL activated NF-kappa-B
• osteoclast differentiation is inhibited by osteoprotegerin which binds RANKL

Additional regulation by:

• parathyroid hormone
• calcitonin
• IL6

More general terms

• osteocyte