Osteoclast

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Bone is the basic unit of the human skeletal system.

Osteoclasts are specialized bone matrix cells that possess the ability to erode bone, a process called resorption, during bone growth and remodeling.

They are large, motile,  multinucleated cells derived from the fusion of cells of the macrophage-monocyte cell line. They measure about 40 μm and over in diameter and contain up to 20 densely packed nuclei. They are found in resorption bays/cavities (a.k.a. Howship lacunae or osteoclastic crypts), which are etched depressions in the bone matrix at sites where bone is being removed.

Osteoclats possess a cytoplasm that is rich in mitochondria and vacuoles, the majority of which function as lysosomes. The cytoplasm also contains a well developed Golgi complex and numerous transport vesicles and microtubule arrays. 

At the site of bone resorption, an active osteoclast develops a specialized structure called the ruffled border, which directly interfaces with the bone matrix. The ruffled border is characterized by numerous fine microvilli and is an extensively folded region of the plasma membrane. A circumferential zone exists between the ruffled border surface and the bone matrix. Within this space, osteoclasts release several organic acids responsible for  degrading the mineral component of bone. They also secrete lysosomal proteolytic enzymes such as cathepsin K, and non-lysosomal such as collagenase which dissolve the organic osteoid matrix proteins.

Osteoclasts play a crucial role in the continuous remodeling and reshaping of bone. They are involved in maintaining calcium homeostasis under the influence of parathyroid hormone (PTH) and calcitonin. The parathyroid hormone stimulates bone resorption and subsequent release of calcium, while calcitonin, a thyroid hormone, inhibits osteoclast activity.