Abstract
The last decade has provided a virtual explosion of data on the molecular biology and function of osteocytes. Far from being the “passive placeholder in bone,” this cell has been found to have numerous functions, such as acting as an orchestrator of bone remodeling through regulation of both osteoclast and osteoblast activity and also functioning as an endocrine cell. The osteocyte is a source of soluble factors not only to target cells on the bone surface but also to target distant organs, such as kidney, muscle, and other tissues. This cell plays a role in both phosphate metabolism and calcium availability and can remodel its perilacunar matrix. Osteocytes compose 90% to 95% of all bone cells in adult bone and are the longest lived bone cell, up to decades within their mineralized environment. As we age, these cells die, leaving behind empty lacunae that frequently micropetrose. In aged bone such as osteonecrotic bone, empty lacunae are associated with reduced remodeling. Inflammatory factors such as tumor necrosis factor and glucocorticoids used to treat inflammatory disease induce osteocyte cell death, but by different mechanisms with potentially different outcomes. Therefore, healthy, viable osteocytes are necessary for proper functionality of bone and other organs. © 2011 American Society for Bone and Mineral Research.
Highlights
The last decade has provided a virtual explosion of data on the molecular biology and function of osteocytes
Over 100 years ago, it was postulated that osteocytes could remodel their extracellular matrix[1]; over 40 years ago, it was thought that osteocytes were responsive to parathyroid hormone,(2) could remodel bone[3] and could express tartrate-specific acid phosphatase[4]; and over 20 years ago, it was said that osteocytes were mechanosensory cells.[5]. Marotti and Palumbo drew beautiful diagrams to illustrate their theories regarding osteocyte function and communication.[6]. Histology was the major tool used by these early pioneers to generate their theories
It was proposed by Manolagas[9] that the matrix-producing osteoblast either can become an osteocyte, a lining cell, or can undergo programmed cell death. His theories were based on the observations of another very early pioneer, Michael Parfitt, who proposed that osteoblasts must die by apoptosis.[10]. Osteocytogenesis has been thought to be a passive process whereby a subpopulation of osteoblasts becomes passively encased in osteoid that passively mineralizes
Summary
Before the introduction of PubMed and easy access to papers, some of the earliest observations regarding osteocytes, like the cells themselves, laid buried and difficult to find. Peter Nijweide was the first to isolate avian osteocytes.[7] Some of the earliest videos of bone cells including osteocytes were recorded by Kumegawa and colleagues.[8] With new technology, such as molecular and transgenic approaches, imaging, cell lines, systems biology, advanced instrumentation, and others, a dramatic increase in information on osteocyte biology has occurred in the last decade, leading to validation of old theories and the generation of new ones.
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