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Review Article Biology of Bone Tissue: This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract Bone tissue is continuously remodeled through the concerted actions of bone cells, which include bone resorption by osteoclasts and bone formation by osteoblasts, whereas osteocytes act as mechanosensors and orchestrators of the bone remodeling process.
This process is under the control of local e. An imbalance between bone resorption and formation can result in bone diseases including osteoporosis. Recently, it has been recognized that, during bone remodeling, there are an intricate communication among bone cells.
For instance, the coupling from bone resorption to bone formation is achieved by interaction between osteoclasts and osteoblasts. Moreover, osteocytes produce factors that influence osteoblast and osteoclast activities, whereas osteocyte apoptosis is followed by osteoclastic bone resorption.
The increasing knowledge about the structure and functions of bone cells contributed to a better understanding of bone biology. It has been suggested that there is a complex communication between bone cells and other organs, indicating the dynamic nature of bone tissue. In this review, we discuss the current data about the structure and functions of bone cells and the factors that influence bone remodeling.
Introduction Bone is a mineralized connective tissue that exhibits four types of cells: Bone exerts important functions in the body, such as locomotion, Stem cells characterization and biomedical importance essay and protection of soft tissues, calcium and phosphate storage, and harboring of bone marrow [ 34 ].
Despite its inert appearance, bone is a highly dynamic organ that is continuously resorbed by osteoclasts and neoformed by osteoblasts. There is evidence that osteocytes act as mechanosensors and orchestrators of this bone remodeling process [ 5 — 8 ].
The function of bone lining cells is not well clear, but these cells seem to play an important role in coupling bone resorption to bone formation [ 9 ].
Bone remodeling is a highly complex process by which old bone is replaced by new bone, in a cycle comprised of three phases: This process occurs due to coordinated actions of osteoclasts, osteoblasts, osteocytes, and bone lining cells which together form the temporary anatomical structure called basic multicellular unit BMU [ 12 — 14 ].
Normal bone remodeling is necessary for fracture healing and skeleton adaptation to mechanical use, as well as for calcium homeostasis [ 15 ]. On the other hand, an imbalance of bone resorption and formation results in several bone diseases.
For example, excessive resorption by osteoclasts without the corresponding amount of nerformed bone by osteoblasts contributes to bone loss and osteoporosis [ 16 ], whereas the contrary may result in osteopetrosis [ 17 ].
Thus, the equilibrium between bone formation and resorption is necessary and depends on the action of several local and systemic factors including hormones, cytokines, chemokines, and biomechanical stimulation [ 18 — 20 ].
Recent studies have shown that bone influences the activity of other organs and the bone is also influenced by other organs and systems of the body [ 21 ], providing new insights and evidencing the complexity and dynamic nature of bone tissue.
In this review we will address the current data about bone cells biology, bone matrix, and the factors that influence the bone remodeling process.
Moreover, we will briefly discuss the role of estrogen on bone tissue under physiological and pathological conditions. These cells show morphological characteristics of protein synthesizing cells, including abundant rough endoplasmic reticulum and prominent Golgi apparatus, as well as various secretory vesicles [ 2223 ].
As polarized cells, the osteoblasts secrete the osteoid toward the bone matrix [ 24 ] Figures 1 a1 band 2 a.
Polarized osteoblasts Ob and giant multinucleated osteoclasts Oc are observed in the bone surface; osteocyte Ot surrounding bone matrix is also observed. Note osteocalcin-positive osteoblasts arrows on the surface of a bony trabecula B. Electron micrographs of portions of alveolar bone of rats.
A layer of bundles of collagen fibrils situated between osteoblasts Ob and calcified bone surface B constitutes the osteoid Otd. Bone lining cells BLC extend some thin cytoplasmic projections arrows towards the osteoid Otd.
Osteoblasts are derived from mesenchymal stem cells MSC. The commitment of MSC towards the osteoprogenitor lineage requires the expression of specific genes, following timely programmed steps, including the synthesis of bone morphogenetic proteins BMPs and members of the Wingless Wnt pathways [ 25 ].
The expressions of Runt-related transcription factors 2, Distal-less homeobox 5 Dlx5and osterix Osx are crucial for osteoblast differentiation [ 2226 ]. Additionally, Runx2 is a master gene of osteoblast differentiation, as demonstrated by the fact that Runx2-null mice are devoid of osteoblasts [ 2627 ].
Once a pool of osteoblast progenitors expressing Runx2 and ColIA1 has been established during osteoblast differentiation, there is a proliferation phase.
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