Four types of cells are found within bone tissue: osteoblasts, osteocytes, osteogenic cells, and osteoclasts ((Figure)).  Osteoblasts/osteocytes develop in mesenchyme. Osteocytes derive from osteoblasts, or bone-forming cells, and are essentially osteoblasts surrounded by the products they secreted. Osteoblasts, which do not divide, synthesize and secrete the collagen matrix and calcium salts. Spongy (Cancellous) Bone Like compact bone, spongy bone, also known as cancellous bone, contains osteocytes housed in lacunae, but they are not arranged in concentric circles. As the name implies, an articulation is where two bone surfaces come together (articulus = “joint”). Like osteoblasts, osteocytes lack mitotic activity. Once osteoblasts get trapped inside the matrix which they secrete themselves, they develop and mature into osteocytes. Bones of the pelvis, skull, spine, and legs are the most commonly affected. spaces in bone matrix that contain osteocytes tiny channels connecting. Osteocytes and bone lining cells are terminally differentiated osteoblasts derived from mature osteoblasts that are no longer involved in active bone formation, and have become entrapped within the canaliculi of the bone matrix that they have produced.  Under normal conditions, osteocytes express high amounts of TGF-β and thus repress bone resorption, but when bone grows old, the expression levels of TGF-β decrease, and the expression of osteoclast-stimulatory factors, such as RANKL and M-CSF increases, bone resorption is then enhanced, leading to net bone loss. They differentiate and develop into osteoblasts. lines cavities within bone, contains bone forming cells . 7. They are found on bone surfaces, are multinucleated, and originate from monocytes and macrophages, two types of white blood cells, not from osteogenic cells. In what ways is the structural makeup of compact and spongy bone well suited to their respective functions? Many people think bones are lifeless.  Skeletal unloading has been shown to induce osteocyte hypoxia in vivo, this is when osteocytes undergo apoptosis and recruit osteoclasts to resorb bone. They are derived from osteoprogenitor cells, some of which differentiate into active osteoblasts. The epiphyses, which are wider sections at each end of a long bone, are filled with spongy bone and red marrow. , Clinically important research of gel based in vitro 3D model for the osteocytic potentiality of human CD34+ stem cells has been described. Osteoclast: Bone cell responsible for breaking down bone matrix . They become osteocytes, the cells of mature bone, when they get trapped in the matrix.  When osteocytes were experimentally destroyed, the bones showed a significant increase in bone resorption, decreased bone formation, trabecular bone loss, and loss of response to unloading. Articulations are places where two bones meet. This cross-section of a flat bone shows the spongy bone (diploë) lined on either side by a layer of compact bone.  The cell also exhibits a reduced size endoplasmic reticulum, Golgi apparatus and mitochondria, and cell processes that radiate largely towards the bone surfaces in circumferential lamellae, or towards a haversian canal and outer cement line typical of osteons in concentric lamellar bone. A typical long bone shows the gross anatomical characteristics of bone. They possess a dendritic morphology.  The process of osteocytogenesis is largely unknown, but the following molecules have been shown to play a crucial role in the production of healthy osteocytes, either in correct numbers or specific distributions: matrix metalloproteinases (MMPs), dentin matrix protein 1 (DMP-1), osteoblast/osteocyte factor 45 (OF45), Klotho, TGF-beta inducible factor (TIEG), lysophosphatidic acid (LPA), E11 antigen, and oxygen. (1990) distinguish three cell types from osteoblast to mature osteocyte: type I preosteocyte (osteoblastic osteocyte), type II preosteocyte (osteoid osteocyte), and type III preosteocyte (partially surrounded by mineral matrix). A long bone has two parts: the diaphysis and the epiphysis. Functions include support, protection, aid in movement, being a reservoir for minerals such as calcium and phosphate, for hematopoiesis, and fat storage. The walls of the diaphysis are compact bone. Blood vessels and nerves enter the bone through the nutrient foramina to nourish and innervate bones. There are three general classes of bone markings: (1) articulations, (2) projections, and (3) holes. Compact bone is dense so that it can withstand compressive forces, while spongy (cancellous) bone has open spaces and supports shifts in weight distribution. Which cells do not originate from osteogenic cells? The trabeculae may appear to be a random network, but each trabecula forms along lines of stress to provide strength to the bone. Diagram of Blood and Nerve Supply to Bone. marks site of growth in length during childhood. X-rays may show bone deformities or areas of bone resorption. A long bone has two parts: the diaphysis and the epiphy… (Figure) describes the bone markings, which are illustrated in ((Figure)). Osteocytes are simply osteoblasts trapped in the matrix that they secrete. Osteocytes are responsible for mechanosensing and mechanotransduction in bone and play a crucial role in bone homeostasis. Morphology and histological staining. These cells contain an average half-life of 25 years. When occurring in the skull, Paget’s disease can cause headaches and hearing loss. supplies blood to marrow.  Sclerostin is inhibited by parathyroid hormone (PTH) and mechanical loading.  Osteocyte apoptosis is thought to be related to decreased mechanotransduction, which possibly leads to the development of osteoporosis. (a) This cross-sectional view of compact bone shows the basic structural unit, the osteon. Osteon, the chief structural unit of compact (cortical) bone, consisting of concentric bone layers called lamellae, which surround a long hollow passageway, the Haversian canal (named for Clopton Havers, a 17th-century English physician).The Haversian canal contains small blood vessels responsible for the blood supply to osteocytes (individual bone cells). , Palumbo et al. Osteoblasts form new collagen matrix and mineralize it, osteoclasts break down bone, osteocytes regulate the formation and breakdown of bone, and osteogenic cells divide and differentiate to form new osteoblasts. Holes are openings or depressions in the bones. , List of human cell types derived from the germ layers, "Dynamics of the Transition from Osteoblast to Osteocyte", "A Review of the Impact of Implant Biomaterials on Osteocytes", https://en.wikipedia.org/w/index.php?title=Osteocyte&oldid=994440348, Creative Commons Attribution-ShareAlike License, This page was last edited on 15 December 2020, at 19:06. The answer is still unknown, but hereditary factors seem to play a role. The osteocytes are located inside spaces called lacunae (singular = lacuna), found at the borders of adjacent lamellae. They are aligned parallel to the long axis of the bone. In addition, blood levels of an enzyme called alkaline phosphatase are typically elevated in people with Paget’s disease. (Figure) reviews the bone cells, their functions, and locations.  Osteocytes die as a consequence of senescence, degeneration/necrosis, apoptosis (programmed cell death), and/or osteoclastic engulfment.  Sclerostin, the product of the SOST gene, is the first mediator of communication between osteocytes, bone forming osteoblasts and bone resorbing osteoclasts, critical for bone remodeling. In this region, the epiphyses are covered with articular cartilage, a thin layer of cartilage that reduces friction and acts as a shock absorber. The periosteum covers the entire outer surface except where the epiphyses meet other bones to form joints ((Figure)). Which of the following are incapable of undergoing mitosis? ... and nerves in place, forms the dermis of the skin, and the connective tissue layer of mucous membranes. Thats far from the truth.Bones are living, dynamic organs that serve many functions. These salt crystals form when calcium phosphate and calcium carbonate combine to create hydroxyapatite, which incorporates other inorganic salts like magnesium hydroxide, fluoride, and sulfate as it crystallizes, or calcifies, on the collagen fibers. Four types of cells are found within bone tissue. If the outer layer of a cranial bone fractures, the brain is still protected by the intact inner layer. If the articular cartilage at the end of one of your long bones were to deteriorate, which is actually what happens in osteoarthritis, you would experience joint pain at the end of that bone and limitation of motion at that joint because there would be no cartilage to reduce friction between adjacent bones and there would be no cartilage to act as a shock absorber. Each osteon is composed of concentric rings of calcified matrix called lamellae (singular = lamella).  The cell undergoes a dramatic transformation from a polygonal shape to a cell that extends dendrites toward the mineralizing front, followed by dendrites that extend to either the vascular space or bone surface. Instead, the lacunae and osteocytes are found in a lattice-like network of matrix spikes called trabeculae (singular = … The trabeculae may appear to be a random network, but each trabecula forms along lines of stress to provide strength to the bone. The diaphysis is the tubular shaft that runs between the proximal and distal ends of the bone.  Only osteocytes express sclerostin, which acts in a paracrine fashion to inhibit bone formation. Part 2: The Major Connective Tissues. A. osteocytes B. osteoclasts C. osteoblasts D. lacunae E. chondrocytes. The microscopic structural unit of compact bone is called an osteon, or Haversian system. The osteocytes in spongy bone are nourished by blood vessels of the periosteum that penetrate spongy bone and blood that circulates in the marrow cavities. This cavity system provides the food supply and communication for osteocytes. Sclerostin antagonizes the activity of BMP (bone morphogenetic protein), a cytokine that induces bone and cartilage formation. As with the other markings, their size and shape reflect the size of the vessels and nerves that penetrate the bone at these points. 6. Osteogenic cells develop into osteoblasts. site of blood cell production in fetus and young children. Bone is hard and many of its functions depend on that characteristic hardness.  Oxygen deprivation that occurs as the result of immobilization (bed rest), glucocorticoid treatment, and withdrawal of oxygen have all been shown to promote osteocyte apoptosis.  The percentage of dead osteocytes in bone increases with age from less than 1% at birth to 75% after age 80. Running down the center of each osteon is the central canal, or Haversian canal, which contains blood vessels, nerves, and lymphatic vessels. These surfaces tend to conform to one another, such as one being rounded and the other cupped, to facilitate the function of the articulation. , Although osteocytes are relatively inert cells, they are capable of molecular synthesis and modification, as well as transmission of signals over long distances, in a way similar to the nervous system. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone. An osteocyte, an oblate shaped type of bone cell with dendritic processes, is the most commonly found cell in mature bone tissue, and can live as long as the organism itself. By the end of this section, you will be able to: Bone tissue (osseous tissue) differs greatly from other tissues in the body. They destroy bone through a rapid, transient (relative to osteoclasts) mechanism called osteocytic osteolysis. , Osteocytes appear to be enriched in proteins that are resistant to hypoxia, which appears to be due to their embedded location and restricted oxygen supply. Osteoblasts are cells that make new bone. Osteocytes are mature bone cells found within the calcium matrix. The differences between compact and spongy bone are best explored via their histology. In addition, the spaces in some spongy bones contain red marrow, protected by the trabeculae, where hematopoiesis occurs. The cell responsible for bone resorption, or breakdown, is the osteoclast.  As the osteoblast transitions to an osteocyte, alkaline phosphatase is reduced, and casein kinase II is elevated, as is osteocalcin. An osteocyte is a type of cell that makes up bone. The results confirm that the human CD34+ stem cells possess unique osteogenic differentiation potential and can be used in the early regeneration of injured bone. Osteocytes are embedded within a hard matrix in bone for life, only being released by fracture or during remodeling.  If osteoblasts and osteocytes are incapable of mitosis, then how are they replenished when old ones die? The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release. These are the attachment points for tendons and ligaments.  Osteocyte size has been shown to covary with genome size; and this relationship has been used in paleogenomic research. What causes the osteoclasts to become overactive? Each osteon consists of lamellae, which are layers of compact matrix that surround a central canal called the Haversian canal. (Micrograph provided by the Regents of University of Michigan Medical School © 2012). The two layers of compact bone and the interior spongy bone work together to protect the internal organs. The outer surface of bone, except in regions covered with articular cartilage, is covered with a fibrous membrane called the periosteum. The open spaces of the trabeculated network of spongy bone allow spongy bone to support shifts in weight distribution, which is the function of spongy bone. Still, most doctors feel that the benefits of bisphosphonates more than outweigh the risk; the medical professional has to weigh the benefits and risks on a case-by-case basis. Osteogenic cells are undifferentiated and develop into osteoblasts.  The adult human body has about 42 billion of them. However, in a small percentage of cases, bisphosphonates themselves have been linked to an increased risk of fractures because the old bone that is left after bisphosphonates are administered becomes worn out and brittle.  The embedded "osteoid-osteocyte" must do two functions simultaneously: regulate mineralization and form connective dendritic processes, which requires cleavage of collagen and other matrix molecules. Osteoclasts develop from monocytes and macrophages and differ in appearance from other bone cells. It is derived from osteoblasts and embedded in the calcified matrix of bone. Although bone cells compose a small amount of the bone volume, they are crucial to the function of bones. epiphysis. Structural Organization of the Human Body, Elements and Atoms: The Building Blocks of Matter, Inorganic Compounds Essential to Human Functioning, Organic Compounds Essential to Human Functioning, Nervous Tissue Mediates Perception and Response, Diseases, Disorders, and Injuries of the Integumentary System, Exercise, Nutrition, Hormones, and Bone Tissue, Calcium Homeostasis: Interactions of the Skeletal System and Other Organ Systems, Embryonic Development of the Axial Skeleton, Development and Regeneration of Muscle Tissue, Interactions of Skeletal Muscles, Their Fascicle Arrangement, and Their Lever Systems, Axial Muscles of the Head, Neck, and Back, Axial Muscles of the Abdominal Wall, and Thorax, Muscles of the Pectoral Girdle and Upper Limbs, Appendicular Muscles of the Pelvic Girdle and Lower Limbs, Basic Structure and Function of the Nervous System, Circulation and the Central Nervous System, Divisions of the Autonomic Nervous System, Organs with Secondary Endocrine Functions, Development and Aging of the Endocrine System, The Cardiovascular System: Blood Vessels and Circulation, Blood Flow, Blood Pressure, and Resistance, Homeostatic Regulation of the Vascular System, Development of Blood Vessels and Fetal Circulation, Anatomy of the Lymphatic and Immune Systems, Barrier Defenses and the Innate Immune Response, The Adaptive Immune Response: T lymphocytes and Their Functional Types, The Adaptive Immune Response: B-lymphocytes and Antibodies, Diseases Associated with Depressed or Overactive Immune Responses, Organs and Structures of the Respiratory System, Embryonic Development of the Respiratory System, Digestive System Processes and Regulation, Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder, Chemical Digestion and Absorption: A Closer Look, Regulation of Fluid Volume and Composition, Fluid, Electrolyte, and Acid-Base Balance, Anatomy and Physiology of the Male Reproductive System, Anatomy and Physiology of the Female Reproductive System, Development of the Male and Female Reproductive Systems, Maternal Changes During Pregnancy, Labor, and Birth, Adjustments of the Infant at Birth and Postnatal Stages. It is known that the cytokine receptor activator of nuclear factor-κB ligand (RANKL) is required for osteocyte formation but, until now, it was thought that RANKL was supplied by osteoblasts and their progenitors even though the exact cell type involved had not been identified. These cells are derived from osteoblasts, which are bone-forming cells.In general, they make up the majority of the mass of adult bone tissue in humans. Osteocyte is a cell characteristic of mature bone tissue. Flat bones consist of two layers of compact bone surrounding a layer of spongy bone. Osteocytes embedded in the bone matrix resorb the bone made by osteoblasts and chondrocytes. Spaces in bone matrix that contain osteocytes tiny. , Osteocyte specific proteins such as sclerostin have been shown to function in mineral metabolism, as well as other molecules such as PHEX, DMP-1, MEPE, and FGF-23, which are highly expressed by osteocytes and regulate phosphate and biomineralization. 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