Endochondral ossification is one of the two essential processes during fetal development of the mammalian skeletal system by which bone tissue is created. Unlike intramembranous ossification, the other process by which bone tissue is created, cartilage is present during endochondral ossification. Endochondral ossification is an essential process during the rudimentary formation of long bones, the growth of the length of long bones, the natural healing of bone fractures; the cartilage model will grow in length by continuous cell division of chondrocytes, accompanied by further secretion of extracellular matrix. This is called interstitial growth; the process of appositional growth occurs when the cartilage model grows in thickness due to the addition of more extracellular matrix on the peripheral cartilage surface, accompanied by new chondroblasts that develop from the perichondrium. The first site of ossification occurs in the primary center of ossification, in the middle of diaphysis. Then: Formation of periosteum The perichondrium becomes the periosteum.
The periosteum contains a layer of undifferentiated cells which become osteoblasts. Formation of bone collar The osteoblasts secrete osteoid against the shaft of the cartilage model; this serves as support for the new bone. Calcification of matrix Chondrocytes in the primary center of ossification begin to grow, they stop secreting collagen and other proteoglycans and begin secreting alkaline phosphatase, an enzyme essential for mineral deposition. Calcification of the matrix occurs and osteoprogenitor cells that entered the cavity via the periosteal bud, use the calcified matrix as a scaffold and begin to secrete osteoid, which forms the bone trabecula. Osteoclasts, formed from macrophages, break down spongy bone to form the medullary cavity. About the time of birth in mammals, a secondary ossification center appears in each end of long bones. Periosteal buds carry mesenchyme and blood vessels in and the process is similar to that occurring in a primary ossification center; the cartilage between the primary and secondary ossification centers is called the epiphyseal plate, it continues to form new cartilage, replaced by bone, a process that results in an increase in length of the bone.
Growth continues until the individual is about 20 years old or until the cartilage in the plate is replaced by bone. The point of union of the primary and secondary ossification centers is called the epiphyseal line; the growth in diameter of bones around the diaphysis occurs by deposition of bone beneath the periosteum. Osteoclasts in the interior cavity continue to resorb bone until its ultimate thickness is achieved, at which point the rate of formation on the outside and degradation from the inside is constant. During endochondral ossification, five distinct zones can be seen at the light-microscope level. During fracture healing, cartilage is formed and is called callus; this cartilage develops into new bone tissue through the process of endochondral ossification. It has been shown that biomimetic bone like apatite inhibits formation of bone through endochondral ossification pathway via hyperstimulation of extracellular calcium sensing receptor. Intramembranous ossification Ossification
Anatomical terms of bone
Many anatomical terms descriptive of bone are defined in anatomical terminology, are derived from Greek and Latin. A long bone is one, cylindrical in shape, being longer than it is wide. However, the term describes the shape of a bone, not its size, relative. Long bones are found in the legs, as well as in the fingers and toes. Long bones function as levers, they are responsible for the body's height. A short bone is one, cube-like in shape, being equal in length and thickness; the only short bones in the human skeleton are in the carpals of the wrists and the tarsals of the ankles. Short bones provide support as well as some limited motion; the term “flat bone” is something of a misnomer because, although a flat bone is thin, it is often curved. Examples include the cranial bones, the scapulae, the sternum, the ribs. Flat bones serve as points of attachment for muscles and protect internal organs. Flat bones do not have a medullary cavity. An irregular bone is one that does not have an classified shape and defies description.
These bones tend to have more complex shapes, like the vertebrae that support the spinal cord and protect it from compressive forces. Many facial bones the ones containing sinuses, are classified as irregular bones. A sesamoid bone is a round bone that, as the name suggests, is shaped like a sesame seed; these bones form in tendons. The sesamoid bones protect tendons by helping them overcome compressive forces. Sesamoid bones vary in number and placement from person to person but are found in tendons associated with the feet and knees; the only type of sesamoid bone, common to everybody is the kneecap, the largest of the sesamoid bones. A condyle is the round prominence at the end of a bone, most part of a joint – an articulation with another bone; the epicondyle refers to a projection near a condyle the medial epicondyle of the humerus. These terms derive from Greek. An eminence refers to a small projection or bump of bone, such as the medial eminence. A process refers to a large projection or prominent bump, as does a promontory such as the sacral promontory.
Both tubercle and tuberosity refer to a projection or bump with a roughened surface, with a "tubercle" smaller than a "tuberosity". These terms are derived from Tuber. A ramus refers to an extension of bone, such as the ramus of the mandible in the jaw or Superior pubic ramus. Ramus may be used to refer to nerves, such as the ramus communicans. A facet refers to a flattened articular surface. A line refers to a long, thin projection with a rough surface. Ridge and crest refer to a narrow line. Unlike many words used to describe anatomical terms, the word ridge is derived from Old English. A spine, as well as referring to the spinal cord, may be used to describe a long, thin projection or bump; these terms are used to describe bony protuberances in specific parts of the body. The Malleolus is the bony prominence on each side of the ankle; these are known as the lateral malleolus. Each leg is supported by two bones, the tibia on the inner side of the leg and the fibula on the outer side of the leg; the medial malleolus is the prominence on the inner side of the ankle, formed by the lower end of the tibia.
The lateral malleolus is the prominence on the outer side of the ankle, formed by the lower end of the fibula. The trochanters are parts of the femur, it may refer to the greater, lesser, or third trochanter The following terms are used to describe cavities that connect to other areas: A foramen is any opening referring to those in bone. Foramina inside the body of humans and other animals allow muscles, arteries, veins, or other structures to connect one part of the body with another. A canal is a long, tunnel-like foramen a passage for notable nerves or blood vessels; the following terms are used to describe cavities that do not connect to other areas: A fossa is a depression or hollow in a bone, such as the hypophyseal fossa, the depression in the sphenoid bone. A meatus is a short canal. A fovea is a small pit on the head of a bone. An example of a fovea is the fovea capitis of the head of the femur; the following terms are used to describe the walls of a cavity: A labyrinth refers to the bony labyrinth and membranous labyrinth, components of the inner ear, due to their fine and complex structure.
A sinus refers to a bony cavity within the skull. A joint, or articulation is the region where adjacent bones contact each other, for example the elbow, shoulder, or costovertebral joint. Terms that refer to joints include: articular process, referring to a projection that contacts an adjacent bone. Suture, referring to an articulation between cranial bones. Bones are described with the terms head, shaft and base The head of a bone refers to the proximal end of the bone; the shaft refers to the elongated sections of long bone, the neck the segment between the head and shaft. The end of the long bone opposite to the head is known as the base; the cortex of a bone is used to refer to its outer layers, medulla used to
Osteoblasts are cells with a single nucleus that synthesize bone. However, in the process of bone formation, osteoblasts function in groups of connected cells. Individual cells cannot make bone. A group of organized osteoblasts together with the bone made by a unit of cells is called the osteon. Osteoblasts are specialized, terminally differentiated products of mesenchymal stem cells, they synthesize dense, crosslinked collagen and specialized proteins in much smaller quantities, including osteocalcin and osteopontin, which compose the organic matrix of bone. In organized groups of connected cells, osteoblasts produce hydroxylapatite, deposited, in a regulated manner, into the organic matrix forming a strong and dense mineralized tissue - the mineralized matrix; the mineralized skeleton is the main support for the bodies of air breathing vertebrates. It is an important store of minerals for physiological homeostasis including both acid-base balance and calcium or phosphate maintenance; the skeleton is a large organ, formed and degraded throughout life in the air-breathing vertebrates.
The skeleton referred to as the skeletal system, is important both as a supporting structure and for maintenance of calcium and acid-base status in the whole organism. The functional part of bone, the bone matrix, is extracellular; the bone matrix consists of mineral. The protein forms the organic matrix, it is synthesized and the mineral is added. The vast majority of the organic matrix is collagen; the matrix is mineralized by deposition of hydroxyapatite. This mineral is hard, provides compressive strength. Thus, the collagen and mineral together are a composite material with excellent tensile and compressive strength, which can bend under a strain and recover its shape without damage; this is called elastic deformation. Forces that exceed the capacity of bone to behave elastically may cause failure bone fractures. Bone is a dynamic tissue, being reshaped by osteoblasts, which produce and secrete matrix proteins and transport mineral into the matrix, osteoclasts, which break down the tissues. Osteoblasts are the major cellular component of bone.
Osteoblasts arise from mesenchymal stem cells. MSC give rise to osteoblasts and myocytes among other cell types. Osteoblast quantity is understood to be inversely proportional to that of marrow adipocytes which comprise marrow adipose tissue. Osteoblasts are found in large numbers in the periosteum, the thin connective tissue layer on the outside surface of bones, in the endosteum. All of the bone matrix, in the air breathing vertebrates, is mineralized by the osteoblasts. Before the organic matrix is mineralized, it is called the osteoid. Osteoblasts buried in the matrix are called osteocytes. During bone formation, the surface layer of osteoblasts consists of cuboidal cells, called active osteoblasts; when the bone-forming unit is not synthesizing bone, the surface osteoblasts are flattened and are called inactive osteoblasts. Osteocytes are connected by cell processes to a surface layer of osteoblasts. Osteocytes have important functions in skeletal maintenance. Osteoclasts break down bone tissue, along with osteoblasts and osteocytes form the structural components of bone.
In the hollow within bones are many other cell types of the bone marrow. Components that are essential for osteoblast bone formation include mesenchymal stem cells and blood vessels that supply oxygen and nutrients for bone formation. Bone is a vascular tissue, active formation of blood vessel cells from mesenchymal stem cells, is essential to support the metabolic activity of bone; the balance of bone formation and bone resorption tends to be negative with age in post-menopausal women leading to a loss of bone serious enough to cause fractures, called osteoporosis. Bone is formed by one of two processes: endochondral ossification or intramembranous ossification. Endochondral ossification is the process of forming bone from cartilage and this is the usual method; this form of bone development is the more complex form: it follows the formation of a first skeleton of cartilage made by chondrocytes, removed and replaced by bone, made by osteoblasts. Intramembranous ossification is the direct ossification of mesenchyme as happens during the formation of the membrane bones of the skull and others.
During osteoblast differentiation, the developing progenitor cells express the regulatory transcription factor Cbfa1/Runx2. A second required transcription factor is Sp7 transcription factor. Osteochondroprogenitor cells differentiate under the influence of growth factors, although isolated mesenchymal stem cells in tissue culture, form osteoblasts under permissive conditions that include vitamin C and substrates for alkaline phosphatase, a key enzyme that provides high concentrations of phosphate at the mineral deposition site. Key growth factors in endochondral skeletal differentiation include bone morphogenetic proteins that determine to a major extent where chondrocyte differentiation occurs and where spaces are left between bones; the system of cartilage replacement by bone has a complex regulatory system. BMP2 regulates early skeletal patterning. Transforming growth factor beta, is part of a superfamily of proteins that include BMPs, which possess common signaling elements in the TGF beta signaling pathway.
TGF-β is important in cartilage differentiation, which precedes bone formation for endochondral ossification. An additional
Intramembranous ossification is one of the two essential processes during fetal development of the gnathostome skeletal system by which rudimentary bone tissue is created. Intramembranous ossification is an essential process during the natural healing of bone fractures and the rudimentary formation of bones of the head. Unlike endochondral ossification, the other process by which bone tissue is created during fetal development, cartilage is not present during intramembranous ossification. Mesenchymal stem cells within mesenchyme or the medullary cavity of a bone fracture initiate the process of intramembranous ossification. A mesenchymal stem cell, or MSC, is an unspecialized cell. Before it begins to develop, the morphological characteristics of a MSC are: a small cell body with a few cell processes that are long and thin. Furthermore, the mesenchymal stem cells are dispersed within an extracellular matrix, devoid of every type of collagen, except for a few reticular fibrils; the process of intramembranous ossification starts when a small group of adjacent MSCs begin to replicate and form a small, dense cluster of cells, a nidus.
Once a nidus has been formed the MSCs within it stop replicating. At this point, morphological changes in the MSCs begin to occur: the cell body is now larger and rounder. All of the cells within the nidus develop into, display the morphologic characteristics of, an osteoprogenitor cell. At this stage of development, changes in the morphology of the osteoprogenitor cells occur: their shape becomes more columnar and the amount of Golgi apparatus and rough endoplasmic reticulum increases. All of the cells within the nidus develop into, display the morphologic characteristics of, an osteoblast; the osteoblasts create an extracellular matrix containing Type-I collagen fibrils, osteoid. The osteoblasts, while lining the periphery of the nodule, continue to form osteoid in the center of the nidus; some of the osteoblasts become incorporated within the osteoid to become osteocytes. At this point, the osteoid becomes mineralized resulting in a nidus consisting of mineralized osteoid that contains osteocytes and is lined by active osteoblasts.
The nidus, that began as a diffuse collection of MSCs, has become rudimentary bone tissue. The first step in the process is the formation of bone spicules which fuse with each other and become trabeculae; the periosteum is formed and bone growth continues at the surface of trabeculae. Much like spicules, the increasing growth of trabeculae result in interconnection and this network is called woven bone. Woven bone is replaced by lamellar bone. Embryologic mesenchymal cells condense into layers of vascularized primitive connective tissue. Certain mesenchymal cells group together near or around blood vessels, differentiate into osteogenic cells which deposit bone matrix constitutively; these aggregates of bony matrix are called bone spicules. Separate mesenchymal cells differentiate into osteoblasts, which line up along the surface of the spicule and secrete more osteoid, which increases the size of the spicule; as the spicules continue to grow, they fuse with adjacent spicules and this results in the formation of trabeculae.
When osteoblasts become trapped in the matrix they secrete. Osteoblasts continue to line up on the surface; as growth continues, trabeculae become woven bone is formed. The term primary spongiosa is used to refer to the initial trabecular network; the periosteum is formed around the trabeculae by differentiating mesenchymal cells. The primary center of ossification is the area where bone growth occurs between the periosteum and the bone. Osteogenic cells that originate from the periosteum increase appositional growth and a bone collar is formed; the bone collar is mineralized and lamellar bone is formed. Osteons are components or principal structures of compact bone. During the formation of bone spicules, cytoplasmic processes from osteoblasts interconnect; this becomes the canaliculi of osteons. Since bone spicules tend to form around blood vessels, the perivascular space is reduced as the bone continues to grow; when replacement to compact bone occurs, this blood vessel becomes the central canal of the osteon.
Flat bones of the face Bones of the skull Clavicles Endochondral ossification Ossification Martin, RB.
Capitulum of the humerus
In human anatomy of the arm, the capitulum of the humerus is a smooth, rounded eminence on the lateral portion of the distal articular surface of the humerus. It articulates with the cupshaped depression on the head of the radius, is limited to the front and lower part of the bone. In non-human tetrapods, the name capitellum is used, with "capitulum" limited to the anteroventral articular facet of the rib. Lepidosaurs show a distinct capitellum and trochlea on the centre of the ventral surface of the humerus at the distal end. In non-avian archosaurs, including crocodiles, the capitellum and the trochlea are no longer bordered by distinct etc.- and entepicondyles and the distal humerus consists two expanded condyles, one lateral and one medial, separated by a shallow groove and a supinator process. Romer homologizes the capitellum in Archosauromorphs with the groove separating the medial and lateral condyles. In birds, where forelimb anatomy has an adaptation for flight, its functional if not ontogenetic equivalent is the dorsal condyle of the humerus.
This article incorporates text in the public domain from page 212 of the 20th edition of Gray's Anatomy Romer, A. S. 1976 Osteology of the reptiles. University of Chicago Press, Chicago. Anatomy figure: 07:02-05 at Human Anatomy Online, SUNY Downstate Medical Center BiowebUW, cached at archive.org
Elastic cartilage or yellow cartilage is a type of cartilage present in the outer ear, Eustachian tube and epiglottis. It contains collagen type II fibers; the principal protein is elastin. Elastic cartilage is histologically similar to hyaline cartilage but contains many yellow elastic fibers lying in a solid matrix; these fibers form bundles. These fibers give elastic cartilage great flexibility so that it is able to withstand repeated bending; the chondrocytes lie between the fibres. It is found in the pinnae. Elastin fibers stain dark purple/black with Verhoeff's stain. Provide support Maintain shape This article incorporates text in the public domain from page 279 of the 20th edition of Gray's Anatomy Histology image: 12_02 at the University of Oklahoma Health Sciences Center - "epiglottis" Histology image: 02901loa – Histology Learning System at Boston University Histology at ucsd.edu Anatomy Atlases - Microscopic Anatomy, plate 03.42
The metaphysis is the narrow portion of a long bone between the epiphysis and the diaphysis. It contains the growth plate, the part of the bone that grows during childhood, as it grows it ossifies near the diaphysis and the epiphyses; the metaphysis may be divided anatomically into three components based on tissue content: a cartilaginous component, a bony component and a fibrous component surrounding the periphery of the plate. The growth plate synchronizes chondrogenesis with osteogenesis or interstitial cartilage growth with appositional bone growth at the same that it is growing in width, bearing load and responding to local and systemic forces and factors. During childhood, the growth plate contains the connecting cartilage enabling the bone to grow. In an adult, the metaphysis functions to transfer loads from weight-bearing joint surfaces to the diaphysis; because of their rich blood supply and vascular stasis, metaphyses of long bones are prone to hematogenous spread of osteomyelitis in children.
Metaphyseal tumors or lesions include osteosarcoma, fibrosarcoma, enchondroma, fibrous dysplasia, simple bone cyst, aneurysmal bone cyst, non-ossifying fibroma, osteoid osteoma. One of the clinical signs of rickets that doctors look for is cupping and fraying at the metaphyses when seen on X-ray. Diaphysis Epiphysis Anatomy photo: Musculoskeletal/bone/structure0/structure2 - Comparative Organology at University of California, Davis - "Bone, structure"