Ossification

Ossification is the process of the synthesis of bone from cartilage. There are two types of ossification— intramembranous and endochondral ossification. Bone may be synthesized by intramembranous ossification, endochondral ossification, or a combination of the two.

Intramembranous ossification is the transformation of the mesenchyme, cells of an embryo into bone. During early development of vertebrate animals, the embryo consists of three primary cell layers: ectoderm on the outside, mesoderm in the middle, and endoderm on the inside. Mesenchyme cells constitute part of the embryo’s mesoderm and develop into connective tissue such as bone and blood. The bones of the skull derive directly from mesenchyme cells by intramembranous ossification.

Endochondral ossification is the gradual replacement of cartilage by bone during development. This process is responsible for formation of most of the skeleton of vertebrate animals. In this process, actively dividing bone-forming cells (osteoblasts) arise in regions of cartilage called ossification centers. The osteoblasts then develop into osteocytes, which are mature bone cells embedded in the calcified (hardened) part of the bone known as the matrix.

Most bones arise from a combination of intramembranous and endochondral ossification. In this process, mesenchyme cells develop into chondroblasts and increase in number by cell division. Then, the chrondroblasts enlarge and excrete a matrix which hardens due to presence of inorganic minerals. Then, chambers form within the matrix and osteoblasts and blood-forming cells enter these chambers. The osteoblasts then secrete minerals to form the bone matrix.

Mature hardened bone is living tissue consisting of an organic component and a mineral component. The organic part mainly consists of proteins such as collagen fibers, an extracellular matrix, and fibroblasts, which have the living cells that produce the collagen and matrix. The mineral part of bone, which is made of hydroxyapatite and calcium carbonate, gives bones their strength and rigidity. During the life of an individual, osteoblasts continually secrete minerals while osteoclasts continually reabsorb the minerals. Bedridden hospital patients and astronauts often show loss of bone because reabsorption by osteoclasts exceeds synthesis by osteoblasts. Bones become more brittle as a person ages because the mineral part of the bones decreases.

See also Osteoporosis; Skeletal system.

Peter A. Ensimger

Ossification

Ossification is the process of the synthesis of bone from cartilage. There are two types of ossification—intramembranous and endochondral ossification. Bone may be synthesized by intramembranous ossification, endochondral ossification, or a combination of the two.

Intramembranous ossification is the transformation of the mesenchyme, cells of an embryo into bone. During early development of vertebrate animals, the embryo consists of three primary cell layers: ectoderm on the outside, mesoderm in the middle, and endoderm on the inside. Mesenchyme cells constitute part of the embryo's mesoderm and develop into connective tissue such as bone and blood . The bones of the skull derive directly from mesenchyme cells by intramembranous ossification.

Endochondral ossification is the gradual replacement of cartilage by bone during development. This process is responsible for formation of most of the skeleton of vertebrate animals. In this process, actively dividing bone-forming cells (osteoblasts) arise in regions of cartilage called ossification centers. The osteoblasts then develop into osteocytes, which are mature bone cells embedded in the calcified (hardened) part of the bone known as the matrix.

Most bones arise from a combination of intramembranous and endochondral ossification. In this process, mesenchyme cells develop into chondroblasts and increase in number by cell division . Then, the chrondroblasts enlarge and excrete a matrix which hardens due to presence of inorganic minerals . Then, chambers form within the matrix and osteoblasts and blood-forming cells enter these chambers. The osteoblasts then secrete minerals to form the bone matrix.

Mature hardened bone is living tissue consisting of an organic component and a mineral component. The organic part mainly consists of proteins such as collagen fibers, an extracellular matrix, and fibroblasts, which have the living cells that produce the collagen and matrix. The mineral part of bone, which is made of hydroxyapatite and calcium carbonate , and gives bones their strength and rigidity. During the life of an individual, osteoblasts continually secrete minerals while osteoclasts continually reabsorb the minerals. Bedridden hospital patients and astronauts often show loss of bone because reabsorption by osteoclasts exceeds synthesis by osteoblasts. Bones become more brittle as a person ages because the mineral part of the bones decreases.

See also Osteoporosis; Skeletal system.

Peter A. Ensimger

os·si·fy / ˈäsəˌfī/ • v. (-fies, -fied) [intr.] turn into bone or bony tissue: these tracheal cartilages may ossify. ∎  [often as adj.] (ossified) fig. cease developing; be stagnant or rigid: ossified political institutions.DERIVATIVES: os·si·fi·ca·tion / ˌäsəfiˈkāshən/ n.

ossification The formation of bone tissue, which is ultimately derived from neural crest cells. The process is unique to vertebrates and differs from dentine formation in that it allows the continued existence of the mineralizing cells (osteoblasts) which reside in the lacunae of bone connected to each other by canalicula. Ossification is carried out by osteoblasts, which become arranged concentric to blood vessels (Haversian canals) to form cylindrical Haversian systems. The resulting bone, together with its series of Haversian systems, is encased in outer lamellae of bone plates and is then known as lamellar bone. Bone tissue may originate from the dermis of the integument giving rise to dermal bone, or occur by endochondrial ossification, which is the invasion and replacement of cartilagenous tissue with osteoblasts and subsequently bone.

ossification The process of bone formation. It is brought about by the action of special cells called osteoblasts, which deposit layers of bone in connective tissue. In intramembranous ossification bones are formed directly in connective tissue (see membrane bone); in endochondral ossification the bones are formed by the replacement of cartilage (see cartilage bone).