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StatPearls . Sweetheart Island (FL): StatPearls Publishing; 2021 Jan-.


Embryology, Bone Ossification

Grant Breeland; Margaret A. Sinkler; Ritesh G. Menezes.

Author Information


Bone ossification, or osteogenesis, is the process of bone formation. This procedure begins between the sixth and also seventh main of embryonic advance and proceeds until about age twenty-five; back this different slightly based upon the individual. There space two types of bone ossification, intramembranous and also endochondral. Each of these processes begins with a mesenchymal organization precursor, yet how it transforms right into bone differs. Intramembranous ossification straight converts the mesenchymal organization to bone and also forms the level bones of the skull, clavicle, and also most that the cranial bones. Endochondral ossification begins with mesenchymal tissue transforming right into a cartilage intermediate, i m sorry is later replaced by bone and forms the remainder the the axial skeleton and the long bones.


The development of the skeleton have the right to be traced ago to 3 derivatives<1>: cranial neural crest cells, somites, and also the lateral plate mesoderm. Cranial neural comb cells form the flat bones the the skull, clavicle, and also the cranial skeleton (excluding a portion of the temporal and occipital bones. Somites type the remainder of the axial skeleton. The lateral key mesoderm forms the lengthy bones

Bone formation requires a theme for development. This template is mostly cartilage, acquired from embryonic mesoderm, but additionally includes undifferentiated mesenchyme (fibrous membranes) in the instance of intramembranous ossification. This structure determines where the bones will develop. By the time of birth, the majority of cartilage has actually undergone replacement by bone, however ossification will continue throughout growth and into the mid-twenties.   

Intramembranous Ossification

This procedure involves the straight conversion the mesenchyme come the bone. It begins when neural crest-derived mesenchymal cells distinguish into specialized, bone-forming cells referred to as osteoblasts. Osteoblasts group into clusters and kind an ossification center. Osteoblasts start secreting osteoid, one unmineralized collagen-proteoglycan matrix that deserve to bind calcium. The binding that calcium come osteoid outcomes in the hardening of the matrix and entrapment that osteoblasts. This entrapment results in the transformation of osteoblasts come osteocytes. As osteoid proceeds to be secreted by osteoblasts, it surrounds blood vessels, developing trabecular/cancellous/spongy bone. These vessels will eventually form the red bone marrow. Mesenchymal cells on the surface ar of the bone kind a membrane referred to as the periosteum. Cells on the inner surface ar of the periosteum distinguish into osteoblasts and secrete osteoid parallel to the of the present matrix, thus forming layers. These layers are jointly called the compact/cortical bone <2>.

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Five steps deserve to summarize intramembranous ossification:

Mesenchymal cells identify into osteoblasts and group into ossification centers
Osteoblasts become entrapped by the osteoid lock secrete, transforming them to osteocytes
Trabecular bone and periosteum form

Endochondral Ossification

This procedure involves the instead of of hyaline cartilage v bone. It starts when mesoderm-derived mesenchymal cells distinguish into chondrocytes. Chondrocytes proliferate rapidly and also secrete an extracellular matrix to form the cartilage design for bone. The cartilage model consists of hyaline cartilage resembling the form of the future bone as well as a neighboring membrane referred to as the perichondrium. Chondrocytes near the center of the bony model begin to undergo hypertrophy and start including collagen X and an ext fibronectin to the matrix that they produce; this transformed matrix allows for calcification. The calcification of the extracellular matrix stays clear of nutrients from getting to the chondrocytes and also causes lock to experience apoptosis. The result cell fatality creates voids in the cartilage design template and enables blood ship to invade. Blood vessels further enlarge the spaces, which at some point combine and become the medullary cavity; they additionally carry in osteogenic cells and trigger the change of perichondrium to the periosteum. Osteoblasts then produce a thickened region of compact bone in the diaphyseal region of the periosteum, called the periosteal collar. The is below that the major ossification center forms. When bone is replacing cartilage in the diaphysis, cartilage proceeds to proliferate in ~ the ends of the bone, boosting bone length. These proliferative locations become the epiphyseal plates (physeal plates/growth plates), which administer longitudinal expansion of bones after birth and also into at an early stage adulthood. After birth, this entire procedure repeats chin in the epiphyseal region; this is where the secondary ossification center forms <3>.

The physeal development plate is separated right into various sections based upon pathologic characteristics.