Bone ossification, or osteogenesis, is the process of bone formation. This process begins between the sixth and seventh weeks of embryonic development and continues until about age twenty-five; although this varies slightly based on the individual. There are two types of bone ossification, intramembranous and endochondral. Each of these processes begins with a mesenchymal tissue precursor, but how it transforms into bone differs. Intramembranous ossification directly converts the mesenchymal tissue to bone and forms the flat bones of the skull, clavicle, and most of the cranial bones. Endochondral ossification begins with mesenchymal tissue transforming into a cartilage intermediate, which is later replaced by bone and forms the remainder of the axial skeleton and the long bones.
The development of the skeleton can be traced back to three derivatives: cranial neural crest cells, somites, and the lateral plate mesoderm. Cranial neural crest cells form the flat bones of the skull, clavicle, and the cranial bones (excluding a portion of the temporal and occipital bones. Somites form the remainder of the axial skeleton. The lateral plate mesoderm forms the long bones
Bone formation requires a template for development. This template is mostly cartilage, derived from embryonic mesoderm, but also includes undifferentiated mesenchyme (fibrous membranes) in the case of intramembranous ossification. This framework determines where the bones will develop. By the time of birth, the majority of cartilage has undergone replacement by bone, but ossification will continue throughout growth and into the mid-twenties.
This process involves the direct conversion of mesenchyme to the bone. It begins when neural crest-derived mesenchymal cells differentiate into specialized, bone-forming cells called osteoblasts. Osteoblasts group into clusters and form an ossification center. Osteoblasts begin secreting osteoid, an unmineralized collagen-proteoglycan matrix that can bind calcium. The binding of calcium to osteoid results in the hardening of the matrix and entrapment of osteoblasts. This entrapment results in the transformation of osteoblasts to osteocytes. As osteoid continues to be secreted by osteoblasts, it surrounds blood vessels, forming trabecular/cancellous/spongy bone. These vessels will eventually form the red bone marrow. Mesenchymal cells on the surface of the bone form a membrane called the periosteum. Cells on the inner surface of the periosteum differentiate into osteoblasts and secrete osteoid parallel to that of the existing matrix, thus forming layers. These layers are collectively called the compact/cortical bone .
Five steps can summarize intramembranous ossification:
This process involves the replacement of hyaline cartilage with bone. It begins when mesoderm-derived mesenchymal cells differentiate into chondrocytes. Chondrocytes proliferate rapidly and secrete an extracellular matrix to form the cartilage model for bone. The cartilage model includes hyaline cartilage resembling the shape of the future bone as well as a surrounding membrane called the perichondrium. Chondrocytes near the center of the bony model begin to undergo hypertrophy and start adding collagen X and more fibronectin to the matrix that they produce; this altered matrix allows for calcification. The calcification of the extracellular matrix prevents nutrients from reaching the chondrocytes and causes them to undergo apoptosis. The resulting cell death creates voids in the cartilage template and allows blood vessels to invade. Blood vessels further enlarge the spaces, which eventually combine and become the medullary cavity; they also carry in osteogenic cells and trigger the transformation of perichondrium to the periosteum. Osteoblasts then create a thickened region of compact bone in the diaphyseal region of the periosteum, called the periosteal collar. It is here that the primary ossification center forms. While bone is replacing cartilage in the diaphysis, cartilage continues to proliferate at the ends of the bone, increasing bone length. These proliferative areas become the epiphyseal plates (physeal plates/growth plates), which provide longitudinal growth of bones after birth and into early adulthood. After birth, this entire process repeats itself in the epiphyseal region; this is where the secondary ossification center forms .
The physeal growth plate is separated into various sections based on pathologic characteristics.
Five steps can summarize endochondral ossification:
Osteochondroprogenitor cells are mesenchymal stem cells that can differentiate into chondrocytes or osteoblasts. The expression of the transcription factors CBFA1/RUNX2 and OSX induce osteoblast differentiation. The expression of transcription factors SOX9, L-SOX5, and SOX6 are necessary for chondrocyte differentiation.
Osteoblasts are responsible for bone deposition. They also regulate osteoclasts. They derive from mesenchymal stem cells. During the embryonic period, they secrete osteoid, an unmineralized matrix, which is subsequently calcified and forms bone. Osteoblasts have a crucial role in maintaining the balance of bone formation and resorption. Osteoblasts secrete RANK ligand (RANKL), which binds to the RANK receptor on pre-osteoclasts and thus induces their differentiation. Osteoblasts also secrete osteoprotegerin (OPG), which prevents RANK/RANKL interaction by binding to RANKL; this prevents osteoclast differentiation. Thus, the balance between RANKL/OPG production by osteoblasts determines osteoclast activity..
Osteoclasts are multinucleated cells that function in bone resorption. They are derived from macrophages and enter the bone through blood vessels. Each osteoclast has numerous processes that extend into the matrix and secrete hydrogen ions, causing acidification and break down of bone. Osteoclast function is under tight control; overactivity results in osteoporosis while decreased activity results in osteopetrosis.
Osteocytes are the most numerous cells present in bone. They form from osteoblasts trapped in osteoid. Their primary function is mechanosensation. Osteocytes connect to each other and their environment via cytoplasmic processes. This communication with each other and the surrounding environment allows them to detect stress and deformation of the bone. Based on this information, osteocytes orchestrate the remodeling of bone.
Several transcription factors are involved in the process of endochondral bone formation. Sox-9 regulates chondrogenesis of several collagen types includes II, IV, and XI. PTHrP delays chondrocyte differentiation in the zone of hypertrophy.
Intramembranous bone formation is controlled by the canonical Wnt and Hedgehog signaling pathway. Beta-catenin enters cells to induce the formation of osteoblasts. Additional transcription factors involved in the process include CBFA1 (Runx2), osterix (OSX), and sclerostin (SOST).
Cleidocranial Dysplasia (CCD)
CCD occurs due to a mutation in CBFA1/RUNX2 (runt-related transcription factor 2) gene, which directs osteoblast differentiation - CCD is an autosomal dominant condition resulting in short stature, patent fontanelles, and supernumerary teeth
CMD occurs due to a mutation in SOX9 (SRY-box 9) gene, which directs chondrocyte differentiation - CMD is an autosomal dominant condition that results in the bowing of long bones, and this condition usually results in neonatal death due to respiratory failure
Osteogenesis Imperfecta (OI)
OI occurs due to a mutation in COL1A1 (collagen type I alpha 1 chain) or COL1A2 (collagen type I alpha 2 chain) genes, which encode the major component of type 1 collagen; this is an autosomal dominant condition that results in very fragile bones
Achondroplasia occurs due to a mutation in FGFR3 (fibroblast growth factor receptor 3) gene, which aids in the formation of collagen and plays a role in the ossification of bone - this mutation prevents adequate bone formation in utero and results in a shortened stature
Acromegaly occurs due to an increased amount of growth hormone and insulin-like growth factor-1. Causes of acromegaly include pituitary tumors and McCune-Albright syndrome. These factors have anabolic effects on cartilage and bone metabolism. The increased factors both cause the increased growth of bone and degenerative changes to cartilage resulting in arthropathy.
Rickets is most commonly caused by a vitamin D deficiency which leads to the softening and weakening of bones in children. The main mechanism is insufficient calcification at the growth plate during bone formation. Symptoms of Rickets disease include bowed legs, spinal curvatures, rachitic rosary, and craniotabes. Rickets results in failure of apoptosis of the hypertrophic chondrocyte in the physeal plate. Eventually, this leads to a cupping appearance of the epiphyseal ends of the bones.
Salter-Harris fractures are fractures of the epiphyseal plate. These types of fractures have the potential to impair bone ossification depending on the location. Injury to the epiphyseal plate can result in decreased longitudinal growth, angular deformity, and altered joint mechanics. The classification is as follows :
Age estimation of the fetus is one of the primary objectives of the fetal autopsy.
Forensic fetal osteology:
|||Jin SW,Sim KB,Kim SD, Development and Growth of the Normal Cranial Vault : An Embryologic Review. Journal of Korean Neurosurgical Society. 2016 May; [PubMed PMID: 27226848]|
|||Percival CJ,Richtsmeier JT, Angiogenesis and intramembranous osteogenesis. Developmental dynamics : an official publication of the American Association of Anatomists. 2013 Aug; [PubMed PMID: 23737393]|
|||Ortega N,Behonick DJ,Werb Z, Matrix remodeling during endochondral ossification. Trends in cell biology. 2004 Feb; [PubMed PMID: 15102440]|
|||Wysokinski D,Pawlowska E,Blasiak J, RUNX2: A Master Bone Growth Regulator That May Be Involved in the DNA Damage Response. DNA and cell biology. 2015 May; [PubMed PMID: 25555110]|
|||Xiong J,Onal M,Jilka RL,Weinstein RS,Manolagas SC,O'Brien CA, Matrix-embedded cells control osteoclast formation. Nature medicine. 2011 Sep 11; [PubMed PMID: 21909103]|
|||Clarke B, Normal bone anatomy and physiology. Clinical journal of the American Society of Nephrology : CJASN. 2008 Nov [PubMed PMID: 18988698]|
|||Bar-Shavit Z, The osteoclast: a multinucleated, hematopoietic-origin, bone-resorbing osteoimmune cell. Journal of cellular biochemistry. 2007 Dec 1 [PubMed PMID: 17955494]|
|||Bonewald LF, The amazing osteocyte. Journal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research. 2011 Feb; [PubMed PMID: 21254230]|
|||Lo Muzio L,Tetè S,Mastrangelo F,Cazzolla AP,Lacaita MG,Margaglione M,Campisi G, A novel mutation of gene CBFA1/RUNX2 in cleidocranial dysplasia. Annals of clinical and laboratory science. 2007 Spring; [PubMed PMID: 17522365]|
|||Lefebvre V,Dvir-Ginzberg M, SOX9 and the many facets of its regulation in the chondrocyte lineage. Connective tissue research. 2017 Jan [PubMed PMID: 27128146]|
|||Jain V,Sen B, Campomelic dysplasia. Journal of pediatric orthopedics. Part B. 2014 Sep [PubMed PMID: 24800790]|
|||Rauch F,Glorieux FH, Osteogenesis imperfecta. Lancet (London, England). 2004 Apr 24; [PubMed PMID: 15110498]|
|||Baujat G,Legeai-Mallet L,Finidori G,Cormier-Daire V,Le Merrer M, Achondroplasia. Best practice & research. Clinical rheumatology. 2008 Mar [PubMed PMID: 18328977]|
|||Lieberman SA,Björkengren AG,Hoffman AR, Rheumatologic and skeletal changes in acromegaly. Endocrinology and metabolism clinics of North America. 1992 Sep [PubMed PMID: 1521515]|
|||Ozkan B, Nutritional rickets. Journal of clinical research in pediatric endocrinology. 2010 [PubMed PMID: 21274312]|
|||Foris LA,Waseem M, Fracture, Salter Harris . 2019 Jan [PubMed PMID: 28613461]|
|||Cepela DJ,Tartaglione JP,Dooley TP,Patel PN, Classifications In Brief: Salter-Harris Classification of Pediatric Physeal Fractures. Clinical orthopaedics and related research. 2016 Nov; [PubMed PMID: 27206505]|
|||Caine D,DiFiori J,Maffulli N, Physeal injuries in children's and youth sports: reasons for concern? British journal of sports medicine. 2006 Sep; [PubMed PMID: 16807307]|
|||Huxley AK,Angevine JB Jr, Determination of gestational age from lunar age assessments in human fetal remains. Journal of forensic sciences. 1998 Nov [PubMed PMID: 9846409]|
|||Huxley AK, Gestational age discrepancies due to acquisition artifact in the forensic fetal osteology collection at the National Museum of Natural History, Smithsonian Institution, USA. The American journal of forensic medicine and pathology. 2005 Sep [PubMed PMID: 16121075]|
|||Castellana C,Kósa F, Estimation of fetal age from dimensions of atlas and axis ossification centers. Forensic science international. 2001 Mar 1 [PubMed PMID: 11230944]|
|||Sakurai T,Michiue T,Ishikawa T,Yoshida C,Sakoda S,Kano T,Oritani S,Maeda H, Postmortem CT investigation of skeletal and dental maturation of the fetuses and newborn infants: a serial case study. Forensic science, medicine, and pathology. 2012 Dec [PubMed PMID: 22392019]|