The adult human skeleton has about 206 different bones which are formed from fusion of significantly more bones in the postnatal developing neonate and child (about 275). Ossification in general continues postnatally, through puberty until mid 20s.
The two major parts of the human skeleton are the axial (80 bones in skull, vertebra, ribs, sternum) and appendicular (126 bones in limbs, shoulders, pelvis) skeletons.
These notes summarise the timecourse of development of some of these bones in humans.
Developing bone and cartilage in the knee.
Two main forms of ossification occur in different bones, intramembranous (eg skull) and endochondrial (eg vertebra) ossification.
Bones within the limb form by endochondrial ossification (begins Carnegie stage 18) throughout embryo. This process is the replacement of cartilage with bone (week 5-12).(More? see limb development).
The process of ossification of bones, as determined postnatally clinically, include a series of stages.
An additional stage has been recently used (Stage 5) which is the disappearance of the epiphyseal scar after total fusion.
Reference: Schmeling A, Schulz R, Reisinger W, Mühler M, Wernecke KD, Geserick G.Studies on the time frame for ossification of the medial clavicular epiphyseal cartilage in conventional radiography. Int J Legal Med. 2004 Feb;118(1):5-8.
Examples of appearance and fusion of some secondary ossification centres in human limb long bones. Proximal is closer to body, distal is further away from the body.
Appearance
Proximal epiphysis gestation week 36 - 4 years
Distal epiphysis 6 months - 10 years
Fusion
Proximal epiphysis 12 - 20 years
Distal epiphysis 11 - 19 years
Appearance
Proximal epiphysis 1 - 12 years
Distal epiphysis Gestation week 36 - 40
Fusion
Proximal epiphysis 11 - 19 years
Distal epiphysis 14 - 19 years
Data from: table 1 - Species comparison of postnatal bone growth and development. Birth Defects Research (Part B) 68:86-110, 2003.
Week 6 - Intramembranous ossification center develops lateral to Meckel's cartilage.
Week 7 - Coronoid process begins differentiating.
Week 8 - Coronoid process fuses with main mandibular mass.
Week 10 (approx) - Both condylar and coronoid processes are recognizable and anterior portion of Meckel's cartilage begins to ossify.
Weeks 12-14 - Secondary cartilages for the condyle, coronoid, and symphysis appear.
Weeks 14-16 - Deciduous tooth germs start to form.
At birth mandible still has separate right and left halves.
Year 1 - Fusion of right and left halves of mandible at the symphysis.
Infancy and childhood - Increase in both size and shape of the mandible; eruption and replacement of teeth.
Year 12-14 - All permanent teeth emerged except third molars.
Data from: table 4 - Species comparison of postnatal bone growth and development. Birth Defects Research (Part B) 68:86-110, 2003.
Stage 3 - 16 years
Stage 4 - 20 years (women), 21 years (men)
Stage 5 - 26 years
Reference: Schmeling A, Schulz R, Reisinger W, Mühler M, Wernecke KD, Geserick G.Studies on the time frame for ossification of the medial clavicular epiphyseal cartilage in conventional radiography. Int J Legal Med. 2004 Feb;118(1):5-8.
Schmeling A, Schulz R, Reisinger W, Mühler M, Wernecke KD, Geserick G.Studies on the time frame for ossification of the medial clavicular epiphyseal cartilage in conventional radiography. Int J Legal Med. 2004 Feb;118(1):5-8.
Zoetis T, Tassinari MS, Bagi C, Walthall K, Hurtt ME. Species comparison of postnatal bone growth and development. Birth Defects Res B Dev Reprod Toxicol. 2003 Apr;68(2):86-110. This paper is not only a review comparison of different species (human, dog, monkey, rat) it provides a good overview of bone development and structure.
Scheuer L and Black, SM. Developmental juvenile osteology. San Diego, Calif. : Academic Press, c2000.
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The process of human skeleton development begins as a cartilagenous or membranous template in the embyro, replacement by ossification through fetal development continuing into the postnatal period where many bone "pieces" fuse into puberty with a burst of growth, into young adult where many bones finally fuse.
The process of bone formation is never completely over, with many bones continuously undergoing remodeling and providing a key repository/store for essential minerals (calcium, phosphorous).
This page summarises the development timecourse of selected skeletal bones (humerus, femur, mandible) in humans.
Please email Dr Mark Hill if you wish to make a comment about this current project.