Bone
Development and Growth
Human Structure and Development ANHB 2212
2008 – Week 12
Avinash Bharadwaj
Bone as a Connective Tissue
 All connective tissue is mesodermal.
 Undifferentiated (not specialised) cells 
 Specialise to produce different kinds of matrix.
 “Osteoprogenitor” cells  osteoblasts.
 Fibres and ground substance
 Osteoid
 Mineralisation  immature (“woven”) bone.
 Replaced by mature, lamellar bone.
Some Significant Features
 Unlike cartilage…
 Bone grows only by “apposition” : on the surface
 In most places…
 Subjected to forces as it develops and grows
 Needs a firm base structure to grow into
 In some areas, can develop in soft tissues
 In either case – bone formation, growth and
maintenance needs blood supply!
“Intramembranous” Ossification
 Around the developing brain –
 Thin “membrane” in the foetus
 Mesenchymal cells differentiate
  Osteoblasts
 … “Ossification centres”
 “Spicules” of bone
 Support for more osteoblasts
 Ossification spreads radially from the
centre.
 Unossified tissue – sutures
 Foetal skull is deformable!
 Sutural growth and obliteration
Endochondral Ossification
Ossification in cartilage
 Cartilage withstands forces better than ‘membrane’
 Muscular forces during intrauterine life
 Muscular and other forces during growth period
 Cartilage provides a mechanism for growth
 Cartilage can grow interstitially as well.
 Most bones in the body develop in this manner.
 Best studied with a long bone as an example.
In this process cartilage is temporary.
It dies and is replaced by bone.
Cartilage is not coverted into bone!
A Cartilage Model
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Mesenchymal cells differentiate  chondroblasts
Miniature cartilage model of bone
Blood vessels invade the middle portion
“Collar” of bone – subperiosteal collar
Chondrocytes die in the centre
Matrix calcified – acts as scaffolding
Osteoblasts deposit bone matrix
“Primary” centre of ossification
Spread from Primary Centre
 Ossification spreads towards the ends
 Calcification of cartilage matrix  death of
cells
 New bone deposited
 Simultaneously, cartilage at the ends
continues to grow  increase in length
 Removal of calcified cartilage matrix
 Osteoclasts (“bone breaker” cells)
 More about their function later
 At birth – Shafts (diaphyses) of all long
bones are bony.
Postnatal Growth
 Bones now subject to more stresses
 The ends need to be ossified
 “Secondary” centres appear at ends  form epiphyses
(First such centres around the knee)
 However, growth must continue
 A plate of cartilage remains between shaft and epiphysis
 epiphyseal plate
 Cartilage grows…
 Replaced by bone on the surface facing the shaft
 “Zones” of epiphyseal plate
 Resting, growth, maturation, hypertrophy, calcification and death,
ossification.
Completion of Growth
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Cartilage stops growing
Remaining cartilage replaced by bone 
Epiphysis unites with shaft
Programmed appearance and fusion of
epiphyses – concept of “bone age”
Hormonal Control
 Growth hormone
 Before and after epiphyseal fusion
 Parathyroid hormone – calcium removal
 Other hormones
Remodelling
 Destruction an integral part of life
history of bone
 Osteoclasts
 Multinucleated large (giant) cells.
 Remodelling during growth
 Metaphysis – junction of epi- and
diaphysis
 Shaft – increase in diameter, resorption
of inner surface
 Flat bones
 Replacement of old osteons,
trabeculation
 Altered muscular and other forces
Bone Repair
 Fracture  bleeding
 Blood clot – fibrous tissue
 Islands of cartilage
 Callus – fibrous + cartilage
 Calcification  strength + death of cartilage
 New bone formation
 Woven bone – bony callus
 Rough surface – remodelling