Events in intra-cartilaginous or enchondral bone
1: Formation of cartilage model : In the region where the bone is to grow
within the embryo, a hyaline cartilage model is developed. The
cartilaginous model is surrounded by a vascular condensed mesenchyme
or perichondrium. For a period this model grows, both appositionally and
2: Appearance of a primary center of ossification: The chondrocytes in
the center of cartilage model hypertrophy, accumulate glycogen in their
cytoplasm and become vacuolated.
3: Formation of primary areolae. Hypertrophy of the chondrocytes
results in enlargement of their lacunae. and reduction in the intervening
cartilage matrix septa, which becomes calcified. The chondrocytes die
and their lacunae are now called primary areolae.
4: Formation of subperiosteal bone collar: The osteogenic cells in the deeper
layer of perichondrium differentiate into osteoblasts and perichondrium is
transformed into periosteum.
Events in enchondral bone formation
• Into osteoblasts and overlying perichondrium is converted
into periosteum. The osteoblasts lay down bone matrix
around the cartilage model in the intramembranous way
making a bone collar around the center of ossification.
• Perichondrium becomes the periosteum.
• The newly formed osteoblasts of periosteum secrete bone
matrix around the cartilage model, forming the
subperiosteal bone collar by intramembranous bone
• The bone collar prevents the diffusion of nutrients to the
hypertrophied chondrocytes within the core of cartilage
model, causing them to die.
• The dead chondrocytes leave empty lacunar spaces
separated by matrix septa. Confluence of these spaces
creates marrow cavity.
• 5: Formation of osteogenic bud: Holes are etched in the
subperiosteal bone collar by osteoclasts.
• Through these holes a subperiosteal bud (osteogenic bud)
composed of osteoprogenitor cells, osteoblasts, osteoclasts
• , hemopoitic cells, and blood vessels, invades the primary center of
ossification (enters the cavities within the cartilage model).
• 6: Secondary areolae and primitive marrow: The osteoclasts erode the
walls of primary areolae which now make larger confluent cavities called
secondary areolae foreshadowing the future medulary cavity. Haemopoitic
tissue, blood vessels osteoblasts and osteoclast fill the secondary areolae.
• 7: Osteoid formation: osteoblasts attach themselves to the residual walls of
the calcified cartilage, laying down osteoid which rapidly changes , firstly
into patches and then into continuous bone lining.
• Further layers of bone are added, enclosing young osteoblasts in lacunae
and narrowing the perivascular spaces. The osteoblasts enclosed in lacunae
• 8 : Calcification. The osteoid is mineralised by deposition of hydroxy
apatite crystals. In this way a woven fibred bone is formed consisting of
atypical Haversian system or primary osteons. The process of bone
formation moves from primary center of ossification to both end of bone
( from diaphysis towards the epiphysis). Osteoblasts elaborate bone matrix
on the surface of calcified cartilage. The bone matrix becomes calcified to
form a calcified cartilage/ calcified bone complex.
• 9: Appositional bone deposition. In the region of the shafts more bone is
laid down at the periphery by bone collar as compared to central bone
formation (interstitial deposition) increases the girth of the bone.
10. Bone erosion and medullary cavity formation:
The osteoclasts erode the bone in the centre, and bone is not reformed in
the centre to make future medullary cavity
11. Secondary osteons or typical Haversian system: Osteoclast erode the
woven fibered bone in the region of the shaft which is destined to become
the compact bone. In that region the vessels get arranged parallel to the
long axis of shaft with surrounding osteoblasts. The osteoid is laid down
with its collagen fibres in concentric lamellae, thus making the typical
these steps comprehensively explain the basic process of endochondral
BONE GROWTH IN LENGTH (Growth plate)
• The continued lengthening of bone depends upon the epiphyseal
plate in the process of endochondral bone formation. Proliferation
occurs at the epiphyseal aspect of growth plate and replacement by
bone takes place at the diaphyseal aspect of the growth plate.
• Histologically the epiphyseal plate is divided into five recognizable
zones. These zones beginning at the epiphyseal side are:
1. Zone of reserve cartilage- chondrocytes randomly distributed
throughout the matrix are mitotically active.
2. Zone of proliferation- chondrocytes, rapidly proliferating forming
rows of isogenous cells that parallel the direction of the bone
3. Zone of maturation and hypertrophy- chondrocytes mature,
hypertrophy, and accumulate glycogen in their cytoplasm.
The matrix between their lacunae narrows with
corresponding growth in the lacunae.
4. Zone of calcification- lacunae become confluent,
hypertrophied chondrocytes die and cartilage matrix
5. Zone of ossification- osteoprogenitor cells invade t6he area
and differentiate into osteoblasts, which elaborate matrix
that becomes calcified on the surface of the calcified
cartilage. This is followed by resorption of the calcified
cartilage/ calcified bone complex.
• Cacification: The osteoid is mineralized by deposition of
hydroxyapatite crystals. This results in the formation of woven
bone with atypical haversian systems.i
• Appositional calcification: The osteoclasts resorb the calcified
cartilage /calcified bone complex and so enlarge the marrow cavity.
• As this process continues the cartilage of the diaphysis is replaced
by bone, except for the epiphyseal plate, which are responsible for
the continued growth of the bone for 18-20 years.
• Secondary centers of ossification: Begin to form at the epiphysis at
each end of the long bone by a process similar to that in diaphysis
except that a bone collar is not formed.
• The osteoprogenitor cells invade the cartilage of the epiphysis,
differentiate into osteoblasts, and begin secreting matrix on the
cartilage scaffold. Gradually the cartilage of epiphysis is replaced by
bone except at (1) articular cartilage and (2) the epiphyseal growth
• The articular surface of the bone remains cartilaginous
throughout the life.
• As long as the rate of mitotic activity in the proliferative zone
equals the rate of resorption in the zone of ossification, the
epiphyseal plate remains the same width, the bone continues
to grow longer. At about the 20th year of age, the rate of
mitosis decreases in the zone of proliferation and the zone of
ossification overtakes the zone of proliferation and cartilage
reserve. The cartilage of the epiphyseal plate is replaced by a
plate of calcified cartilage/calcified bone complex, which
becomes resorbed by osteoclastic activity and marrow cavity
of the diaphysis becomes confluent with the marrow cavity of
the epiphysis. Once the growth plate is resorbed, the growth
in length is no longer possible.
• Bone growth in width: the growth of diaphysis in girth takes
place by appositional growth. The osteogenic layer of the cells
of the periosteum proliferate and differentiate into
osteoblasts that elaborate bone matrix on the subperiosteal
bone surface. This process occurs throughout the growth
period of the bone So that in the mature long bone the shaft
is built via subperiosteal intramembranous bone formation.
Intramembranous ossification is the process in which ossification starts
directly in a mesenchymal membrane without having passed through
cartilaginous stage. This is direct formation of bone in highly
vascular cellular sheets of mesenchymal tissue containing osteoprogenitor
cells. Most of the flat bones of skull like frontal, parietal temporal, occipital
bone and mandible develop through intramembranous ossification. This
process also contributes to the growth of short bones and the thickening
of long bones. In the early embryonic life, the region in which a membrane
bone is to develop, shows condensation of the mesenchymal cells. Blood
vessels rapidly proliferate within the condensed area so that it becomes
more vascular. Further steps are
1. Differentiation and proliferation of osteoprogenitor cells in the
2. Formation of osteoblasts from osteoprogenitor cells in the center of
membrane to form primary ossification centre..
3. Secretion of osteoid by osteoblasts around themselves and capillaries. The
osteoid consists of matrix and collagen fibres but latter are disposed off
irregularly in conformity with irregular capillary network.
4. Calcification of the osteoid: The collagen fibers of the osteoid are
randomly oriented and so they form a network of spicules and trabeculae
whose surfaces are populated by osteoblasts. Osteoid is calcified and
islands of developing bone surround elongated cavities contaning
capillaries and bone marrow cells.
5. Osteoblasts are entrapped within bone , now being called osteocytes and
they occupy lacunae. Also they send out cytoplasmic processes which pass
through canaliculi and via these all osteocytes maintain contact with each
6. Primary osteons or primary Haversian system
As matrix secretion, calcification and enclosure of osteoblasts proceed,
the trabeculae thicken and intervening spaces become narrower. Where
bone remains trabecular, the process of calcification becomes slow and
spaces between trabeculae are occupied by hemopoitic tissue. Where
compact bone is forming, trabeculae continue to thicken and vascular
spaces to narrow. The collagen fibers of the matrix secreted on the walls
of narrowing spaces between trabeculae, become organized as parallel,
spiral or longitudinal bundles around a central canal. The enclosed cells
occupy concentric sequential rows. These irregular, interconnected
7. FORMATION OF SECONDARY OSTEONS
The woven bone is eroded by osteoclasts and
replaced by regular lamellar bone in trabeculae
and typical Haversian systems or secondary
osteons in compact regions. The process
Removal of woven bone
Rearrangement of blood vessels parallel each other and bone surface.
Secretion of Osteoid around vessels
Mineralization of Osteoid.
8. Marrow Space: bone is not replaced after removal by osteoclasts
in between bony septa and these spaces get occupied by marrow.
9. The process extends in all directions in same steps i.e early
formation of woven fibered bone and its replacement by the
lamellar bone later.
10. Periosteum and bone deposition from periphery: As the
ossification proceeds inside, on the surface highly vascular
connective tissue mesenchymal membrane condenses to form
periosteum, containing in its deep part the osteoprogenitor cells.
These cells differentiate into osteoblasts and latter start laying
down bone at the periphery and also send in fresh generations of
11. On the inner aspect of flat bones of skull, condensation of
mesenchyme forms endosteum. During remodelling a process
meant for reshaping the bone and continued through out the life,.
FACTORS AFFECTING THE OSSIFICATION
REMODELING AND APPLIED ASPECTS
excess of this hormone leads to gigantism with
specially large hands, feet and facial skeleton
less than normal secretion leads to dwarfism.
high levels cause endosteal and trabecular bone
deposition whereas decreased levels cause bone.
Absropiton reflecting osteoporosis after menopause
when estrogen levels fall.
high levels lead to premature fusion of epi and
diaphysis leading to short stature.
in low levels (hypogonadism) fusion is delayed and is
specially characterized by excessively long limbs.
iv. Parathyroid hormone:
hyperparathyroidism leads to sub periosteal and
endosteal bone absorption by osteoclasts, a
condition being referred to as osteitis fibrosa
V. Thyroid hormones
T3 and T4 (tri-iodothyronin and tetra iodo-thyronin
respectively) are important for remodeling to adult mature
B. DIETARY ELEMENTS
Proper intake of calcium is of explicit significance but adequate intake does
not necessarily means adequate availability as absorption of calcium is
dependent upon availability of vit. D as well.
I. Vit. D.
Vit. D is not only important for Ca absorption but also for normal
mineralization of bone. Prolonged deficiency (even if Ca intake is more than
normal) leads to following defects (but this is to be remembered that these
malformations can result either due to vit. D or calcium deficiency or both)
In adults this is demineralization and loss of bone tissue leading to fragile
bones and hence fracture are common in old age.
In this condition bones are fragile with regions of deformable, uncalcified
During growth deficiency of Ca or Vit. D causes failure of
calcification of cartilage and thickening of growth plates. Poorly
calcified cartilage is partially absorbed by osteoclasts, osteoid
fails to ossify specially in metaphysial regions and as the child
starts walking the body weight and gravitational traction
deforms the bones. Another sign is compression of costal
cartilages, leading to "pigeon chest".
II. Vitamin C
This is essential for normal collagen and proteoglycans
synthesis. In deficiency, growth plates thin out, ossification
almost ceases and whatsoever bone is formed is thin and
fragile. Fracture healing is also delayed. Scurvy is the name
given to gross deficiency of this vitamin.
III. Vitamin A:
Correct deposition and removal of bone, both are dependant
upon this vitamin. Deficiency retards growth specially at base of
skull, with resultant narrowing of foramina, at times to the
extent that there is pressure atrophy (degeneration) of emerging
cranial nerves. Cranial cavity and vertebral canal fail to expand
with CNS growth, impairing nervous functions. (Recall small
headed imbecile beggars seen at times in streets.
Excess this vitamin leads to erosion of growth plates which may
be completely lost with cessation of longitudinal growth.
iii) PHOSPHATES, PROTEINS AND GENERAL DIET:
Mention has been made of many minerals as trace elements in
bone. Deficiency of trace elements is rare but at times does
occur. Adequate protein intake is of utmost importance for
collagen and other proteins' deposition in the bone, specially the
"essential amino acids" must be taken for normal osteoid
BONE INJURY AND HEALING
When a bone fractures, following four stages are seen in healing process:
– Soft callus formation
– Hard callus formation
First step is characterized by swelling, haematoma formation and pain.
As the dead tissue is absorbed by phagocytes, next stage follows.
Second step is characterized by formation of soft tissue blastema
consisting of glycoproteins, collagen, osteoblasts, fibroblasts and
chondroblasts and the area is highly vascular. Soft callus helps to
immobilize the broken ends.
Third step involves formation of woven bone and its gradual
replacement by lamellar bone.
First 3 steps take 2 - 3 months.
Remodelling starts after that and continues for several years.
SIGNIFICANCE OF PERIOSTEAL SUPPLY:
Role of periosteal supply is clear from the fact that fractures of
lower end of tibia have delayed healing, as no muscle is
attached to lower 1/3 of tibia, hence periosteal vessels are
scanty. (Cunningham's manual of dissection, vol. I)
However all those elements mentioned above for normal bone
formation are operative in bone healing process as well.
First 3 steps take 2 - 3 months.
Remodelling starts after that and continues for several
SIGNIFICANCE OF PERIOSTEAL SUPPLY:
• Role of periosteal supply is clear from the fact that
fractures of lower end of tibia have delayed healing, as
no muscle is attached to lower 1/3 of tibia, hence
periosteal vessels are scanty. (Cunningham's manual of
dissection, vol. I)
• However all those elements mentioned above for
normal bone formation are operative in bone healing
process as well.