Mineralization

Although a continuous process, mineralization has
been divided into primary and secondary phases. Primary
mineralization lasts several days and is responsible
for 70% of total mineralization, while secondary
mineralization occurs over several following months
and completes the total process. The primary phase
is under the control of chondrocytes and osteoblasts,
whereas the secondary phase is most likely governed
by the chemical composition of the fl uid surrounding
the matrix.
Mineral crystals appear to be formed initially in
extracellular matrix vesicles, which have been found
at sites of rapid calcifi cation, namely embryonic bone,
calcifying epiphyseal growth cartilage and healing
fractures. The vesicles are formed by budding from the
plasma membranes of chondrocytes and osteoblasts,
and are active at the commencement of primary mineralization.
The newly released matrix vesicles undergo
maturation, exhibiting increased alkaline phosphatase
and phospholipase A2 activities, and increasing in diameter
as discrete hydroxyapatite crystals form on the
inner leafl et of the vesicle membrane. Rupture of the
membrane releases the crystals which adhere to each
other and serve as foci for continued crystal deposition
in the extracellular matrix. The vesicles also release
proteases which facilitate matrix calcifi cation by
degrading surrounding proteoglycan aggregates.
3.8.5 Bone formation: ossifi cation
Ossifi cation begins around the sixth or seventh week
of embryonic life and continues throughout adulthood.
There are two types of ossifi cation, which differ
by the initial material on which the bone is formed.
Intramembranous ossifi cation refers to the formation
of bone directly within mesenchymal connective tissue.
Endochondral ossifi cation refers to the replacement
of cartilage by bone in a miniature cartilage
‘model’. In both types of ossifi cation, woven bone is
formed initially.
Intramembranous ossifi cation
The frontal and parietal bones of the skull are examples
of bones that develop by intramembranous ossifi
cation. This process also contributes to the growth
of short bones and the thickening of long bones.
During the eighth week of life in the human fetus,
an area of mesenchyme becomes richly vascularized.
This prompts mesenchymal cells to differentiate into
a variety of cell types, including osteoblasts. The
osteoblasts secrete the osteoid and when they are
completely surrounded by it, they become osteocytes.
Within a few days, minute crystals of calcium salts are
deposited in an orderly fashion upon collagen fi brils,
and the matrix calcifi es. Lacunae and canaliculi are
formed around the osteocytes and their cytoplasmic
processes. As the bone matrix forms, it develops into
trabeculae that fuse with one another to create the
open latticework appearance of cancellous bone.
The spaces between trabeculae fi ll with vascularized
connective tissue which differentiates into red bone
marrow. The mesenchyme located near the endosteal
surface transforms into the endosteum and the
remaining envelope of connective tissue at the periosteal
surface develops into the periosteum. Eventually,
most surface layers of the cancellous bone are replaced
by cortical bone, but cancellous bone remains in the
centre of the bone.
Endochondral ossifi cation
Most bones of the body are formed by endochondral
ossifi cation. This is a slow process which is not
achieved until the bone has reached its full size and
Background physiology and functional anatomy 59
growth has ceased. The process is best observed in a
long bone.
At the site where the bone is going to form, mesenchymal
cells cluster together in the shape of the future
bone. The mesenchymal cells differentiate into chondroblasts
that produce the cartilage model. In addition,
a membrane called the perichondrium develops
around the cartilage model. The cartilage model
grows in length by continual cell division of chondrocytes
accompanied by further secretion of cartilage
matrix by the daughter cells. This pattern of growth
from within is called interstitial growth. The increase
in thickness of the cartilage model is due mainly to
appositional growth. This refers to the addition of
more matrix to its periphery by new chondroblasts
that develop from the perichondrium.
As the cartilage model continues to grow, chondrocytes
in its mid-region hypertrophy, probably because
they accumulate glycogen for ATP production. Some
hypertrophied cells burst, releasing their contents,
and changing the pH of the matrix. The resultant
chemical changes trigger calcifi cation. Once the
cartilage becomes calcifi ed, other chondrocytes die
because nutrients no longer diffuse quickly enough
through the matrix. The lacunae of the cells that have
died are now empty, and the thin partitions between
them break down forming small cavities.
In the meantime, a nutrient artery penetrates the
perichondrium and then the developing bone through
a hole in the mid-region of the model. This stimulates
osteoprogenitor cells in the perichondrium to differentiate
into osteoblasts. The cells lay down a thin sheet
of cortical bone under the perichondrium called the
periosteal bone collar. Once the perichondium starts
to form bone, it is known as the periosteum.
Near the mid-region of the model, the periosteum
sends out osteogenic buds into the disintegrating calcifi
ed cartilage through holes made by osteoclasts in
the bone collar. The buds, containing blood capillaries
and osteoprogenitor cells, enter the spaces left by the
dead and degenerating chondrocytes. The invading
osteoprogenitor cells proliferate and develop into
osteoblasts, which begin to deposit bone matrix over
the remnants of calcifi ed cartilage, forming cancellous
bone. This region of bone deposition is called the
primary ossifi cation centre, in which ossifi cation proceeds
inward from the external surface of the model.
As the ossifi cation centre expands towards the ends of
the model, osteoclasts break down the newly formed
cancellous bone. This bone-resorbing activity leaves a
hollow cavity, the medullary cavity, in the core of the
diaphysis along its length. The cavity then fi lls with
red bone marrow. The longitudinal expansion of the
primary ossifi cation centre is accompanied by a widening
of the periosteal bone collar in the same direction.
The bone collar also thickens, providing support
to the central zone of resorbing cartilage prior to its
replacement by bone.
At about the time of birth, epiphyseal arteries enter
the epiphyses and secondary ossifi cation centres develop.
Osteoprogenitor cells invade the area via vascular
osteogenic buds originating from the diaphysis.
Cartilage removal and bone matrix deposition follow.
Bone formation is similar to that in the primary ossifi
cation centre. One difference, however, is that no
medullary cavities are formed in the epiphyses. Secondary
ossifi cation proceeds outward in all directions
from the centre of the epiphysis until there is almost
complete replacement of cartilage by cancellous bone.
Two regions at each epiphysis, the articular surface
and the epiphyseal plate, do not undergo secondary
ossifi cation. Articular cartilage persists throughout
adult life and, in the absence of a perichondrium, no
equivalent of a bone collar is formed here. The epiphyseal
plate is converted to cancellous bone much later
in life, when growth of the long bone is complete.