Bone and cartilage formation - ossification Flashcards
What are the 2 mechanishms of bone formation?
- Endochondral
- Intramembranous
What differentiates endochondral and intramembranous ossification?
Endochondral bone formation takes place when cartilage is replaced by
bone.
Intramembranous bone formation occurs directly within mesenchyme
When does bone formation occur?
- the initial formation of bones in an embryo and foetus
- the growth of bones during infancy, childhood, and adolescence until their adult sizes are reached
- the remodelling of bone
- the repair of fractures throughout life.
Which bones are formed through intramembranous ossification?
Intramembranous ossification is the simpler of the two methods of
bone formation.
• The flat bones of the skull, most of the facial bones, mandible, and the
medial part of the clavicle are formed in this way.
• Also, the “soft spots” that help the foetal skull pass through the birth
canal later harden as they undergo intramembranous ossification,
which occurs as follows
Name the 4 stages of intramembranous ossification
- Development of the ossification centre
- Calcification
- Formation of trableculae
- Development of the periosteum
Explain ‘development of the ossification centre’?
At the site where the bone will develop, specific chemical messages cause the mesenchymal cells to cluster together and differentiate, first into osteogenic cells and then into osteoblasts.
The site of such a cluster is called an ossification
centre.
Osteoblasts secrete the organic extracellular matrix of bone
until they are surrounded by it.
Explain ‘calcification’
the secretion of extracellular matrix stops, and the
cells, now called osteocytes, lie in lacunae and extend their narrow
cytoplasmic processes into canaliculi that radiate in all directions.
Within a few days, calcium and other mineral salts are deposited and
the extracellular matrix hardens or calcifies (calcification)
Explain ‘formation of trabelculae’
As the bone extracellular matrix forms, it
develops into trabeculae that fuse with one another to form spongy
bone around the network of blood vessels in the tissue. Connective
tissue that is associated with the blood vessels in the trabeculae
differentiates into red bone marrow.
Explain ‘development of the periosteum’
In conjunction with the formation of trabeculae, the mesenchyme condenses at the periphery of the bone and develops into the periosteum.
Eventually, a thin layer of compact bone replaces the surface layers of the spongy bone, but spongy bone remains in the centre. Much of the newly formed bone is remodelled (destroyed and reformed) as the bone is transformed into its adult size
and shape.
What are the stages of endochondral ossification?
- Development of the cartilage model
- Growth of cartilage model
- Development of the primary ossification centre
- Devlopment of the medullary (marrow) cavity
- Development of the secondary ossification centres
- Formation of articular cartilage and epiphyseal (growth) plate
Explain ‘development of the cartilage model’
At the site where the bone is going to form, specific chemical messages cause the mesenchymal cells to crowd together in the general shape of the future bone, and then develop into chondroblasts.
The chondroblasts secrete cartilage
extracellular matrix, producing a cartilage model consisting of hyaline
cartilage. A covering called the perichondrium develops around the
cartilage model.
Explain ‘growth of the cartilage model’
Once chondroblasts become deeply
buried in the cartilage extracellular matrix, they are called
chondrocytes.
The cartilage model grows in length by continual cell
division of chondrocytes, accompanied by further secretion of the
cartilage extracellular matrix.
This type of cartilaginous growth, called
interstitial (endogenous) growth (growth from within), results in an
increase in length.
In contrast, growth of the cartilage in thickness is
due mainly to the deposition of extracellular matrix material on the
cartilage surface of the model by new chondroblasts that develop
from the perichondrium. This process is called appositional
(exogenous) growth.
As the cartilage model continues to grow,
chondrocytes in its mid-region hypertrophy (increase in size) and the
surrounding cartilage extracellular matrix begins to calcify.
Other
chondrocytes within the calcifying cartilage die because nutrients can
no longer diffuse quickly enough through the extracellular matrix. As
these chondrocytes die, the spaces left behind by dead chondrocytes
merge into small cavities called lacunae.
Explain ‘development of the primary ossification centre’
Primary ossification proceeds inward from the external surface of the bone. A nutrient artery penetrates the perichondrium and the calcifying cartilage model
through a nutrient foramen in the mid-region of the cartilage model,
stimulating osteogenic cells in the perichondrium to differentiate into
osteoblasts.
Once the perichondrium starts to form bone, it is known
as the periosteum. Near the middle of the model, periosteal capillaries
grow into the disintegrating calcified cartilage, inducing growth of a
primary ossification centre, a region where bone tissue will replace
most of the cartilage.
Osteoblasts then begin to deposit bone
extracellular matrix over the remnants of calcified cartilage, forming
spongy bone trabeculae. Primary ossification spreads from this
central location toward both ends of the cartilage model
Explain ‘development of the medullary (marrow) cavity’
As the primary ossification
centre grows toward the ends of the bone, osteoclasts break down
some of the newly formed spongy bone trabeculae. This activity
leaves a cavity, the medullary (marrow) cavity, in the diaphysis (shaft).
Eventually, most of the wall of the diaphysis is replaced by compact
bone.
Explain ‘development of the secondary ossification centres’
When branches of the
epiphyseal artery enter the epiphyses, secondary ossification centres
develop, usually around the time of birth. Bone formation is similar to
what occurs in primary ossification centres.
However, in the
secondary ossification centres spongy bone remains in the interior of
the epiphyses (no medullary cavities are formed here). In contrast to
primary ossification, secondary ossification proceeds outward from
the centre of the epiphysis toward the outer surface of the bone.
