Week 5 – Bone Formation 1
Ossification (ossification) – process by which bone forms that occurs in four principal situations:
a. Initial formation of bones in an embryo and fetus
b. The growth of bones during infancy, childhood, and adolescence until their adult sizes are reached
c. The remodeling of bone (replacement of old bone by new bone tissue throughout life)
d. The repair of fractures throughout life
Initial Bone Formation in an Embryo and Fetus
The embryonic “skeleton,” initially composed of mesenchyme, is where cartilage formation and ossification occur
during the 6 week of embryonic development.
Two types of formation – both replace pre-existing CT with bone; no differences in mature bones
1. Intramembranous ossification – bone forms directly within mesenchyme
2. Endochondral ossification – bone forms within hyaline cartilage that develops from mesenchyme
- The simpler process that forms the skull, facial bones, mandible, medial part of the clavicle, and fontanels
ONE–D EVELOPMENT OF THE OSSIFICATION CENTER
- At the site where bone will develop, chemical messages cause the mesenchymal cells to cluster together
and differentiate ossification center
- First into osteogenic cells, then to osteoblasts
- Osteoblasts secrete the organic matrix of bone until they are surrounded by it
- The secretion of matrix stops
- The cells, now osteocytes, lie in lacunae and extend their narrow cytoplasmic processes into caniculi that
radiate in all directions.
- Within a few days, calcium and other mineral salts are deposited
- Matrix calcifies
THREE–F ORMATION OF TRABECULAE
- As the bone matrix forms, it develops into trabeculae that fuse to form spongy bone around the network
of blood vessels in the tissue.
- CT that is associated with the blood vessels in the trabeculae differentiate into red bone marrow.
FOUR–D EVELOPMENT OF THE PERIOSTEUM
- At the same time as trabeculae, the mesenchyme condenses at the periphery of the bone and develops
into the periosteum.
- A thin layer of compact bone eventually replaces the surface layers of spongy bone
- Much of the newly formed bone is remodeled (destroyed and reformed).
The replacement of cartilage by bone; best observed in a long bone Week 5 – Bone Formation 2
ONE–D EVELOPMENT OF THE CARTILAGE MODEL
- At the site where bone will form, chemical messages cause the mesenchymal cells to crowd together in
the general shape of the future bone
- They then develop into chondroblasts
- Chondroblasts secrete cartilage extracellular matrix that produces a cartilage model consisting of hyaline
- Periochondrium (a covering) develops around the cartilage model
TWO–G ROWTH OF THE CARTILAGE MODEL
- When chondroblasts become deeply buried in the cartilage matrix, they’re called chondrocytes.
- The cartilage model grows in length by continual cell division of chondrocytes, accompanied by further
secretion of the cartilage matrix.
o Cartilaginous growth in length interstitial (endogenous) growth
o Growth of cartilage in width is mainly due to deposition of matrix on the cartilage surface of the
model by new chondroblasts that develop from the periochondrium appositional
- As the model continues to grow, chondrocytes in the mid-region hypertrophy (increase in size) and the
surrounding cartilage matrix starts to calcify.
- Other chondrocytes within calcifying cartilage die because nutrients can no longer diffuse quickly enough
through the matrix
- As they die, the spaces left behind by dead chondrocytes merge into small cavities called lacunae
THREE–D EVELOPMENT OF THE PRIMARY OSSIFICATION CENTER
- Primary ossification proceeds INWARD
- Nutrient artery penetrates the perichondrium and the calcifying cartilage model through a nutrient
foramen in the midregion of the cartilage model, stimulating osteogenic cells in the perichondrium to
differentiate into osteoblasts
- Once the perichondrium starts to form bone, it is called the periosteum.
- Near the middle of the model, periosteal capillaries grow into the disintegrating calcified cartilage,
inducing growth of a primary ossification center
o A region where bone tissue will replace most of the cartilage
- Osteoblasts then deposit bone matrix over the remnants of calcified cartilage forms spongy bone
- Primary ossification spreads from this central location toward both ends of the cartilage model
FOUR–D EVELOPMENT OF THE MEDULLARY (MARROW ) CAVITY
- As the primary ossification center grows towards the ends of the bones, osteoclasts break down some of
the newly formed spongy bone trabeculae
- This leaves a cavity, the medullary (marrow) cavity, in the diaphysis.
- Most of the wall of the diaphysis will eventually be replaced by compact bone.
FIVE–D EVELOPMENT OF THE SECONDARY OSSIFICATION CENTERS
- When branches of the epiphyseal artery enter the epiphyses, secondary ossification centers develop,
usually around birth. Week 5 – Bone Formation 3
- Bone formation is similar to what occurs in primary ossification centers, but in the secondary ossification
centers spongy bone remains in the interior of the epiphyses (no medullary cavities are formed here)
- Secondary ossification proceeds OUTWARD
SIX–F ORMATION OF ARTICULAR CARTILAGE AND THE EPIPHYSEAL GROWTH PLATE
- The hyaline cartilage that covers epiphyses becomes articular cartilage
- Prior to adulthood, hyaline cartilage remains between the diaphysis and epiphysis as the epiphyseal
growth plate, the region responsible for the lengthwise growth of long bones.
Bone Growth during Infancy, Childhood, and Adolescence
Bones grow in thickness by appostitional growth