Intramembranous and Endochondral Osteogenesis

Study Notes

Bone formation directly from mesenchymal tissue, called mesenchyme, which forms membranes.
Osteoblasts, bone-forming cells, lay down unmineralized bone matrix called osteoid.
Osteoid quickly mineralizes due to the deposition of calcium phosphate crystals.
Mineralization is facilitated by the enzyme alkaline phosphatase, secreted by osteoblasts.
This method produces membrane bones, like flat skull bones, facial bones, and the clavicle.

A miniature cartilaginous model of the bone is formed first.
Primary center of ossification develops in the center of the cartilaginous model.
Secondary centers of ossification appear later in the bone ends, not simultaneously.
Hyaline cartilage is not converted to bone; it is replaced by osseous tissue.

Bone formation is incomplete at birth.
Intramembranous bones have remaining membrane for growth.
Endochondral bones retain some cartilage, including the epiphyseal cartilage (growth plate) between the diaphysis and epiphysis of long bones.
Cartilage at bone ends allows for lengthening during childhood and early adulthood.
Bone also grows in diameter due to osteoblasts in the periosteum.
Bone tissue is removed from the medullary cavity by osteoclasts to accommodate diameter growth.
Closure of the epiphysis occurs during adulthood when the epiphyseal cartilage stops growing and fuses with the diaphysis.

Long bones ossify according to a rule: the epiphysis with the first-appearing secondary center of ossification fuses with the diaphysis last.
This epiphysis is the growing end.
Nutrient foramina are directed away from the growing end.
For example, the humerus's nutrient foramen runs downwards, the radius and ulna's run upwards, the femur's runs upwards, and the tibia's runs downwards.
This can be remembered with the anatomical proverb: "from the knee I flee, toward the elbow, I go."
The fibula is an exception to this rule.

Located at bone ends transmitting body weight or under pressure during movement.
They are articular and form part of joints.
Examples: head of the femur and humerus.

Non-articular and don't form joints.
Provide attachment sites for tendons and ligaments.
Ossify later than pressure epiphyses.
Examples: greater and lesser trochanters of the femur, greater and lesser tubercles of the humerus.

These represent bones that were once independent in animals but fused in humans.
Example: the coracoid process of the scapula.

Deviations from the normal and not always present.
Examples: the head of the first metacarpal and bases of some other metacarpals.
Normally, metacarpals have one secondary ossification center, but aberrant epiphyses can occur.

Bone development is called osteogenesis or ossification.
Two types of bone formation occur in the embryo: intramembranous osteogenesis and intracartilaginous or endochondral osteogenesis.