Intramembranous Ossification Steps


1.    Starting with connective tissue mesenchyme cells. …differentiate into osteoblasts within a connective tissue (near arterioles), form ossification centers

2.    Clusters of osteoblasts form osteoid that becomes mineralized

3.    Small struts of bone (trabeculae) radiate out from the ossification centers to join with neighboring bone struts

4.     Trabeculae just deep to periosteum thicken, becoming compact lamellar bone (the rest stays trabecular--> spongy bone- diploë)


Examples of bones formed by this process:  lower jaw (mandible), clavicle, roof of the skull



Endochondral Ossification Steps


1.    Starting with a cartilage model. …Chondrocytes enlarge and calcify creating primary ossification centers

2.    Blood vessels invade the perichondrium, mesenchymal cells become osteoblasts, the perichondrium is converted into a periosteum and the inner layer produces bone

3.    Osteoblasts secrete osteiod, creating a bone collar, cartilage in the middle calcifies and deteriorates

4. Blood and lymph vessels, nerves and osteoblasts and osteoclasts invade, erode calcified cartilage and replace with spongy bone

5.    Secondary ossification in epiphyses, osteoclasts create a marrow cavity and osteoblasts created more spongy bone


Examples of bones formed by this process:  most long bones of the body, short, irregular.



Bone Repair involves these steps:


1.    A fracture hematoma forms

2.    Fibrocartilage forms a collar, granulation tissue reorganizes blood flow

    a. Cells of the periosteum and endosteum divide rapidly

    b. External callus encircles the bone at the level of the fracture and an internal callus organizes within the marrow cavity

    c. Fibroblasts make collagen fibers to span and reconnect bone ends

    d. Cells at the center of the callus differentiate into chondrocytes and cells at the edges of the callus differentiate into osteoblasts

3.    Osteoblasts replace the central cartilages of spongy bone and turn fibrocartilagenous  callus to bony callus

4.    Osteoclasts and osteoblasts remodel the region of the injury