Bone development

Osteogenesis/Ossification

The process of forming bone tissue

2 types of osteogenesis/ossification

Intramembranous ossification and Endochondral ossification

What is intramembranous ossification replacing

Forms bone directly within a fibrous connective tissue membrane formed by mesenchymal cells

Step 1 of intramembranous ossification

ossification centers appear in the fibrous connective tissue

Step 1 of intramembranous ossification cellularly

Centrally located mesenchymal cells differentiate into osteoblasts and form trabeculae of spongey bone

Step 2 of intramembranous ossification

Osteoid secreted within fibrous membrane calcifies

Step 2 of intramembranous ossification cellularly

Osteoblasts secrete osteoid which then calcifies. Some osteoblasts become trapped and convert to osteocytes

Step 3 of intramembranous ossification

Spongy bone and periosteum form

Step 3 of intramembranous ossification cellularly

Blood vessels enter the tissue. Mesenchyme condenses into periosteum. Osteoid is secreted around the blood vessels trabecular/woven bone formation.

Step 4 of intramembranous ossification

Compact bone replaced woven bone and red marrow appears

Step 4 of intramembranous ossification cellularly

Spongy bone just deep to periosteum thickens and forms compact bone. Spongy bone internally and vascular tissue produces red marrow that persists into adulthood.

What is endochondral ossification replacing

Forms bone by replacing an existing hyaline cartilage model

Examples for intramembranous ossification

Flat bones of cranium and clavicles

Examples for endochondral ossification

All bones of the post cranial skeleton except the clavicle

Step 1 of endochondral ossification

Bone collar forms around the diaphysis of the hyaline cartilage model

Step 1 of endochondral ossification cellularly

Osteoblasts in the osteogenic layer of the periosteum make Osteoid surrounding the hyaline model (primary ossification center)

Step 2 of endochondral ossification

Cartilage in center of diaphysis calcifies and develops cavities

Step 2 of endochondral ossification cellularly

Chondrocytes in diaphysis start to hypertrophy which signals surrounding cartilage to calcify. Chondrocytes become isolated from nutrients and die- leaving cavities. Ends of model continue to elongate.

Step 3 of endochondral ossification

Periosteum bud invades internal cavity- begins to form spongy bone

Step 3 of endochondral ossification cellularly

Periosteal bud- nutrient artery, vein, nerve fibers, red marrow elements, osteogenic cells, and osteoclasts. Osteoblasts form Osteoid around remaining cartilage forming spongy bone

Step 4 of endochondral ossification

Diaphysis elongates and medullary cavity forms

Step 4 of endochondral ossification cellularly

Osteoclast break down newly formed spongy bone and clear medullary cavity. Hyaline cartilage model at epiphysis continues to grow and diaphysis continues to elongate. Ossification chases the cartilage. Some secondary ossification centers start to form

Step 5 of endochondral ossification

Epiphyses ossify

Step 5 of endochondral ossification cellularly

At birth most epiphyses are still cartilage models- shortly after birth they become secondary ossification centers. Process is similar to primary ossification centers except no medullary cavity forms. Eventually the epiphyseal plate ossifies leaving behind the epiphyseal line