Skeletal System - 6.4 BONE DEVELOPMENT INC

Bone formation in the fetus follows two processes:

Intramembranous ossification and endochondral ossification.

Intramembranous ossification

Starts within embryonic connective tissue membranes.

Endochondral ossification

Starts with a cartilage model.

Intramembranous ossification

Many skull bones, part of the mandible (lower jaw), and the diaphyses of the clavicles (collarbones) develop by.

Centers of ossification

The locations in the membrane where intramembranous ossification begins.

Centers of ossification

Expand to form a bone by gradually ossifying the membrane.

Fontanels or soft spots

Are the larger, membrane-covered spaces between the developing skull bones that have not yet been ossified.

Osteoblasts activity

Osteoblasts within the center of ossification begin producing bone matrix.

Spongy bone formation

Additional osteoblasts gather on the surfaces of the trabeculae and produce more bone, thereby causing the trabeculae to become larger and longer.

Spongy bone

Forms as the trabeculae join together.

Compact bone formation

Cells within the spaces of the spongy bone specialize to form red bone marrow, and cells surrounding the developing bone specialize to form the periosteum.

The end products of intramembranous bone formation

Are bones with outer compact bone surfaces and spongy centers.

The formation of cartilage

Begins at approximately the end of the fourth week of embryonic development.

Endochondral ossification

Bones of the base of the skull, part of the mandible, the epiphyses of the clavicles, and most of the remaining skeletal system develops.

Cartilage bone formation

A hyaline cartilage model is produced by cells called chondrocytes and has the approximate shape of the future bone.

Hyaline cartilage model

Is produced by cells called chondrocytes and has the approximate shape of the future bone.

Bone collar formation

The osteoblasts produce compact bone on the surface of the cartilage model.

Interstitial and appositional cartilage growth

The cartilage model increases in size as a result.

Interstitial growth

Occurs as a result of continuous cell division of the chondrocytes within the cartilage model.

Appositional growth

Occurs as a result of cell division at the surface of the cartilage model, and under the perichondrium.