Chapter 6 - Bone Formation

Perichondrium

mesenchymal covering that develops around the cartilage model

Bone formation (4 Principle Situations)

(1) the initial formation of bones in an embryo and fetus (2) the growth of bones during infancy, childhood, and adolescence until their adult sizes are reached (3) the remodeling of bone (replacement of old bone by new bone tissue throughout life) (4) the repair of fractures (breaks in bones) throughout life.

Steps in Endochondrial Ossification

1. Development of Cartilage model 2. Growth of Cartilage model 3. Development of Ossification Center 4. Development of Medullary (marrow) 5. Development of Secondary Centers 6. Formation of Articular Cartilage

Steps in Intramembranous Ossification

1. Development of Ossification Center 2. Calcification 3. Formation of Trabeculae 4. Development of Periosteum

Development of the secondary ossification centers (Endochondrial)

Fifth Step; these occur in the epiphyses of the bone; When branches of the epiphyseal artery enter the epiphyses, secondary ossification centers develop, usually around the time of birth; spongy bone remains in interior of epiphyses; proceeds outward to bone surface

Development of Cartilage model (Endochondrial)

First step; At site of bone formation, specific chemical messages cause the mesenchymal cells to crowd together in the general shape of the future bone, and then develop into chondroblasts. The chondroblasts secrete cartilage extracellular matrix, producing a cartilage model consisting of hyaline cartilage. Perichondrium develops around the cartilage model.

Growth of Cartilage Model (Endochondrial)

Second step; growth occurs by cell division of chondrocytes; Interstial/Appositional growth

Calcification (Intramembranous)

Second step; the secretion of extracellular matrix stops; osteocytes, lie in lacunae and extend their narrow cytoplasmic processes into canaliculi that radiate in all directions. Within a few days, calcium and other mineral salts are deposited and the extracellular matrix hardens or calcifies.

Primary Ossification Center

a region where bone tissue will replace most of the cartilage; spreads from this central location toward both ends of the cartilage model.

Intramembranous ossification

bone forms directly within mesenchyme, which arranged in sheet-like layers that resemble membranes; simpler of the two methods + Forms: flat bones of the skull, most of the facial bones, mandible (lower jawbone), and the medial part of the clavicle (collar bone)

Endochondral ossification

bone forms within hyaline cartilage that develops from mesenchyme; Ex.) replaces cartilages of embryonic skull

Formation of articular cartilage and the epiphyseal (growth) plate (Endochondrial)

both structures consist of hyaline cartilage; The hyaline cartilage that covers the epiphyses becomes the articular cartilage; epiphyseal (growth) plate - responsible for lengthwise growth

Cartilage model

consists of hyaline cartilage

Intramembranous & Endochondral ossification

involve the replacement of a preexisting connective tissue with bone, do not lead to differences in the structure of mature bones, but are simply different methods of bone development

Periosteum

known as this once perichondrium starts to form bone

Ossification

the process by which bone forms

Development of Ossification center (Intramembranous)

First step; At sites of bone development, specific chemical messages cause the mesenchymal cells to cluster together and differentiate, first into osteogenic cells and then into osteoblasts; ossification center; Osteoblasts secrete the organic extracellular matrix of bone until they are surrounded by it.

Development of the medullary (marrow) cavity (Endochondrial)

Fourth Step; bone breakdown by osteoclasts forms the medullary cavity; as the primary ossification center grows toward the ends of the bone, osteoclasts break down some of the newly formed spongy bone trabecular; most of the wall of the diaphysis is replaced by compact bone.

Development of the Periosteum (Intramembranous)

Fourth step; the mesenchyme at the periphery of the bone condenses and develops into the periosteum. Eventually, a thin layer of compact bone replaces the surface layers of the spongy bone, but spongy bone remains in the center. Much of the newly formed bone is remodeled as the bone is transformed into its adult size and shape.

Formation of Trabeculae (Intramembranous)

Third step; As the bone extracellular matrix forms, it develops into this that fuse with one another to form spongy bone around the network of blood vessels in the tissue. CT that is associated with the blood vessels in this differentiates into red bone marrow.

Development of primary ossification center (Endochondrial)

Third step; in this region of the diaphysis, bone tissue replaced most of the cartilage; proceeds inward from external surface; periosteum; primary ossification center

Interstitial (endogenous) growth

growth from within; results in an increase in length

Appositional (exogenous) growth

growth of the outer surface