Chapter 6.5 - Bone Formation

Intramembranous Ossification

Bone forms directly within mesenchyme, which is arranged in sheetlike layers that resemble membranes.

Zone of Calcified Cartilage

Few cells thick and consists mostly of chondrocytes that are dead because the extracellular matrix around them as calcified.

Minerals needed for bone growth and remodelling

Large amounts of calcium and phosphorus, small amounts of magnesium, fluoride and manganese.

Factors Affecting Bone Growth and Bone Remodelling

Minerals, vitamins and horomones

Zone of Proliferating Cartilage

Slightly larger chondrocytes in this zone are arranged like stacks of coins. Condrocytes undergo interstitial growth as they divide and secrete extracellular matrix. Chondrocytes divide to replace those that die at diaphyseal side of the epiphyseal plate.

Vitamins needed for bone growth and remodelling

Vitamin A (stimulates osteoblast activity), Vitamin C (synthesis for collagen), Vitamin D (increases absorption of calcium from foods), Vitamins K and B12 (synthesis of bone proteins)

Epiphyseal Line

When the epiphyseal cartilage cells stop dividing and bone replaces all remaining cartilage, epiphyseal plate fades leaving this bony structure behind.

Four Zones of the Epiphyseal Plate

Zone of resting cartilage, zone of proliferating cartilage, zone of hypertrophic cartilage and zone of calcified cartilage.

Formation of Trabeculae

As the bone extracellular matrix forms, it develops into trabeculae that fuse with one another to form spongy bone around the network of blood vessels in the tissue. Connective tissue associated with the blood vessels in the trabeculae differentiate into red bone marrow.

Development of the Ossification Center

At the site where the bone will develop, specific chemical messages cause the cells of the mesenchyme to cluster together and differentiate, first into osteoprogenitor cells and then into osteoblasts.

Development of the Medullary (Marrow) Cavity

Bone breakdown by osteoclasts forms the medullary cavity.

Secondary Ossification Center

Bone formation similar to primary ossification center, however Spongy bone remains in the interior of epiphyses (no medullary cavities are formed here). Proceeds growth outward from the center of the epiphysis toward the outer surface of the bone.

Endochondral ossification

Bone forms within hyaline cartilage and develops from mesenchyme. Also the replacement of cartilage by bone.

Appositional (exogenous) Growth

Deposition of extracellular matrix material on the cartilage surface of the model by new chondroblasts that develop from the perichondrium, resulting in growth at the outer surface, increasing cartilage thickness.

Six Stages of Endochondral Ossification

Development of the cartilage model, growth of the cartilage model, development of primary ossification center, development of the medullar (marrow) cavity, development of secondary ossification centers, and formation of articular cartilage and epiphyseal plate

Four stages of Intramembranous Ossification

Development of the ossification enter, calcification, formation of trabeculae and development of the periosteum.

Development of the Cartilage Model

Mesenchymal cells develop into chondroblasts which form the cartilage model.

Primary Ossification Center

Region where bone tissue will replace most of the cartilage. Proceeds growth inward from the external surface of the bone.

Periosteum

Result of perichondrium forming bone.

Bone Deposition

The addition of minerals and collagen fibers to bone by osteoblasts.

Interstitial (endogenous) growth

The cartilage model grows in length by continual cell division of chondrocytes accompanied by further secretion of the cartilage extracellular matrix. This results in an increase in length.

Bone Resorption

the removal of minerals and collagen fibers from bone by osteoclasts

Cartilage model

Cartilage extracellular matrix secreted by chondroblasts consisting of hyaline cartilage.

Perichondrium

Covering that develops around the cartilage model.

Growth of Cartilage Model

Growth occurs by cell division of chondrocytes.

Development of Primary Ossification Center

In this region of the diaphysis bone tissue has replaced most of the cartilage.

4 Principle Situations of Bone Formation

Initial formation of bones in an embryo and fetus, the growth of bones during infancy, childhood and adolescence, the remodelling of bone and the repair of fractures.

Horomones needed for bone growth and remodelling

Insulin-like growth factors, human growth hormone (hGH), thyroid hormones (T3 and T4), insulin, sex hormones

Zone of Hypertrophic Cartilage

Layer consists of large, maturing chondrocytes arranged in columns.

Zone of Resting Cartilage

Layer nearest epiphysis consists of small, scattered chondrocytes. Anchors the epiphyseal plate to the epiphysis of the bone.

Epiphyseal (growth) Plate

Layer of hyaline cartilage in the metaphysis of a growing bone that consists of four zones.

Development of the Periosteum

The mesenchyme condenses at the perphery of the bone and develops into the periosteum.

Bone Remodelling

The ongoing replaement of old bone tissue by new bone tissue.

Ossification / Osteogenesis

The process by which bone forms.

Calcification

The secretion of extracellular matrix stops, and the cells, now called osteocytes, lie in lacunae and extend their narrow ctoplasmic processes into canaliculi that radiate in all directions. Other mineral salts are deposited and the extracellular matrix hardens or calcifies.

Ossification Center

The site where the bone will develop.

Development of Secondary Ossification Centers

These occurs in the epiphyses of the bone.