Bones and Muscles, Part 2: Bone Development, Growth, and Remodeling

Bones and Muscles, Part 2

Bone Development

Bone development, also known as ossification or osteogenesis, is the process by which bone tissue forms. This process begins in the second month of embryonic development and continues throughout life, with postnatal bone growth occurring until early adulthood.

• Ossification (osteogenesis): Formation of bone tissue from precursor cells.

• Initiation: Begins in month 2 of development.

• Postnatal growth: Continues until early adulthood.

• Lifelong process: Bone remodeling and repair persist throughout life.

Formation of the Bony Skeleton

The bony skeleton forms from fibrous membranes and hyaline cartilage during embryonic development. Two main types of ossification are involved: endochondral and intramembranous.

• Endochondral ossification: Most bones (except clavicles) form by replacing hyaline cartilage.

• Intramembranous ossification: Forms flat bones such as cranial bones and clavicles from mesenchymal cells.

Endochondral Ossification

• Occurs in all bones except the clavicles.

• Begins in the second month of development.

• Uses hyaline cartilage as a model for bone construction.

• Primary ossification center forms in the center of the shaft.

Steps in Endochondral Ossification

1. Bones form from hyaline cartilage models.

2. Cartilage calcifies and develops cavities.

3. Periosteal bud invades cavities, bringing blood vessels, nerves, and osteogenic cells.

4. Diaphysis elongates and medullary cavity forms.

5. Epiphyses ossify; cartilage remains only at epiphyseal plates and articular surfaces.

Intramembranous Ossification

• Formed by mesenchymal cells (a type of stem cell).

• Produces flat bones: parietal, occipital, temporal, and clavicle.

Steps in Intramembranous Ossification

1. Ossification centers are formed.

2. Osteoid is secreted and calcifies.

3. Woven bone and periosteum form.

4. Lamellar bone replaces woven bone; red marrow appears.

Postnatal Bone Growth

After birth, bones continue to grow in length and thickness. Long bones grow lengthwise due to activity at the epiphyseal plate, while bones increase in thickness through appositional growth.

• Epiphyseal plate: Maintains constant thickness during growth.

• Appositional growth: Increases bone thickness.

• Bones stop growing during adolescence (except some facial bones).

Growth in Length of Long Bones

Long bone growth occurs at the epiphyseal plate, which consists of several zones with distinct cellular activities.

• Interstitial growth: Growth from within the cartilage.

• Epiphyseal plate zones (from epiphysis to diaphysis):

Zone	Main Activity
Resting (quiescent) zone	Relatively inactive; reserve cartilage
Proliferation (growth) zone	Rapid cell division; new cells move upward
Hypertrophic zone	Older chondrocytes enlarge; cartilage lacunae erode
Calcification zone	Matrix calcifies; chondrocytes die
Ossification (osteogenic) zone	Osteoblasts deposit new bone; medullary cavity enlarges

Growth in Length: Stages

1. Chondrocytes divide and enlarge in the proliferation zone.

2. Older chondrocytes in the hypertrophic zone enlarge and erode.

3. Matrix calcifies in the calcification zone; chondrocytes die.

4. Osteoblasts deposit new bone in the ossification zone.

Stopping of Lengthening

• Near end of adolescence, chondroblasts divide less often.

• Epiphyseal plate thins, then is replaced by bone (epiphyseal closure).

• Bone lengthening ceases: females ~18 years, males ~21 years.

Growth in Width (Thickness) of Bones

Bones grow in thickness through appositional growth, which involves the addition of new bone tissue to the surface.

• Osteoblasts beneath the periosteum secrete bone matrix on external bone surface.

• Osteoclasts remove bone on endosteal surface.

• Usually more bone is built than broken down, resulting in thicker, stronger bones.

• Bone thickening responds to increased stress from muscle activity or added weight.

Hormonal Regulation of Bone Growth

Bone growth is regulated by several hormones, which coordinate the timing and rate of growth and closure of growth plates.

• Growth hormone: Most important for stimulating epiphyseal plate activity.

• Thyroid hormone: Modulates activity of growth hormone.

• Sex hormones: Testosterone (males) and estrogen (females) promote growth spurts and closure of epiphyseal plates at puberty.

• Excesses or deficits of these hormones can cause abnormal skeletal growth.

Bone Remodeling

Bone remodeling is a continuous process involving bone deposit and bone resorption, which maintains bone strength and mineral homeostasis.

• About 5-7% of bone mass is recycled each week.

• Spongy bone replaced every 3-4 years; compact bone every 10 years.

• Bone remodeling: Consists of both bone deposit (by osteoblasts) and bone resorption (by osteoclasts).

• Occurs at surfaces of both periosteum and endosteum.

Bone Deposit and Calcification

• New bone matrix is deposited by osteoblasts.

• Osteoid seam: Band of unmineralized bone matrix marking new bone formation.

• Calcification front: Abrupt transition between osteoid seam and older mineralized bone.

Summary Table: Types of Ossification

Type	Location	Process	Examples
Endochondral	Most bones (except clavicle)	Replaces hyaline cartilage	Long bones, vertebrae
Intramembranous	Flat bones of skull, clavicle	Forms from mesenchymal cells	Parietal, occipital, temporal bones

Key Terms and Definitions

• Ossification: Process of bone tissue formation.

• Epiphyseal plate: Growth plate in long bones where lengthening occurs.

• Osteoblast: Bone-forming cell.

• Osteoclast: Bone-resorbing cell.

• Periosteum: Membrane covering outer surface of bone.

• Endosteum: Membrane lining inner surface of bone.

• Mesenchymal cell: Stem cell that can differentiate into bone, cartilage, or other tissues.

Relevant Equations

• Bone mass turnover rate: $\text{Bone mass replaced per year} = \frac{\text{Total bone mass} \times \text{percentage replaced}}{100}$

Example: If total bone mass is 5 kg and 7% is replaced each week, then weekly replacement is $5 \times 0.07 = 0.35$ kg.

Additional info: Some details about the cellular and molecular mechanisms of ossification and bone remodeling have been inferred for completeness.