Bone Development, Growth, and Remodeling: Study Notes for Anatomy & Physiology

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.

• Begins: In month 2 of development.

• Postnatal growth: Continues until early adulthood.

• Lifelong: 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 fibrous membranes.

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.

• Begins at the primary ossification center in the center of the shaft.

• Secondary ossification centers appear in the epiphyses.

Steps in Endochondral Ossification

1. Bones form from hyaline cartilage models.

2. Cartilage calcifies and develops cavities.

3. Periosteal bud invades cavities, leading to formation of spongy bone.

4. Diaphysis elongates and medullary cavity forms.

5. Epiphyses ossify; hyaline cartilage remains only in epiphyseal plates and articular cartilage.

Intramembranous Ossification

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

• Originates from mesenchymal cells (a type of stem cell).

1. Ossification centers are formed.

2. Osteoid is secreted and calcifies.

3. Woven bone is formed.

4. Lamellar bone replaces woven bone, and 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

Growth in length occurs at the epiphyseal plate, which consists of several zones. Interstitial growth maintains the thickness of the plate as cartilage is replaced by bone.

• Zones of the epiphyseal plate:

  1. Resting (quiescent) zone: Relatively inactive; reserve cartilage.

  2. Proliferation (growth) zone: Rapidly dividing cells; new cells move upward.

  3. Hypertrophic zone: Older chondrocytes enlarge; cartilage lacunae erode.

  4. Calcification zone: Surrounding cartilage matrix calcifies; chondrocytes die.

  5. Ossification (osteogenic) zone: Chondrocyte deterioration leaves long spicules of calcified cartilage; osteoblasts replace with spongy bone.

Growth in Length: Stopping and Remodeling

• Lengthening stops at end of adolescence when chondroblasts divide less often.

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

• Females: closure around 18 years; males: around 21 years.

Growth in Width (Thickness) of Bones

Bones grow in thickness through appositional growth, which occurs throughout life. Osteoblasts in the periosteum secrete bone matrix, while osteoclasts remove bone on the endosteal surface.

• Growth in width is stimulated by increased stress from muscle activity or added weight.

• Usually more bone is built than broken down, leading to thicker, stronger bones.

Hormonal Regulation of Bone Growth

Bone growth is regulated by several hormones, which coordinate the activity of growth and closure of the epiphyseal plate.

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

• Thyroid hormone: Modulates activity of growth hormone.

• Testosterone (males) and estrogen (females): Promote growth spurts and closure of epiphyseal plate at puberty.

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

Bone Remodeling

Bone remodeling is a continuous process where old bone is replaced by new bone tissue. About 5-7% of bone mass is recycled each week.

• Spongy bone: Replaced every 3-4 years.

• Compact bone: Replaced every 10 years.

• Bone remodeling: Consists of bone deposit and bone resorption.

• Occurs at surfaces of both periosteum and endosteum.

• Remodeling units: packets of adjacent osteoblasts and osteoclasts coordinate the process.

Bone Deposit and Calcification

• New bone matrix is deposited by osteoblasts.

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

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

Summary Table: Types of Ossification

Type	Location	Process	Examples
Endochondral Ossification	Most bones (except clavicles)	Bone replaces hyaline cartilage	Long bones, vertebrae
Intramembranous Ossification	Flat bones of skull, clavicles	Bone develops from fibrous membrane	Parietal, occipital, temporal bones, clavicle

Key Equations and Terms

• Ossification: $\text{Mesenchymal cells} \rightarrow \text{Osteoblasts} \rightarrow \text{Bone tissue}$

• Bone Remodeling Rate: $\text{Bone mass replaced per week} \approx 5\%$

Example: During adolescence, increased growth hormone and sex hormones stimulate rapid bone growth, leading to increased height and bone mass.

Additional info: The notes have been expanded to include definitions, processes, and examples for clarity and completeness.