Bone Structure and Growth: Key Concepts in Anatomy & Physiology

Bone Structure and Growth

Trabeculae: Structure and Function

The trabeculae are the lattice-like, branching structures found within spongy bone (also called cancellous bone). They play a crucial role in bone strength and metabolic activity.

• Function: Trabeculae provide structural support and help distribute mechanical stress throughout the bone. Their arrangement allows bones to withstand forces from multiple directions.

• Composition: Made of lamellae containing osteocytes within lacunae, interconnected by canaliculi.

• Example: The trabeculae in the vertebrae help resist compression forces during movement.

Bone Development: Intramembranous vs. Endochondral Ossification

Bone formation occurs through two primary processes: intramembranous ossification and endochondral ossification. Each process is responsible for the development of specific bone types.

• Intramembranous Ossification: Direct formation of bone from mesenchymal tissue without a cartilage precursor.

  • Bones formed: Flat bones such as the skull, mandible, and clavicle.

• Endochondral Ossification: Bone develops by replacing hyaline cartilage.

  • Bones formed: Most bones of the body, especially long bones like the femur and humerus.

• Comparison Table:

Ossification Type	Process	Bones Formed
Intramembranous	Mesenchymal cells differentiate directly into osteoblasts	Flat bones (skull, clavicle)
Endochondral	Cartilage model replaced by bone	Long bones, most other bones

Long Bone Lengthening: Cellular Mechanisms

Long bones lengthen primarily through the activity of specific cell types at the epiphyseal plate (growth plate).

• Chondrocytes: Cartilage cells that proliferate and enlarge, forming new cartilage.

• Osteoblasts: Bone-forming cells that replace cartilage with bone tissue.

• Process: Chondrocytes in the epiphyseal plate divide and enlarge, then die and are replaced by osteoblasts that deposit bone matrix.

Zones of the Epiphyseal Plate

The epiphyseal plate consists of distinct zones, each with specific cellular composition and function, facilitating bone growth.

• Zone of Resting Cartilage: Small, inactive chondrocytes; anchors plate to epiphysis.

• Zone of Proliferation: Rapidly dividing chondrocytes; responsible for lengthening.

• Zone of Hypertrophy: Enlarged chondrocytes; cells mature and prepare for calcification.

• Zone of Calcification: Chondrocytes die, matrix becomes calcified.

• Zone of Ossification: Osteoblasts deposit new bone matrix.

Mechanisms of Bone Growth: Longitudinal vs. Appositional

Bone growth occurs in two main ways: longitudinal (lengthwise) and appositional (widthwise).

• Longitudinal Growth: Occurs at the epiphyseal plate; increases bone length via chondrocyte proliferation and subsequent ossification.

• Appositional Growth: Occurs at the periosteum; increases bone diameter by adding new bone tissue to the surface via osteoblasts.

• Comparison Table:

Growth Type	Location	Cells Involved	Result
Longitudinal	Epiphyseal plate	Chondrocytes, osteoblasts	Bone length increases
Appositional	Periosteum	Osteoblasts, osteoclasts	Bone diameter increases

Hormonal Regulation of Bone Growth

Bone growth is regulated by several hormones, with growth hormone (GH) playing a central role.

• Growth Hormone (GH): Secreted by the anterior pituitary gland; stimulates growth of bone and cartilage.

• Target Cells: GH stimulates chondrocytes in the epiphyseal plate for longitudinal growth and osteoblasts in the periosteum for appositional growth.

• Additional Hormones: Thyroid hormone and sex hormones (estrogen, testosterone) also influence bone growth.

• Equation: $\text{Bone Growth Rate} \propto \text{GH Concentration} \times \text{Chondrocyte Activity}$

Additional info: Sex hormones accelerate bone growth during puberty and eventually lead to closure of the epiphyseal plate, ending longitudinal growth.