Human Anatomy: Histology of Bone Structures

Types of Bone Structure

• Macroscopic structure of mature bone includes compact (cortical) and spongy (cancellous) bone.
• Microscopic structure of bone can be classified into lamellar bone characterizing mature bone and woven bone typical of immature bone.
• Lamellar bone features regular alignment of collagen fibers, found in both compact and spongy bone of healthy adults.
• Woven bone displays irregular collagen fiber alignment, present during development and in pathologic conditions such as healing fractures.

Components of Compact Bone

• Compact bone comprises cylindrical units called osteons or Haversian systems.
• Each osteon contains:
  • Haversian canals at the center housing blood vessels and nerves.
  • Concentric lamellae (4-15 layers) surrounding the canal with collagen fibers oriented parallel within lamellae and perpendicular between adjacent lamellae.
  • Osteocytes in lacunae with processes extending through canaliculi.
• Volkmann’s canals connect Haversian canals transversely.
• Periosteum: the outer fibrous layer includes blood vessels and nerves, allowing vascularization through Volkmann’s canals.

Bone Formation Processes

• Two primary methods of bone formation:
  • Intramembranous ossification: Bone develops from mesenchyme, primarily in flat bones.
  • Endochondral ossification: Bone forms from hyaline cartilage, predominant in long, short, and irregular bones.

Intramembranous Bone Formation

• Occurs in flat bones of the skull, beginning in fetal life.
• Involves:
  • Mesenchymal cells condensing into a vascularized membrane-like structure.
  • Differentiation into osteoprogenitor cells, which then become osteoblasts.
  • Osteoblasts secrete and mineralize osteoid, ultimately leading to the formation of osteocytes as they get encapsulated.

Endochondral Bone Formation

• Takes place in long, short, and irregular bones, starting in fetal development.
• Mesenchymal cells differentiate into chondroblasts forming a hyaline cartilage model, which is later replaced by bone.
• Key regions in the epiphyseal growth plate include:
  • Zone of Proliferating Cartilage: Rapidly dividing chondrocytes.
  • Zone of Maturation and Hypertrophy: Enlarged chondrocytes.
  • Zone of Calcifying Cartilage: Chondrocyte death and calcification.
  • Zone of Ossification: Osteoblast activity leading to bone formation.

Bone Growth

• Appositional growth: Width increase occurs from the periosteum.
• Remodeling allows adaptation to stress and maintains bone density according to Wolff’s law.

Vascular Supply to Bones

• Long bones receive blood from:
  • Nutrient, epiphyseal, metaphyseal, and periosteal arteries.
• Short bones rely primarily on nutrient arteries, while flat and irregular bones receive a combination of nutrient and periosteal arteries.
• Blood vessels penetrate through compact bone into the marrow cavity via Haversian and Volkmann’s canals.

Bone Health and Maintenance

• Vitamins C and D are essential for maintaining bone health.
• Deficiencies in these vitamins can lead to bone defects or impaired growth.

Fracture Repair Process

• Initially involves hemorrhage and formation of a procallus (granulation tissue).
• Soft callus formation involves chondroblasts converting granulation tissue into cartilaginous tissue.
• Bony callus formation replaces the soft callus with woven bone.
• Final remodeling converts woven bone into compact bone, establishing lamellar structure.