Histology of osseous tissue chp 5

Bone Structure and Function

• Bones contain specialized cells known as osteocytes that help sense mechanical stresses applied to the bone.

  • When significant stress is detected, osteocytes signal for increased activity of osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells).

Bone Matrix Composition

• Bone has a hard extracellular matrix consisting of two major components:

  1. Mineral Component: Primarily made of calcium phosphate, providing rigidity.

  2. Organic Component: Composed mainly of collagen fibers, which give flexibility to the bone structure.

Experiments Demonstrating Bone Structure

• Soaking Bone in Acid: Soaking a bone in a weak acid (e.g., vinegar) removes the mineral content, demonstrating that the remaining organic component provides flexibility, making the bone malleable (like a pretzel).

• When the organic material is removed (e.g., through exposure to heat and sunlight), bones become brittle as proteins denature, leading to a more fragile structure.

• Mineralized bone possesses strength comparable to cast iron, making it strong but also vulnerable to fracturing upon excessive force.

Bone Types

1. Compact Bone

   • Comprised of functional units called osteons (Haversian systems), which are circular structures made up of concentric layers known as lamellae.

   • Lamellae contain blood vessels, providing nutrients and oxygen to the osteocytes housed in lacunae within the matrix.

   • Interstitial Lamellae: Remnants of older osteons found between newer ones, indicating continuous bone remodeling.

   • Circumferential Lamellae: Layers that surround the entire bone surface, contributing to bone strength.

2. Spongy Bone

   • Characterized by trabecular structures (spicules) with spaces that house bone marrow, contributing to lightness without sacrificing strength.

   • Important for blood cell formation (red bone marrow) and fat storage (yellow bone marrow).

Stress and Bone Adaptation

• Bone structure adapts based on stress; trabecular patterns align along lines of stress, providing support while minimizing weight.

• Loading Response: Increased activity in osteocytes stimulates remodeling, optimizing bone strength without excessive mass.

Bone Marrow Types

• Red Marrow: Important for blood cell formation; present in various bones and more extensive in children.

• Yellow Marrow: Fatty tissue present in adults, found in long bones.

Bone Development Processes

1. Intramembranous Ossification

   • Involves the transformation of connective tissue sheets into bone, primarily occurring in flat bones. Osteogenic cells transform into osteoblasts, laying down the bone matrix (osteoid).

   • Minerals crystallize within the osteoid to form bone.

2. Endochondral Ossification

   • Occurs in long bones, starting with a cartilage template that is gradually replaced by bone tissue. This process is favored during development.

   • Involves the formation of the primary ossification center, with cartilage deteriorating as blood vessels enter and stimulate bone formation.

   • Continues as growth hormone influences cartilage cell proliferation, enabling elongation until skeletal maturity.

Bone Growth Types

• Appositional Growth: Increase in bone diameter from the outside as new bone tissue is deposited by periosteum-derived osteoblasts, while osteoclasts remove bone from the interior to maintain optimal weight.

• Osteoporosis and Osteopenia: Conditions arising from insufficient bone density, linked to hormonal changes, nutrient deficiencies, and aging. Preventive measures include exercise and adequate nutrient intake.

Bone Fractures

• Classified as open (bone protrudes through skin) or closed (skin remains intact).

• Each type has specific characteristics (e.g., nondisplaced, comminuted, greenstick) affecting treatment approaches.

  • Open Reduction Internal Fixation: Surgical procedure to stabilize complex fractures using plates and screws.

Age and Bone Health

• Bone mass increases during childhood and peaks in early adulthood, followed by gradual declines in density due to age-related changes and hormonal shifts (notably during menopause).

• Osteoporosis is characterized by a significant reduction in bone density, increasing fracture risk.

• Preventive Strategies: Regular exercise, weight-bearing activities, and proper nutrition (calcium and vitamin D) are vital for maintaining bone health as one ages.