Lecture on the Skeletal System

Overview

• Introduction to the skeletal system
• Focus on bones, features, and formation
• Will cover two divisions in subsequent sections

Functions of the Skeletal System

1. Support and Protection

   • Supports body and holds it upright
   • Protects internal organs (e.g., brain, heart)

2. Assistance in Movement

   • Muscles pull on bones to move the body

3. Storage and Release of Ions

   • Crucial for storage and release of calcium, phosphorus, magnesium
   • Calcium: essential for neurotransmitter release, muscle contraction, immune function, blood clotting
   • Skeletal system maintains calcium levels by storing and releasing as needed

4. Hematopoiesis

   • Formation of blood cells (red, white blood cells, platelets) in bone marrow
   • Red bone marrow for blood cells, yellow bone marrow for fat storage (triglycerides)

Calcium Homeostasis

• Controlled by parathyroid hormone (PTH) and calcitonin

• Parathyroid Hormone (PTH):

  • Released when calcium levels drop
  • Increases calcium absorption from bones, urine reabsorption, and vitamin D synthesis

• Calcitonin:

  • Released when calcium levels are high
  • Inhibits osteoclast activity, promotes calcium deposition in bones

• Bone Density Changes with Age

  • Before puberty: more bone formation than resorption
  • Adulthood: balanced
  • Post-menopausal women: increased resorption leads to osteoporosis

Structure of Long Bones

1. Main Parts

   • Diaphysis: central shaft
   • Epiphyses: ends of the bone
   • Metaphysis: neck between diaphysis and epiphyses

2. Coverings and Linings

   • Articular cartilage at joint surfaces
   • Periosteum: external connective tissue
   • Endosteum: lining inside

3. Bone Types

   • Compact bone: dense outer layer
   • Spongy bone: lighter, contains red marrow

Bone Cells

• Osteoprogenitor Cells: Stem cells of bone
• Osteocytes: Mature bone cells
• Osteoblasts: Bone-building cells
• Osteoclasts: Bone-resorbing cells

Microstructure of Bone

• Compact Bone:

  • Organized in concentric circles called Haversian systems or osteons
  • Central canal (Haversian canal) contains blood vessels and nerves
  • Lamellae: concentric rings forming the osteon

• Spongy Bone:

  • Trabeculae network with spaces for marrow
  • Osteocytes in lacunae, connected by canaliculi

Bone Vascularization

• Nutrient arteries and veins enter through nutrient foramina
• Individual blood supply to epiphyses and metaphyses

Bone Formation (Ossification)

1. Intramembranous Ossification:

   • Flat bones, mesenchymal cells form ossification centers
   • Osteoblasts mature into osteocytes, forming trabechulae
   • Compact bone forms outer layers

2. Endochondral Ossification:

   • Replaces cartilage in embryo and fetus, and epiphyseal plate before puberty
   • Mesenchymal cells become chondrocytes, cartilage forms, ossification centers develop

• Bone Growth Zones:
  • Proliferative zone: rapid chondrocyte division
  • Maturation and hypertrophy
  • Calcification and ossification

Bone Thickening

• Bone thickens by osteoblastic activity around blood vessels
• Formation of new Haversian systems

These notes summarize the key points of the skeletal system as discussed in the lecture, focusing on bone structure, function, and development.