6.1 Bone Structure and Function

4 min read•august 28, 2024

Bones are the unsung heroes of our body, providing structure and protection while secretly running a mineral bank and blood cell factory. They're not just rigid sticks holding us up—they're dynamic organs constantly remodeling themselves.

Dive into the world of bone tissue, where compact and spongy bone team up to create a strong yet lightweight skeleton. From osteoblasts building new bone to osteoclasts breaking it down, these cellular workers keep our bones healthy and responsive to our body's needs.

Bone tissue composition and structure

Bone composition and organization

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  • Bone is a dense, porous, mineralized connective tissue that makes up the skeletal system
  • Bone tissue is composed of an organic matrix of collagen fibers and ground substance, which is strengthened by deposits of calcium phosphate crystals, primarily hydroxyapatite
  • The basic structural unit of compact bone is the osteon (Haversian system)
    • Consists of concentric layers of mineralized matrix called lamellae surrounding a central Haversian canal
    • Haversian canal contains blood vessels and nerves

Microstructure of bone

  • Compact bone contains small spaces called lacunae, which house osteocytes
  • Tiny canals called canaliculi radiate from the lacunae to allow for nutrient and waste exchange
  • Spongy bone consists of an irregular lattice of thin columns of bone called trabeculae
    • Trabeculae are adapted to withstand stresses applied in varying directions
    • Spaces between trabeculae are filled with bone marrow (red and yellow)

Functions of the skeletal system

Structural support and protection

  • The skeletal system provides structural support and protection for the body and its internal organs
    • Skull protects the brain, ribcage protects the heart and lungs, vertebrae protect the spinal cord
  • Bones serve as attachment points for muscles, tendons, and ligaments, enabling movement and locomotion
    • Muscle contraction pulls on bones, allowing for movement (walking, running, grasping)

Mineral homeostasis and hematopoiesis

  • The skeletal system acts as a reservoir for minerals, particularly calcium and phosphorus
    • Minerals can be released into the bloodstream when needed for various physiological processes (muscle contraction, nerve impulse transmission)
  • Bones house and protect the bone marrow, which is responsible for hematopoiesis (blood cell production)
    • Red bone marrow produces red blood cells, white blood cells, and platelets

Endocrine regulation

  • The skeletal system plays a role in endocrine regulation
    • Certain bones (e.g., the long bones) secrete hormones such as osteocalcin
      • Osteocalcin helps regulate glucose metabolism and fat deposition
    • Bone cells also respond to hormones (parathyroid hormone, calcitonin) to regulate calcium homeostasis

Compact vs spongy bone

Compact bone characteristics

  • Compact bone is dense and solid, found in the shafts (diaphyses) of long bones and the outer layers of other bones
    • Provides strength and protection
  • Compact bone is organized into osteons (Haversian systems)
  • Compact bone has a slower turnover rate and is less metabolically active compared to spongy bone

Spongy bone characteristics

  • Spongy bone, also called trabecular or cancellous bone, is less dense and more porous than compact bone
    • Consists of a network of trabeculae that create spaces filled with bone marrow
  • Spongy bone lacks the specific structural arrangement of osteons found in compact bone
  • Spongy bone is typically found in the ends (epiphyses) of long bones, the interior of short and flat bones, and the cores of the vertebrae
  • Spongy bone has a higher surface area and is more responsive to metabolic changes compared to compact bone

Bone cell types and roles

Osteoblasts and osteocytes

  • Osteoblasts are bone-forming cells derived from mesenchymal stem cells
    • Secrete the organic matrix (osteoid) and initiate the mineralization process
    • Osteoblasts trapped within the bone matrix become osteocytes
  • Osteocytes are mature bone cells that maintain the bone matrix and regulate mineral homeostasis
    • Sense mechanical stress and communicate with other cells through the canaliculi

Osteoclasts and bone remodeling

  • Osteoclasts are large, multinucleated cells derived from hematopoietic stem cells
    • Resorb and break down bone tissue, playing a crucial role in bone remodeling and calcium homeostasis
    • Osteoclast activity is balanced by osteoblast activity to maintain bone mass
  • Bone lining cells are inactive osteoblasts that cover the surfaces of bone
    • Act as a barrier and mediator for bone remodeling by regulating the movement of ions between the bone and the extracellular fluid

Osteogenic cells

  • Osteogenic cells, also known as osteoprogenitor cells, are undifferentiated cells found in the periosteum and endosteum
    • Can divide and differentiate into osteoblasts when stimulated
    • Play a role in bone growth, repair, and remodeling
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