Anatomy of Compact Bone, Spongy Bone, and Bone Marrow

Compact Bone Anatomy

  • Compact bone is composed of repeating column-shaped structures called osteons.

    • Osteons are the building blocks of compact bone, similar to bricks in a brick wall.
    • They are long, cylinder-shaped structures.

  • Osteons are formed in rings called lamellae.

  • The center of an osteon contains a hollow space called the central canal, which houses blood vessels.

  • Osteocytes are found within osteons, residing in cavities called lacunae.

    • Lacunae are interconnected by small channels called canaliculi.

  • Perforating canals connect the central canals of osteons, running perpendicularly across the bone.

  • Nutrients reach each osteocyte through the central canals, perforating canals, and canaliculi.

Components of an Osteon

  • Osteons: The fundamental building blocks of compact bone, appearing as long columns.

  • Lamellae: Concentric rings of bone tissue that make up each osteon, growing outward from the center.

  • Lacunae: Cavities located between the lamellae, housing osteocytes (formerly osteoblasts).

  • Central Canal: The central hollow space within each osteon, containing blood vessels.

  • Perforating Canals: Canals that run perpendicularly, connecting the central canals of different osteons.

  • Canaliculi: Tiny, spider-like channels connecting lacunae, facilitating nutrient delivery to osteocytes.

Spongy Bone

  • Spongy bone also contains osteons, but they are scattered and less tightly packed compared to compact bone.

  • Spongy bone has many cavities, giving it a spongy appearance.

  • The rods of bone within spongy bone are called trabeculae.

    • Each trabecula is composed of osteons.

  • Trabeculae form along stress points, resulting in higher density and more osteons in high-stress areas.

  • Example: Cross-section of the femur shows varying density of trabeculae depending on stress points.

Bone Marrow Types

  • Red Marrow

    • Named for its color.
    • Found in spongy bone.
    • Hematopoietic, meaning it produces all types of blood cells.
    • Located in flat bones, long bones, epiphyses, and other areas.

  • Yellow Marrow

    • Located in the medullary cavity of long bones.
    • Serves as a fat supply for long-term energy storage.

  • Gelatinous Marrow

    • Formed when yellow marrow degrades with age.
    • Becomes reddish and jelly-like.