Fatty Marrow vs. Bone: Composition, Location, and Function

Distinct Tissues Within the Skeletal System

Bone (Cortical & Cancellous)

• Composition: Collagen matrix (organic, ~30%) + hydroxyapatite crystals (mineral, ~70%).
• Function: Structural support, protection of organs, mineral reservoir (Ca²⁺, PO₄³⁻), lever for muscle attachment.
• Types: Compact (cortical) bone forms outer shell; spongy (cancellous) bone fills ends of long bones and interior of flat bones.

Bone Marrow (Red & Yellow/Fatty)

• Location: Medullary cavities of long bones; spaces of cancellous bone (flat bones: pelvis, sternum, skull, ribs).
• Red marrow (hematopoietic): Produces erythrocytes, leukocytes, platelets; dominant in children.
• Yellow marrow (fatty): Adipocytes, stromal cells, vascular supply; increases with age, replaces red marrow in long bone diaphyses. Can revert to red marrow under hypoxic stress.
• Function: Hematopoiesis, fat storage, mesenchymal stem cell niche.

Key Distinction

Bone is the rigid, calcified connective tissue forming the skeleton. Marrow is the soft, vascular tissue occupying bone cavities. They are anatomically adjacent but functionally distinct.