6.1 Bone Structure and Function

Bone tissue composition and structure

Bone is a mineralized connective tissue that forms the skeletal system. Far from being static or "dead," bone is a dynamic organ that constantly remodels itself in response to mechanical stress, hormonal signals, and the body's mineral needs.

Two main components give bone its properties: an organic matrix of collagen fibers and ground substance (providing flexibility and tensile strength) and inorganic mineral deposits, primarily calcium phosphate in the form of hydroxyapatite crystals (providing hardness and compressive strength). This combination makes bone both strong and somewhat flexible, similar to how rebar and concrete work together in reinforced structures.

Organization of compact bone

The basic structural unit of compact bone is the osteon (also called a Haversian system):

• Each osteon consists of concentric rings of mineralized matrix called lamellae, arranged like tree rings around a central Haversian canal
• The Haversian canal contains blood vessels and nerves that supply the bone tissue
• Perforating (Volkmann's) canals run perpendicular to Haversian canals, connecting neighboring osteons and linking them to the periosteum and medullary cavity

Between the lamellae sit small spaces called lacunae, each housing an osteocyte (mature bone cell). Radiating outward from each lacuna are tiny channels called canaliculi. These form a network that allows osteocytes to exchange nutrients, wastes, and signaling molecules with one another and with the blood supply. Think of canaliculi as the "communication highways" connecting osteocytes throughout the bone.

Spongy bone microstructure

Spongy (trabecular or cancellous) bone has a very different architecture. Instead of osteons, it consists of an irregular lattice of thin columns and plates of bone called trabeculae.

• Trabeculae are oriented along lines of mechanical stress, making spongy bone well-adapted to resist forces from multiple directions
• The spaces between trabeculae are filled with bone marrow (red marrow for blood cell production, yellow marrow for fat storage)
• Because trabeculae are thin enough for nutrients to diffuse directly from the marrow, spongy bone does not need Haversian systems

Functions of the skeletal system

Structural support and protection

The skeleton provides the rigid framework that supports soft tissues and maintains body shape. Specific bones are shaped to protect vital organs:

• The skull encases and shields the brain
• The ribcage protects the heart and lungs
• The vertebral column surrounds and guards the spinal cord

Bones also serve as attachment points for muscles, tendons, and ligaments. When skeletal muscle contracts, it pulls on bone across a joint, producing movement. Every action from walking to grasping a pencil depends on this lever system.

Mineral homeostasis and hematopoiesis

The skeleton acts as the body's primary reservoir for calcium and phosphorus, storing about 99% of the body's calcium. When blood calcium levels drop, minerals can be released from bone into the bloodstream to support critical processes like muscle contraction, nerve impulse transmission, and blood clotting. When calcium is abundant, bone tissue absorbs and stores the excess.

Bones also house and protect bone marrow, the site of hematopoiesis (blood cell production). Red bone marrow produces red blood cells, white blood cells, and platelets. In adults, red marrow is concentrated in flat bones (sternum, pelvis, skull) and the epiphyses of long bones, while yellow marrow (mostly fat storage) fills the medullary cavities of the diaphyses.

Endocrine regulation

Bone tissue participates in endocrine signaling in two key ways:

• Osteoblasts secrete osteocalcin, a hormone that helps regulate blood glucose levels and fat deposition. This connection between bone and metabolism is a relatively recent discovery and an active area of research.
• Bone cells respond to circulating hormones to maintain calcium homeostasis. Parathyroid hormone (PTH) stimulates osteoclasts to break down bone and release calcium when blood levels are low. Calcitonin (from the thyroid gland) inhibits osteoclast activity, promoting calcium deposition into bone when blood levels are high.

Compact vs. spongy bone

A useful way to remember the relationship: compact bone forms the hard outer shell (the cortex), while spongy bone fills the interior where lightweight strength and marrow space are needed. Most bones in the body contain both types.

Because spongy bone has a much higher surface-area-to-volume ratio, it responds more quickly to metabolic demands. This is why conditions like osteoporosis tend to affect spongy bone first.

Bone cell types and roles

Four main cell types work together to build, maintain, and remodel bone tissue. Understanding their origins and functions is essential for understanding bone physiology.

Osteogenic (osteoprogenitor) cells

Osteogenic cells are undifferentiated stem cells found in the periosteum (outer bone covering) and endosteum (inner lining of the medullary cavity). They can divide by mitosis and differentiate into osteoblasts when stimulated by growth factors or mechanical stress. These cells are critical for bone growth, fracture repair, and ongoing remodeling throughout life.

Osteoblasts

Osteoblasts are bone-forming cells derived from mesenchymal stem cells (via osteogenic cells). Their job is to build new bone:

1. They secrete the unmineralized organic matrix called osteoid (mainly collagen and proteoglycans)
2. They then initiate mineralization by depositing hydroxyapatite crystals into the osteoid
3. As osteoblasts become surrounded by the matrix they've produced, they become trapped and mature into osteocytes

Osteocytes

Osteocytes are the most abundant bone cells. They are mature osteoblasts that have become embedded within lacunae in the mineralized matrix. Despite appearing "trapped," they are highly active:

• They sense mechanical stress (acting as mechanoreceptors) and signal other cells to adjust bone architecture accordingly
• They communicate with neighboring osteocytes and surface cells through gap junctions in the canaliculi
• They help regulate mineral homeostasis by controlling the flow of calcium and phosphate between bone and blood

Osteoclasts

Osteoclasts are large, multinucleated cells derived from hematopoietic stem cells (the same lineage that produces monocytes and macrophages). They are the bone-resorbing cells:

• They attach to bone surfaces and secrete acids and enzymes that dissolve both the mineral and organic components of the matrix
• This process, called bone resorption, releases stored calcium and phosphate into the blood
• Osteoclast activity must be carefully balanced with osteoblast activity to maintain healthy bone mass. When resorption outpaces formation, bone density decreases (as in osteoporosis)

Bone lining cells

Bone lining cells are inactive osteoblasts that cover bone surfaces not currently undergoing remodeling. They form a protective barrier and regulate the movement of ions between bone tissue and extracellular fluid. When remodeling signals arrive, bone lining cells can retract to expose the bone surface to osteoclasts, or they can be reactivated as osteoblasts.