6.3 Bone Structure

Bone Anatomy and Physiology

Bones are complex organs with distinct regions, each built to handle specific mechanical and metabolic demands. Understanding bone structure means knowing how the anatomy, cell types, and tissue organization all connect to support movement, protection, and mineral balance.

Key Anatomical Features of Bones

A long bone (like the femur or humerus) is the best model for learning bone anatomy because it clearly shows each structural region.

• Diaphysis is the shaft or main portion of a long bone, composed primarily of dense compact bone. Its thick walls make it strong enough to resist bending and torsion during movement.
• Epiphysis refers to the rounded ends of a long bone. Each epiphysis is covered by smooth articular cartilage (hyaline cartilage) that reduces friction at joints, and its interior is filled with spongy bone.
• Metaphysis is the flared region between the diaphysis and epiphysis. During growth, this is where the epiphyseal plate (growth plate) sits, allowing the bone to elongate. In adults, the plate ossifies into the epiphyseal line.
• Medullary cavity is the hollow space running through the diaphysis. In adults, it contains yellow bone marrow (mostly fat storage). Red bone marrow, which produces blood cells, is found primarily in the spongy bone of the epiphyses.
• Periosteum is a tough fibrous membrane covering the outer surface of bone everywhere except at joint surfaces (where articular cartilage covers instead). It contains blood vessels, nerves, and osteoblasts, making it critical for bone growth, repair, and pain sensation.
• Endosteum is a thin connective tissue membrane lining the medullary cavity and the inner surfaces of spongy bone. It also contains osteoblasts and osteoclasts, supporting bone remodeling from the inside.

Common Bone Markings and Functions

Bone markings are surface features that fall into two broad categories: those that form joints (articulations) and those that serve as attachment points or passageways. You'll need to recognize these on diagrams and specimens.

• Foramen — a hole or opening that allows passage of blood vessels and nerves (e.g., the nutrient foramen on the diaphysis)
• Fossa — a shallow or deep depression that may provide space for muscle attachment or articulation with another bone (e.g., the mandibular fossa of the temporal bone)
• Condyle — a large, rounded articular projection at the end of a bone (e.g., the femoral condyles that articulate with the tibia at the knee)
• Tuberosity — a roughened, raised area serving as an attachment point for muscles, tendons, or ligaments (e.g., the tibial tuberosity, where the patellar ligament attaches)
• Crest — a prominent, narrow ridge of bone (e.g., the iliac crest of the hip bone, where several abdominal and hip muscles attach)
• Spine — a sharp, slender projection (e.g., the spinous process of a vertebra, which you can feel along your back)

Cellular Composition of Bone Tissue

Bone tissue contains three main cell types, each with a distinct role. Think of them as a construction crew: one builds, one maintains, and one demolishes.

• Osteoblasts are bone-forming cells. They secrete osteoid, the unmineralized organic matrix, and then initiate its mineralization by depositing calcium and phosphate. Once an osteoblast becomes surrounded by the matrix it produced, it matures into an osteocyte.
• Osteocytes are the most abundant bone cells. They sit in small spaces called lacunae and extend cytoplasmic processes through tiny channels called canaliculi. This network allows osteocytes to communicate with each other and sense mechanical stress, helping regulate mineral homeostasis and signaling when remodeling is needed.
• Osteoclasts are large, multinucleated cells that break down (resorb) bone tissue. They respond to hormonal signals (like parathyroid hormone) and mechanical signals to reshape bone and release stored minerals into the blood.

The extracellular matrix is what gives bone its unique mechanical properties. It has two components:

• Organic component (osteoid) — primarily type I collagen fibers, which provide flexibility and tensile strength (resistance to stretching and twisting)
• Inorganic component (mineral) — primarily hydroxyapatite $Ca_{10}(PO_4)_6(OH)_2$, a crystalline calcium phosphate salt that provides hardness and compressive strength (resistance to being crushed)

This combination is what makes bone both strong and somewhat flexible. Without collagen, bone would be brittle and shatter easily. Without mineral, bone would be rubbery and bend under load.

Compact vs. Spongy Bone Structure

These two tissue types appear in every bone but in different proportions and locations.

Compact bone is dense and solid. It forms the walls of the diaphysis and the outer shell of all bones. Its structural unit is the osteon (Haversian system):

1. A central Haversian canal runs lengthwise through the osteon, carrying blood vessels and nerves.
2. Concentric rings of bone matrix called lamellae surround the central canal, like tree rings.
3. Osteocytes sit in lacunae between the lamellae.
4. Canaliculi radiate outward from each lacuna, connecting osteocytes to each other and to the central canal for nutrient and waste exchange.
5. Perforating (Volkmann's) canals run perpendicular to Haversian canals, connecting adjacent osteons and linking them to the periosteum and medullary cavity.

Spongy (cancellous) bone is porous and lighter. It's found in the epiphyses of long bones and the interior of flat bones (like the skull) and irregular bones (like vertebrae). Instead of osteons, spongy bone is organized into an open lattice of bony plates and rods called trabeculae. These trabeculae align along lines of mechanical stress, giving strength where it's needed most without adding excess weight. The spaces between trabeculae are filled with bone marrow.

Bone density refers to the mineral content per unit volume of bone. Higher mineral density generally means greater resistance to fracture. Bone density is clinically measured using DEXA scans and is a key indicator for conditions like osteoporosis.

Blood Supply and Nerves in Bones

Bone is living tissue with high metabolic demands, so it requires a rich blood supply.

• Nutrient arteries enter the bone through the nutrient foramen, penetrate the diaphysis, and supply the medullary cavity and the inner two-thirds of compact bone.
• Periosteal arteries supply the outer one-third of compact bone from the surface.
• Metaphyseal and epiphyseal arteries supply the ends of long bones.
• Venous drainage follows a similar pattern in reverse.

The periosteum is richly innervated with sensory nerve fibers, which is why bone injuries and periosteal damage are extremely painful. Nerves in bone also include autonomic fibers that regulate blood flow and influence bone metabolism. Sensory nerves in and around bone contribute to proprioception, your body's sense of limb position and movement.

This vascular and nerve supply is what makes bone repair possible after a fracture and allows ongoing remodeling throughout life.

Bone Function and Regulation

Bone structure directly supports several critical functions:

• Support — the skeleton provides the rigid framework that holds the body upright and anchors soft tissues
• Protection — bones shield vital organs (the skull protects the brain, the rib cage protects the heart and lungs, the vertebral column protects the spinal cord)
• Mineral reservoir — bone stores about 99% of the body's calcium and 85% of its phosphorus, releasing or absorbing these minerals as needed to maintain blood calcium homeostasis

Osteogenesis (bone formation) occurs during embryonic development and continues throughout life as part of bone remodeling and fracture repair. Bones are constantly being broken down by osteoclasts and rebuilt by osteoblasts in response to mechanical loading, hormonal signals, and metabolic needs.

The biomechanics of bone refers to how bones respond to mechanical forces. Bones subjected to regular stress (like weight-bearing exercise) become denser and stronger over time, while bones that are unloaded (during prolonged bed rest, for example) lose density. This adaptive response is described by Wolff's Law: bone remodels in response to the mechanical demands placed on it.