1. Support

Provides structural support for the entire body. Individual bones and groups of bones provide a framework for the attachment of soft tissues and organs.

2. Storage

Calcium salts in bone represent a mineral reserve that maintains normal concentrations of calcium and phosphate ions in body fluids. Bones also store lipids as energy reserves in areas filled with yellow marrow.

3. Blood Cell Production

Red blood cells, white blood cells, and other blood elements are produced within red marrow, which fills the internal cavities of many bones. This is called hematopoiesis.

4. Protection

Skeletal elements surround soft tissues and organs: the ribs protect the heart and lungs; the skull encloses the brain; the vertebrae shield the spinal cord; the pelvis cradles digestive and reproductive organs.

5. Movement

Many bones function as levers that change the magnitude and direction of forces generated by skeletal muscles. Movements range from the delicate motion of a fingertip to powerful changes in body position.

• Diaphysis — central shaft; surrounds the marrow cavity containing bone marrow (soft, fatty tissue)
• Epiphyses — expanded ends covered by articular cartilages; each articulates with an adjacent bone at a joint
• Periosteum — outer surface covering; tendon and ligament fibers intermingle with it; provides route for circulatory and nervous supply; participates in growth and repair
• Endosteum — cellular lining covering the spongy bone of the marrow cavity and inner surfaces; active during growth and repair
• Epiphyseal line — remnant of epiphyseal cartilage visible in adults after growth ends

Structure

The basic functional unit is the osteon (Haversian system). Within each osteon, osteocytes are arranged in concentric layers around a central canal (Haversian canal) containing blood vessels. Lamellae are cylindrical and oriented parallel to the long axis of the central canal.

• Lamellae — narrow sheets of calcified matrix
• Lacunae — small pockets between lamellae housing osteocytes
• Canaliculi — small channels radiating through matrix, interconnecting lacunae to blood vessels; contain cytoplasmic extensions of osteocytes
• Perforating canals — link blood vessels of central canals to those of the periosteum and marrow cavity

Function

Found where stresses come from a limited range of directions. Limb bones are built to withstand forces applied at either end. Osteons run parallel to the long axis, so the bone does not bend under axial force. A force applied to the side, however, can break the bone.

Structure

Lamellae form rods or plates called trabeculae. Frequent branching creates an open network. Canaliculi from lacunae end at the exposed surfaces of trabeculae — nutrients and wastes diffuse between marrow and osteocytes. No osteons.

Function

Found where bones are not heavily stressed or where stresses arrive from many directions. Present in epiphyses of long bones, where stresses are transferred across joints. Spongy bone is lighter than compact bone, reducing weight the muscles must move. Its trabecular network supports and protects red bone marrow.

Osteocytes

Mature bone cells. The most abundant cell type in bone (~98% of bone mass is matrix). Maintain normal bone structure by recycling calcium salts in the surrounding matrix and assisting in repairs.

Osteoclasts (clast = break)

Giant cells with 50 or more nuclei. Secrete acids and enzymes that dissolve bony matrix and release stored minerals — a process called osteolysis or resorption. Regulate calcium and phosphate concentrations in body fluids. Associated with the endosteum and cellular layer of the periosteum.

Osteoblasts (blast = precursor)

Cells responsible for producing new bone — a process called ossification. Produce new bone matrix and promote calcium salt deposition. When an osteoblast becomes completely surrounded by calcified matrix, it differentiates into an osteocyte.

Elevations and Projections

Depressions and Openings

Bone develops directly within sheets or membranes of connective tissue — no cartilage model. Begins in the deeper layers of the dermis.

1. Osteoblasts differentiate from connective tissue stem cells after the organic matrix they secrete becomes calcified
2. The site where ossification first occurs = ossification center
3. New bone branches outward; some osteoblasts become trapped in bony pockets and change into osteocytes
4. Blood vessels grow into the area and become trapped in developing bone
5. At first resembles spongy bone; further remodeling around trapped blood vessels can produce osteons typical of compact bone

Bone replaces existing hyaline cartilage. Used for most bones of the skeleton. The cartilages form first as miniature models of the future bone.

1. Chondrocytes enlarge within the cartilage model; surrounding matrix calcifies; chondrocytes die as calcified matrix slows nutrient diffusion
2. Bone forms at the shaft surface: blood vessels invade the perichondrium; inner layer cells differentiate into osteoblasts and produce bone matrix
3. Primary ossification center forms: blood vessels invade the inner cartilage; migrating fibroblasts differentiate into osteoblasts; spongy bone forms in the center of the shaft and spreads toward each end
4. Marrow cavity forms: osteoclasts break down some spongy bone; epiphyseal cartilages (epiphyseal plates) continue growing at the ends, lengthening the bone
5. Secondary ossification centers form in the epiphyses; epiphyses fill with spongy bone; a thin cap of original cartilage remains as articular cartilage

Epiphyseal Growth and Closure

As long as cartilage growth keeps pace with osteoblast invasion, the epiphyseal cartilage persists and the bone grows longer. At puberty, sex hormones cause osteoblasts to produce bone faster than cartilage expands — the epiphyseal cartilages narrow and disappear.

In adults, the former location of epiphyseal cartilage is marked by the epiphyseal line visible on x-rays. The end of epiphyseal growth = epiphyseal closure.

While the bone elongates, its diameter also enlarges through appositional growth. Cells of the periosteum develop into osteoblasts and produce additional bony matrix on the outer surface of the shaft. Simultaneously, the inner surface is eroded by osteoclasts, and the marrow cavity gradually enlarges.

Minerals

Normal bone growth depends on adequate calcium and phosphate. During prenatal development, minerals are absorbed from the mother's bloodstream — the maternal skeleton often loses bone mass during pregnancy.

Vitamin D3

Obtained from dietary supplements or synthesized by epidermal cells exposed to UV radiation. After processing in the liver, the kidneys convert a derivative into calcitriol — a hormone that stimulates absorption of calcium and phosphate in the digestive tract. Vitamin D3 deficiency in children causes rickets (soft, flexible bones; bowlegged appearance).

Vitamins A and C

Also essential for normal bone growth and maintenance. Vitamin C deficiency causes scurvy — reduced osteoblast activity leads to weak, brittle bones.

Hormones

Growth hormone, thyroid hormones, sex hormones, and hormones involved in calcium metabolism are all essential to normal skeletal growth and development.

As one osteon forms through osteoblast activity, another is destroyed by osteoclasts. Regional and local differences in turnover rate exist — spongy bone in the femoral head may be replaced 2–3 times per year, while compact bone in the shaft remains largely untouched.

Remodeling and Mechanical Stress

Regular mineral turnover allows each bone to adapt to new stresses. Heavily stressed bones become thicker and stronger with more pronounced surface ridges. Unstressed bones become thin and brittle — using a crutch while wearing a cast causes the unstressed leg to lose up to one-third of its bone mass within a few weeks.

Calcium is the most abundant mineral in the human body. A typical body contains 1–2 kg of calcium, 99 percent of which is deposited in the skeleton.

Why Calcium Is Tightly Regulated

• +30% above normal: neurons and muscle cells become relatively unresponsive
• -35% below normal: neurons become so excitable that convulsions may occur
• -50% below normal: generally causes death
• Daily fluctuations greater than 10% are very unusual in healthy individuals

Hormonal Regulation

Bones usually heal after severe damage as long as blood supply remains and cells of the endosteum and periosteum survive. Repair takes 4 months to over a year.

1. Fracture hematoma forms — many blood vessels are broken; a large blood clot closes off injured vessels; dead bone extends from the break in both directions
2. Calluses form — cells of the periosteum and endosteum undergo mitosis; daughter cells migrate into the fracture zone; they form an external callus (cartilage at center) and internal callus (spongy bone)
3. Osteoblasts replace cartilage — the central cartilage of the external callus is replaced with spongy bone; external and internal calluses form a continuous brace at the fracture site
4. Remodeling completes — fragments of dead bone and callus spongy bone are removed; only living compact bone remains; the bone may be slightly thicker than normal at the fracture site

Osteopenia (penia = lacking) = inadequate ossification. All of us become slightly osteopenic as we age. Bone mass reduction begins between ages 30 and 40 — osteoblast activity begins to decline while osteoclast activity continues at previous levels.

• Women lose roughly 8% of skeletal mass per decade
• Men lose roughly 3% per decade
• Epiphyses, vertebrae, and jaws are most affected

Osteoporosis

A reduction in bone mass severe enough to compromise normal function. Over age 45: 29% of women and 18% of men have osteoporosis. Accelerates after menopause due to declining estrogens. Vertebrae may collapse, distorting articulations and putting pressure on spinal nerves.

Forms the longitudinal axis of the body. Supports and protects the brain, spinal cord, and organs of the ventral body cavity. Provides extensive surface area for muscles that adjust positions of the head, neck, and trunk, perform respiratory movements, and stabilize the appendicular skeleton.

Includes the bones of the limbs and the pectoral and pelvic girdles that attach limbs to the trunk.

Each person has 12 pairs of ribs regardless of sex. They protect the heart and lungs and serve as base for respiratory muscles.

Sternum — Three Parts

• Manubrium — broad superior part; articulates with clavicles and first rib; has jugular notch
• Body — elongated middle section
• Xiphoid process — slender inferior tip; last to ossify; can break from impact and damage the liver (CPR precaution)

Protect and support the entrances to the digestive and respiratory tracts. Provide sites for muscles controlling facial expressions and food manipulation. Only the mandible is movable.

Hyoid Bone (associated bone)

U-shaped; suspended below the skull by ligaments from styloid processes of the temporal bones. Functions as base for muscles associated with the larynx, tongue, and pharynx. The only bone that does not articulate with another bone.

Paranasal Sinuses

Air-filled chambers in the frontal, sphenoid, ethmoid, palatine, and maxillary bones. They lighten the skull, provide extensive mucous epithelium, and drain into the nasal cavities. Incoming air is humidified and warmed; foreign particles are trapped in mucus.

Fontanelles (Infant Skull)

Areas of fibrous connective tissue between incompletely ossified cranial bones at birth. Allow skull distortion during delivery. By about age 4, fontanelles disappear and skull growth is completed.

Four Spinal Curves

• Primary curves (present at birth): thoracic curve + sacral curve — create the C-shape of an infant
• Secondary curves (develop after birth): cervical curve (develops as infant learns to hold head up) + lumbar curve (develops when child learns to stand). Bring body weight in line with the body axis.
• All four curves fully developed by age 10

Abnormal Curvature

• Kyphosis — exaggerated thoracic curve (humpback)
• Lordosis — exaggerated lumbar curve (swayback)
• Scoliosis — abnormal lateral curve

All vertebrae share: a vertebral body, a vertebral arch, and articular processes.

• Vertebral body — most massive, weight-bearing portion; separated from adjacent bodies by intervertebral disc of fibrocartilage
• Vertebral arch — forms posterior margin of vertebral foramen; walls = pedicles; roof = laminae
• Vertebral foramen — successive foramina form the vertebral canal (enclosing spinal cord)
• Transverse processes — project laterally from pedicles; muscle attachment sites
• Spinous process — projects posteriorly from where laminae fuse; forms bumps felt along midline of back
• Articular processes — arise at junction of pedicles and laminae; superior and inferior on each side; contact at articular facets
• Intervertebral foramina — gaps between pedicles of successive vertebrae; nerve passage to/from spinal cord

Special Cervical Vertebrae

• Atlas (C1) — holds up the head; articulates with occipital condyles; nodding "yes" occurs here
• Axis (C2) — has the dens (odontoid process) projecting upward; atlas rotates around it; shaking head "no" occurs here

Sacrum and Coccyx

• Sacrum — 5 fused vertebrae; protects reproductive, digestive, excretory organs; articulates with pelvic girdle at sacroiliac joints; sacral promontory = obstetric landmark; fusion complete at ages 25–30
• Coccyx — 3–5 fused vertebrae; attachment for muscle closing anal opening

Intervertebral Discs

Pads of fibrocartilage between vertebral bodies. Consist of tough outer fibrocartilage layer + soft, elastic gelatinous core. Act as shock absorbers; account for roughly one-quarter of spinal column length above the sacrum. Water content decreases with age, causing height decrease. If gelatinous core ruptures through outer layer = herniated disc.

Consists of two clavicles and two scapulae. Only direct connections between pectoral girdle and axial skeleton are clavicle-manubrium articulations. Scapulae have no bony or ligamentous connection to the thoracic cage — supported entirely by skeletal muscles.

Clavicle (Collarbone)

S-shaped; sternal end articulates with manubrium; acromial end articulates with the acromion of the scapula. Relatively small and fragile — clavicle fractures are common. Most heal rapidly without a cast.

Scapula (Shoulder Blade)

Broad flat triangle with superior, medial, and lateral borders. Key landmarks:

• Glenoid cavity (glenoid fossa) — shallow cup where scapula articulates with the humerus (shoulder joint)
• Coracoid process — anterior projection; attachment for ligaments and tendons
• Acromion — large posterior process; articulates with distal clavicle; can be felt at tip of shoulder
• Scapular spine — crosses posterior surface; divides it into supraspinous fossa (above) and infraspinous fossa (below)
• Subscapular fossa — depression in anterior surface

Humerus (arm — shoulder to elbow)

Head articulates with scapula's glenoid cavity. Key landmarks: greater tubercle (lateral contour of shoulder), lesser tubercle (anterior), surgical neck (common fracture site), deltoid tuberosity (deltoid muscle attachment), medial/lateral epicondyles, trochlea (pulley; medial, articulates with ulna), capitulum (lateral, articulates with radius), coronoid fossa, olecranon fossa.

Radius and Ulna (forearm)

Radius = lateral (thumb side); Ulna = medial. Connected by interosseous membrane. Ulna: olecranon (point of elbow), trochlear notch (articulates with humerus trochlea), coronoid process, styloid process. Radius: head articulates with capitulum of humerus; radial tuberosity (biceps attachment); styloid process. Pronation = rotating palm to face back; supination = rotating palm to face forward.

Wrist and Hand

8 carpal bones in 2 rows; 5 metacarpal bones (palm); 14 phalanges (fingers) — 4 fingers have 3 each (proximal, middle, distal); thumb (pollex) has 2 (proximal, distal).

Consists of two hip bones (coxal bones). Much more firmly attached to the axial skeleton than the pectoral girdle. Each hip bone = fusion of ilium + ischium + pubis.

• Ilium — most superior and largest; iliac crest (muscle/ligament attachment); broad curved surface for muscle attachment above acetabulum
• Ischium — posteroinferior; ischial tuberosity (supports body weight when sitting)
• Pubis — anterior; pubic symphysis (fibrocartilage pad connecting the two pubic bones; limits movement); obturator foramen enclosed by ischium-pubis fusion
• Acetabulum — cup-shaped socket where all three bones meet; articulates with the femoral head

The Pelvis as a Whole

Pelvis = 2 hip bones + sacrum + coccyx. Extensive ligament network stabilizes it. Interacts with both appendicular and axial skeletons.

Sex Differences in Pelvic Structure

Female differences are adaptations for supporting the developing fetus and easing passage of the newborn. The sacral promontory is an important obstetric landmark.

Femur (thigh)

Longest and heaviest bone in the body. Head articulates with acetabulum. Greater and lesser trochanters = large projections at neck-shaft junction for large tendon attachments. Linea aspera = posterior ridge for adductor muscles. Medial and lateral condyles form part of the knee joint.

Patella (kneecap)

Forms within the quadriceps femoris tendon. Glides over patellar surface between femoral condyles. Patellar ligament attaches it to tibial tuberosity.

Tibia and Fibula (leg)

Tibia = large medial bone (shinbone); medial and lateral condyles articulate with femoral condyles; tibial tuberosity; medial malleolus (medial ankle support). Fibula = slender lateral bone; does NOT participate in knee joint or bear weight; lateral malleolus (lateral ankle stability); important muscle attachment surface.

Ankle and Foot

7 tarsal bones; only talus articulates with tibia/fibula. Most standing weight passes through calcaneus (heel bone) — Achilles tendon attaches here. 5 metatarsals; 14 phalanges — great toe (hallux) has 2, other toes have 3 each.

• Suture — fibrous; bones interlocked and bound by dense CT; found between skull bones
• Gomphosis — fibrous; ligament binds each tooth within a bony socket (alveolus) in the jaw
• Synchondrosis — cartilaginous; rigid cartilage connection; example: first ribs to sternum; also epiphyseal cartilage during growth

• Syndesmosis — fibrous; connected by a ligament; example: distal tibia-fibula articulation
• Symphysis — cartilaginous; bones separated by a broad fibrocartilage disc or pad; examples: intervertebral discs, pubic symphysis

Components of a Synovial Joint

• Articular cartilages — cover bony surfaces; no perichondrium; matrix contains more water than other cartilages; prevent bone-to-bone contact
• Joint capsule (articular capsule) — fibrous; surrounds the entire joint; continuous with the periostea of articulating bones
• Synovial membrane — lines inner surfaces of joint cavity; produces synovial fluid
• Synovial fluid — provides lubrication; reduces friction between moving surfaces
• Menisci — fibrocartilage pads in some complex joints (e.g., knee); act as shock absorbers and conform to articulating surface shape
• Fat pads — protect articular cartilages; act as packing material when joint cavity changes shape
• Bursae — small packets of connective tissue containing synovial fluid; reduce friction where tendons/ligaments rub against other tissues; may also occur as tubular sheaths around tendons

Gliding

Two opposing surfaces slide past each other. Occurs between carpal bones, tarsal bones, and between clavicles and sternum. Movement can occur in almost any direction but is slight. Rotation prevented by joint capsule and ligaments.

Angular Movements

Rotation

Turning around the longitudinal axis of the body or a limb. Lateral (external) rotation = turning away from midline. Medial (internal) rotation = turning toward midline.

Pronation — rotation of forearm so palm faces back (posterior); radius rolls across anterior surface of ulna.

Supination — rotation returning palm to face forward (anatomical position).

Special Movements

Vertebrae (axis to sacrum) articulate two ways:

• Gliding joints between superior and inferior articular processes — permit small flexion and rotation movements
• Symphyseal joints between vertebral bodies — separated by intervertebral discs

After physical maturity, the gelatinous core of intervertebral discs begins to degenerate. If stresses are sufficient, the core can rupture through the fibrocartilage layer and protrude beyond the intervertebral space = herniated disc (not "slipped disc" — the disc does not actually slip). Intervertebral discs account for roughly one-quarter of the length of the spinal column above the sacrum. Disc water content decreases with age, reducing height.

Ball-and-socket joint. Glenoid cavity (scapula) + head of humerus. The most frequently dislocated joint in the body.

• The joint capsule is relatively loose — extends from scapular neck to humerus; this oversized capsule allows extensive range of motion
• Bursae are especially large and numerous; bursitis causes restricted motion and pain
• The rotator cuff — group of muscles covering anterior, superior, and posterior surfaces of the capsule — does more to stabilize the shoulder than all its ligaments combined
• Range of motion limited primarily by surrounding muscles, not joint structure

Two articulations: humerus-ulna (primary) and humerus-radius. The humerus-ulna hinge joint provides stability and limits motion.

Extremely stable because: bony surfaces of humerus and ulna interlock; joint capsule is very thick; capsule is reinforced by stout ligaments. Permits only flexion and extension. If you fall with a partially flexed elbow, muscle contractions extending the elbow may fracture the ulna at the center of the trochlear notch.

Ball-and-socket diarthrosis. Head of femur + acetabulum. Compared to the shoulder, the hip is extremely stable because: almost complete bony socket; dense, strong joint capsule enclosing femoral head AND neck; three broad external reinforcing ligaments; one internal ligament (ligamentum teres) inside the acetabulum; massive surrounding muscles.

Fractures of the femoral neck or between trochanters are more common than hip dislocations. Total range of motion is considerably less than the shoulder. Flexion is the most important normal hip movement.

Functions as a hinge joint but is far more complex than the elbow. Femoral condyles roll across the tibia — contact points are constantly changing. Combines three separate articulations: medial femur-tibia condyle + lateral femur-tibia condyle + patella-femur. No single unified joint capsule; no common synovial cavity.

• Medial and lateral menisci — fibrocartilage pads between femoral and tibial surfaces; act as cushions; conform to articulating surface shape as femur changes position
• Patellar ligament — quadriceps tendon continuation; attaches patella to tibial tuberosity; supports front of knee
• Fibular and tibial collateral ligaments — reinforce lateral and medial surfaces; stabilize joint at full extension
• Anterior and posterior cruciate ligaments (ACL/PCL) — inside the joint capsule; cross each other; limit anterior and posterior femur movement

Complete dislocation is extremely rare because of extensive ligamentous support. Subjected to much greater forces than the elbow.

Integumentary → Skeletal: Synthesizes vitamin D3 when exposed to UV radiation. Vitamin D3 is converted by the liver and kidneys to calcitriol, which stimulates calcium and phosphate absorption in the digestive tract — essential for bone mineralization and maintenance.

Skeletal → Integumentary: Provides structural support for the skin and other soft tissues of the integument.

Skeletal → Muscular: Bones serve as levers; provide attachment sites for tendons and muscles; work with muscles to maintain body position and produce controlled movements. Bones transmit muscular forces to produce movement.

Muscular → Skeletal: Muscle contractions apply mechanical stress to bones, stimulating bone remodeling and maintaining bone mass. About 85% of body heat generated by muscular activity helps maintain body temperature that supports enzymatic activity in bone metabolism.

Endocrine → Skeletal: Multiple hormones regulate skeletal function:

• Parathyroid hormone (PTH) — elevates blood calcium by mobilizing calcium from bone (increases osteoclast activity)
• Calcitriol (from kidneys, stimulated by vitamin D3) — elevates blood calcium by increasing GI absorption
• Calcitonin (thyroid) — depresses blood calcium; promotes calcium deposition in bone
• Growth hormone — stimulates bone elongation during development
• Sex hormones (estrogen/androgens) — stimulate osteoblast activity; accelerate growth at puberty; decline leads to osteoporosis (especially post-menopausal estrogen decline)
• Thyroid hormones — essential for normal skeletal growth and development

Skeletal → Cardiovascular: Red bone marrow produces red blood cells, white blood cells, and platelets (hematopoiesis) — all of which enter the cardiovascular system. Calcium stored in bone is essential for cardiac muscle contraction and blood clotting.

Cardiovascular → Skeletal: Blood vessels run through Haversian canals, perforating canals, and the periosteum — delivering oxygen and nutrients to osteocytes and removing wastes. Blood pressure drives filtration that delivers nutrients to bone.

Skeletal → Lymphatic: Red bone marrow produces lymphocytes, which are the primary cells of the immune/lymphatic system. Lymphatic vessels drain excess fluid from around bone tissue.

Digestive → Skeletal: Absorbs calcium, phosphate, and other minerals from food — directly supplying the raw materials needed for ossification and remodeling. Calcitriol (activated by kidneys) stimulates this absorption. Adequate dietary calcium (especially during growth) is essential to prevent osteoporosis.

Urinary → Skeletal: The kidneys convert the liver-processed derivative of vitamin D3 into calcitriol — the active hormone that regulates calcium absorption. Kidneys also regulate blood levels of calcium and phosphate through selective reabsorption or excretion. PTH and calcitonin act on the kidneys to regulate these processes.

Skeletal → Nervous: The skull protects the brain; vertebrae shield the spinal cord. Together the cranial cavity and vertebral canal house and protect the entire central nervous system. Calcium ions (stored in bone) are essential for normal neuron function — dramatic deviations in blood calcium cause neurological symptoms ranging from unresponsiveness (hypercalcemia) to convulsions and death (hypocalcemia).

Nervous → Skeletal: Nerves running through intervertebral foramina carry signals to and from the spinal cord. Sensory nerves in the periosteum detect pain from bone injury (periosteum is highly sensitive). Neural signals control the muscles that apply mechanical stress to bones, maintaining bone mass through use.