Bone and Bone Structure

• Skeletal system includes

  – Bones of the skeleton

  – Cartilages, ligaments, and other connective tissues
• Primary functions of the skeletal system 

  – Support

  – Storage of minerals and lipids 

  – Protection

  – Leverage

  – Blood cell production
• Bones are classified by their 

  – Shape

  – Structure
• Bone shapes 

  – Sutural

  – Irregular 

  – Flat

  – Long

  – Short

  – Sesamoid
• Sutural bones (Wormian bones)

  – Small, flat, irregularly shaped bones 

  – Number varies among individuals

  – Between flat bones of the skull (knit together)
• Irregular bones

  – Have complex shapes

  – Examples: spinal vertebrae, pelvic bones
• Short bones 

  – Boxy

  – Examples: carpal bones and tarsal bones
• Flat bones

  – Thin with parallel surfaces

  – Examples: bones of skull roof, sternum, ribs, and scapulae
• Long bones

  – Long and slender

  – Found in arms, legs, palms, soles, fingers, toes
• Sesamoid bones

  • Usually small, round, and flat
  • Develop within tendons near joints of knees, hands, and feet
  • Location and number vary between individuals
  • Example: patellae
• Bone markings (surface features) 

  – Projections: process and ramus & trochanter, crest, spine, line, tubercle, tuberosity, head, neck, facet, condyle, trochlea

  • Where muscles, tendons, and ligaments attach

  • At articulations with other bones 

  – Openings and depressions: sinus, foramen, fissure, meatus, canal & sulcus, fossa

  • For passage of blood vessels and nerves
• Structure of a long bone

  • Diaphysis (shaft)

  • Wall of compact bone

  • Central space called medullary cavity (marrow cavity)

  • Epiphysis (wide part at each end)

  • Mostly spongy bone (trabecular bone)

  • Metaphysis

  • Where diaphysis and epiphysis meet
• Structure of flat bones

  • For example, parietal bones of the skull
  • Consist of spongy bone between two layers of compact bone (cortex)
  • Within the cranium, the layer of spongy bone is called the diploë
• Bone tissue

  – Dense, supportive connective tissue

  – Contains specialized cells

  – Solid extracellular matrix with collagen fibers
• Characteristics of bone

  • Dense matrix due to deposits of calcium salts
  • Osteocytes (bone cells) within lacunae organized around blood vessels
  • Canaliculi
  • Narrow passageways that allow for exchange of nutrients, wastes, and gases
  • Periosteum
  • Covers outer surfaces of bones (except at joints) 
  • Consists of outer fibrous and inner cellular layers
• Bone matrix

  – Calcium phosphate, Ca3(PO4)2 makes up almost 2/3 of bone mass

  • Interacts with calcium hydroxide, Ca(OH)2, to form

  crystals of hydroxyapatite, Ca10(PO4)6(OH)2

  – Incorporates other calcium salts such as calcium

  carbonate (CaCO3) and ions (e.g., magnesium)

  – A bone lacking a calcified matrix looks normal, but is very flexible
• Bone tissue

  - Bone matrix

  – Matrix proteins

  • About 1/3 of bone mass is collagen fibers
• Bone tissue - Bone cells

  – Make up only 2% of bone mass 

  – Four types

  • Osteogenic cells 

  • Osteoblasts

  • Osteocytes

  • Osteoclasts
• Osteogenic cells (osteoprogenitor cells)

  – Mesenchymal cells that divide to produce osteoblasts

  – Located in inner cellular layer of periosteum and in endosteum

  – Assist in fracture repair
• Osteoblasts

  – Immature cells that produce new bone matrix during

  osteogenesis (ossification)

  – Osteoid — matrix produced by osteoblasts that has not yet become calcified

  – Osteoblasts surrounded by bone matrix become osteocytes
• Osteocytes

  – Mature bone cells that do not divide

  – Live in lacunae between layers of matrix

  – Have cytoplasmic extensions that pass through canaliculi

  – Two major functions

  • Maintain protein and mineral content of matrix 

  • Help repair damaged bone
• Osteoclasts

  • Absorb and remove bone matrix
  • Large, multinucleate cells
  • Secrete acids and protein - digesting enzymes
  • Dissolve bone matrix and release stored minerals
  • This osteolysis is important in homeostasis
  • Derived from the same stem cells that producemonocytes and macrophages
• Osteon

  — functional unit of compact bone 

  – Central canal contains blood vessel(s)

  – Perforating canals

  • Perpendicular to surface of bone

  • Carry blood vessels into deep bone and marrow

  – Lamellae — layers of bone matrix

  • Concentric lamellae surround central canal
  • Interstitial lamellae fill spaces between osteons
  • Circumferential lamellae are at outer and inner bone surfaces
• Spongy bone lacks osteons

  – Matrix forms an open network of trabeculae

  • Lacks capillaries and venules

  – Red bone marrow fills spaces between trabeculae

  • Forms blood cells

  • Contains blood vessels that supply nutrients to osteocytes by diffusion

  – Yellow bone marrow

  • Found in other sites of spongy bone 

  • Stores fat
• Weight-bearing bones

  – Trabeculae in epiphysis of femur transfer forces from

  pelvis to compact bone of femoral shaft 

  • Causing tension on the lateral side

  • Medial side of shaft compresses
• Periosteum — membrane that covers outside of bones

  - Except within joint cavities

  - Outer, fibrous layer and inner, cellular layer

  - Fibers are interwoven with those of tendons

  - Perforating fibers — fibers that become incorporated into bone tissue

  - Increase strength of attachments
• Functions of periosteum

  – Isolates bone from surrounding tissues

  – Provides a route for blood vessels and nerves 

  – Participates in bone growth and repair
• Endosteum — incomplete cellular layer that lines medullary cavity

  – Active during bone growth, repair, and remodeling 

  – Covers trabeculae of spongy bone

  – Lines central canals of compact bone

  – Consists of flattened layer of osteogenic cells
• Fractures

  – Cracks or breaks in bones due to physical stress

  –  Open (compound) or closed (simple)

  § Major types of fractures

  – Transverse, displaced, compression, spiral, epiphyseal, comminuted, greenstick, Colles, Pott’s
• Fractures are repaired in four steps 

  1. Fracture hematoma formation 

  2. Callus formation

  3. Spongy bone formation

  4. Compact bone formation
• Fracture hematoma formation

  – Production of a large blood clot 

  – Establishes a fibrous network 

  – Bone cells in the area die
• Callus formation

  - Cells of endosteum and periosteum divide and migrate into fracture zone

  - Calluses stabilize the break

  - Internal callus develops in medullary cavity

  - External callus of cartilage and bone surrounds break
• Spongy bone formation

  – Osteoblasts replace central cartilage of external callus with spongy bone
• Compact bone formation

  – Repaired bone may be slightly thicker and stronger than normal, often weaker with a scar
• Bones become thinner and weaker with age

  – Osteopenia — inadequate ossification (reduction of bone mass)

  • Begins between ages 30 and 40

  • Women lose 8% of bone mass per decade 

  • Men lose 3%

  – Epiphyses, vertebrae, and jaws are most affected

  • Results in fragile limbs, reduced height, and tooth loss
• Osteoporosis —severe loss of bone mass 

  – Over age 45, occurs in

  – Compromises normal function

  • 29% of women 

  • 18% of men
• Hormones and bone loss

  – Sex hormones help maintain bone mass

  – In women, osteoporosis accelerates after menopause
• Cancer and bone loss

  – Cancerous tissues release osteoclast-activating factor

  • Stimulates osteoclasts

  • Produces severe osteoporosis, don’t know until they have spontaneous bone fractures