Bones and Bone Structure

The Skeletal System

Axial and Appendicular Divisions

  • The skeletal system consists of approximately 206206 bones.
  • Axial Skeleton (80 bones):
    • Bones of the skull, thorax, and vertebral column.
    • Forms the longitudinal axis of the body.
  • Appendicular Skeleton (126 bones):
    • Bones of the limbs and girdles that attach them to the axial skeleton.
  • Associated cartilages and ligaments are also part of the skeletal system.

Functions of the Skeletal System

  • Support.
  • Storage of minerals and lipids.
  • Production of blood cells.
  • Protection.
  • Leverage.

Bone Classification

  • Six categories of bone based on shape:
    • Flat bones
    • Sutural bones
    • Long bones
    • Irregular bones
    • Sesamoid bones
    • Short bones
Flat Bones
  • Thin, roughly parallel surfaces.
  • Examples: cranial bones, sternum, ribs, scapulae.
  • Protect underlying soft tissues.
  • Provide surface area for skeletal muscle attachment.
Sutural Bones (Wormian Bones)
  • Irregular bones formed between cranial bones.
  • Number, size, and shape vary.
Long Bones
  • Relatively long and slender.
  • Examples: various bones of the limbs.
Irregular Bones
  • Complex shapes with short, flat, notched, or ridged surfaces.
  • Examples: vertebrae, bones of the pelvis, facial bones.
Sesamoid Bones
  • Small, flat, and somewhat shaped like a sesame seed.
  • Develop inside tendons of the knee, hands, and feet.
  • Individual variation in location and number.
Short Bones
  • Small and boxy.
  • Examples: bones of the wrist (carpals) and ankles (tarsals).

Bone Markings

  • Also known as surface features.
  • Related to particular functions.
    • Elevations/projections: Muscle, tendon, and ligament attachment; at joints where adjacent bones articulate.
    • Depressions/grooves/tunnels: Sites for blood vessels or nerves to lie alongside or penetrate the bone.

Long Bone Features

  • Epiphysis: Expanded area at each end of the bone.
    • Consists largely of spongy bone (trabecular bone).
    • Outer covering of compact bone (cortical bone) - strong, organized bone.
    • Articular cartilage: Covers portions of the epiphysis that form articulations.
  • Metaphysis: Connects epiphysis to shaft.
  • Diaphysis: Shaft.
    • Contains medullary cavity (marrow cavity).
      • Filled with two types of marrow:
        • Red bone marrow (involved in red blood cell production).
        • Yellow bone marrow (adipose tissue; important as an energy reserve).

Blood Supply

  • Growth and maintenance require an extensive blood supply.
  • Vascular features:
    • Nutrient artery and nutrient vein (commonly one of each per bone).
      • Nutrient foramen: Tunnel providing access to marrow cavity.
    • Metaphyseal artery and metaphyseal vein.
      • Carry blood to/from metaphysis.
      • Connect to epiphyseal arteries/veins.

Bone Cells

  • Osteogenic cells (osteoprogenitor cells):
    • Mesenchymal (stem) cells that produce cells that differentiate into osteoblasts.
    • Important in fracture repair.
    • Locations: inner lining of periosteum, lining endosteum in medullary cavity, lining passageways containing blood vessels.
  • Osteoblasts:
    • Produce new bony matrix (osteogenesis or ossification).
      • Produces unmineralized matrix (osteoid).
      • Then assists in depositing calcium salts to convert osteoid to bone.
    • Become osteocytes once surrounded by bony matrix.
  • Osteocytes:
    • Mature bone cells that cannot divide.
    • Maintain protein and mineral content of surrounding matrix.
    • Occupy lacunae (pockets).
      • Separated by layers of matrix (lamellae).
      • Interconnected by canaliculi.
  • Osteoclasts:
    • Remove and remodel bone matrix.
    • Release acids and proteolytic enzymes to dissolve matrix and release stored minerals.
      • Process called osteolysis.

Bone Matrix

  • Collagen fibers account for ~13{\frac{1}{3}} bone weight.
    • Provide flexibility.
  • Calcium phosphate (Ca<em>3(PO</em>4)2Ca<em>3(PO</em>4)_2) accounts for ~23\frac{2}{3} bone weight.
    • Interacts with calcium hydroxide (Ca(OH)<em>2Ca(OH)<em>2) to form crystals of hydroxyapatite (Ca</em>10(PO<em>4)</em>6(OH)2Ca</em>{10}(PO<em>4)</em>6(OH)_2) salts.
      • Incorporates other salts (calcium carbonate, CaCO3CaCO_3) and ions (Na+,Mg2+,F−Na^+, Mg^{2+}, F^−).
      • Provides strength.

Compact Bone

  • Functional unit is osteon (Haversian system).
    • Organized concentric lamellae around a central canal.
      • Osteocytes (in lacunae) lie between lamellae.
      • Central canal contains small blood vessels.
      • Canaliculi connect lacunae with each other and central canal.
    • Strong along its length.

Long Bone Organization

  • Periosteum outermost layer
  • Compact bone outer bone tissue layer
    • Circumferential lamellae (circum-, around + ferre, to bear) at outer and inner surfaces
    • Interstitial lamellae fill spaces between osteons
    • Osteons
      • Connected by perforating canals (perpendicular to surface)
  • Spongy bone innermost layer

Spongy Bone

  • Lamellae form struts and plates (trabeculae) creating an open network.
    • No blood vessels in matrix.
      • Nutrients reach osteons through canaliculi open to trabeculae surfaces.
    • Red bone marrow is found between trabeculae.

Appositional Bone Growth

  • Increases bone diameter of existing bones.
  • Osteogenic cells differentiate into osteoblasts that add bone matrix under periosteum.
    • Adds successive layers of circumferential lamellae.
    • Trapped osteoblasts become osteocytes.
  • Deeper lamellae recycled and replaced by osteons.
  • Osteoclasts remove matrix at the inner surface to enlarge the medullary cavity.

Periosteum

  • Wraps the superficial layer of compact bone.
  • Two layers:
    • Fibrous outer layer.
    • Cellular inner layer.
  • Functions:
    • Isolates bone from surrounding tissues.
    • Route for blood and nervous supply.
    • Actively participates in bone growth and repair.
  • Perforating fibers allow for strong attachment.

Endosteum

  • Incomplete cellular layer lining medullary cavity.
  • Active during bone growth, repair, remodeling.
  • Covers spongy bone and lines central canals.
  • Where the layer is incomplete, exposed matrix is remodeled by osteoclasts and osteoblasts.
    • Osteoclasts in shallow depressions called osteoclastic crypts (Howship’s lacunae).

Endochondral Ossification

  • Initial skeleton of embryo formed of hyaline cartilage.
  • Cartilage gradually replaced by bone through endochondral ossification.
  • Uses cartilage as a small model.
  • Bone grows in diameter and length.
    • Diameter growth involves appositional bone deposition.
Steps in Endochondral Ossification
  1. Cartilage model enlarges.
    • Chondrocytes near the center of the shaft enlarge.
    • Enlarged chondrocytes die and disintegrate.
    • Disintegration leaves cavities within cartilage.
  2. Blood vessels grow around the edge of the cartilage model.
    • Cells of perichondrium convert to osteoblasts.
    • Osteoblasts form a superficial layer of bone along the shaft.
  3. Blood vessels penetrate cartilage and enter the central region.
    • Entering fibroblasts differentiate into osteoblasts.
    • Begin spongy bone production at the primary ossification center.
    • Bone formation spreads along the shaft toward both ends.
  4. Growth continues along with remodeling.
    • The medullary cavity is created.
    • Osseous tissue of the shaft thickens.
    • Cartilage near the epiphyses is replaced by shafts of bone.
    • Bone grows in length and diameter.
  5. Capillaries and osteoblasts migrate into the epiphyses.
    • Create secondary ossification centers.
  6. Epiphyses fill with spongy bone.
    • Articular cartilage remains exposed to the joint cavity.
    • Epiphyseal cartilage (epiphyseal plate) separates epiphysis from diaphysis.
  7. Bone grows in length at the epiphyseal cartilage.
    • Chondrocytes actively produce more cartilage on the epiphyseal side.
    • Osteoblasts actively replace cartilage with bone on the diaphyseal side.
    • Epiphyses are pushed away by continued production of new cartilage.

Bone Growth at Puberty

  • At puberty, hormones stimulate increased bone growth, and epiphyseal cartilage is replaced.
    • Osteoblasts produce bone faster than chondrocytes produce cartilage.
    • Epiphyseal cartilage narrows until it disappears.
      • Process called epiphyseal closure.
      • Leaves epiphyseal line in adults.

Intramembranous Ossification

  • Begins when mesenchymal (stem) cells differentiate into osteoblasts within embryonic or fibrous connective tissue.
  • Normally occurs in deeper layers of the dermis.
  • Bones called dermal bones or membrane bones.
  • Examples: roofing bones of the skull, lower jaw, collarbone, sesamoid bones (patella).
Steps of Intramembranous Ossification
  1. Mesenchymal cells cluster.
    • Differentiate into osteoblasts.
      • Secrete osteoid matrix.
        • Osteoid matrix becomes mineralized, forming bone matrix.
          • Location in tissue where ossification begins is ossification center.
  2. Bone grows out in small struts (spicules).
    • Osteoblasts become trapped in pockets and mature into osteocytes.
    • Mesenchymal cells produce more osteoblasts.
  3. Blood vessels enter the area.
    • Bone spicules meet and fuse.
    • Blood vessels are trapped in developing bone.
  4. Continued deposition of bone by osteoblasts close to blood vessels.
    • Results in spongy bone with interwoven blood vessels.
  5. Remodeling around blood vessels produces osteons of compact bone.
    • Connective tissue around bone organizes into a fibrous layer of the periosteum.
    • Osteoblasts near bone surface remain as a cellular layer of the periosteum.

Intramembranous Ossification in Development

  • Begins during the eighth week of embryonic development.
  • Ossification centers and progressing bone formation can be seen at 10 weeks.
  • At 16 weeks, most of the bones of the adult skeleton can be identified.

Abnormalities of Bone Growth and Development

Disorders Causing Shortened Bones
  • Pituitary growth failure
    • Inadequate growth hormone production
    • Reduced epiphyseal cartilage activity; abnormally short bones
    • Rare in the United States due to treatment with synthetic growth hormone
  • Achondroplasia
    • Epiphyseal cartilage of long bones grows slowly
      • Replaced by bone early in life
    • Short, stocky limbs result
    • Trunk is normal size
    • No effects on sexual or mental development
Disorders Causing Lengthened Bones
  • Marfan syndrome
    • Inherited metabolic condition
    • Excessive cartilage formation at epiphyseal cartilages
    • Results in a very tall person with long, slender limbs
    • Affects other connective tissues throughout the body
      • Commonly causes cardiovascular problems
  • Gigantism
    • Overproduction of growth hormone before puberty
      • Can reach heights of over 2.72.7 m (8 ft. 11 in.)
    • Puberty often delayed
    • The most common cause is a pituitary tumor
    • Treated by surgery, radiation, or medications suppressing growth hormone release
  • Acromegaly
    • Overproduction of growth hormone after epiphyseal plates close
      • Bones get thicker, not longer, especially those in the face, jaw, and hands
    • Alterations in soft-tissue structure change physical features

Bones as Mineral Reservoirs

  • Minerals: Inorganic ions contributing to the osmotic balance of body fluids; vital in many physiological processes
  • The Importance of Calcium
    • The most abundant mineral in the body
      • 1–21–2 kg (2.2–4.42.2–4.4 lb)
      • ~9999 percent deposited in the skeleton
    • A variety of physiological functions (muscle contraction, blood coagulation, nerve impulse generation)
      • Concentration variation greater than 30–3530–35 percent affects neuron and muscle function
      • Normal daily fluctuations are < 1010 percent

Maintaining Calcium Levels

  • Controlled by activities of:
    • Intestines: Absorb calcium and phosphate under hormonal control
    • Bones: Osteoclasts erode matrix and release calcium, osteoblasts use calcium to deposit new matrix
    • Kidneys: Varying levels of calcium and phosphate loss in urine under hormonal control
  • The primary hormones regulating calcium ion metabolism are parathyroid hormone, calcitriol, and calcitonin
Factors That Increase Blood Calcium Levels
  • Parathyroid hormone (PTH)
    • Secreted from the parathyroid glands
    • Responses:
      • In bones: osteoclasts are stimulated to erode matrix, releasing stored calcium
      • In intestines: calcitriol effects enhanced and calcium absorption increased
      • In kidneys: increased release of hormone calcitriol, stimulating calcium reabsorption in kidneys

Calcium and the Skeleton

  • As a calcium reserve, the skeleton has a primary role in calcium homeostasis.
  • Has a direct effect on the shape and strength of bones
    • The release of calcium into the blood weakens bones.
    • The deposition of calcium salts strengthens bones.

Bone Fractures

  • Crack or break due to extreme mechanical stress
  • Most heal as long as the blood supply and cellular parts of the periosteum and endosteum survive
  • Repair involves four steps
Steps in Fracture Repair
  1. Fracture hematoma formation
    • A large clot closes injured vessels; develops within several hours
  2. Callus formation
    • Internal callus: network of spongy bone unites inner edges of fracture
    • External callus: composed of cartilage and bone stabilizes outer edges of fracture
  3. Spongy bone formation
    • The cartilage of external callus replaced by spongy bone
    • Bone fragments and dead bone are removed and replaced
    • Ends of fracture held firmly in place
  4. Compact bone formation
    • Spongy bone replaced by compact bone
    • Remodeling over time eliminates evidence of fracture
General Categories of Fractures
  • Closed or simple:
    • Completely internal (no break in the skin)
      • Only seen on x-rays
  • Open or compound:
    • Projects through the skin
      • More dangerous due to infection and uncontrolled bleeding
Specific Types of Fractures
  • Transverse fractures: break shaft across long axis
  • Spiral fractures: produced by twisting stresses, spread along the length of the bone
  • Displaced fractures: produce new and abnormal bone arrangements; non-displaced fractures retain normal alignment
  • Compression fractures: occur in vertebrae subjected to extreme stresses, often associated with osteoporosis
  • Greenstick fractures: one side of shaft broken, one side bent; generally occurs in children because long bones have yet to fully ossify
  • Comminuted fractures: shatter affected area producing fragments
  • Epiphyseal fractures: occur where bone matrix is calcifying
    • A clean transverse fracture of this type heals well
    • If not monitored, breaks between epiphyseal plate and cartilage can stop growth at site
  • Pott’s (bimalleolar) fracture: occurs at the ankle and affects both medial malleolus and lateral malleolus
  • Colles fracture: Break in distal radius