Notes on Bones and Bone Tissue

Bones and Bone Tissue

Introduction

  • The skeletal system consists of:
    • Skeletal bones
    • Cartilage
    • Ligaments
    • Connective tissue (stabilizes the skeleton)
  • Bones are dynamic organs with several tissue types.

Cartilage

  • Connective tissue made up of chondrocytes only.
  • Not innervated (no nerve supply).
  • Avascular (no blood supply).
  • Springy in nature.
  • Located in:
    • External ear, nose, larynx
    • Between vertebrae
    • Pubic symphysis
    • Articular surfaces: covers the ends of most bones at joints
    • Articular discs: within moveable joints (meniscus)
    • Costal: connects the ribs to the sternum

Cartilage Types

  1. Hyaline:
    • Only collagen fibers
    • Contains lacunae
    • Most abundant type
  2. Elastic:
    • Collagen and elastic fibers
    • Important in areas that bend
  3. Fibrocartilage:
    • Resists pulling forces

Cartilage Growth

  • Stops growing in late teens and doesn’t heal well in adults.
  • Two types of growth:
    1. Appositional Growth:
      • Growth from the outside.
      • Perichondrium: girdle of dense connective tissue surrounds cartilage and secretes matrix, forming new cartilage.
    2. Interstitial Growth:
      • Growth from within.
      • Chondrocytes within the cartilage divide.

Bones

Functions

  • Support: Provides framework for attachment of other organs.
  • Mineral Storage:
    • Calcium ions: Stores 98% of the body’s Ca^{2+}
    • Phosphate ions
  • Blood cell production:
    • Bone marrow produces erythrocytes, leukocytes, and platelets.
    • Red bone marrow: Makes blood cells
    • Yellow bone marrow: Fat storage
  • Movement:
    • Bones act as levers to move bone (ligaments).
    • Muscles attached to bone via tendons.
    • Muscles pull on bones, providing leverage.
  • Protection:
    • Ribs protect heart and lungs.
    • Skull protects brain.
    • Vertebrae protect spinal cord.
    • Pelvic bones protect reproductive organs.

Bone Shapes

  • Shape indicates function.
  • Seven broad categories:
    • Sutural bones: Small, flat, oddly shaped bones found between the flat bones of the skull in the suture line. They develop from separate centers of ossification, regarded as a type of flat bone.
    • Irregular bones: Various shapes, e.g., vertebrae and hips.
    • Short bones: Cuboidal-like, e.g., sesamoid bones.
    • Pneumatized bones: Hollow or contain numerous air pockets, such as the ethmoid.
    • Flat bones: Thin and flat, e.g., cranial bones, ribs, sternum, scapula. Have thin, roughly parallel surfaces of compact bone. They provide protection for underlying soft tissues and offer an extensive surface area for the attachment of skeletal muscles.
      • Internal and external tables relatively thick layers of compact bone.
      • Diploë layer of spongy bone between the tables.
    • Long bones: Columnar-like, e.g., limbs, fingers, toes. Relatively long and slender. They have a diaphysis, two metaphyses, two epiphyses, and a medullary (marrow) cavity
    • Sesamoid bones: Usually small, round, and flat. They develop inside tendons and are most often encountered near joints at the knee, the hands, and the feet. Everyone has sesamoid patellae, or kneecaps.

Anatomy of Skeletal Elements

  • Bony markings include:
    • Projections
    • Depressions
    • Fossa
    • Openings: Sinuses/canals/fissures/foramen
    • Processes: Trochanter/crest/spine/line/tubercle/tuberosity/head/neck/facet/condyle/trochlea

Long Bone Anatomy

  • Compact bone: Dense with smooth outer layer.
  • Spongy layer: Inner layer made of trabeculae (bony network with open spaces filled with bone marrow).
  • Diaphysis: Shaft. Center of shaft = medullary cavity filled with marrow (red & yellow).
  • Epiphyses: Ends.
  • Epiphyseal line: Growth line between the diaphysis & epiphyses.
  • Epiphyseal plate: Origin of bone lengthening, made of hyaline cartilage.

Osseous Tissue Types

  1. Compact Bone (Dense Bone):
    • Dense and solid.
    • Forms the walls of bone.
  2. Spongy Bone (Trabecular or Cancellous Bone):
    • Open network of plates.
    • Surrounds the medullary cavity.
    • Medullary cavity consists of bone marrow.
Compact Bone Details
  • Consists of osteons (basic functional unit of bone).
  • Consists of:
    • Central canal
    • Canaliculi
    • Osteocytes
    • Lacunae
    • Lamellae
  • Looks solid but has passages for vessels and nerves.
  • Strong structural support.
  • Osteon: Important structural component that bears weight and looks like tree rings.
  • Concentric lamellae: Layers of bone matrix (collagen and minerals).
  • Central canal: Core of osteon; vessels in canal provide nutrients to bone.
  • Osteocytes: Cells that make up osteon; mature bone cells necessary for maintaining bone matrix.
    • Occupy the lacunae in bone matrix; spider-shaped.
    • Leg-like structures form gap junctions (canaliculi).
Spongy Bone Details
  • Arranged in parallel struts; forms branching plates called trabeculae.
  • Trabeculae form an open network, making it lightweight, deep, and porous.
  • Lacks osteons.
  • Flexibility.
  • Branching plates of osteocytes and linear layers of lamellae.
  • Vessels are not in central canal; nutrients come from capillaries of the endosteum.

Functional Differences Between Compact and Spongy Bone

  • Compact Bone:
    • Conducts stress from one end of the long bone to the other.
    • Generates tremendous strength from end to end.
    • Weak strength when stress applied to the side.
    • Osteon arrangement is parallel to the bone axis.
  • Spongy Bone:
    • Trabeculae create strength to deal with stress from the side.
    • Trabeculae are oriented along the stress lines.
    • Has extensive cross-bracing.
    • Supports yellow marrow in the shaft.
    • Supports red marrow in the epiphysis.

Bone Structure

  • Epiphysis (ends of the long bones): Consists of red marrow.
  • Diaphysis (shaft of the long bones): Medullary cavity of the diaphysis consists of yellow marrow.
  • Metaphysis: Narrow growth zone between epiphysis and diaphysis.
  • Periosteum:
    • Outer surface of the bone.
    • Isolates and protects the bone from surrounding tissue.
    • Provides a route and a place for attachment for circulatory and nervous supply.
    • Actively participates in bone growth and repair.
    • Attaches the bone to the CT network of deep fascia.
  • Endosteum:
    • Inner surface of bone; lines the medullary cavity.
    • Consists of osteoprogenitor cells.
    • Actively involved in repair and growth.

Periosteum and Endosteum

  • Periosteum: Outer lining (CT membrane covers the entire outside of the bone, except for articular cartilage).
    • Two layers:
      1. Superficial: Resists tension during bending.
      2. Deep: Osteogenic (remodels bone).
  • Endosteum: Inner lining (CT membrane that covers the internal bone surface); osteogenic.
  • Osteoblasts: Cells that generate bone.
    • Osteoblasts become osteocytes when surrounded by bone matrix.
  • Osteoclasts: Cells that resorb bone.

Bone Vascularization and Innervation

  • Well-vascularized (3-11% of blood is in your skeleton).
  • In diaphysis, nutrient foramen made up of nutrient artery and nutrient vein.
  • In epiphyses, epiphyseal veins and arteries.
  • Well innervated.
  • Short, flat, and irregular bones are similar to long bones but lack medullary cavity.
    • Bone marrow is within spongy cavity.

Bone Cells

  • Osteocytes:
    • Mature bone cells.
    • Maintain the protein and mineral content of the matrix.
    • Cause release of calcium ions from the bone to the blood.
    • Sit in depressions (lacunae).
    • Matrix layer associated with osteocytes = lamellae.
    • Small channels extending from the osteocytes to the bone capillaries = canaliculi.
  • Osteoblasts:
    • Immature bone cells.
    • Found on the inner and outer surfaces of bones.
    • Produce osteoid (involved in making the matrix).
    • Involved in making new bone (osteogenesis).
  • Osteoprogenitor cells:
    • Bone stem cells.
    • Found on the innermost layer of the periosteum and inner lining of the endosteum.
    • Differentiate to form new osteoblasts.
    • Heavily involved in the repair of bones after a break.
  • Osteoclasts:
    • Multinucleated cells.
    • Secrete acids that dissolve the bones, releasing stored calcium ions and phosphate ions into the blood (=osteolysis).

Histological Differences of Bone

  • Matrix
  • Osteons
  • Osteocytes
  • Osteoblasts
  • Osteoclasts
  • Osteoprogenitor cell
  • Lamellae
  • Lacunae
  • Canaliculi
  • Central canals

Bone Development and Growth

  • Before six weeks of development, the skeleton is hyaline cartilage.
  • Cartilage cells are replaced by bone cells (ossification).
  • Osteogenesis: Bone formation.
  • Calcification: Deposition of calcium ions into the bone tissue.
  • Two types of ossification:
    • Intramembranous ossification: Involved in the development of clavicle, mandible, skull, and face.
    • Endochondral ossification: Involved in the development of limbs, vertebrae, and hips.

Appositional Growth

  • Increasing the Diameter of a Developing Bone:
    • Inner layer of the periosteum differentiates to form osteoblasts and adds bone matrix to the surface.
    • This forms circumferential lamellae to the outer surface.
    • Osteons form.
    • Bone continues to enlarge in diameter.

Epiphyseal Plate

  • Area of cartilage in the metaphysis.
  • Cartilage near the diaphysis is converted to bone.
  • The width of this zone gets narrower as we age.
  • Marks the former location of the epiphyseal cartilage.

Blood Vessels Associated with Long Bones

  • Four major sets of blood vessels:
    • Nutrient vessels
    • Metaphyseal vessels
    • Epiphyseal vessels
    • Periosteal vessels

Bone Innervation

  • Nerves penetrate the bone with the nutrient artery.
  • Innervates throughout the periosteum, endosteum, medullary cavity, and epiphysis.

Factors Regulating Bone Growth

  • Nutrition
  • Calcium ions
  • Phosphate ions
  • Magnesium ions
  • Citrate
  • Carbonate ions
  • Sodium ions
  • Vitamins A, C, D (calcitriol)
  • Hormones:
    • Parathyroid gland releases parathyroid hormone:
      • Stimulates osteoclasts and osteoblasts
      • Increases calcium ion absorption from the small intestine to the blood
      • Reduces the rate of calcium ion loss from the kidneys
    • Thyroid gland releases calcitonin:
      • Inhibits osteoclasts
      • Increases rate of calcium ion loss in the urine
      • Removes calcium ions from blood and adds it to bone
    • Thyroid gland releases thyroxine:
      • Maintains normal activity in the epiphyseal region
    • Estrogen and testosterone:
      • Stimulate osteoblast activity, producing bone faster than epiphyseal cartilage expansion
      • Ultimately, the epiphyseal cartilage narrows, and bone growth ceases (about age 25)
    • Pituitary gland releases growth hormone (somatotropin):
      • Stimulates bone growth
      • Maintains normal activity of the epiphyseal cartilage

Bone Maintenance, Remodeling, and Repair

Remodeling of Bone

  • Realignment of teeth can change the shape of tooth sockets.
  • Increased muscular development.
  • Bone changes occur due to stress.
    • Different features develop on the bone (attachment of ligaments and tendons).
  • Stressed bones become thicker and stronger.
  • Inactivity of bones can cause degeneration.
    • After a few weeks, unstressed bones can lose ~1/3 of their mass.

Injury and Repair (Fractures)

  • Transverse fractures: Break transverse to the long axis.
  • Displaced fractures: Produce new and abnormal bone arrangements.
  • Compression fractures: Bones “jam” together.
  • Spiral fractures: Bones twist along the length of the bone.
  • Epiphyseal fractures: Fractures within the epiphyseal region.
  • Comminuted fractures: The fractured area shatters into many bony fragments.
  • Greenstick fractures: Only one edge of the bone breaks while the other edge bends.
  • Colles fracture: A break at the distal portion of the radius.
  • Pott fracture: Occurs at the ankles and affects both the tibia and the fibula.
  • When a bone is broken, it bleeds, forming a network of spongy bone.
  • Osteoblasts are overly activated, resulting in an enlarged callused area that is stronger and thicker than normal bone.

Aging and the Skeletal System

  • When we’re young, osteoblast activity balances osteoclast activity.
  • When we get older, osteoblast activity declines.
  • Osteoclast activity maintains previous level of activity.
  • When osteoclast activity is faster than osteoblast activity, bones become porous.
  • As women age, estrogen levels drop, osteoclast control is lost, osteoclasts are overactive, and bones become porous (osteoporosis).

Integration with Other Systems

  • Bones are not inert; they are dynamic structures.
  • Attached to muscles and undergo remodeling.
  • Under physiological control via the endocrine system.
  • Digestive and excretory system provides calcium and phosphate needed for growth.
  • Serves as a store of calcium, phosphate, and other minerals.

Supplemental Resources (Bone Development and Growth)

Nutrient Vessels

  • Enter the diaphysis and branch toward the epiphysis.
  • Enter through the nutrient foramen of the bone.
  • Penetrates the shaft and enters the medullary cavity.
  • Divides into ascending and descending branches to go toward the epiphysis regions.
  • Vessels branch to form perforating vessels.

Metaphyseal Vessels

  • Supply nutrients to the diaphyseal edge of the epiphysis.

Epiphyseal Vessels

  • Supply nutrients to the cavities of the epiphysis.

Periosteal Vessels

  • Supply nutrients to the superficial osteons.

Intramembranous Ossification

  • Mesenchymal cells differentiate to form osteoblasts.
  • Osteoblasts begin secreting a matrix.
  • Osteoblasts become trapped in the matrix.
  • Osteoblasts differentiate and form osteocytes.
  • More osteoblasts are produced, thus move outward.
  • Blood vessels are trapped by the formation of spicules.

Endochondral Ossification

  • Chondrocytes near the center of the diaphysis increase in size.
  • Blood vessels begin to grow around the cartilage.
  • The perichondrial cells begin to form the periosteum.
  • A thin layer of compact bone begins to form.
  • Cartilage cells die and are replaced by osteoblasts.
  • Osteoblasts begin to form spongy bone (primary ossification center).
  • The cartilage in the metaphysis region is invaded by osteoblasts.
  • An increase in bone length and diameter begins.
  • Osteoblasts begin to migrate into the epiphysis region (secondary ossification center).
  • Osteoblasts begin to replace cartilage with bone.
  • This results in pushing the epiphysis away from the diaphysis thus resulting in longer bones.