Skeletal System: Introduction

Two parts of skeleton

• Axial skeleton

• Appendicular skeleton

Functions of skeleton

• Support

• Protection

• Movement

• Regulation of minerals

• Production of blood cells

• Fat storage

Stuctures of bone

• Diaphysis

• Epiphysis

• Metphyses

• Articular Cartilage

• Periosteum

• Medullary cavity

• Endosteum

Diaphysis (Part of bone)

• Body/shaft

• Main portion

• Long, cylindrical

Epiphysis (Part of bone)

• Proximal/distal ends of the bone

Metaphyes (part of bone)

• Between the diaphysis and the epiphysis

• Contains an epiphyseal (growth) plate

• Hyline cartilage on top of bone

• Allows the diaphysis of bone to grow

If no growth turns to bone resulting in epiphseal line

Articular cartilage (part of bone)

• A thin layer of hyline on top of the epiphysis to form an articulation (joint)

• Reduces friction, absorbs shock, and has freely movable joints.

• Lacks perichondria and blood vessels, with limited damage repair.

Periosteum (Part of bone)

• Connective tissue sheath

  • Outer fibrous layer (dense, irregular)

  • Inner osteogenic layer (osteoprogenitor cells)(helps bone grow in thickness/NOT length)

• Protects bone, gives nutrients, and provides attachment for ligaments/tendons

• Periosteum attached by sharpeys fibres (thick bundles of collagen that extend) to the bone extracellular matrix)

Medullary Cavity (Part of bone)

• Marrow cavity, hollow, cylindrical within the diaphysis

• Contains fatty yellow bone morrow with blood vessels

• Min weight of bone

Endosteum (Part of bone)

• Thin membrane

• Lines medullary cavity and spongy bone

• Single layer amount of connective tissue

Calcification

• Initiated by osteoblasts

• Mineral salts form into crystals within a framework of collagen fibers

Hardness and flexibility of bones

• Hardness: Crystal/minerals

• Flexibillity: Collagen fibers

Four types of bone cells

1) Osteoprogenitor Cells

2) Osteoblasts

3) Osteocytes

4) Osteoclasts

Osteoprogenitor Cells

• Unspecialized bone cells (stem cells)(from mesenchyme)

• Develops into osteoblasts

• Found along the inner osteogenic layer of periosteum, in endosteum, in canals within bone that have blood vessels.

Osteoblasts

• Synthesize/secrete collagen fibers + other organic components to build extracellular matrix of bone tissue

• Initiate calcification

• Becomes osteocytes b/c as osteoblasts build bone they get trapped in their secretions.

Osteocytes

• Mature bone cells

• Maintain daily metabolism, exchange nutrients/wastes into blood

• Do NOT undergo division

Osteoclasts

• Derived from white blood cells, concentrated in the endosteum.

• Face’s bone surface

• Deeply folded, releases lysosomal enzymes/acids that digest protein/mineral components of extracellular bone matrix.

• Help regulate blood calcium.

Compact (dense) Bone (Functions)

• Protection

• Support

• Resists dtrains from movement/weight

Bones change with

Lines of strain

• Learn to walk

• Physical activity

Compact bone (circular)

Osteons (or the haversian systems)

Osteons (Composed of)

• Concentric bone lamellae around the osteonic canal

• Lacunae

• Canaliculi

• Interstitial bone lamellae

• Perforating (Vokmann’s) canals

• Circumferential bone lamellae

Concentric bone lamellae around osteonic canal (Compact bone)

• Surrounds a small network of blood vessels/nerves in osteonic cancal

Lacunae (Compact bone)

• Between concentric bone lamellae (in those small spaces)

• Contains osteocytes

Canaliculi (Compact bone)

• All directions (little vessels-like structures) from lacumae

• Filled with extracullualr fluid

• Has osteocytes (can communicate through gap junctions)

Interstitial bone lamellae (Compact bone)

• Areas between neighbouring osteons containing hone lamellae.

• Also have bone lacunae with osteocytes and Canaliculi

• Are older osteons that have been slightly destroyed during the rebuilding of bone.

Perforating (Vokmanns) canals (Compact bone)

• Where blood vessels, nerves from periosteum penetrate the compact bone

• Connect with medullary cavity, periosteum, osteonic canals

Circunferential bone lamellae (Compact bone)

• Around outer/inner circumference of the bone

• Types

  • External circumferenial lamellae (Directly deep to periosteum (connected to peeriosteum by sharpey fibers)

  • Internal circumferenial lamellae (line medullary cavity)

Spongy bone (always found)

• Located in interior of bone

• Protected by covering og compact bone

Spongy bone (Composed of)

• Trabeculae

• Endosteum

• Red/yellow bone marrow

• Osteocytes

• Bone lacumae

• Bone canaliculi

Trabeculae (Spongy bone)

Bone lamellae in irregular patterns (thin columns)

Endostrum (Spongy bone)

• Line trabeculae spaces

Red/Yellow bone marrow

• Fill trabecular spaces

• Both have blood vessels

Bone formation (types)(principal situations)

1) Initial Formation (embryo to fetus)

2) Growth of bone during infancy/childhood, adolescence

3) Remodeling bones

Mesenchyme

• Embryonic ‘skeleton’

• Site where cartilage + ossification occur within 6th week (2 patterns of bone formation)

2 Patterns of bone formation

• Intramembrane ossification

• Endochondral ossification

Intramembrane Ossification (simple)

• Mesenchyme to bone

Endochondral ossification (simple)

• Hyline cartilage to bone

Intramembrane Ossification (steps)

1) Development of the ossification centre

2) Calcification

3) Formation of bone Trabeculae

4) Development of Periosteum

Development of ossification (Intramembrane Ossification)

• First step

• Chemical messages cause mesenchyme cells to cluster and differentiate. Become osteoprogenitor cells into osteoblasts (such clusters become an ossification centre).

Calcification (Intramembrane ossification)

• Second step

• Secretion of extracellular matrix stops, cells NOW osteocytes (in bone lacunae). Extend into canaliculi.

• After few days, calcium/minerals salts harder/calcifies

Formation of Bone Trabeculae (Intramembrane ossification)

• Third step

• Bone extracellular matrix forms trabeculae

• Keep on fusing to form spongy bone. Usually around blood vessels.

Development of periosteum (intramembrane ossification)

• Four step

• Mesenchyme condenses at the periphery of the bone, and develops periosteum

• Compact bone replaces surface layers of spongy bone.

Endochondral Ossification (Steps)

1) Development of the cartilage model

2) Growth of cartilage model

3) Development of Primary ossification

4) Development of medullary cavity

5) Development of secondary ossification centers

6) Formation of articular cartilage

Development of cartilage model (Endochondral)

• First Steps

• Chemical message causes the Mesenchyme to crowd into the general shape of bone.

• Develop chondroblasts that secrete cartilage extracellular matrix (hyaline cartilage)

Growth of cartilage model (Endochondral ossification)

• Second step

• One chondroblast, deeply buried, is called a chondrocyte

• Chondrocytes develop from within (a process called interstitial growth). Result: increased length.

Development of the primary ossification center (Endochondral ossification)

• Thrid Step

• Primary ossification proceeds inward (from the external surface)

• The nutrient artery penetrates the perichondrium and stimulates osteoprogenitor cells in the perichondrium to differentiate into osteoblasts.

• Once the perichondrium forms bone = periosteum. Will start califying and inducing primary ossification center (which will replace most cartilage to bone)

• Osteoblasts start to form spongy bone trabeculae.

Development of the medullary cavity (Endochondral ossification)

• Fourth Step

• The primary ossification center grows towards the outer. Osteoclasts break down the center (spongy trabeculae). Leaving a cavity.

Development of secondary ossification centers (Endochondral ossification)

• Fifth step

• Secondary ossification centers develops

• Spongy bone remain interior of epiphyses

• Secondary ossification proceeds outward

Types of growth

• Interstitial growth (length)

• Appositional growth (width)

Interstitial Growth Zones (Types)

• Zone of resting cartilage

• Zone of proliferating cartilage

• Zone of hypertrophic cartilage

• Zone of calcified cartilage

Zone of resting cartilage (Interstitial growth)

• First growth

• Anchor epiphyseal plate to epiphysis of bone

Zone of proliferating cartilage (interstitial growth)

• Second growth

• Divide, replace those that die at diaphysial side of epiphyseal plate

Zone of hypertrophic cartilage (interstitial growth)

• Third growth

• Large, maturing chondrocytes arranged in columns

Zone of calcified cartilage (interstitial growth)

• Fourth growth

• Dead/calcified chondrocytes

• Osteoblasts invade and make bone (endochondral ossification)

Appositional (Thickness) growth (steps)

1) Form ridges

2) Ridges larger

3) New concentric bone lamellae

4) Osteon forming

Remodeling of bone

• Osteoclast attaches to the bone surface and forms a leakproof seal that, once broken, it releases lysosomal enzymes and acids

• Enzymes break down fibres/organic substances

• Acids dissolve the bone minerals

• Substance is ingested by osteoclasts by endocytosis (exits by exocytosis) on the other side of the ruffled border

Factors affecting remodeling/growth

• Normal bone metabolism

• Minerals

• Vitamins

• Hormones

Fracture and repair of bone (steps)

1) Reactive Phase

2a) Reparative phase: fibrous cartilage callus formation

2b) Reparative phase: Bony callus formation

3) Bone remodelling phase

Reactive Phase (Fracture and repair of bone)

• First step

• Early information

• Forms a fracture hematoma

• Nearby bone cells die

• Phagocytes/osteoclasts begin to remove dead/damage tissue.

Reparative phase: fibrous cartilage callus formation (Facture and repair of bone)

• Second step (A)

• Blood vessels grow into the fracture hematoma, and phagocytes begin to clean up the dead bone cells

• Fibroblasts produce collagen

• Start to produce cartilage

Reparative Phase: Bony callus formation (Fracture and repair of bone)

• Second step (B)

• Osteogenitor cells develop into osteoblasts. Produce spongy bone trabeculae.

Bone Remodelling Phase (Fracture and repair of bone)

• Third step

• Original fragments of broken bone are resorbed by osteoclasts

• Compact bone replaces spongy bone

Fracture and remodelling bone (Simplifed)

1) Fracture occurs

2) Phagocytes begin their work by removing dead bone tissue

3) Chondroblasts from fibrocartilage at the site of fracture

4) The fibrocartilage is converted to spongy bone tissues by action of osteoblasts

5) Bone remodelling occurs

6) Dead bone is absorbed by the action of osteoclasts

7) Spongy bone is converted to compact bone.

Osteoporotic Bone

It is not dense bone (more than usual).

Long Bones

Arms, legs, fingers

Short Bones

Carpel

Flat Bones

Shoulder, wings of pelvis

Irregular Bones

Vertebrae

Seasamoid Bones

Knee cap

Sutural Bones

In skull

Fissure

Narrow slit between adjacent parts of bones through which blood vessels or nerves pass

Foramen

Opening through which blood vessels, nerves, or ligaments pass

Fossa

Shallow depression

Sulcus

Furrow along the bone surface that accommodates a blood vessel, nerve, or tendon

Meatus

Tubelike opening

Condyle

Large, round protuberance with a smooth articular surface at end of the bone

Facet

Smooth, flat, slightly concave or convex articular surface

Head

Usually, a rounded articular projection is supported on the neck (constricted portion) of the bone.

Crest

Prominent ridge or elongated projections

Epicondyle

Typically roughened projection above condyle

Line

Long, narrow ridge or border (less prominent than crest)

Spinous process

Sharp, slender projection

Trochanter

Very large projection

Tubercle

Variably-sized rounded projection

Tuberosity

Variably-sized projection that has a rough, bumpy surface

Depressions and openings (types)

• Fissue

• Foramen

• Fossa

• Sulcus

• Meatus

Processes

Processes that form a joint

• Condyle

• Facet

• Head

Process that forms attachment points for connective tissue

• Crest

• Epicondyle

• Line

• Spinous process

• Trochanter

• Tuberosity

• Tuberosity