MCB 244 Skeletal System

Types of bone 

compact and spongy

Compact bone

• dense or cortical bone

• 80% of bone mass

Spongy bone

• cancellous or trabecular bone

• located internal to compact bone

• appears porous

• 20% of bone mass

Cartilage

• semirigid CT, more flexible than bone

• types: hyaline cartilage and fibrocartilage

• structures composed of dense regular CT

____ connect bone to bone, ____ connects muscle to bone

ligaments; tendons

Hyaline Cartilage

attaches ribs to sternum, covers ends of some bones, within growth plates, model for bone formation

Fibrocartilage

weight-bearing cartilage that withstands compression (intervertebral discs, pubic symphysis, menisci of knee)

Coastal cartilage

located in the ribs

Where is fibrocartilage found

thorax, cartilage of inverterbral disc, and pubic symphysis

Where is hyaline cartilage found

in bone sockets of shoulder, arm, wrist, fingers, femur, knees, and toes

Where is articular cartilage found

Kids: at the neck followed by epiphyseal plate at each end

Adults: joints of ankles and shoulders

Function of bones

support, protection, levers for movement, storage of mineral and energy reserves, and hematopoiesis

Hematopoiesis

blood cell production; occurs in red bone marrow CT

What are the classifications of bones based on shape 

• long bones

• short bones

• flat bones

• irregular bones

Long bones

greater in length than width

ex: femur, humerus

Short bones

length nearly equal to width

ex: carpal and tarsals

Flat bones

flat, thins surfaces, may be slightly curved

ex: cranial

Irregular Bone

elaborate sometimes complex shapes

ex: vertebrae

Regions of a long bone

• Diaphysis 

• Medullary (marrow) cavity

• Epiphysis

• Articular Cartilage

• Metaphysis

• Epiphyseal plate

Diaphysis

• elongated, usually cylindrical shaft

• provides leverage and weight support

• compact bone with thin spicules of spongy bone extending inward

Medullary (marrow) cavity

• hollow, cylindrical space within the diaphysis

• contains red bone marrow in children

• contains yellow bone marrow in adults

Epiphysis

knobby region at each end of long bone (2 long bones can meet end-to-end at their epiphyses)

• proximal epiphysis

• distal epiphysis

Proximal epiphysis

end of the bone closest to the body trunk 

Distal epiphysis

end farthest from trunk; outer thing layer composed of compact bone; inner region composed of spongy bone

Articular cartilage

• covers the joint surface

• thin layer of hyaline cartilage

• reduces friction

• absorbs shock in moveable joints

Metaphysis

region where bone widens and transfers weight between the diaphysis and epiphysis

Epiphyseal plate

located in metaphysis

• growth plate

• thin layer of hyaline cartilage

• provides lengthwise bone growth

• in adults, the epiphyseal line, is the remnant of the epiphyseal plate

Periosteum

tough sheath covering outer surface of bone

• outer fibrous layer

• inner cellular layer

• perforating fibers

Outer fibrous layer of Periosteum

• dense irregular CT

• protects bone from surrounding structures

• anchors blood vessels and nerves to bone surface

• attachment site for ligaments and tendons

Inner cellular layer

• includes osteoprogenitor cells, osteoblasts, osteoclasts

• attached to bone by numerous collagen fibers

Endosteum

• covers all internal surfaces of bone within medullary cavity

• thin layer of CT containing osteoprogenitor cells, osteoblasts, and osteoclasts 

What anatomical similarities do short, flat, and irregular bones have that differ from long bones

• external surface composed of compact bone covered by periosteum

• interior composed of spongy bone

  • Diploe: spongy bone in flat or skull bone

• no medullary cavity

  

Blood supply of bone 

• bone is hihgly vascularized 

• vessels enter from periosteum 

• nutrient foremen 

  • small opening or hole in bone

  • artery entrance and vein exit here

Nerves that supply bones

• accompany blood vessels through foramen 

• innervate bone, periosteum, endosteum, and marrow cavity 

• mainly sensory nerves 

Bone marrow

soft CT of bone

• red and yellow bone marrow

Red bone marrow (myeloid tissue)

• hematopoietic

• reticular CT, developing blood cells, and adipocytes

• in children: located in spongy bone and medullary cavity of long bones

• in adults: located only in selected areas of axial skeleton (skull, vertebrae, ribs, sternum, ossa coxae, proximal epiphyses of humerus and femur)

Yellow bone marrow

• produce of red bone marrow degeneration as children mature 

• fatty substance 

• may convert back to red bone marrow 

  • during severe anemia: condition with reduced erythrocytes (RBC)

  • facilitates production of additional erthrocytes

Bone CT (osseous CT)

primary component of bone; composed of cells and extracellular matrix 

4 types of cells found in bone CT

• osteoprogenitor cells

• osteoblasts

• osteocytes

• osteoclasts

Osteoprogenitor Cells

• stem cells derived from mesenchyme

• cellular division yields another stem cell and a “committed cell” that matures to become an osteoblast

• located in periosteum and endosteum

Osteoblasts

• form from osteprogenitor stem cells 

• synthesize and secrete osteoid

  • initial semisolid organic form of bone matrix

  • osteoid later calcifies

• becomes entrapped within the matrix

• differentiate into osteocytes

Osteocytes

• mature bone cells derived from osteoblasts

• detect stress on bone; trigger new bone formation

Osteoclasts 

• large, multinuclear, phagocytic cells 

• derived from fused bone marrow cells 

• ruffled border increases surface area exposed to bone 

• located within/adjacent to depression/pit on bone surface 

• resorption lacuna-involved in bone resorption (bone breakdown) 

Composition of bone matrix

made up of organic and inorganic components

Organic components of bone matrix

• osteoid produced by osteoblasts contain:

  • collagen protein

  • semisolid ground substance of proteoglycans and glycoproteins

• give bone tensile strength by resisting stretching, and contribute to bone flexibility

Inorganic components of bone matrix 

• salt crystals- primarily made of calcium phosphate, which interacts with calcium hydroxide to form hydroxyapatite crystals 

• the crystals incorporate other substances like calcium carbonate, sodium, magnesium, fluoride during bone calcification 

• crystals deposit around long axis of collagen fibers in ECM 

• harden matrix and account for rigidity in bones 

What happens if organic and inorganic proportions of bones aren’t correct

inadequate organic: brittle bones

inadequate inorganic: soft bones

Bone formation

• begins with secretion of osteoid

• Calcification occurs, deposition of hydroxyapatite crystals: Ca and PO3 ions precipate out, form crystals 

Requires: 

• vitamin D 

• vitamin C 

• calcium and phosphate for calcification

Bone resorption

• bone matrix is destroyed by substance released from osteoclasts

• lysosomes within osteoclasts release proteolytic enzymes, which digest organic matrix components

• calcium and phosphate dissolved by hydrochloric acid

• Freed Ca and PO3 ions enter blood

• occurs when blood calcium levels are low

Osteitis Deformans

• results from disruption between osteoclast and osteoblast function

• characterized by excessive bone resorption followed by excessive bone deposition

• large osteoclasts resorb bone at higher rate

• newly deposited bone poorly formed

• most commonly affected bones: pelvis, skull, vertebrae, femur, tibia

Osteons

• make up compact bone

• small cylindrical structures

• basic functional and structural unit of a mature compact bone

• oriented parallel to bone diaphysis

• appears like a bull’s eye target

Components of Osteon

• central (haversian) canal

• concentric lamellae

• osteocytes

• canaliculi

Central (Haversian) canal

• cylindrical channel at center of osteon and paralel to it

• blood vessels and nerves extend through channel

Concentric Lamellae

• rings of bone CT

• surround central canal

• collagen fibers

  • 90 degree from previous and next lamellae

  • give bone strength and resilience

Canaliculi

• tiny, interconnecting channels within bone CT

• extend from each lacuna, travel through lamellae and connect to lacunae and central canal

• house osteocytes projections that allow intercellular contact 

• allow exchange of nutrients, minerals, gases, and wastes between blood vessels and osteocytes 

Osteocytes

• mature bone cells 

• found in small spaces between concentric lamellae

• maintain bone matrix 

non-osteon structures in long bone

• perforating (volkmann) canals

• circumferential lamellae

• interstitial lamellae

Perforating canals

• perpendicular to central canal

• connect central canals within different osteons

Circumferential lamellae

• external: rings of bone run immediately internal to periosteum

• Internal: rings of bone run internal to the endosteum 

• both run the entire circumference of the bone 

Interstitial lamellae

components of compact bone between osteons or partially resorbed osteons

Sponge bone components

trabeculae

parallel lamellae

Trabeculae 

• open lattice of narrow rods and plates of bones

• bone marrow fills spaces

• meshwork of crisscrossing bars 

• resistance of stresses 

Parallel lamellae

• bone matrix

• osteocytes between lamellae

• canaliculi radiate from lacunae

Structure of hyaline cartilage

• cells scattered through matrix of protein fibers

• embedded in gel-like ground substance

• include proteoglycans but not calcium

• resilient and flexible

• high percentage of water

• highly compressile and good shock absorber

• avascular and contains no nerves

Components of hyaline cartilage

• chondroblasts 

• chrondrocytes

• perichondrium

Chondroblasts

cells that produce cartilage matrix

Chondrocytes

• chondroblasts encases within the matrix

• occupy small spaces, lacunae

• maintain the matrix

Perichondrium

• dense irregular CT

• covers cartilage and helps maintain its shape 

Interstitial growth in cartilage

1. chondrocyte within lacuna begins to exhibit mitoic activity

2. 2 chondroblasts grow

3. each cell produces new matrix and beings to separate

4. cartilage continues to grow internally

Appositional Growth in cartilage

1. mitotic activity occurs within the perichondrium

2. new undifferentiated stem cells and committed cells that differentiate into chondroblasts are formed. Chondroblasts create new matrix at periphery

3. as result of matrix formation, the chondroblasts push apart and become chondrocytes. Chondrocytes continue to produce more matrix.

Ossification (osteogenesis)

• formation and development of bone CT

• begins in the embryo

• continues through childhood and adolescence 

• by 8th-12th week of embryonic development, skeleton beings to form 

  • intramembraneous ossification

  • endochondral ossification

Intramembranous ossification

• bone growth within a membrane; called dermal ossification 

• produces flat bones of skull, some facial bones, mandible, central part of clavicle

Steps of intramembranous ossification

1. ossification centers form within thickened regions of mesenchyme beginning at 8th week of development

2. osteioid undergoes calcification

3. woven bone and surrounding periosteum form

4. lamellar bone replaces woven bone, as compact bone and spongy bone form

Endochondral ossification

• begins with hyaline cartilage model

• produces most bones of skeleton, including bones of upper and lower limbsm pelvis, vertebrae, ends of calvicle

Steps in long bone development in a limb 

1. fetal hyaline cartilage model develops 

2. cartilage calcifies, a periosteal bone collar forms 

3. primary ossification center forms in diaphysis

4. secondary ossification centers form in epiphyses 

5. bone replaces cartilage, except articular cartilage and epiphyseal plates

6. lengthwise growth continues until epiphyseal plates ossify and form epiphyseal lines 

Epiphyseal plate intersistial growth

• dependent upon cartilage growth in epiphyseal plate

• plate is divided into 5 phases beginnings with zone 1, zone 2, zone 3, zone 4, and zone 5

What are the zones of cartilage in the bone

zone 1: resting cartilage

zone 2: zone of proliferating cartilage

zone 3: hypertonic cartilage

zone 4: calcificed cartilage

zone 5: zone of ossification

Zone 1 of resting cartilage

• zone closest to epiphysis

• small chondrocytes distributed throughout matrix

• resembles mature hyaline cartilage

• secures epiphysis to epiphyseal plate

Zone 2 of proliferating cartilage

• chondrocytes undergo rapid mitotic division

• align into longtiudinal columns of flattened lacunae

• columns parallel to diaphysis

Zone 3 of hypertonic cartilage 

• chondrocytes cease dividing 

• cells greatly enlarge (hypertrophy)

• walls of lacunae become thin

Zone 4 of calcified cartilage

• composed of 2-3 layers of chondrocyte

• minerals are deposited between columns of lacunae

• destroys chondrocytes

Zone 5 of ossification

• walls breaks down between lacunae in columns 

• spaces invaded by capillaries and osteoprogenitor cells 

• new bone matrix deposited on the calcified cartilage matrix 

Where does bone growth take place

• zone 2 and 3

• pushes zone of resting cartilage towards epiphysis

Appositional bone growth

• occurs within periosteum

• bone matrix deposited within layers parallel to surface

• osteoclasts resorb bone matrix along medullary cavity

Axial skeleton

• composed of bones along central body axis 

• skull, vertebral column, thoracic cage 

Appendicular skeleton

• bones of upper and lower limbs

• girdles of bones attach limbs to axial skeleton

• pectoral girdle holds upper limbs in place

• pelvic girdle holds lower limbs in place

What are the cranial bones

• frontal bone

• parietal bone (2)

• temporal bone (2)

• occipital bone

• sphenoid bone

• ethmoid bone

Facial bones

• zygomatic bone (2)

• lacrimal bone (2)

• nasal bone (2)

• vomer

• inferior nasal concha (2)

• palatine bones (2)

• maximillae (2)

• mandible 

Auditory ossicles

• malleus (2)

• incus (2)

• stapes (2)

• hypoid bone 

Vertebral column bones

cervical vertebrae (7)

thoracic vertebrae (12)

lumbar vertebrae (5)

sacrum

coccyx

Thoracic Cage

sternum

ribs (24)

Pectoral girdle 

clavicle (2)

scapula (2)

Upper limb bones

humerus (2)

radius (2)

Carpals (16)

Metacarpais (10)

Phalanges (28)

Ulna (2)

Pelvis girdle

os coxae (2)

Lower limb bones

femur (2)

patella (2)

tibia (2)

fibula (2)

tarsals (14)

metatarsais (10)

phalanges (28)

Articulating surfaces of bone markings

condyle, facet, head, trochlea

Openings and spaces of bone markings

canal, fissure, foramen, meatus, sinus

Condyle

large, smooth, rounded, oval structure 

facet

small, flat, shallow surface

head

prominent, round epiphysis