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5The Skeletal System: Osseous Tissue and Skeletal Structure

PowerPoint® Lecture Presentations prepared bySteven BassettSoutheast Community College Lincoln, Nebraska


Introduction

• The skeletal system is made of:• Skeletal bones• Cartilage• Ligaments• Connective tissue to stabilize the skeleton

• Bones are dynamic organs, which consist of several tissue types

• Bone tissue, or osseous tissue, is the major component of the skeletal system.


Introduction

• Functions of the skeletal system• Support

• Provides the framework for the attachment of other

organs

• Storage of minerals• Calcium ions: 98% of the body’s calcium ions are

in the bones• Phosphate ions

• Blood cell production• Bone marrow produces erythrocytes, leukocytes,

and platelets


Introduction

• Functions of the skeletal system (continued)• Leverage

• Muscles pull on the bones to produce movement

• Protection • Ribs protect heart and lungs• Skull protects the brain• Vertebrae protect the spinal cord• Pelvic bones protect the reproductive organs


Structure of Bone

• Bones (osseous tissue)• Supporting connective tissue

• Specialized cells• Solid matrix

• Outer lining• Called the periosteum, Continuous with the deep

fascia

• Inner lining• Called the endosteum


Structure of Bone

• The Histological Organization of Mature Bone• Matrix

• Calcium phosphate: • eventually converts to hydroxyapatite crystals• Hydroxyapatite crystals resist compression

• Collagen fibers• Make up 1/3 of the bone matrix• Contribute to the tensile strength of bones

• Bone cells• Contribute only 2% of the bone mass


Structure of Bone

• The Cells of Mature Bone• Osteocytes

• Mature bone cells• Maintain the protein and mineral content of the matrix• They are contained in lacunae

• Osteoblasts • Immature bone cells• Found on the inner and outer surfaces of bones• Produce osteoid, which is involved in making the

matrix• Osteoblasts are involved in making new bone. This is

a process called osteogenesis• Osteoblasts can convert to osteocytes


Structure of Bone

• The Cells of Mature Bone (continued)• Osteoprogenitor cells

• Found on the inner and outer surfaces of bones• Differentiate to form new osteoblasts• Heavily involved in the repair of bones after a break

• Osteoclasts

• Giant multinucleated cells, secrete acids, which dissolve the bones thereby causing the release of stored calcium ions and phosphate ions into the blood

• This process is called osteolysis


Figure 5.1a Histological Structure of a Typical Bone

Osteocyte: Mature bone cellthat maintains the bone matrix

Osteoblast

Matrix

Matrix

Canaliculi Osteocyte

Osteoid

Osteoblast: Immature bonecell that secretes organiccomponents of matrix

The cells of bone

Endosteum Osteoprogenitor cell

Medullarycavity

Osteoprogenitor cell:Stem cell whose divisions produceosteoblasts

MatrixOsteoclast

Medullarycavity

Osteoclast: Multinucleate cellthat secretes acids and enzymesto dissolve bone matrix


Figure 5.1c Histological Structure of a Typical Bone

Canaliculi

Concentriclamellae

Osteon

Lacunae

Osteons

A thin section throughcompact bone; in thisprocedure the intact matrixand central canals appearwhite, and the lacunae andcanaliculi are shown in black.

LM × 220

Central canal


Figure 5.1d Histological Structure of a Typical Bone

A single osteon at highermagnification

Osteon LM × 343

Canaliculi

Concentriclamellae

Osteon

Lacunae

Central canal


Figure 5.1b Histological Structure of a Typical BoneOsteon

Lacunae

Centralcanals

Lamellae

A scanning electronmicrograph of several osteonsin compact bone

Osteons SEM × 182


Structure of Bone

• Two types of osseous tissue• Compact bone (dense bone)

• Compact bones are dense and solid• Forms the walls of bone outlining the medullary cavity• Medullary cavity consists of bone marrow• its basic functional unit is the osteon

• Osteon consists of:• Central canal• Canaliculi• Osteocytes• Lacunae• Lamellae

• Spongy bone (porous/ trabecular bone)• Spongy bone forms an open network of struts and plates (trabeculae).

Trabeculae does not contain central canal within.


Figure 5.2a-c The Internal Organization in Representative Bones

Spongy bone

Blood vessels

Compact bone

Medullary cavity

Endosteum

Periosteum

Gross anatomyof the humerus

Compactbone

Spongybone

Medullarycavity

Concentriclamellae

Interstitiallamellae

Capillary

Small veinCircumferentiallamellae

Osteons

Periosteum

Collagen fiberorientation

Concentriclamellae

Centralcanal

Endosteum

The organization of collagenfibers within concentric lamellae

Trabeculae ofspongy bone

Centralcanal

Diagrammatic view of the histologicalorganization of compact and spongy bone

Perforatingcanal

ArteryVein


Figure 5.2d The Internal Organization in Representative Bones

Canaliculiopening

on surface

Location and structure of spongy bone. The photo shows a sectional view of the proximal end of the femur.

Trabeculae ofspongy bone

LamellaeEndosteum


Structure of Bone

• Structural Differences • Compact bone

• Consists of osteons• Makes up the dense, solid portion of bone

• Spongy bone• Trabeculae are arranged in parallel struts• Trabeculae form branching plates• Trabeculae form an open network• Creates the lightweight nature of bones


Structure of Bone

• Functional Differences • Compact bone

• Conducts stress from one area of the body to another area of the body

• Generates tremendous strength from end to end• Weak strength when stress is applied to the side

• Spongy bone• Trabeculae create strength to deal with stress from

the side


Facts about compact and spongy bones• The structural unit of compact bone is

osteon.• The structural unit of spongy bone is

trabeculae.• The layers of bone in compact bone is

called lamellea. There are concentric, interstitial and circumferential lamellae.


Figure 5.3a Anatomy of a Representative Bone

Articularsurface ofhead of femur

Epiphysis

Metaphysis

Diaphysis(shaft)

Metaphysis

Epiphysis

Medullary cavity

Posterior view Sectional view

The femur, or thigh bone, in superficial and sectional views. The femur has adiaphysis (shaft) with walls of compact bone and epiphyses (ends) filled withspongy bone. A metaphysis separates the diaphysis and epiphysis at eachend of the shaft. The body weight is transferred to the femur at the hip joint.Because the hip joint is off center relative to the axis of the shaft, the bodyweight is distributed along the bone so that the medial portion of the shaft iscompressed and the lateral portion is stretched.

Spongy bone

Compact bone


Figure 5.3b Anatomy of a Representative Bone

An intact femur chemically clearedto show the orientation of thetrabeculae in the epiphysis


Figure 5.3c Anatomy of a Representative Bone

Articular surface of head of femur

A photograph showing theepiphysis after sectioning

Compact bone

Medullary cavity

Cortex

Trabeculaeof spongy

bone


Structure of Bone

• Organization of Compact and Spongy Bone• Epiphysis

• Each end of the long bones• Made of the spongy bone.

• Diaphysis• Shaft of the long bones• contains marrow cavity, which holds the red or yellow bone marrow. Red

bone marrow is the source of producing RBCs, WBCs, and platelets.

• Metaphysis • Narrow growth zone between the epiphysis and the diaphysis (formerly epiphyseal plate). Epiphyseal plate is contributing in longitudinal

growth of the bone in prepubertal age. After puberty it becomes ossified and the longitudinal growth of the bone stops.

Metaphysis connecting the epiphysis to diaphysis.


Figure 5.3a Anatomy of a Representative Bone

Articularsurface ofhead of femur

Epiphysis

Metaphysis

Diaphysis(shaft)

Metaphysis

Epiphysis

Medullary cavity

Posterior view Sectional view

The femur, or thigh bone, in superficial and sectional views. The femur has adiaphysis (shaft) with walls of compact bone and epiphyses (ends) filled withspongy bone. A metaphysis separates the diaphysis and epiphysis at eachend of the shaft. The body weight is transferred to the femur at the hip joint.Because the hip joint is off center relative to the axis of the shaft, the bodyweight is distributed along the bone so that the medial portion of the shaft iscompressed and the lateral portion is stretched.

Spongy bone

Compact bone


Structure of Bone

• The Periosteum and Endosteum• 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 connective tissue network

of the deep fascia


Structure of Bone

• The Periosteum and Endosteum• Endosteum

• Inner surface of bone• Lines the medullary cavity• Consists of osteoprogenitor cells• Actively involved in repair and growth


Figure 5.4ab Anatomy and Histology of the Periosteum and Endosteum

The endosteum is an incompletecellular layer containing osteoblasts,osteoprogenitor cells, and osteoclasts.

Joint capsule

Osteoblasts

Osteoid

Osteocyte

Osteoprogenitorcell

Endosteum

Giantmultinucleate

osteoclast

Bone matrix

Compact bone

Fibrous layerof periosteum

Cellular layerof periosteum

Endosteum

The periosteum contains outer (fibrous) and inner (cellular) layers. Collagen fibers of the periosteum are continuous with those of the bone, adjacent joint capsules, and attachedtendons and ligaments.

Circumferentiallamellae

Cellular layerof periosteum

Fibrous layerof periosteum

Canaliculi

Lacuna

Osteocyte

Perforatingfibers


Bone Development and Growth

• Osteogenesis• Bone formation

• Calcification• The deposition of calcium ions into the bone

tissue



Before six weeks of development the skeleton is cartilage. Osteogenesis is bone formation.

Ossification is bone replacing existing tissue The two types of Ossification:

Intramembraneous ossification, such as flat bones. Endochondreal ossification, such as long bones.

This ossification begins with hyaline cartilage.

Calcification is the process of depositing calcium salts

into tissues.


Figure 5.5 Histology of Intramembranous Ossification

Mesenchymal cells aggregate, differentiate into osteoblasts,and begin the ossification process. The bone expands as aseries of spicules that spread into surrounding tissues.

Bone matrixOsteoblast

Osteoid

Embryonic connective tissue

Mesenchymal cellBlood vessel

Osteocyte in lacuna

Blood vessel Osteoblasts Spicules LM × 32

As the spicules interconnect, theytrap blood vessels within the bone.

Osteocytesin lacunae

Bloodvessels

Osteoblastlayer

Blood vessel

Over time, the boneassumes the structure ofspongy bone. Areas of spongy bone may later beremoved, creatingmedullary cavities.Through remodeling,spongy bone formed in thisway can be converted tocompact bone.

LM × 32


Figure 5.7 Anatomical and Histological Organization of Endochondral Ossification

As the cartilage enlarges,chondrocytes near thecenter of the shaftincrease greatly in size.The matrix is reduced to aseries of small struts thatsoon begin to calcify. Theenlarged chondrocytesthen die and disintegrate,leaving cavities within thecartilage.

Blood vessels growaround the edges of thecartilage, and the cells ofthe perichondrium convertto osteoblasts. The shaftof the cartilage thenbecomes ensheathed in asuperficial layer of bone.

Blood vessels penetrate thecartilage and invade the centralregion. Fibroblasts migratingwith the blood vesselsdifferentiate into osteoblastsand begin producing spongybone at a primary center ofossification. Bone formationthen spreads along the shafttoward both ends.

Epiphysis

Diaphysis

Blood vessel

Medullarycavity

Primaryossificationcenter

Superficialbone

Spongybone

Boneformation

Medullarycavity

Metaphysis

SeeFigure 5.9

Enlargingchondrocytes within

calcifying matrix

Hyaline cartilage

Remodeling occurs as growthcontinues, creating a medullarycavity. The bone of the shaftbecomes thicker, and the cartilagenear each epiphysis is replaced byshafts of bone. Further growthinvolves increases in length (Steps 5and 6) and diameter (see Figure 5.9).

Steps in the formation of a long bone from a hyaline cartilage model

Epiphysealcartilage matrix

Cartilage cellsundergoing division

Zone of proliferation

Zone of hypertrophy

Medullarycavity

Osteoblasts Osteoid

Epiphyseal cartilage

Light micrograph showing thezones of cartilage and theadvancing osteoblasts at anepiphyseal cartilage

LM × 250

Soon the epiphyses are filled withspongy bone. An articular cartilageremains exposed to the joint cavity;over time it will be reduced to a thinsuperficial layer. At each metaphysis,an epiphyseal cartilage separates theepiphysis from the diaphysis.

Articular cartilage

Spongybone

Epiphysealcartilage

Diaphysis

Capillaries and osteoblastsmigrate into the epiphyses,creating secondaryossification centers.

Hyaline cartilage

Epiphysis

Metaphysis

Periosteum

Compactbone

Secondaryossification

center


Figure 5.8 Epiphyseal Cartilages and Lines

X-ray of the hand of a young child. The arrowsindicate the locations of the epiphyseal cartilages.

X-ray of the hand of an adult. The arrows indicate thelocations of epiphyseal lines.


Figure 5.6 Fetal Intramembranous and Endochondral Ossification

Intramembranousossificationproduces theroofing bones ofthe skull

Temporalbone

Mandible

Clavicle

Scapula

Humerus

Femur

Vertebrae

Hip bone(ilium)

Endochondralossificationreplaces cartilagesof embryonic skull

Primary ossificationcenters of thediaphyses (bonesof the lower limb)

Futurehip bone

At 10 weeks the fetal skull clearly showsboth membrane and cartilaginous bone,but the boundaries that indicate the limitsof future skull bones have yet to beestablished.

At 16 weeks the fetal skull shows the irregularmargins of the future skull bones. Mostelements of the appendicular skeleton formthrough endochondral ossification. Note theappearance of the wrist and ankle bones at 16weeks versus at 10 weeks.

Parietal bone

Frontal bone

Metacarpal bones

Phalanges

Radius

Ulna

Cartilage

FibulaTibia

Phalanx

Metatarsal bones

Ribs



Factors Regulating Bone Growth Ions

Calcium, phosphate, magnesium, citrate, carbonate, sodium

Vitamins Vitamins A and C Vitamin D derivatives

Parathyroid hormone (PTH) acts to increase the overall availability of calcium ions in the blood. Decreased levels of blood calcium stimulates the secretion of PTH.

Increased osteoclast activity is the direct result of PTH levels. Calcitonin is the antagonist of PTH. Growth hormone and thyroxine increase osteoblast activity leading to

bone growth. Sex hormones increase bone growth dramatically during puberty.


Remodeling and bone maintenance


Bone Maintenance, Remodeling, and Repair

• Injury and Repair• When a bone is broken, bleeding occurs• A network of spongy bone forms• Osteoblasts are overly activated, thus

resulting in enlarged callused area• This area is now stronger and thicker than

normal bone


Clinical Note 5.3 Fractures and Their Repair (Part 3 of 4)

Fracturehematoma

Deadbone

Bone fragments

Spongy bone ofexternal callus

Periosteum

Immediately after the fracture,extensive bleeding occurs.Over a period of severalhours, a large blood clot, orfracture hematoma, develops.

Repair of afracture

An internal callus forms asa network of spongy boneunits the inner edges, andan external callus ofcartilage and bonestabilizes the outer edges.


Clinical Note 5.3 Fractures and Their Repair (Part 4 of 4)

Internalcallus

Externalcallus

Externalcallus

A swelling initially marksthe location of the fracture.Over time, this region willbe remodeled, and little evidence of the fracturewill remain.

The cartilage of the externalcallus has been replaced by bone,and struts of spongy bone nowunite the broken ends. Fragmentsof dead bone and the areas ofbone closest to the break havebeen removed and replaced.


Anatomy of Skeletal Elements

• There are seven broad categories of bones according to their shapes

• Long bone: humerus• Flat bone: cranial bones or scapula.• Wormian bone: develops between the sutures of

cranium.• Pneumatized bone: ethmoid bone.• Irregular bone: vertebrae.• Short bones: carpals and tarsals.• Sesamoid bone: patella


Figure 5.11 Shapes of Bones

Irregular Bones

Suturalbone

Sutures

Pneumatized BonesSutural Bones

Air cellsEthmoid

Vertebra

Carpalbones

Humerus

External tableParietal bone

Internaltable

Diploë(spongy bone)

Patella

Short Bones

Flat Bones

Long Bones

Sesamoid Bones


Integration with other systems

The skeletal system is closely linked to other body systems.The skeletal system is a reservoir for calcium, phosphate, and other minerals.The growth and development of bones is under control of several factors including diet, hormones, muscle attachments and mechanical stress. Lack of any of these factors causes weak bones and increases the chance of break in the bone or Fracture.The most common type of fracture in children is called Green stick fracture.


Anatomy of Skeletal Elements

• Bone markings include:• Projections• Depressions• Fissures• Foramina• Canals (meatuses)


Figure 5.12a Examples of Bone Markings (Surface Features)Trochanter

Head

Neck

Femur

Facet

Tubercle

Condyle


Figure 5.12b Examples of Bone Markings (Surface Features)

Fissure

Ramus

Process

Foramen

Skull, anterior view


Figure 5.12c Examples of Bone Markings (Surface Features)

Canal

Sinuses

Meatus

Skull, sagittal section


Figure 5.12d Examples of Bone Markings (Surface Features)

Humerus

Condyle

Trochlea

Fossa

Tuberosity

Neck

SulcusHead

Tubercle


Figure 5.12e Examples of Bone Markings (Surface Features)

Crest

Pelvis

Spine

Line

Foramen

Fossa

Ramus


Table 5.1 Common Bone Marking Terminology