Chapter 6: Bones and Skeletal Tissues

Axial skeletonAppendicular skeleton

Hyaline cartilagesElastic cartilagesFibrocartilages

Cartilages

Bones of skeleton

EpiglottisLarynx

TracheaCricoidcartilage

Lung

Respiratory tube cartilagesin neck and thorax

Thyroidcartilage

Cartilage inexternal ear

Cartilages innose

ArticularCartilageof a joint

Costalcartilage

Cartilage inIntervertebraldisc

Pubicsymphysis

Articular cartilageof a joint

Meniscus (padlikecartilage in knee joint)

Skeletal Cartilages

• Contain no blood vessels or nerves• Dense connective tissue girdle of

perichondrium contains blood vessels for nutrient delivery to cartilage

• 3 types:1. Hyaline cartilages

– Provide support, flexibility, and resilience– Most abundant type

2. Elastic cartilages– Similar to hyaline cartilages, but contain

elastic fibers3. Fibrocartilages

– Collagen fibers—have great tensile strength

Growth of Cartilage

•Appositional– Cells secrete matrix against the external face of

existing cartilage

•Interstitial– Chondrocytes divide and secrete new matrix,

expanding cartilage from within

•Calcification of cartilage occurs during– Normal bone growth– Old age

Bones of the Skeleton

Two main groups, by location• Axial skeleton: long axis of body:

skull, vertebral column, rib cage• Appendicular skeleton: upper

and lower limbs that attach to axial skeleton

Types of bonesLong bones: longerthan wide

Short bones•Cube shaped bones (in wrist and ankle)•Sesamoid bones (within tendons, e.g., patella)

Flat bones: thin, flat, slightly curved

Irregular bones:complicated shapes

Functions of Bones

• Support for the body and soft organs• Protection for brain, spinal cord, and vital organs• Movement: Levers for muscle action• Storage of minerals (calcium and phosphorus) • Storage of growth factors (like insulin-like growth

factor) in bone matrix• Blood cell formation (hematopoiesis) in marrow

cavities • Triglyceride (energy) storage in bone cavities

Bone Markings

Bulges, depressions, and holes serve as– Sites of attachment for muscles,

ligaments, and tendons– Joint surfaces– Conduits for blood vessels and nerves

Bone Markings: Projections

• Sites of muscle and ligament attachment– Tuberosity—rounded projection– Crest—narrow, prominent ridge – Trochanter—large, blunt,

irregular surface– Line—narrow ridge of bone– Tubercle—small rounded

projection– Epicondyle—raised area above

a condyle– Spine—sharp, slender

projection– Process—any bony prominence

Bone Markings

Projections that help to form joints– Head: bony expansion carried on a narrow

neck– Facet: Smooth, nearly flat articular surface– Condyle: Rounded articular projection– Ramus: Armlike bar

Bone Markings: Depressions and Openings• Meatus: Canal-like

passageway• Sinus: Cavity within a

bone• Fossa: Shallow,

basinlike depression• Notch: indentation at

the edge of a structure

• Groove: Furrow• Fissure: Narrow,

slitlike opening• Foramen: Round or

oval opening through a bone

Bone Textures

• Compact bone– Dense outer

layer• Spongy

(cancellous) bone– Honeycomb of

trabeculae

Spongy bone

Compact bone

Structure of a Long Bone

• Diaphysis (shaft)– Compact bone collar surrounds medullary

(marrow) cavity– Medullary cavity in adults contains fat (yellow

marrow) • Epiphyses

– Expanded ends – Spongy bone interior – Epiphyseal line (remnant of growth plate) – Articular (hyaline) cartilage on joint surfaces

Proximalepiphysis

Epiphysealline

Articularcartilage

Spongy bone

Compact boneMedullarycavity

Compact bone

Diaphysis

Distalepiphysis

Membranes of Bone

•Periosteum– Outer fibrous layer– Inner osteogenic layer

• Osteoblasts (bone-forming cells)• Osteoclasts (bone-destroying cells)• Osteogenic cells (stem cells)

– Nerve fibers, nutrient blood vessels, and lymphatic vessels enter the bone via nutrient foramina

– Secured to underlying bone by Sharpey’s fibers• Endosteum

– Delicate membrane on internal surfaces of bone– Also contains osteoblasts and osteoclasts

Yellow bone marrow

Endosteum

Compact bone

Periosteum

Perforating(Sharpey’s) fibers

Nutrientarteries

Trabeculae

Bone marrowbetween trabeculae

Spongy bone called diploë in flat bones

Structure of short, irregular and flat bones

•Periosteum-covered compact bone on the outside

•Endosteum-covers the trabeculae

Location of Hematopoietic Tissue (Red Marrow)

•Red marrow cavities of adults– Trabecular cavities of the heads of the

femur and humerus– Trabecular cavities of the diploë of flat

bones•Red marrow of newborn infants

– Medullary cavities and all spaces in spongy bone

Osteogenic cell Osteoblast

Stem cells in periosteum and endosteum that give rise to osteoblasts

Matrix-synthesizingcell responsiblefor bone growth

Microscopic Anatomy of Bone

Microscopic Anatomy of Bone

Osteocyte

Mature bone cellthat maintains the

bone matrix

Osteoclast

Bone-resorbing cell

Structuresin thecentralcanal

Artery withcapillariesVeinNerve fiber

LamellaeCollagenfibersrun indifferentdirections

Twistingforce

Compact Bone: Haversian system, or osteon—structural unit

•Central (Haversian) canal

•Contains blood vessels and nerves

•Lamellae

•Weight-bearing

•Column-like matrix tubes

Endosteum lining bony canalsand covering trabeculae

Perforating (Volkmann’s) canal: Perpendicular to central canal. Connects blood vessels and nerves of the periosteum with central canal

Perforating (Sharpey’s) fibersPeriosteal blood vesselPeriosteum

Lacuna (withosteocyte)

Lacunae

Lamellae

NerveVein

ArteryCanaliculi

Osteocytein a lacuna

Circumferentiallamellae

Osteon

Central(Haversian) canal

Centralcanal

Interstitial lamellae

Lamellae

Compactbone

Spongy bone

Microscopic Anatomy of Bone: Spongy Bone

• Trabeculae– Align along lines of stress to resist stress– No osteons– Contain irregularly arranged lamellae,

osteocytes, and canaliculi– Capillaries in endosteum supply nutrients

Composition of Bone

Organic• Osteogenic cells, osteoblasts, osteocytes,

osteoclasts• Osteoid—organic bone matrix secreted by

osteoblasts– Ground substance (proteoglycans, glycoproteins)– Collagen fibers

• Provide tensile strength and flexibilityInorganic

• Hydroxyapatites (mineral salts)– 65% of bone by mass– Mainly calcium phosphate crystals– Responsible for hardness and resistance to

compression

Bone Development

• Osteogenesis (ossification)—bone tissue formation

• Stages– Bone formation—begins in the 2nd month of

development– Postnatal bone growth—until early adulthood– Bone remodeling and repair—lifelong

Two Types of Ossification

1. Intramembranous ossification– Membrane bone develops from fibrous

membrane– Forms flat bones, e.g. clavicles and cranial

bones

2. Endochondral ossification– Cartilage (endochondral) bone forms by

replacing hyaline cartilage– Forms most of the rest of the skeleton

Mesenchymalcell

Collagenfiber

Ossificationcenter

Osteoid (boneMatrix)

Osteoblast

Ossification centers appear in the fibrousconnective tissue membrane.• Selected centrally located mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center.

1Intramembranous ossification

Osceocytes

Osteoid

Newly calcifiedbone matrix

Osteoblast Bone matrix (osteoid) is secreted within thefibrous membrane and calcifies.• Osteoblasts begin to secrete osteoid, which is calcified

within a few days.• Trapped osteoblasts become osteocytes.

2

Mesenchyme condensingto form the periosteum

Blood vessel

Trabeculae ofwoven bone

• Osteoid laid down between blood vessels in a random manner. The result is a network of trabeculae called woven bone.• Vascularized mesenchyme condenses and becomes the periosteum.

3

Fibrousperiosteum

Osteoblast

Plate ofcompact bone

Diploë (spongybone) cavitiescontain redmarrow

• Trabeculae just deep to the periosteum thicken, and are later replaced with mature lamellar bone, forming compact bone plates.• Spongy bone (diploë), consisting of distinct trabeculae, persists internally and its vascular tissue becomes red marrow.

Woven bone and periosteum form.

Lamellar bone replaces woven bone, just deep to the periosteum. Red marrow appears.4

Bone collar forms around hyaline cartilagemodel.

Cartilage in the center of the diaphysis calcifies and then developscavities.

Periosteal bud invades the internal cavities and spongy bonebegins to form.

Diaphysis gets longer, medullary cavity forms, ossification continues. 2o

ossification center develops.

Epiphyses ossify. After, hyalinecartilage is onlyin the epiphysealplates and articularcartilages.

Hyalinecartilage

Area ofdeterioratingcartilage matrix

Epiphysealblood vessel

Spongyboneformation

Epiphysealplatecartilage

Secondaryossificationcenter

Bloodvessel ofperiostealbud

Medullarycavity

Articularcartilage

Childhood toadolescence

Birth

Week 9 Month 3 Spongybone

Bonecollar

Primaryossificationcenter1

2

3 4 5

Endochondral ossification•Uses hyaline cartilage blueprint•Hyaline cartilage breaks downBefore ossification

Postnatal Bone Growth

How bones widen (appositional growth):• Osteoblasts beneath periosteum secret bone

matrix• Osteoclasts on bone surface remove bone

How bones widen• Cartilage divide and hypertrophy and are

eventually replaced by bone (see next slide)

Hormonal regulation of bone growth • Growth hormone stimulates epiphyseal plate

activity• Thyroid hormone modulates activity of growth

hormone• Testosterone and estrogens (at puberty)

– Promote adolescent growth spurts– End growth by inducing epiphyseal plate

closure

Articularcartilage

Bone of epiphysis

Epiphyseal plate

Bone of diaphysis

Marrow cavity

Cartilage

Calcified cartilageBone

GH stimulates the lengthening of bones at the epiphyseal plate.

GH stimulates osteoblast activity & the proliferation of epiphyseal cartilage.

New bone tissue replaces cartilage in this region.

GH stimulates bone thickness by activating osteoblasts under the periosteum.

CartilageCalcified cartilageBone

Restingchondrocytes

Ep

iph

yse

al p

late

Dia

ph

ysi

s

Bone of epiphysis

Chondrocytesundergoingcell division

chondrocytesenlarging

Calcification of extracellular matrix(chondrocytes die)

Dead chondrocytes clearedaway by osteoclasts

Osteoblasts swarming up from diaphysis and depositing bone over persisting remnants of disintegrating cartilage

Causes thickening ofepiphyseal plate

Bone remodeling:continuous deposition and resorption of bone

Why? 1. To make bones stronger 2. To maintain Ca 2+ homeostasis• Calcium is necessary for: transmission of nerve

impusles, muscle contraction, blood coagulation, secretion by glands, cell division

• Primarily controlled by parathyroid hormone (PTH) Blood Ca2+ levels

Parathyroid glands release PTH

PTH stimulates osteoclasts to degrade bone matrix

and release Ca2+

Blood Ca2+ levels • Calcitonin is secreted by C cells in the thyroid gland to

prevent plasma Calcium from being too high

Load here (body weight)

Head offemur

Compressionhere

Point ofno stress

Tensionhere

Response to Mechanical Stress• Wolff’s law: A bone grows or

remodels in response to forces or demands placed upon it

• Observations supporting Wolff’s law:– Handedness (right or left

handed) results in bone of one upper limb being thicker and stronger

– Curved bones are thickest where they are most likely to buckle

– Trabeculae form along lines of stress

– Large, bony projections occur where heavy, active muscles attach

• Bone fractures may be classified by four “either/or” classifications:

1. Position of bone ends after fracture:• Nondisplaced—ends retain normal position• Displaced—ends out of normal alignment

2. Completeness of the break• Complete—broken all the way through• Incomplete—not broken all the way through

3. Orientation of the break to the long axis of the bone:• Linear—parallel to long axis of the bone• Transverse—perpendicular to long axis of the

bone4. Whether or not the bone ends penetrate the skin:

• Compound (open)—bone ends penetrate the skin

• Simple (closed)—bone ends do not penetrate the skin

All fractures can be classified based on these criteria:–Location–External appearance–Nature of the break

Some common types of fractures

Comminuted: Bone broken into 3 or more pieces.Common in people with brittle bones, such as elderly

Compression: Bone is crushed. Common in porous bones (i.e. osteoporotic bone) subjected to fall or other trauma

More types of fractures

Spiral: Ragged break from twisting force on a bone,

common in sports injury

Epiphyseal: Epiphysis separates from the diaphysis along the epiphyseal plate. Tends to occur where cartilage cells

are dying and calcification of the

matrix is occurring

Depressed: Broken bone is pressed inward. Typical in a skull fracture.

Greenstick: Bone breaks incompletely, like a broken twig. Only one side of the

shaft breaks. Common in kids

Stages in the Healing of a Bone Fracture

A hematoma forms.

Hematoma

1 Fibrocartilaginouscallus forms.

2

Externalcallus

NewbloodvesselsSpongybonetrabecula

Internalcallus(fibroustissue andcartilage)

Bony callus forms.

Bonycallus ofspongybone

3 Bone remodeling occurs.4

Healedfracture

Homeostatic Imbalances• Osteomalacia and rickets

– Calcium salts not deposited– Rickets (childhood disease) causes bowed legs and

other bone deformities– Cause: vitamin D deficiency or insufficient dietary

calcium• Paget’s disease

– Excessive and haphazard bone formation and breakdown, usually in spine, pelvis, femur, or skull

– Pagetic bone has very high ratio of spongy to compact bone and reduced mineralization

– Unknown cause (possibly viral)– Treatment includes calcitonin and biphosphonates

Osteoporosis•Loss of bone mass—bone resorption outpaces deposit•Spongy bone of spine and neck of femur become most susceptible to fracture•Risk factors: Lack of estrogen, calcium or vitamin D; petite body form; immobility; low levels of TSH; diabetes mellitus

Treatment and prevention:•Calcium, vitamin D, and fluoride supplements• Weight-bearing exercise throughout life•Hormone (estrogen) replacement therapy slows bone loss •Some drugs (Fosamax, SERMs, statins) increase bone mineral density

Developmental Aspects of Bones

• Embryonic skeleton ossifies predictably so fetal age easily determined from X rays or sonograms

• At birth, most long bones are well ossified (except epiphyses)

• Nearly all bones completely ossified by age 25• Bone mass decreases with age beginning in 4th

decade• Rate of loss determined by genetics and

environmental factors • In old age, bone resorption predominates