Bones and Skeletal TissuesChapter 6

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Skeletal Cartilages

• Contain no blood vessels or nerves• Perichondrium (dense irregular connective

tissue girdle) contains blood vessels for nutrient delivery to cartilage

• Types– Hyaline– Elastic– Fibrocartilage

Figure 6.1

Axial skeleton

Appendicular skeleton

Hyaline cartilages

Elastic cartilages

Fibrocartilages

Cartilages

Bones of skeleton

EpiglottisLarynx

TracheaCricoidcartilage Lung

Respiratory tube cartilagesin neck and thorax

ThyroidcartilageCartilage in

external earCartilages innose

ArticularCartilageof a jointCostalcartilage

Cartilage inIntervertebraldisc

Pubicsymphysis

Articular cartilageof a joint

Meniscus (padlikecartilage inknee joint)

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

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Figure 6.1

Cartilage inexternal ear

Cartilages innose

ArticularCartilageof a jointCostalcartilage

Cartilage inIntervertebraldisc

Pubicsymphysis

Articular cartilageof a joint

Meniscus (padlikecartilage inknee joint)

Bones of the Skeleton

Classification of Bones by Shape

• Long bones

• Short bones

• Flat bones

• Irregular bones

Functions of Bones

• Support• Protection• Movement• Mineral & Growth Factor

Storage• Blood cell formation • Triglyceride storage• Hormone Production

Bone Structure• Bones are organs!–Multiple tissue types• Bone (osseous) tissue, nervous tissue, cartilage,

fibrous connective tissue, muscle and epithelial cells (in its blood vessels)

Bone Texture• Compact– Dense outer layer; smooth

and solid• Spongy (trabecular)– Honeycomb of flat pieces of

bone (trabeculae) deep to compact

– Space b/w trabeculae filled with red or yellow bone marrow

Structure of Short, Irregular, and Flat Bones

• Periosteum covered compact bone on the outside

• Endosteum covered spongy bone within – diploë

• Bone marrow b/w the trabeculae

• Hyaline cartilage on articular surfaces

Structure of Typical Long Bone

• Diaphysis– Tubular shaft forms long axis– Compact bone surrounds medullary cavity

• Epiphyses (bone ends)– Compact bone outside; spongy bone inside– Articular cartilage covers articular surfaces

• Epiphyseal line– b/w diaphysis and epiphysis– Remnant of epiphyseal plate

Membranes of Bone

• Periosteum–Outer fibrous layer– Inner osteogenic layer–Contains nerve fibers, nutrient blood

vessels, and lymphatic vessels that enter the bone via nutrient foramina– Secured to underlying bone by Sharpey’s

fibers

Membranes of Bone

• Endosteum– Delicate membrane on

internal surfaces of bone

– Contains osteogenic cells

Hematopoietic Tissue (Red Marrow)

• Infants (long bones)– Medullary cavities and

spongy bone• Adults (long bones)– Little red marrow

• Red marrow in flat and some irregular bones is most active

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Bone Markings

• Projections, depressions, and holes– Sites of attachment for muscles, ligaments, and

tendons– Joint surfaces– Passageways for blood vessels and nerves

Bone Markings: Projections• Sites of muscle and

ligament attachment– Tuberosity– Crest– Trochanter– Line– Tubercle– Epicondyle– Spine– Process

• Projections that help to form joints– Head– Facet– Condyle– Ramus

Bone Markings: Depressions and Openings

• Passages for blood vessels and nerves– Meatus– Sinus– Fossa– Groove– Fissure– Foramen

Microscopic Anatomy of Bone

Microscopic Anatomy of Bone: Compact Bone

• Haversian system (or osteon)– Lamellae– Central

(Haversian) canal

• Perforating (Volkmann’s) canals• Lacunae• Canaliculi

Microscopic Anatomy of Bone: Compact Bone

Microscopic Anatomy of Bone: Spongy Bone

• Trabeculae– Align along lines of stress– No osteons– Irregularly arranged lamellae, osteocytes, and

canaliculi– Capillaries in endosteum supply nutrients

Chemical Composition of Bone• Organic– Bone cells– Osteoid—organic bone matrix secreted by

osteoblasts• Ground substance, collagen fibers

• Inorganic– Hydroxyapatites (mineral salts)• 65% of bone by mass• Mainly calcium phosphate crystals

Bone Development

• Ossification (osteogenesis): process of bone tissue formation– Formation of bony skeleton• Begins in 2nd month of development

– Postnatal bone growth• Until early adulthood

– Bone remodeling and repair• Lifelong

Types of Ossification

• Endochondral ossification– Bone forms by replacing hyaline cartilage– Majority of skeleton

• Intramembranous ossification– Bone develops from fibrous membrane– Bones called membrane bones– Forms flat bones, e.g. clavicles and cranial bones

Endochondral Ossification

• Forms most all bones inferior to base of skull (except clavicles)

• Begins late in 2nd month of development• Uses hyaline cartilage models • Hyaline cartilage must be broken down before

ossification

Figure 6.9

Hyalinecartilage

Area ofdeterioratingcartilage matrix

Epiphysealblood vessel

Spongyboneformation

Epiphysealplatecartilage

Secondaryossificationcenter

Bloodvessel ofperiostealbud

Medullarycavity

Articularcartilage

Childhood toadolescence

BirthWeek 9 Month 3

Spongybone

BonecollarPrimaryossificationcenter

1 2 3 4 5

Intramembranous Ossification

• Forms cranial bones of the skull and clavicles• Begins within fibrous connective tissue membranes

formed by mesenchymal cells• Ossification centers appear• Osteoid is secreted• Woven bone and periosteum form• Lamellar bone replaces woven bone & red marrow

appears

Figure 6.9 Intramembranous ossification.

Fibrous periosteum

Osteoblast

Plate ofcompact bone

Diploë (spongy bone)cavities contain redmarrow

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

Mesenchymalcell

Collagen fibril

Ossification center

Osteoid

Osteoblast

1 Ossification centers appear in the fibrous connective tissue membrane.

Osteoblast

Osteoid

Osteocyte

Newly calcifiedbone matrix

2 Osteoid is secreted within the fibrous membrane and calcifies.

Mesenchymecondensingto form the periosteum

Trabeculae ofwoven bone

Blood vessel

3 Woven bone and periosteum form.

Postnatal Bone Growth

• Interstitial growth: – length of long bones

• Appositional growth: – thickness and remodeling of all bones by

osteoblasts and osteoclasts on bone surfaces

Interstitial (Longitudinal) Growth

• Epiphyseal plate cartilage organizes into 5 important functional zones: – Resting (quiescent) zone– Proliferation (growth)– Hypertrophic– Calcification– Ossification (osteogenic)

Figure 6.10

Calcified cartilagespicule

Osseous tissue(bone) coveringcartilage spicules

Resting zone

Osteoblast depositingbone matrix

Proliferation zoneCartilage cells undergo mitosis.

Hypertrophic zoneOlder cartilage cells enlarge.

Ossification zoneNew bone formation is occurring.

Calcification zoneMatrix becomes calcified; cartilage cells die; matrix begins deteriorating.

1

2

3

4

Appositional Growth

• Growth in Width– Osteoblasts active in periosteum– Osteoclasts active in the endosteum– Building > Breaking down = thicker stronger bone

Figure 6.11

Bone growth Bone remodeling

Articular cartilage

Epiphyseal plate

Cartilagegrows here.

Cartilageis replacedby bone here.Cartilagegrows here.

Bone isresorbed here.

Bone isresorbed here.

Bone is addedby appositionalgrowth here. Cartilage

is replacedby bone here.

Hormonal Regulation of Bone Growth

• Growth hormone • Thyroid hormone • Testosterone and

Estrogen

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Bone Remodeling

• Bone is constantly being “recycled” • Occurs @ surface of periosteum and

endosteum• Deposit– Injury or needed strength, requires good diet– Osteoid seam and Calcification front

• Resorption– Osteoclasts secrete: lysosomal enzymes, acids– Dissolved matrix is transcytosed

Control of Remodeling

• What controls continual remodeling of bone?– Hormonal mechanisms

that maintain calcium homeostasis in the blood

– Mechanical and gravitational forces

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Hormonal Control of Blood Ca2+

• Most calcium in the body is in the bones• Less that 1.5g in blood – tightly regulated

narrow range• Calcium is necessary for– Transmission of nerve impulses– Muscle contraction– Blood coagulation– Secretion by glands and nerve cells– Cell division

Figure 6.12

Osteoclastsdegrade bonematrix and release Ca2+

into blood.

Parathyroidglands

Thyroidgland

Parathyroidglands releaseparathyroidhormone (PTH).

StimulusFalling bloodCa2+ levels

PTH

Calcium homeostasis of blood: 9–11 mg/100 mlBALANCEBALANCE

Hormonal Control of Blood Ca2+

• May be affected to a lesser extent by calcitonin

Blood Ca2+ levels

Parafollicular cells of thyroid release calcitonin

Osteoblasts deposit calcium salts

Blood Ca2+ levels

Response to Mechanical Stress

• Wolff’s law: A bone grows or remodels in response to forces or demands placed upon it

Hormones and Mechanical Stress

• Hormones – – when remodeling occurs– As a response to what???

• Mechanical Stress– Where the remodeling occurs

Classification of Bone Fractures • Bone fractures may be classified by four

“either/or” classifications

1. Position of bone ends after fracture:• Nondisplaced or Displaced

2. Completeness of the break• Complete or Incomplete

3. Orientation of the break to the long axis of the bone:• Linear or transverse

4. Whether or not the bone ends penetrate the skin• Compound (open) or Simple (closed)

Common Types of Fractures

• In addition to the previous classification, all fractures can be described in terms of– Location– External appearance– Nature of the break

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Table 6.2

Table 6.2

Table 6.2

Fracture Healing

1. Hematoma forms2. Fibrocartilaginous

callus forms

Fracture Healing

3. Bony callus formation4. Bone remodeling

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

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Homeostatic Imbalances

• Osteoporosis– Loss of bone mass– 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; smoking

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)

Developmental Aspects of Bones

• 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