Copyright © 2010 Pearson Education, Inc. Chapter 6 Bones And Skeletal Tissues Shilla Chakrabarty,...

Copyright © 2010 Pearson Education, Inc.

Chapter 6

Bones And Skeletal Tissues

Shilla Chakrabarty, Ph.D.


Bone Development

• Osteogenesis (ossification)—bone tissue formation

• Stages

• Before week 8, fetal skeleton is constructed entirely from fibrous membranes and hyaline cartilage

• Bone formation—begins in the 2nd month of development

• Postnatal bone growth continues until early adulthood

• Bone remodeling and repair occurs throughout life


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 all bones below the base of the skull, except the clavicles

Copyright © 2010 Pearson Education, Inc. Figure 6.8, (1 of 4)

Mesenchymalcell

CollagenfiberOssificationcenter

Osteoid

Osteoblast

Ossification centers appear in the fibrousconnective tissue membrane.• Selected centrally located mesenchymal cells cluster

and differentiate into osteoblasts, forming anossification center.

1

Intramembranous Ossification: Step 1


Osteoid

Osteocyte

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



Mesenchymecondensingto form theperiosteum

Blood vessel

Trabeculae ofwoven bone

Woven bone and periosteum form.• Accumulating osteoid is laid down between embryonic

blood vessels in a random manner. The result is a network(instead of lamellae) of trabeculae called woven bone.

• Vascularized mesenchyme condenses on the external faceof the woven bone and becomes the periosteum.

3



FibrousperiosteumOsteoblast

Plate ofcompact bone

Diploë (spongybone) cavitiescontain redmarrow

Lamellar bone replaces woven bone, just deep tothe periosteum. Red marrow appears.• Trabeculae just deep to the periosteum thicken, and are later

replaced with mature lamellar bone, forming compact boneplates.

• Spongy bone (diploë), consisting of distinct trabeculae, per-sists internally and its vascular tissue becomes red marrow.

4



Endochondral Ossification

• Uses hyaline cartilage models

• Requires breakdown of hyaline cartilage prior to ossification

• Formation of a long bone typically begins at the primary ossification center, which is a region in the center of the hyaline cartilage shaft

• In preparation for ossification,

1. Perichondrium covering hyaline cartilage is invaded by blood vessels

2. Change in vascularity causes mesenchymal cells to specialize into osteoblasts

3. Process of ossification begins

Copyright © 2010 Pearson Education, Inc. Figure 6.9

Bone collarforms aroundhyaline cartilagemodel.

Cartilage in thecenter of thediaphysis calcifiesand then developscavities.

The periostealbud inavades theinternal cavitiesand spongy bonebegins to form.

The diaphysis elongatesand a medullary cavityforms as ossificationcontinues. Secondaryossification centers appearin the epiphyses inpreparation for stage 5.

The epiphysesossify. Whencompleted, hyalinecartilage remains 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

BonecollarPrimaryossificationcenter

1 2 3 4 5

Endochondral Ossification


Postnatal Bone Growth

• Interstitial growth :

• length of long bones throughout infancy and youth

• Appositional growth:

• thickness and remodeling of all bones by osteoblasts and osteoclasts on bone surfaces


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

Growth in Length of Long Bones

Functional Zones Of Epiphyseal Plate Cartilage


Hormonal Regulation of Bone Growth

• During infancy and childhood, Growth hormone from the pituitary 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

NOTE: Excesses or deficits of any of these hormones can result in obviously abnormal skeletal growth

Example: Hypersecretion of growth hormone in children results in excessive height, while deficits in growth hormone or thyroid hormone will produce characteristic type of dwarfism.


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.

Long Bone Growth And Remodeling During Youth


Bone Remodeling

• Bone deposit and bone modeling in adult skeleton occurs at the surfaces of the periosteum and endosteum

• This bone remodeling is coupled with and coordinated by packets of adjacent osteoblasts and osteoclasts

• In healthy young adults, total bone mass remains constant, suggesting that bone deposit and bone resorption occur at an equal rate

NOTE: Resorption does not occur uniformly.

Example: The distal part of the femur is fully altered every 5-6 months, while the shaft is altered much more slowly


Bone Deposit

• Occurs wherever bone is injured or added strength is needed

• Requires a diet rich in protein; vitamins C, D, and A; calcium; phosphorus; magnesium; and manganese

• New matrix deposits (osteoid) are marked by an osteoid seam, an unmineralized band of bone matrix

• The abrupt transition zone between the osteoid seam and the older mineralized bone is known as the calcification front

• Newly formed osteoid must mature for a week before it can calcify

• Local concentrations of calcium and phosphate ions are critical factors for calcification of osteoid


Bone Resorption

• Osteoclasts are giant multinucleate cells that secrete

• Lysosomal enzymes (digest organic matrix)

• Hydrochloric acids (convert calcium salts into soluble forms)

• Dissolved matrix is transcytosed across osteoclast, enters interstitial fluid and then blood


Control of Remodeling

Bone remodeling is controlled by:

•Hormonal mechanisms that maintain calcium homeostasis in the blood

•Mechanical and gravitational forces acting on the skeleton


Hormonal Control of Blood Ca2+

• Calcium is necessary for

• Transmission of nerve impulses

• Muscle contraction

• Blood coagulation

• Secretion by glands and nerve cells

• 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


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



• 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

• Leptin has also been shown to influence bone density by inhibiting osteoblasts


Response to Mechanical Stress

• Response of bones to mechanical stress (muscle pull) and gravity keeps the bones strong where stressors act

• 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


Load here (body weight)

Head offemur

Compressionhere

Point ofno stress

Tensionhere

Bone Anatomy And Bending Stress


Classification of Bone Fractures

• 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


Classification of Bone Fractures

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 bone

4. 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


Common Types of Fractures

• All fractures can be described in terms of

• Location

• External appearance

• Nature of the break

Copyright © 2010 Pearson Education, Inc. Table 6.2




Stages in the Healing of a Bone Fracture1. Hematoma forms

• Torn blood vessels hemorrhage

• Clot (hematoma) forms

• Site becomes swollen, painful, and inflamed

Figure 6.15, step 1A hematoma forms.1

Hematoma


Figure 6.15, step 1

A hematoma forms.1

Hematoma


Stages in the Healing of a Bone Fracture

2. Fibrocartilaginous callus forms

• Phagocytic cells clear debris

• Osteoblasts begin forming spongy bone within 1 week

• Fibroblasts secrete collagen fibers to connect bone ends

• Mass of repair tissue now called fibrocartilaginous callus

Fibrocartilaginouscallus forms.

2

Externalcallus

Newbloodvessels

Spongybonetrabecula

Internalcallus(fibroustissue andcartilage)


Stages in the Healing of a Bone Fracture3. Bony callus formation

• New trabeculae form a bony (hard) callus

• Bony callus formation continues until firm union is formed in ~2 months

Bony callus forms.3

Bonycallus ofspongybone


Stages in the Healing of a Bone Fracture

Bone remodelingoccurs.4

Healedfracture

4. Bone remodeling

• Occurs in response to mechanical stressors over several months

• Final structure resembles original


Hematoma Externalcallus

Bonycallus ofspongyboneHealedfracture

Newbloodvessels

Spongybonetrabecula

Internalcallus(fibroustissue andcartilage)

A hematoma forms. Fibrocartilaginouscallus forms.

Bony callus forms. Boneremodelingoccurs.

1 2 3 4


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


Homeostatic Imbalances

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


Osteoporosis: Treatment and Prevention

• Calcium, vitamin D, and fluoride supplements

• Weight-bearing exercise throughout life

• Hormone (estrogen) replacement therapy (HRT) slows bone loss

• Some drugs (Fosamax, SERMs, statins) increase bone mineral density


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


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)


Parietal bone

RadiusUlna

Humerus

Femur

Occipital bone

Clavicle

Scapula

Ribs

Vertebra

Hip bone

Tibia

Frontal boneof skull

Mandible


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

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