APPENDICULAR APPENDICULAR SKELETONSKELETON

Dr. Mujahid KhanDr. Mujahid Khan

CompositionComposition

The appendicular skeleton consists of pectoral girdles The appendicular skeleton consists of pectoral girdles and limb bonesand limb bones

Mesenchymal bones form during the fifth week in the Mesenchymal bones form during the fifth week in the limb budslimb buds

Chondrification of mesenchymal bone models occurs in Chondrification of mesenchymal bone models occurs in the sixth weekthe sixth week

Clavicle initially develops from intramembranous Clavicle initially develops from intramembranous ossification ossification

Later forms growth cartilages at both endsLater forms growth cartilages at both ends

CompositionComposition

The models of pectoral girdle and upper limb The models of pectoral girdle and upper limb bones appear slightly before those of the pelvic bones appear slightly before those of the pelvic girdle and lower limbsgirdle and lower limbs

The bone models appear in a proximodistal The bone models appear in a proximodistal sequencesequence

Ossification begins in the long bones by the Ossification begins in the long bones by the eighth weekeighth week

Initially occurs in the diaphysisInitially occurs in the diaphysis

Primary OssificationPrimary Ossification

By 12 weeks primary ossification centers appear By 12 weeks primary ossification centers appear in almost all bones of the limbsin almost all bones of the limbs

The clavicle begin to ossify before any other The clavicle begin to ossify before any other bone in the bodybone in the body

The femora are the next bones to show traces of The femora are the next bones to show traces of ossificationossification

First indication of ossification in cartilaginous First indication of ossification in cartilaginous model appear in the center of the future shaft, model appear in the center of the future shaft, called primary center of ossification called primary center of ossification

Primary OssificationPrimary Ossification

Primary centers appear at different times in Primary centers appear at different times in different bonesdifferent bones

Most of them develop between 7 and 12 weeksMost of them develop between 7 and 12 weeks

Virtually all primary centers of ossification are Virtually all primary centers of ossification are present at birthpresent at birth

The part of the bone ossified from a primary The part of the bone ossified from a primary center is the diaphysiscenter is the diaphysis

Secondary OssificationSecondary Ossification

Secondary ossification centers of the bones at Secondary ossification centers of the bones at knee are the first to appearknee are the first to appear

The centers for the distal end of femur and The centers for the distal end of femur and proximal end of tibia appear during 34 to 38 proximal end of tibia appear during 34 to 38 weeksweeks

Consequently they are present at birthConsequently they are present at birth

Most secondary centers of ossification appear Most secondary centers of ossification appear after birth, called epiphysisafter birth, called epiphysis

Secondary OssificationSecondary Ossification

The bone forms from the primary center in The bone forms from the primary center in the diaphysis do not fuse with that formed the diaphysis do not fuse with that formed from the secondary centers in the from the secondary centers in the epiphysis until the bone grows to its adult epiphysis until the bone grows to its adult lengthlength

The delay enables lengthening of the bone The delay enables lengthening of the bone to continue until the final size is reachedto continue until the final size is reached

Secondary OssificationSecondary Ossification

During bone growth, epiphysial plate During bone growth, epiphysial plate intervenes between the diaphysis and intervenes between the diaphysis and epiphysisepiphysis

The epiphysial plate is eventually replaced The epiphysial plate is eventually replaced by bone development on each of its two by bone development on each of its two sides, diaphysial and epiphysialsides, diaphysial and epiphysial

When this occurs, growth of the bone When this occurs, growth of the bone ceasesceases

Limb DevelopmentLimb Development

The limb buds appear as elevations of the The limb buds appear as elevations of the ventrolateral body wall by end of 4ventrolateral body wall by end of 4thth week week

The limb buds form deep to a thick band of The limb buds form deep to a thick band of ectodermectoderm

The upper limb buds are visible by 26 to The upper limb buds are visible by 26 to 27 days27 days

Lower limb buds appear 2 days laterLower limb buds appear 2 days later

Limb BudLimb Bud

Each limb bud consists of a mass of Each limb bud consists of a mass of mesenchyme covered by ectodermmesenchyme covered by ectoderm

The mesenchyme is derived from the somatic The mesenchyme is derived from the somatic layer of lateral mesodermlayer of lateral mesoderm

The limb buds elongate by the proliferation of The limb buds elongate by the proliferation of the mesenchymethe mesenchyme

The upper limb buds appear low on the The upper limb buds appear low on the embryo’s trunkembryo’s trunk

Limb BudLimb Bud

The early stages of limb development are alike The early stages of limb development are alike for the upper and lower limbsfor the upper and lower limbs

Development of upper limb buds occurs 2 days Development of upper limb buds occurs 2 days before that of lower limb budsbefore that of lower limb buds

The upper limb buds develop opposite the The upper limb buds develop opposite the caudal cervical segmentscaudal cervical segments

Lower limb buds form opposite the lumbar and Lower limb buds form opposite the lumbar and upper sacral segmentsupper sacral segments

Limb BudLimb Bud

At the apex of each limb bud the ectoderm At the apex of each limb bud the ectoderm thickens to form an apical ectodermal ridge thickens to form an apical ectodermal ridge (AER)(AER)

AER exerts an inductive influence on the limb AER exerts an inductive influence on the limb mesenchyme that initiates growth of limbs in mesenchyme that initiates growth of limbs in proximal-distal axisproximal-distal axis

Mesenchymal cells aggregate at the posterior Mesenchymal cells aggregate at the posterior margin of the limb bud to form the zone of margin of the limb bud to form the zone of polarizing activity (ZPA)polarizing activity (ZPA)

Digital RaysDigital Rays

By the end of 6By the end of 6thth week, mesenchymal week, mesenchymal tissue in the hand plates has condensed to tissue in the hand plates has condensed to form digital raysform digital rays

These mesenchymal condensations or These mesenchymal condensations or finger buds outline the pattern of the digitsfinger buds outline the pattern of the digits

During the 7During the 7thth week, similar condensations week, similar condensations of mesenchyme form digital rays and toe of mesenchyme form digital rays and toe buds in the foot platesbuds in the foot plates

Digital RaysDigital Rays

AER induces development of the AER induces development of the mesenchyme into the mesenchymal mesenchyme into the mesenchymal primordia of the bones in the digitsprimordia of the bones in the digits

The intervals between the digital rays are The intervals between the digital rays are occupied by loose mesenchymeoccupied by loose mesenchyme

Soon the intervening regions of Soon the intervening regions of mesenchyme break down forming notches mesenchyme break down forming notches between the digital raysbetween the digital rays

Digital RaysDigital Rays

As the tissue breakdown progresses, separate As the tissue breakdown progresses, separate digits are formed by the end of 8digits are formed by the end of 8 thth week week

Programmed cell death (apoptosis) is Programmed cell death (apoptosis) is responsible for the tissue breakdown in the responsible for the tissue breakdown in the interdigital regionsinterdigital regions

Blocking these cellular and molecular events Blocking these cellular and molecular events could account for syndactyly, webbing or fusion could account for syndactyly, webbing or fusion of the fingers or toesof the fingers or toes

Final Stages of Limb Final Stages of Limb DevelopmentDevelopment

As the limbs elongate in the 5As the limbs elongate in the 5thth week, week, chondrification centers appearchondrification centers appear

By the end of 6By the end of 6thth week, the entire limb week, the entire limb skeleton is cartilaginousskeleton is cartilaginous

Osteogenesis of long bones begins in the Osteogenesis of long bones begins in the 77thth week from primary ossification centers week from primary ossification centers in the middle of the cartilaginous models of in the middle of the cartilaginous models of long boneslong bones

Final Stages of Limb Final Stages of Limb DevelopmentDevelopment

Primary ossification centers are present in all long Primary ossification centers are present in all long bones by the 12bones by the 12thth week week

Ossification of the carpal (wrist) bones begins Ossification of the carpal (wrist) bones begins during the first year after birthduring the first year after birth

As the long bones form, myoblasts aggregate and As the long bones form, myoblasts aggregate and form a large muscle mass in each limb budform a large muscle mass in each limb bud

In general this muscle mass separates into dorsal In general this muscle mass separates into dorsal (extensor) and ventral (flexor) components(extensor) and ventral (flexor) components

Final Stages of Limb Final Stages of Limb DevelopmentDevelopment

The mesenchyme in the limb bud gives The mesenchyme in the limb bud gives rise to bones, ligaments, and blood rise to bones, ligaments, and blood vesselsvessels

From dermomyotome regions of somites, From dermomyotome regions of somites, myogenic precursor cells also migrate into myogenic precursor cells also migrate into the limb budthe limb bud

Later they differentiate into myoblasts or Later they differentiate into myoblasts or precursors of muscle cellsprecursors of muscle cells

Rotations of LimbsRotations of Limbs

The cervical and lumbosacral myotomes The cervical and lumbosacral myotomes contribute to the muscles of the pectoral contribute to the muscles of the pectoral and pelvic girdles, respectivelyand pelvic girdles, respectively

Early in the seventh week the limbs extend Early in the seventh week the limbs extend ventrallyventrally

The developing upper limbs rotate in The developing upper limbs rotate in opposite directions and to different opposite directions and to different degreesdegrees

Rotations of LimbsRotations of Limbs

The upper limbs rotate laterally through 90 degrees on The upper limbs rotate laterally through 90 degrees on their longitudinal axistheir longitudinal axis

Now the future elbows point dorsallyNow the future elbows point dorsally

Extensor muscles lie on the lateral and posterior aspects Extensor muscles lie on the lateral and posterior aspects of the limbof the limb

The lower limbs rotate medially through 90 degreesThe lower limbs rotate medially through 90 degrees

Now the future knees face ventrallyNow the future knees face ventrally

Extensor muscles lie on the anterior aspect of the lower Extensor muscles lie on the anterior aspect of the lower limb limb

Cutaneous Innervation Cutaneous Innervation of Limbsof Limbs

Motor axons arising from the spinal cord Motor axons arising from the spinal cord enter the limb buds during the fifth weekenter the limb buds during the fifth week

Grow into dorsal and ventral muscle Grow into dorsal and ventral muscle massesmasses

Sensory axons enter the limb buds after Sensory axons enter the limb buds after the motor axons and use them for the motor axons and use them for guidanceguidance

Cutaneous Innervation Cutaneous Innervation of Limbsof Limbs

Neural crest cells, the precursors of Neural crest cells, the precursors of schwann cells, surround the motor and schwann cells, surround the motor and sensory nerve fibers in the limbssensory nerve fibers in the limbs

Form the neurolemmal and myelin sheathsForm the neurolemmal and myelin sheaths

A dermatome in this area of skin supplied A dermatome in this area of skin supplied by a single spinal nerve and its spinal by a single spinal nerve and its spinal ganglionganglion

Cutaneous Innervation Cutaneous Innervation of Limbsof Limbs

During the 5During the 5thth week, the peripheral nerves grow week, the peripheral nerves grow from the developing limb plexuses into from the developing limb plexuses into mesenchyme of limb budsmesenchyme of limb buds

The spinal nerves are distributed in segmental The spinal nerves are distributed in segmental bands, supplying both dorsal and ventral bands, supplying both dorsal and ventral surfaces of the limb budssurfaces of the limb buds

As the limbs elongate, the cutaneous distribution As the limbs elongate, the cutaneous distribution of the spinal nerves migrates along the limbs of the spinal nerves migrates along the limbs

Cutaneous Innervation Cutaneous Innervation of Limbsof Limbs

The original dermatomal pattern changes during The original dermatomal pattern changes during growth of the limbsgrowth of the limbs

An orderly sequence of distribution can still be An orderly sequence of distribution can still be recognized in the adultrecognized in the adult

When the limbs descend they carry their nerves When the limbs descend they carry their nerves with them with them

This explains the oblique course of the nerves This explains the oblique course of the nerves arising from the brachial and lumbosacral arising from the brachial and lumbosacral plexusesplexuses

Blood Supply to LimbsBlood Supply to Limbs

The limb buds are supplied by branches of the dorsal The limb buds are supplied by branches of the dorsal intersegmental arteriesintersegmental arteries

They arise from the aorta and form a fine capillary They arise from the aorta and form a fine capillary network in the mesenchymenetwork in the mesenchyme

The primordial vascular pattern consists of a primary The primordial vascular pattern consists of a primary axial artery and its branchesaxial artery and its branches

The vascular pattern changes as the limbs developThe vascular pattern changes as the limbs develop

This occurs by vessels sprouting from existing vesselsThis occurs by vessels sprouting from existing vessels

Blood Supply to LimbsBlood Supply to Limbs

The new vessels coalesce with other sprouts to The new vessels coalesce with other sprouts to form new vesselsform new vessels

The primary axial artery becomes the brachial The primary axial artery becomes the brachial artery and common interosseous artery in the artery and common interosseous artery in the forearmforearm

In the thigh the primary axial artery is In the thigh the primary axial artery is represented by the profonda femoris arteryrepresented by the profonda femoris artery

In the leg it is represented by the anterior and In the leg it is represented by the anterior and posterior tibial arteriesposterior tibial arteries

Anomalies of LimbsAnomalies of Limbs

Minor limb anomalies are common and can be Minor limb anomalies are common and can be corrected surgicallycorrected surgically

The most critical period of limb development is The most critical period of limb development is from 24 to 36 days after fertilizationfrom 24 to 36 days after fertilization

Exposure to teratogen before day 33 may cause Exposure to teratogen before day 33 may cause severe limb defectssevere limb defects

Major limb anomalies appear about twice in Major limb anomalies appear about twice in 1000 newborns1000 newborns

THE ENDTHE END