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