Appendicular

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Appendicular limbby dr.saro0ona
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The appendicular skeleton consists of
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• Mesenchymal bones form in the limb buds during
  the(5) fifth week
• mesenchymal bone models in the limbs undergo
  Chondrification to form hyaline cartilage bone
  models in the (6 )sixth week.
• The clavicle initially develops by
  intramembranous ossification and it later forms
  growth cartilages at both ends
• The bone models appear in a proximodistal
  sequence
• Paterning in the developing limbs is regulated by
  homeobox containing (hox) genes

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• in the eighth week ,Ossification begins in the
  long bones Initially occurs in the diaphysis
  from the primary ossification centers
• By 12 weeks primary ossification centers have
• appeared in nearly all bones of the limbs.
• The clavicles begin to ossify before any other
  bone in the body.
• The femora are the next bones to show traces of
  ossification
• First indication of ossification in cartilaginous
  model appear in the center of the future shaft,
  called primary center of ossification

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Primary Ossification

• Primary centers appear at different times in
  different bones .But ,Most of them develop
  between 7th and 12 weeks
• Virtually all primary centers of ossification are
  present at birth
• The part of the bone ossified from a primary
  center is the diaphysis

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Secondary Ossification

• Secondary ossification centers of the bones at
  knee are the first to appear in utero
• The centers for the distal end of femur and
  proximal end of tibia appear during 34 to 38
  weeks(last month)
• Consequently they are present at birth
• Most secondary centers of ossification appear
  after birth,
• The part of bone ossified from the secondary
  centers called epiphysis

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• The bone forms from the primary center in the
  diaphysis do not fuse with that formed from the
  secondary centers in the epiphysis until the bone
  grows to its adult length
• The delay enables lengthening of the bone to
  continue until the final size is reached

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• During bone growth, epiphysial plate intervenes
  between the diaphysis and epiphysis
• The epiphysial plate is eventually replaced by
  bone development on each of its two sides,
  diaphysial and epiphysial
• When this occurs, growth of the bone ceases

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Limb Development

• Limb development begins with the activation of a
  group of mesenchymal cells from the somatic layer
  of lateral mesoderm.
• Limb buds first appear as elevations (swelling )
  of the ventrolateral body wall toward the end of
  the 4th week ( 28 days). Homeobox- containing
  genes ( Hox ) regulate the limb development.

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• The limb buds appear as elevations of the
  ventrolateral body wall by end of 4th week
• The limb buds form deep to a thick band of
  ectoderm
• The upper limb buds are visible by 26 to 27 days
• Lower limb buds appear 2 days later

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Limb Bud

• Limb buds elongate by the proliferation of the
• mesenchyme.
• The upper limb buds appear disproportionately low
  on the embryos trunk because of the early
  development of the cranial half of the embryo.
• The early stages of limb development are alike
• for the upper and lower limbs
• Because of their form function, there are many
  distinct differences between the development of
  hand and foot

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• At the apex of each limb bud the ectoderm
  thickens to form and apical ectodermal ridge
  (AER)
• The AER, a multilayered epitheleal structure , is
  induced by the underlying mesenchyme
• Bone morphogenic protein signaling is required 4
  its formation
• AER exerts an inductive influence on the limb
  mesenchyme that initiates growth of limbs in
  proximal-distal axis

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• Expermental studies show that expression of
  endogenous fibroblast growth factors T-box
  genes (tbx-4 and tbx-5) in the AER are essential
  for this process
• Mesenchymal cells aggregate at the posterior
  margin of the limb bud to form the zone of
  polarizing activity (ZPA)
• Fibroblast growth factors from AER activiate the
  zona of polarizing activity, which cause
  expression of the sonic hedgehos(shh) genes
• It has been suggested that shh secretions
  (morphogens) control the paterning of the limb
  along the anterior posterior axis

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Digital Rays

• By the end of 6th week, mesenchymal tissue in the
  hand plates has condensed to form digital rays
• These mesenchymal condensations or finger buds
  outline the pattern of the digits
• During the 7th week, similar condensations of
  mesenchyme form digital rays and toe buds in the
  foot plates
• At the tip of each digital ray ,a part of AER
  induces development of the mesenchyme into the
  mesenchymal primordia of the bones(phalanges) in
  the digits

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• The intervals between the digital rays are
  occupied by loose mesenchyme
• Soon the intervening regions of mesenchyme break
  down forming notches between the digital rays
• As the tissue breakdown progresses, separate
  digits are formed by the end of 8th week
• Programmed cell death (apoptosis) is responsible
  for the tissue breakdown in the interdigital
  regions, and is probably mediated by bone
  morphogenetic proteins, signaling molecules of
  the transforming growth factor B (????)
• Blocking these cellular and molecular events
  could account for syndactyly, webbing or fusion
  of the fingers or toes

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Final Stages of Limb Development

• As the limb elongate, mesenchymal models of the
  bones are formed by cellular aggregation
• Chondrification centers appear later in the 5th
  week. By the end of the 6th week, the entire limb
  skeleton is cartilaginous.
• Osteogenesis of long bones begins in the 7th week
  from primary ossification centers in the middle
  of the cartilaginous models of long bones
• ossification centers are present in all long
  bones by the 12th week
• Ossification of the carpal (wrist) bones begins
  during the first year after birth

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• From the dermomyotome regions of the somites,
  myogenic precursor cells migrate into the limb
  bud and later differentiate into myoblasts (
  precursors of muscle cells ).
• As the long bones form, myoblasts aggregate and
  form a large muscle mass in each limb bud
• In general this muscle mass separates into dorsal
  (extensor) and ventral (flexor) components
• The mesenchyme in the limb bud gives rise to
  bones, ligaments, and blood vessels
• The cervical and lumbosacral myotome contribute
  to the muscles of the pectoral and pelvic
  girdles.

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• Early in the seventh week the limbs extend
  ventrally

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• The developing upper limbs rotate in opposite
  directions and to different degrees
• Rotations of Limbs
• The upper limbs rotate laterally through 90
  degrees on their longitudinal axis
• Now the future elbows point dorsally
• Extensor muscles lie on the lateral and posterior
  aspects of the limb
• The lower limbs rotate medially through 90
  degrees
• Now the future knees face ventrally
• Extensor muscles lie on the anterior aspect of
  the lower limb

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• Developmentally, the radius and tibia are
  homologous bones
• Ulna and fibula , just as the thumb and great toe
  are homologous digits
• Synovial joints appear at the beginning of the
  fetal period , conceding functional
  differentiation of limb muscles and their
  innervations

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Cutaneous Innervations of Limbs

• There is strong relationship between the growth
  and rotation of the limb and the cutaneous
  segmental nerve supply of limbs.
• Motor axons arising from the spinal cord enter
  the limb buds during the fifth week Grow into
  dorsal and ventral muscle masses.
• Sensory axons enter the limb buds after the motor
  axons and use them for guidance.
• Neural crest cells, the precursors of schwann
  cells, surround the motor and sensory nerve
  fibers in the limbs Form the neurolemmal and
  myelin sheaths.

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• During the 5th week, the peripheral nerves grow
  from the developing limb plexuses into mesenchyme
  of limb buds.
• The spinal nerves are distributed in segmental
  bands, supplying both dorsal and ventral surfaces
  of the limb buds.
• A dermatome is the area of skin supplied by a
  single spinal nerve and its ganglion, however ,
  cutaneous nerve areas and dermatomes show
  considerable overlapping
• As the limbs elongate, the cutaneous distribution
  of the spinal nerves migrates along the limbs and
  no longer reaches the surface in the distal part
  of the limbs

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• The original dermatomal pattern changes during
  growth of the limbs
• An orderly sequence of distribution can still be
  recognized in the adult
• In the upper limb, observe that the area supplied
  by C5 and C6 adjoin the areas supplied by T2, T1
  and C8 but the overlap between them is minimal at
  the ventral axial line.
• A cutaneous nerve area is the area of skin
  supplied by a peripheral nerve.

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• Because there is overlapping of dermatomes , a
  particular area of skin is not exclusively
  innervated
• by a single segmental nerve.
• The limb dermatomes may be traced progressively
  down the lateral aspect of the upper limb and
  back up its medial aspect.
• A comparable distribution of dermatosomes occur
  in the lower limb , which may be traced down the
  ventral aspect and then up the dorsal aspect of
  the
• lower limbs.
• When the limbs descend they carry their nerves
  with them
• This explains the oblique course of the nerves
  arising from the brachial and lumbosacral
  plexuses

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Blood supply to limbs

• The limb buds are supplied by branches of the
  intersegmental arteries which arise from the
  aorta and form a fine capillary network
  throughout the mesenchyme.
• The primordial vascular pattern consists of a
  primary axial artery and its branches which drain
  into a peripheral marginal sinus.
• Blood in the marginal sinus drain into a
  peripheral vein
• The vascular pattern changes as the limbs
  develop, chievly gy angiogenesis (sprouting from
  existing vessels)

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• The new vessels coalesce with other sprouts to
  form new vessels
• The primary axial artery becomes the brachial
  artery in the arm and the common interosseous
  artery in the forearm which has anterior and
  posterior interosseous branches.
• The ulnar and radial arteries are terminal
  branches of the brachial artery.
• As the digits form, the marginal sinus breaks up
  and final venous pattern is represented by
  basilic and cephalic veins and their tributaries
  develop.

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• In the thigh, the primary axial artery is
  represented by the deep artery of the thigh
  ( profunda femoris ).
• In the leg, the primary axial artery is
  represented by the anterior and posterior tibial
  arteries.

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• anomalies of the limbs
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