Transcript PPTX, 3.71MB

DEVELOPMENT OF THE
LIMBS
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Development of the Limb Bud
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Limb development is divided into four stages:
The bud stage (initial outgrowth),
The paddle stage (dorsoventral flattening),
The plate stage (relative expansion of the distal end), and
Rotation stage (rotation around the proximodistal axis).
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Limb Bud
• Limbs will not form just anywhere along the body axis. Rather,
there are discrete positions where limb fields are generated.
• The upper buds develop at the lower cervical and upper
thoracic levels C5-C8. The lower buds develop at the lumbar
and upper sacral regions L3-L5.
• The upper limb buds are visible by day 26 or 27, and the lower
limb buds appear 1 or 2 days later.
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Limb Bud…
• Each limb bud consists of an outer ecto- dermal cap and an
inner mesodermal core.
• The ectodermal cells at the end of each limb bud proliferate to
form the apical ectodermal ridge (AER).
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Limb Bud…
• The AER, however, does not remain passive, for without the
AER cells the limb does not develop. Thus, the ectodermal tissue
signals the underlying mesoderm. The mesodermal cells closest to
the apical ectodermal ridge proliferate, and as this proliferation
continues the cells further from the ectoderm begin to
differentiate.
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Limb Bud…
• The pattern of digit formation is dependent upon a region of
posterior limb bud mesoderm located beneath the apical
ectodermal ridge. This region is called the zone of polarizing
activity (ZPA).
• The ZPA determines the polarity of the developing digits in
part by influencing the overlying apical ectodermal ridge. The
factors that are involved in this activity include sonic hedgehog,
which influences both the AER and the ZPA and fibroblast
growth factor - fgf2 and fgf4.
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Limb Bud…
• As the mesoderm within the limb proliferates, the limb
elongates and the mesenchyme segregates into superficial,
intermediate and deep regions.
• Mesenchymal cells aggregate throughout the cores of the
elongating limb buds to form the limb skeleton.
Chondrification centers appear and initially the entire limb
skeleton is cartilaginous.
• Osteogenesis, mostly by the endochondral process, begins with
the clavicle and extends throughout fetal life and childhood.
Primary ossification centers start in the shaft of long bones; an
epiphyseal plate of cartilage occurs between the primary and
secondary (epiphyseal) ossification centers.
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Hand and footplate
• The distal end of each limb bud becomes flattened as paddle-shaped
limb plates.
• Five mesenchymal condensations, the digital rays, appear in each plate.
These eventually chondrify and ossify to form the long bones of the
hands and feet
• Loose mesenchyme, lying between the digital rays, eventually
degenerates, with the overlying ectoderm, leaving interdigital clefts and
thus delimiting and defining the fingers and toes.
28 days
36 days
46 days
49 days
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52 days
56 days
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Development of the musculature and
nerve supply of the limbs
• There is evidence that the limb muscles develop from
somites and that the cells migrate into the limb bud.
Generally, the muscles differentiate on the flexor and
extensor sides of the axial skeleton of the limbs, in the
proximo-distal direction.
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Rotation of the Limbs
• Initially the limb buds extend laterally with the thumb
and great toe facing superiorly. The flexor surface faced
ventrally. The limbs then shift into a more ventral
position with both thumb and great toe still facing
superiorly, but the flexor surfaces now facing medially.
Both limbs then rotate.
• The upper extremity rotates laterally through 90o on its
longitudinal axis. As a result the elbow joint is located
dorsally and operates in the ventral direction. The flexor
and extensor compartments are located ventrally and
dorsally respectively.
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Rotation of the Limbs…
• In the lower limb, the rotation of almost 90o occurs in
the medial direction. The patella comes to lie anteriorly
and the knee operates in the dorsal direction. The flexor
and extensor compartments are located in the dorsal
and ventral aspects of the lower limb respectively.
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(Fig. 10)
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Anomalies of the limbs
• Amelia, absence of a limb
• Meromelia absence of part of a limb.
 Suppression of limb bud development during the early part of
the fourth week results in absence of the limbs, amelia. Arrest or
disturbance of differentiation or growth of the limbs during the
fifth week results in various types of meromelia/phocomelia
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Cleft Hand and Cleft Foot
• In lobster-claw deformities, there is absence of one or more
central digits, resulting from failure of development of one or
more digital rays. The hand or foot is divided into two parts that
oppose each other like lobster claws.
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BRACHYDACTYLY
• Shortness of the digits (fingers or toes) is the result of
reduction in the length of the phalanges. This anomaly is
usually inherited as a dominant trait and is often associated
with shortness of stature.
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POLYDACTYLY
• The term supernumerary digits refers to the presence of more than
the usual number of fingers or toes. Often the extra digit is
incompletely formed and lacks normal muscular development.
• If the hand is affected, the extra digit is most commonly medial or
lateral rather than central.
• In the foot, the extra toe is usually on the lateral side. Polydactyly
is inherited as a dominant trait.
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SYNDACTYLY
• Syndactyly is the most common anomaly of the hand or foot.
Cutaneous syndactyly (simple webbing between digits) is the
most common limb anomaly. It is more frequent in the foot than in
the hand.
• Cutaneous syndactyly results from failure of the webs to
degenerate between two or more digits.
• Osseous syndactyly
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CONGENITAL CLUBFOOT
• A club foot, or congenital talipes equinovarus (CTEV), is a congenital
deformity involving one foot or both. The affected foot appears rotated
internally at the ankle.
• Without treatment, persons afflicted often appear to walk on their
ankles, or on the sides of their feet. It is a common birth defect,
occurring in about one in every 1,000 live births. Approximately 50% of
cases of clubfoot are bilateral.
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=THE END=
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