Transcript 12. Appendicular & limb2009-06

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.
Limb buds is covered by thick band of ectoderm.
The upper limb buds are visible by day 26 or 27
and the lower limb buds are appear a day or 2
later.
Limb buds elongate by the proliferation of the
mesenchyme. The upper limb buds appear low on
the embryo’s trunk because of the early
development of the cranial half of the embryo.
The upper limb buds develop opposite the caudal
cervical segments and the lower limb buds form
opposite the lumbar and upper sacral segments.
At 32 days, the upper limb buds are paddleshaped and the lower limb buds are flipper- like.
At the apex of each limb bud the ectoderm thickens to form an apical ectodermal
ridge ( AER ). This ridge exerts an inductive influence on the limb mesenchyme to
initiates growth and development of limbs in a proximal- distal axis.
Mesenchymal cells aggregate the posterior margin of the limb to form the zone of
polarizing activity ( ZPA ). Fibroblast growth factors from the AER activate the ZPA
The distal ends of the flipper- like
limb buds flatten into paddle- like
hand and foot plates.
By the end of the 6th week,
mesenchymal tissue in the hand
plates has condensed to form
digital rays.
The intervals between the digital
rays are occupied by loose
mesenchyme. These mesenchyme
break down forming notches
between the digital rays.
6 week
During the 7th week, similar condensations of mesenchyme form digital rays and
toe buds in the foot plates.
As the tissue breakdown progresses, separate digits are formed by the end of 8th
week.
Programmed cell death ( apoptosis ) is responsible for tissue breakdown in the
interdigital regions and is mediated by bone morphogenetic proteins.
At the tip of each digital ray,
a part of the apical
ectodermal ridge (AER )
induces development of the
mesenchyme into the
mesencymal primordia of
the bones ( phalanges ) in
the digits
Blocking of cellular and molecular apoptosis could account for
syndactyly , webbing or fusion of the fingers or toes.
In most bones like the long
bones in the limbs, the
condensed mesenchyme
which develops from
mesoderm and neural crest
undergoes chondrificaion to
form hyaline cartilage bone
models.
Chondrification centers
appear later in the 5th week.
By the end of the 6th week,
the entire limb skeleton is
cartilaginous.
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 bone, ligaments and blood
vessels.
The cervical and lumbosacral myotome
contribute to the muscles of the
pectoral and pelvic girdles.
Cutaneous Innervation of Limbs
Motor axons arising from the spinal cord enter the limb buds during the 5th week and grow
into the dorsal and ventral muscle masses.
Sensory axons enter the limb buds after the motor and use them for guidance.
Neural crest cells ( precursors of Schwann cells ) surround the motor and sensory nerve
fibers in the limbs and form the neurolemmal and myelin sheaths.
A dermatome is the area of skin supplied by a single spinal nerve and its ganglion.
A cutaneous nerve area is the area of skin supplied by a peripheral nerve.
During the 5th week, the peripheral nerves grow from the developing limb plexuses ( brachial
and lumbosacral ) into the mesenchyme of the limb buds .
The spinal nerves are distributed in segmental bands supplying both dorsal and ventral
surfaces of the limb buds.
Although the original dermatomal pattern changes during growth of the limbs, the
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.
Originally the flexor aspect of the limbs is ventral and the extensor aspect dorsal
and the preaxial and postaxial borders are cranial and caudal respectively.
Early in the 7th week the limbs extend ventrally. The developing upper and lower
limbs rotate in opposite directions and to different degrees.
The upper limbs rotate laterally 90 degrees on their longitudinal axes. So, the
future elbows point dorsally and the extensor muscle lie on the lateral and
posterior aspects of the limb.
The lower limbs rotate medially through 90 degrees. So, the future knees face
ventrally and the extensor muscles lie on the anterior aspect of the lower limb.
Embryo about 52 days.
The fingers are
separated and the toes
are beginning to
separate. Note that the
feet are fan- shaped.
Embryo about 56 days.
All regions of the limbs
are apparent and the
digits in the hands and
feet are separated.
It should now be clear
that the radius & tibia ;
ulna & fibula and thumb &
great toe are homologous
digits.
Joints
They develop from interzonal mesenchyme between the primordia of bones.
In a fibrous joint the intervening mesenchyme differentiates into dense fibrous connective
tissue.
In a cartilagenous joint the mesenchyme between the bones differentiates into cartilage.
In a synovial joint, a synovial cavity is formed within the intervening mesenchyme by
breakdown of the cells. The mesenchyme gives rise to the synovial membrane and the
capsular and other ligaments of the joint.
Blood supply to limbs
The limb buds are supplied by branches of the
dorsal 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.
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.
Blood in the marginal sinus drains into a
peripheral vein.
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.
Exposure to thalidomide before 33 day may cause
The most critical period of limb
development is from 24 to 36 days after
fertilization.
Thalidomide is absolutely
contraindicated in women of
childbearing age. This drug is used as a
sedative and antinauseant. It is a potent
human teratogen during the embryonic
period.
Major limb
anomalies
appear about 2 in
1000 newborns.
Most are caused
by genetic
factors.
Molecular have
implicated gene
mutation ( Hox
gene ) in some
cases of limb
defects.
Several
congenital
anomalies of
arterial pattern
may be found in
these defects.
Congenital Clubfoot
Any deformity of the foot involving the
talus ( heel ) is called clubfoot or
talipes.
It is uncommon, occurring about once
in 1000 births.
As the children develop, they tends to
walk on the ankle ( talus ) rather than
on the sole of the foot.
Talipes equinovarus, the most common type of clubfoot. It occurs twice in males.
The sole is turned medially ( inverted foot ) with hyperextension and incurving of
the feet.
It results from abnormal positioning or restricted movement of the fetus’s lower
limbs in utero.
Hereditary factors and environmental factors are involved in most cases.
Clubfoot appears to follow a multifactorial pattern of inheritance ( interaction of
genetic and environmental factors ).
Development of Appendicular Skeleton
It consists of the pectoral and pelvic girdles
and limb bones.
The model of pectoral girdle and upper limb
bones appear slightly before those of the
pelvic girdle and lower limbs.
The clavicle initially develops by
intramembranous ossification and it later
forms growth cartilages at both ends.
The clavicles begin to ossify before any other
bone in the body.
The femora are the next bones to show traces
of ossification
Ossification begins in the long bones by the
8th week of embryonic development and
initially occurs in the diaphyses of the bones
from primary centers of ossification.
By 12 weeks primary ossification centers have
appeared in nearly all bones of the limbs.
The first indication of ossification
in the cartilaginous model of a long
bone is visible near the center of
the future shaft ( primary center of
ossification ).
The part of a bone ossified from a
primary center is the diaphysis
Primary centers appear at different
times in different bones. Most of
these centers appear between the
7th and 12th weeks of development.
All primary centers of ossification
are present at birth.
The part of a bone ossified from a
secondary center is the epiphysis.
The epiphysial plate ( cartilage )
intervenes between the diaphysis and the
epiphysis. This plate is response for
lengthening of the bone until the final size
is reached.
The epiphysial plate is replaced by bone
development on each of its 2 sides. When
this occurs growth of bone ceases.
The secondary ossification centers of the
bones at the knee are the first to appear.
The centers for the distal end of the femur
and the proximal end of the tibia appear
during the last month of intrauterine life.
Most secondary centers of ossification
appear after birth.