Download Appendicular Skeleton2009-06-04 08:555.0 MB

APPENDICULAR
SKELETON
Dr. Mujahid Khan
Composition

The appendicular skeleton consists of pectoral girdles
and limb bones

Mesenchymal bones form during the fifth week in the
limb buds

Chondrification of mesenchymal bone models occurs in
the sixth week

Clavicle initially develops from intramembranous
ossification

Later forms growth cartilages at both ends
Composition

The models of pectoral girdle and upper limb
bones appear slightly before those of the pelvic
girdle and lower limbs

The bone models appear in a proximodistal
sequence

Ossification begins in the long bones by the
eighth week

Initially occurs in the diaphysis
Primary Ossification

By 12 weeks primary ossification centers appear
in almost all bones of the limbs

The clavicle 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
Primary Ossification

Primary centers appear at different times in
different bones

Most of them develop between 7 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
Secondary Ossification

Secondary ossification centers of the bones at
knee are the first to appear

The centers for the distal end of femur and
proximal end of tibia appear during 34 to 38
weeks

Consequently they are present at birth

Most secondary centers of ossification appear
after birth, called epiphysis
Secondary Ossification
 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
Secondary Ossification
 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
Limb Development
 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
Limb Bud

Each limb bud consists of a mass of
mesenchyme covered by ectoderm

The mesenchyme is derived from the somatic
layer of lateral mesoderm

The limb buds elongate by the proliferation of
the mesenchyme

The upper limb buds appear low on the
embryo’s trunk
Limb Bud

The early stages of limb development are alike
for the upper and lower limbs

Development of upper limb buds occurs 2 days
before that of lower limb buds

The upper limb buds develop opposite the
caudal cervical segments

Lower limb buds form opposite the lumbar and
upper sacral segments
Limb Bud

At the apex of each limb bud the ectoderm
thickens to form an apical ectodermal ridge
(AER)

AER exerts an inductive influence on the limb
mesenchyme that initiates growth of limbs in
proximal-distal axis

Mesenchymal cells aggregate at the posterior
margin of the limb bud to form the zone of
polarizing activity (ZPA)
Digital Rays
the end of 6th week, mesenchymal
tissue in the hand plates has condensed to
form digital rays
 By
 These
mesenchymal condensations or
finger buds outline the pattern of the digits
the 7th week, similar condensations
of mesenchyme form digital rays and toe
buds in the foot plates
 During
Digital Rays
 AER
induces development of the
mesenchyme into the mesenchymal
primordia of the bones in the digits
 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
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

Blocking these cellular and molecular events
could account for syndactyly, webbing or fusion
of the fingers or toes
Final Stages of Limb
Development
the limbs elongate in the 5th week,
chondrification centers appear
 As
the end of 6th week, the entire limb
skeleton is cartilaginous
 By
 Osteogenesis
of long bones begins in the
7th week from primary ossification centers
in the middle of the cartilaginous models of
long bones
Final Stages of Limb
Development

Primary 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

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
Final Stages of Limb
Development
 The
mesenchyme in the limb bud gives
rise to bones, ligaments, and blood
vessels
 From
dermomyotome regions of somites,
myogenic precursor cells also migrate into
the limb bud
 Later
they differentiate into myoblasts or
precursors of muscle cells
Rotations of Limbs
 The
cervical and lumbosacral myotomes
contribute to the muscles of the pectoral
and pelvic girdles, respectively
 Early
in the seventh week the limbs extend
ventrally
 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
Cutaneous Innervation
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
Cutaneous Innervation
of Limbs
 Neural
crest cells, the precursors of
schwann cells, surround the motor and
sensory nerve fibers in the limbs
 Form
A
the neurolemmal and myelin sheaths
dermatome in this area of skin supplied
by a single spinal nerve and its spinal
ganglion
Cutaneous Innervation
of Limbs

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

As the limbs elongate, the cutaneous distribution
of the spinal nerves migrates along the limbs
Cutaneous Innervation
of Limbs

The original dermatomal pattern changes during
growth of the limbs

An orderly sequence of distribution can still be
recognized in the adult

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
Blood Supply to Limbs

The limb buds are supplied by branches of the dorsal
intersegmental arteries

They arise from the aorta and form a fine capillary
network in the mesenchyme

The primordial vascular pattern consists of a primary
axial artery and its branches

The vascular pattern changes as the limbs develop

This occurs by vessels sprouting from existing vessels
Blood Supply to Limbs

The new vessels coalesce with other sprouts to
form new vessels

The primary axial artery becomes the brachial
artery and common interosseous artery in the
forearm

In the thigh the primary axial artery is
represented by the profonda femoris artery

In the leg it is represented by the anterior and
posterior tibial arteries
Anomalies of Limbs

Minor limb anomalies are common and can be
corrected surgically

The most critical period of limb development is
from 24 to 36 days after fertilization

Exposure to teratogen before day 33 may cause
severe limb defects

Major limb anomalies appear about twice in
1000 newborns
THE END