APPENDICULAR SKELETONDr. Mujahid Khan

CompositionThe 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

CompositionThe 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 OssificationBy 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 OssificationPrimary 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 OssificationSecondary 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 OssificationThe 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 OssificationDuring 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 DevelopmentThe 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 BudEach 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 embryos trunk

Limb BudThe 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 BudAt 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 RaysBy 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

Digital RaysAER 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 RaysAs 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 DevelopmentAs the limbs elongate in the 5th week, chondrification centers appear

By the end of 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

Final Stages of Limb DevelopmentPrimary 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 DevelopmentThe 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 LimbsThe 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 LimbsThe 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 LimbsMotor 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 LimbsNeural crest cells, the precursors of schwann cells, surround the motor and sensory nerve fibers in the limbs

Form the neurolemmal and myelin sheaths

A dermatome in this area of skin supplied by a single spinal nerve and its spinal ganglion

Cutaneous Innervation of LimbsDuring 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 LimbsThe 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 LimbsThe 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 LimbsThe 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 LimbsMinor 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