DEVELOPMENT OF LIMBS
Aims and Objectives
At the end of lecture, students should be able to:
- Know and define the site and time of appearance of upper and lower limb bud
- Define apical ectoderrmal ridge (AER)
- Understand mesenchymal proliferation under the influence of AER and differentiation into cartilaginous models of future limb bones
- Understand and define the source of mesoderm forming the limb muscles
- Know the hand plate and formation of digital rays resulting into digits
- Define and understand the rotation of upper limb with final position of flexor and extensor limb muscles
- Define common congenital anomalies of limbs
The parts of the vertebrate limb
►The resulting limb is divided into four basic
►elements; a root consisting of the scapula and clavicle
►in the forelimb and hip bones in the hind limb, a
►proximal part consisting of a single
►skeletal element called the Stylopod (in the forelimb
► this is the Humerus, in the hind limb the femur),
►a middle portion, the Zeugopod with two elements
►(ulna and radius in the forelimb or fibula and tibia
►in the hind limb) and a distal part, the Autopod
►composed of a carpus (hand) or tarsus (foot) with a
►variable number of digits.
THE DEVELOPMENT OF LIMBS
►The limb bud appear from ventro-lateral body wall during 5th week of development at the level of lower cervical and first two thoracic segments
►The mesenchymal core was initially derived from somatoplrure of lateral plate mesoderm
►This mesenchyme give rise to bones & connective tissue covered by a layer of cuboidal ectoderm
►The ectoderm at tip of limb ,thickened to form Apical Ectodermal Ridges
►Ridges exerts the inductive influence on underlying mesoderm
►The mesenchyme adjacent to AER remains as undifferenciated , rapidly proliferating cells
►The cells far away from from inductive influence begins to differentiate into cartilage &muscles
►The development of limb proceeds to proximo-distal direction
►During sixth week the limb bud flattens to form the handplate & footplate
►The first circular constriction separate these plates from proximal segment
►Second constriction divide the proximal segment into two segments
►Fingers & toes are formed when cell death in AER separate ridges into five parts
►The formation of finger is dependent upon their continuing outgrowth under the inductive influence of five segments of ridge ectoderm ,condensation of mesenchyme to form cartilaginous digital rays & death of intervening tissue between rays
►The development of U/L & L/l is similar , morphogens of L/L appear two days later than U/L
►During 7th week the limbs rotate in opposite direction
►The U/L rotates 90 degrees laterally so that, the extensor muscle lie on lateral and posterior surface & thumb lie laterally
►The L/L rotates 90 degrees medially ,so that extensor muscles lie anteriorly & big toe faces medially
►When the external shape is being established , mesenchyme in buds begin to condense ,by the end of sixth week the first hyaline cartilage model of bones can be recognized
►Ossification of bones of extremities, the endochondral ossification begins by the end of embryonic period.
►From primary centres of ossification in shaft or diaphysis ,endochondral ossification progresses gradually towards the end of cartilaginous “model.”
►At birth diaphysis of bone is completely ossified but the epiphysis remains cartilaginous,shortly ossification centres appear in epiphysis
►Temporarily the cartilaginous plate remains between diaphysis & epiphysis
►This cartilaginous plate plays an important role in growth in length of long bones
►On both sides of plate , endochondral ossification proceeds, when the bone acquire a full length , the epiphyseal plate disappear & the epiphysis then join the shaft of long bone
►In long bones epiphyseal plate is found on both extremeties , whereas in smaller bones ,it is found only on one extremity
CINICAL COORRELATES
►The bone age of child can be studied by the radiologist to sort out whether the child has reached his or her proper maturation age
►The ossification centers in hand &wrist of children are studied
►Prenatal analysis of fetal bones by U/S provide information about fetal growth & gestational age
►MEROMELIA : is partial absence of limbs
►AMELIA is total absence of limb
►PHOCOMELIA where long bones absent or rudimentary
►MICROMEDIA all segments of bones are present but are very short
►These abnormalities are may be due to teratogenic insults e.g. thalidomide (sleeping pills or antinauseant falum during 4-5th week of development may lead to absence or gross deformities of long bones, intestinal atresia & cardiac anomalies
►Other category of limb development is presence of extra fingers and toes- POLYDACTLY
►The extra digit lack proper muscular connection excessive number of bones are present and may be bilateral
►ECTODACTYLY- absence of digit, such as thumb-umblitical
►SYNDACTYLY- abnormal fusion of fingers or toes
►CLEFT HAND AND FOOT (lobster claw deformity, abnormal cleft between 2nd & 4th metacarpal bones & soft tissues)
►The 3rd metacarpal & phalangeal bones are about always absent, thumb & index fingers as well as 4-5th fingers may be fused
►The two parts of hands are some what opposed to each other and act like lobster claw
►CLUB FOOT: is usually present in combination with syndactyly
►The sole of foot is turned inwards, the foot is adducted and planter flexed
►Common in males, hereditary.
►Congenital absence of deficiency of radius
►Genetic
►Associated with craneisyntosis-radial aplasia syndrome
►Digital defects along with it are absent thumb and short corner ulna usually present
►AMNIOTIC BANDS: may cause rung constrictions of limbs or digits & amputation
►CONGENITAL HIP DISLOCATIONS:
►Consists of underdevelopment of Acetabulum & head of femur
►Common and occurs in female
►Abnormality of bones occur precaudally whereas dislocation may occur after birth
►Baby with breech deliveries are affected.