Msk3: Development Of The Limbs

Question 0

Describe the formation of digits

Answer 0

Apoptosis of interdigital tissue- sculpted out

Condensation of mesenchyme further to form cartilagenous digital Rays

Question 1

Summarise prenatal limb development

Answer 1

First limb buds appear on antero- ventral wall and are visible at the end of the 4th week
The buds are developed from proliferation of somatic layer of lateral plate mesoderm covered by cuboidal ectoderm
An apical ectodermal ridge (AER) develops from apex which organises growth direction from proximal to distal.
The fetus then undergoes patterning, digit formation, limb rotation, and primitive development of bones, muscles and neurovascualture.

Question 2

What is the function of the AER

Answer 2

The apical ectodermal ridge organises the limb structure and their development.
Growth from proximal to distal where ends paddle at 6 weeks
AER regresses and separates into five parts
Also acts as a boundary between dorsal and ventral limb ectoderm- patterning

Question 3

Describe how the limbs orientate through development

Answer 3

Limbs initially extend ventrally,
In 7th week they rotate
Upper limbs initially have thumbs up and elbows lateral,
then 90o laterally so thumbs are facing dorsal laterally and out and elbows posteriorly, (extensors now lateral and posterior)
Lower limbs from soles facing towards each other and knees out medially 90o to soles down knees forward (extensors now anterior aspect)

Question 4

Describe the development of bone structure

Answer 4

By the end of the 6th week hyaline cartilage model is developed: mesenchyme cells become chondrocytes,
Joints develop where chondrogenesis is arrested
Endochronal ossification begins at end of 12 th week- end of embryonic period. Primary ossification centres developed by birth. Secondary (epiphyseal plates) finish later.
If damaged: affect growth

Question 5

Describe the envelop meant if musculature

Answer 5

As bones developing myoblasts aggregate and develop a large muscle mass in each limb bud
Separates into at first dorsal flexor and ventral extensor compartments and then into
dorsal (extensor) and ventral (flexor) compartments, think about rotation

Question 6

Describe the innervation of the limbs

Answer 6

As soon as buds form (4 wks) ventral primary rami from spinal nerves extend to mesenchyme cells. Needs to happen early for complete function.
UL bud is opposite the caudal cervical spine segments
LL bud is opposite the lumbar and sacral

Nerves group to form large dorsal and ventral nerves
Ventral/anterior: medial/lateral cords- flexors (ulna median nerves)
Dorsal/posterior: posterior cords- extensors (radial)

Question 7

Explain the developmental basis of some common abnormalities of the limbs

Answer 7

Amelia- absence of a limb
Meromelia- partial absence of limbs (phocomelia from thalidomide, AER interfered with)
Polydactyly- extra digits, recessive
Syndactyly (cutaneous and Osseous)- fusion of digits, from soft tissue or from bones fusing

Question 8

Describe how congenital hip displacement can occur

Answer 8

Now termed Developmental dysplasia of the hip (DDH) as doesn’t always present at birth.
Either the shape of the socket (acetabulum) , the head of the femur or the supporting structures around them are shaped wrong.
Can be mild to severe.
Breech presentation can be a risk factor- strains new joints