Embryology

Question 1

What day of development do limb buds appear

Answer 1

Day 24 - upper limb buds appear

1-2 days later - lower limb buds appear

Question 2

The apical ectodermal ridge (AER)

Answer 2

Thickened ectoderm at the apex of the limb bud

The mesenchyme adjacent to the AER consists of undifferentiated, rapidly proliferating cells - “the progress zone”

Question 3

Formation of cartilaginous precursors of limb bones

Answer 3

Proximal mesenchyme of the limb bud (further away from the AER) differentiates into a cartilage bone model by chondrification
(6th week)

Question 4

Ossification of limb bones

Answer 4

Endochondral ossification - cartilage bone model ossifies to form bone

(except clavicle = membranous ossification)

Question 5

Formation of hands and feet

Answer 5

Distal ends of the limb bud flatten into paddle-like hand plates and flipper-like foot plates

Question 6

Formation of fingers/ toes

Answer 6

1. Mesenchymal tissue condenses to form digital rays (UL=6th week, LL=7th week)
2. Notches form btw digital rays
3. Webbed fingers / toes
4. Separate digits

Question 7

The breakdown of interdigital regions (btw fingers and toes) is mediated by…

Answer 7

Apoptosis, mediated by bone morphogenic proteins

Question 8

Rotation of the upper limb

Answer 8

90 degrees laterally

so extensors are lateral and posterior and thumbs are lateral

Question 9

Rotation of the lower limb

Answer 9

90 degrees medially

so extensors are anterior and big toes are medial

Question 10

Development of nerve supply to the limbs

Answer 10

Peripheral nerves grow into the mesenchyme from developing limb plexuses (brachial and lumbosacral)

In the 5th week