9.21.16 Limb Development Flashcards
(34 cards)
___ appear toward the end of the 4th week as slight bulges of the ventrolateral body wall.
Limb buds
Most of the initial growth of the limbs occurs during weeks ___.
4-8
The upper limb buds develop approximately ___ days before the lower limb buds.
2
The tissues of the limb buds are derived from what two main sources?
Mesoderm and ectoderm
What is the AER?
Apical ectodermal ridge
What does the AER do?
Exerts an inductive influence on the limb mesenchyme, promoting growth and development of the limbs.
How do limb buds elongate?
By proliferation of the mesenchyme within them
Limb elements arise from what three things?
- Paraxial mesoderm (somites): dermatome (dermis) and myotome (limb muscles)
- Lateral plate mesoderm (bones and blood vessels)
- Neural crest cells (Schwann cells and melanocytes)
What is the first step of limb patterning?
Four paddle-like limb buds develop from the ventrolateral body wall
What is the second step of limb patterning?
A circular groove appears, separating a hand plate from the upper limb bud and a foot plate from the lower one.
During the third step of limb patterning, a mesodermal condensation in the hand and foot plates results in the appearance of ___.
Digital rays
During the four step of limb patterning, a thin membrane intervening the rays (webs) breaks down and the digits become separated by ___.
Notches
Finally, a second circular constriction divides each limb imto three segments. What are these?
Upper limb: arm, forearm, hand
Lower limb: thigh, leg, foot
The four limb buds are extended at a ___ angle to the trunk.
Right
What three axes do the limbs grow in?
Proximal-distal, anterior-posterior, dorsal-ventral
What are the 2 borders and 2 surfaces of each limb?
Borders: preaxial (cephalic) near the big digit and postaxial (caudal)
Surfaces: ventral (flexor) and dorsal (extensor)
During the fifth week, peripheral nerves grow from the developing ___ into the mesenchyme of the limbs.
Limb plexuses
What are the two limb plexuses?
Brachial and lumbosacral
Limb outgrowth initiated by the AER at the tip of the limb buds proceeds from ___ to ___.
Proximal (shoulder or hip); distal (hand or foot)
The proximo-distal growth and patterning is dependent on ___ signaling from the AER.
FGF
Anything that disrupts FGF signaling and/or formation and maintenance of the AER will result in ___.
Arrested limb development
Proximo-distal patterning of the limb elements is also dependent on ___ genes to specify each of the elements. Disruption of these genes will result in loss of specific limb elements.
HOX
Describe the limb rotation of the upper limb during weeks 6-8.
90 degrees laterally:
- Flexors (anterior) and extensors (posterior)
- Radius (lateral) and ulna (medial)
- Thumb (lateral) and elbow joint points backward
Describe the limb rotation of the lower limb during weeks 6-8.
90 degrees medially:
- Flexors (posterior) and extensors (anterior)
- Fibula (lateral) and tibia (medial)
- Big toe (medial) and knee joint points forward
