Development Of The Limbs Flashcards
What is a limb bud made up of?
Mesenchymal cells surrounded by ectoderm
What is bone derived from?
Paraxial mesoderm
When and where do limb buds appear?
After embryonic folding
On the ventral lateral wall
What appear first? Upper or lower limb buds?
Upper limb buds, two days before lower limb buds.
In which direction do the limb buds grow?
Ventral
What drives elongation of the limb buds?
Proliferation of the mesenchymal cells
In which week do limb buds appear?
4th
What happens to the notochord?
Becomes marooned by axial skeleton formation
What is the thickened ectoderm at the apex of the limb bud called?
The apical ectodermal ridge
What does the AER do?
Induces development of the hand and foot plates
Drives elongation of the limb bud
Drives limb development in the proximal-distal axis
What does the AER secrete and what does this do?
Signalling molecules
Keeps the underlying mesoderm from differentiating so that they just multiply to extend the limb bud
What is the significance of the signals being diffusion limited?
As the limb bud extends, the more proximal mesenchyme isn’t affected and can differentiate into skeletal components
How does the ectoderm begin dorsalisation and ventralisation?
The AER - marks the boundary between the dorsal and ventral limb ectoderm. Initiates signalling within the ectoderm for this giving palmar and dorsal characteristics
What generates asymmetry of the limbs?
Zone of polarising activity
What are digital rays? Explain their function
They appear on the flattened paddle. AER is maintained only over the tips of these rays which causes their slight elongation. Apoptosis of the cells between the rays occurs.
What is syndactyly?
Fusion of digits by connective tissue or bone
What is polydactyly?
How does it occur?
Extra digits.
Hereditary recessive
What is Amelia?
Complete absence of a limb
What is Meromelia?
Partial absence of one or more limb structures
How does musculature develop?
Myogenic precursors migrate into the limb bud from somites either side of the axis
Coalesce into two common muscle masses around the skeletal elements.
Individual muscles then split from common masses.
Which mass makes up the flexor and which the extensor?
Ventral = flexor Dorsal = extensor
How is it that in the lower limb, the extensor and flexors are in lateral and medial compartments, but in the upper limb, they are anterior and posterior?
As the limbs grow, they start to rotate.
Upper limb rotates laterally and lower limb medially.
What is a dermatome?
A strip of skin supplied by a single spinal nerve
What is a myotome?
A muscle/group of muscles supplied by a single spinal nerve
Where, in relation to the spine, do the upper limb buds appear?
Opposite the caudal cervical spine segments
Where do the lower limb buds appear?
Opposite the lumbar and sacral spine segments
What does preaxial mean?
Thumb and big toe
What does postaxial mean?
Little finger and toe
