Embryology Flashcards
When does limb development occur?
4 to 8 weeks
When are limb buds visible?
End of week 4
Which germ layer contribute to the development of limbs?
ectoderm and mesoderm
What do the buds initially consist of?
Lateral plate mesoderm covered by a layer of ectoderm
What does the mesoderm core condense into?
The mesenchyme which will form the bone and connective tissue of the limbs
What are the skeletal muscles of the limbs derived from?
Somite- blocks of paraxial mesoderm
What is the ventral part of the somite?
sclerotome which forms the vertebral column
What is dorsolateral part of the somite?
dermomyotome which divides into the dermatome and myotome.
What two components foes the myotome differentiate into?
Dorsal epimere: muscles which are innervated bu the dorsal rami of spinal nerves
Ventral hypomere: muscles of thoracic and abdominal wall and muscles of limbs which are innervated by the central ram of spinal nerves
What does limb skeletal muscle arise from?
hypomere
What two parts does the hypomeres divide into?
Posterior condensation and anterior condensation
What does the posterior condensation form?
Extensors and supinators of the upper limbs
Extensions and abductors of lower limb
What does the anterior condensation form?
Flexor pronator of the upper limbs
Flexor and abductors of the lower limb
Which branches of the central primary rami is the upper limb innervated by?
Brachial plexus C5-T1
Which branches of the central primary ram is the lower limb innervated by?
lumbrosacral plexus L4-S3
When does limb rotation occur?
6-8 weeks
What is the rotation of the limbs?
upper limbs 90 laterally so flexors lie anteriorly
lower limbs 90 medially so flexors lie posteriorly
Which 3 axes does the limb develop along?
proximodistal- growing out form the body
craniocaudal- in the embryonic position it determines the fingers ending up in the correct position
dorsoventral- ensures the ventral surface is soft and the dorsal (nails and knuckles) are hard
What controls the proximodistal outgrowth?
The lateral plate mesoderm secretes fibroblast growth factor 10
This induces a thickening of the overlying ectoderm alone the top of the limb bud- apical ectodermal ridge
The AER express Fgf 4 and 8 which cause apid proliferation of the cells underlying AER
How is the dorsocentral growth controlled?
The dorsal ectoderm expressed Wnt7 and then ventral ectoderm expresses engrained-1 which inhibits Wnt7
How is craniocaudal patterning controlled?
It is determined by the zone of polarising activity (ZPA) in the causal part of the limb bud where sonic hedgehog (Shi) is expressed. The concentration of Shh decreases as it moves crani ally. Low concs. Forms the thumb etc.
When does digit formation occur?
Week 6
How are fingers and toes formed?
Programmed cell apoptosis in the AER
What is Amelia?
complete absence of a limb- early loss of Fgf signalling
