Embryology

Question 1

What is visible at the end of week4 and where?

Answer 1

• Outpouchings from venterolateral body wall

- Hindlimb bud

Question 2

What do the 3 germ layers make?

Answer 2

• Mesoderm= Muscles
• Endoderm= Organs
• Ectoderm= Nervous system

Question 3

What are the origins of the limb buds?

Answer 3

• Originally core of tissue from lateral plate mesoderm

- Covered in ectoderm

Question 4

What does the mesoderm core differenitate into? What does this later form?

Answer 4

• Mesenchyme

- Forms bones & connective tissue of limbs

Question 5

What is skeletal muscle derived from?

Answer 5

Paraxial mesoderm

Question 6

What does the paraxial mesoderm form?

Answer 6

• Somites each side of neural tube
• Differentiate into structures inc. muscles of limbs
• Migrate from somites into developing limb bud

Question 7

Describe somite differentiation

Answer 7

• Divide into ventral (Sclerotome)= vertebral column
• Dorsolateral part forms dermomyotome divides into dermatome & myotome
• Dermatome gives rise to dermis of skin
• Myotome gives rise to skeletal muscle

Question 8

What 2 components are formed from myotome differentiation?

Answer 8

• Dorsal epimere= back muscles inn: dorsal rami of spinal nerves
• Ventral hypomere= thoracoabdominal muscles & limb muscles inn: ventral rami of spinal nerves

Question 9

What is the hypomere divided into?

Answer 9

• Posterior condensation: Extensors & abductors

- Anterior condensation: Flexors& adductors

Question 10

When does limb rotation occur and in what direction?

Answer 10

• 6-8weeks

- Rotate 90degrees medially (flexors lie posteriorly)

Question 11

Along what 3 axes is limb development controlled?

Answer 11

• Proximodistal= from hip to foot
• Craniocaudal
• Dorsoventral= Sole of foot is ventral

Question 12

How is proximodistal outgrowth controlled?

Answer 12

• Lateral plate mesoderm secretes fgf10
• Induces thickening of overlying ectoderm along tip of limb bud (AER)
• AER expresses fgf4&8 causing rapid proliferation of mesenchymal cells in progress zone
• Progress zone maintains outgrowth of limb

Question 13

How is dorsoventral patterning controlled?

Answer 13

-Dorsal ectoderm expresses wnt7 ventral ectoderm expresses engrailed-1 inhibiting wnt7

Question 14

How is craniocaudal patterning controlled?

Answer 14

• Determined by small region of mesenchyme in caudal limb bud- ZPA
• ZPA expresses shh protein
• Shh diffuses ZPA in cranial direction
• High conc of shh induces formation of caudal structures (little toe)

Question 15

What is:

• Amelia
• Meromelia
• Phocomelia

Answer 15

A= Complete loss of limb (early fgf loss)
M= Partial limb loss (late fgf loss)
P= Digits develop prematurely

Question 16

What digit malformations can occur?

Answer 16

• Polydactyly=extra digits (ectopic shh expression)
• Syndactyly= fused digits (apoptosis failure)
• Ectrodactyly= Split foot (failure of middle AER to form)