Week 19 - Development of the Lower Limb

Question 1

What is organogenesis?

Answer 1

The period encompassing all of the development of organs during the first 8 weeks of embryonic development.

Question 2

What are morphogens?

Answer 2

Factors that regulate development.

Question 3

What are teratogens?

Answer 3

Factors that can disrupt developmental processes.

Question 4

What germ layers and tissues are involved in limb derivation?

Answer 4

Somatic mesoderm from the lateral plate and somites.

Question 5

When does limb development initiate?

Answer 5

Around the end of week 4.

Question 6

What are the two types of folding that occur during embryonic development?

Answer 6

• Lateral folding
• Rostro-caudal folding

Question 7

What is the AER in limb development?

Answer 7

Apical ectodermal ridge that induces limb bud formation.

Question 8

When is the forelimb initiated during gestation?

Answer 8

Day 26-27.

Question 9

What are HOX genes responsible for in limb development?

Answer 9

The placement of limb buds.

Question 10

What is the role of TBX4 and TBX5 in limb development?

Answer 10

• TBX4 induces the hindlimb
• TBX5 induces the forelimb

Question 11

What do FGF8 and FGF10 do in limb development?

Answer 11

• FGF8 is expressed in the ectoderm and signals to the mesoderm
• FGF10 is expressed in the mesoderm and signals to the AER

Question 12

What are the three zones of limb segmentation?

Answer 12

• Stylopod
• Zeugopod
• Autopod

Question 13

What is the ZPA and its function?

Answer 13

Zone of Polarizing Activity that expresses Sonic Hedgehog (SHH) to polarize the limb.

Question 14

What happens if WNT7a or LMX-1 is lost?

Answer 14

Results in a bi-ventral limb.

Question 15

What is chondrogenesis?

Answer 15

The process of cartilage formation induced by BMPs.

Question 16

What occurs at the end of week 6 in limb development?

Answer 16

Separation of digits begins in the hand plate and foot plate.

Question 17

What is Amelia?

Answer 17

Absence of limb development.

Hemimelia is half absent limb

Question 18

What is Meromelia?

Answer 18

Partial absence of limb development.

Question 19

What is Phocomelia?

Answer 19

Absence of the zeugopod.

Question 20

What is Sirenomelia?

Answer 20

Fusion of limb buds due to failure of mesoderm migration.

Question 21

What is Polydactyly?

Answer 21

Extra digits resulting from disruption of ZPA induction.

Question 22

What is Syndactyly?

Answer 22

Failure of digital rays to undergo complete apoptosis.

Question 23

What is Brachydactyly?

Answer 23

Congenitally shortened digits.

Question 24

What is Ectrodactyly?

Answer 24

Split hand/foot malformation due to absent central AER rays.

Question 25

What is the significance of the chick model in developmental biology?

Answer 25

Allowed for surgical alteration and monitoring of limb development.

Question 26

When do the upper and lower limbs rotate, and in which directions?

Answer 26

* Upper limb rotates dorsally * Lower limb rotates ventrally

Question 27

What is the timeline for limb development by the end of week 8?

Answer 27

Limbs have developed externally but require continued differentiation during fetal period.

Question 28

What is the impact of Thalidomide during pregnancy?

Answer 28

Linked to increased incidence of phocomelia.

Question 29

What are the effects of limb rotation on muscle positioning?

Answer 29

Flexor and extensor muscles are on opposing sides of the limbs.

Question 30

Fill in the blank: The limb is derived from both somatic mesoderm from the lateral plate and from the ______.

Answer 30

[somites]