Limb development

Question 1

Which process forms the mesoderm?

Answer 1

Gastrulation

Question 2

Describe how mesoderm is formed during gastrulation

Answer 2

Invading epiblast cells travel through the primitive streak to form the mesoderm

Question 3

What happens as a result of gastrulation?

Answer 3

Formation of the 3 definitive germ layers

Question 4

Name the 3 germ layers

Answer 4

Ectoderm
Mesoderm
Endoderm

Question 5

What happens to the mesoderm when it is formed?

Answer 5

Undergoes further changes to form paraxial, intermediate and lateral plate mesoderm

Question 6

What does the paraxial mesoderm give rise to?

Answer 6

Somites

Question 7

Which part of the mesoderm is important for limb development?

Answer 7

Lateral plate mesoderm

Question 8

Describe the first signs of limbs

Answer 8

Small outpocketings from the body

Question 9

When do the first signs of the limbs appear?

Answer 9

During week 5

Question 10

What do the limb buds contain?

Answer 10

Mesenchyme

Question 11

Where is the mesenchyme in the limb buds made?

Answer 11

From lateral plate mesoderm

Question 12

What can be found at the distal area of the limb bud?

Answer 12

Thickened area called the apical ectodermal ridge (AER)

Question 13

Why is the apical ectodermal ridge important?

Answer 13

Keeps cells at the distal region in a state of proliferation and undifferentiated

Question 14

What happens to cells proximal to the AER as the limb grows?

Answer 14

Able to differentiate

Question 15

What controls the spatial patterning of the limb?

Answer 15

Hox genes

Question 16

What does the limb mesenchyme produce?

Answer 16

All the connective tissues of the limb including tendons, ligaments, fascia, cartilage and bone

Question 17

How are the skeletal elements laid down?

Answer 17

As cartilage in a proximal to distal sequence

Question 18

How is the hand/foot plate formed?

Answer 18

Paddle shaped feature

Question 19

What begins to form from the hand/foot plate?

Answer 19

Digital rays

Question 20

How are fingers and toes formed?

Answer 20

Programmed cell death of tissue between each digital ray

Question 21

What happens after the fingers and toes are formed?

Answer 21

All of the cartilages undergo endochondral ossification to form bone

Question 22

When does ossification of long bones stop?

Answer 22

Adulthood

Question 23

Where do the limb muscles originate?

Answer 23

Somites

Question 24

Describe how limb muscles form

Answer 24

Limb muscle cells delaminate from the somite and migrate into the limb bud

Once in the limb, muscle cells congregate close to the skeletal elements

Muscle cells organise themselves into ventral and dorsal muscle masses which undergo further cleavage to form the individual muscles of the limb

Question 25

Which nerves migrate into the limb to innervate muscles

Answer 25

Ventral primary rami of spinal nerves

Question 26

What is phocomelia?

Answer 26

Where parts of the limb are absent

Question 27

What is syndactyly?

Answer 27

Where there is fusion of the digits

Question 28

What causes phocomelia?

Answer 28

lack of cell proliferation

Question 29

What causes syndactyly?

Answer 29

lack of cell death