Limb Development

Question 1

Describe the order of limb pattterning

Answer 1

Cingulum —> stylopod —> zeugopod —> auto pod

Question 2

What are some examples of a stylopod

Answer 2

Humerus/femur

Question 3

What are some examples of a zeugopod

Answer 3

Ulna + radius

Tibia + fibula

Question 4

What are some examples of auto pods

Answer 4

Wrist + fingers

Ankle + toes

Question 5

What does the mesenchyme core form and what layers are associated with it?

Answer 5

Is undifferentiated tissue

Lateral plate mesoderm and paraxial mesoderm

Question 6

What does the lateral plate mesoderm form

Answer 6

Cartilage, bones, general CT

Question 7

What does the paraxial mesoderm form

Answer 7

Somites, skeletal muscle

Question 8

What is the ectoderm cover and what are some things it forms?

Answer 8

Is the outer germ layer

Out-pocketing of ventral body wall: epidermis of skin, nails, hair

Spinal nerves that will form nervous plexuses

Question 9

In what fashion do elements of the limb develop

Answer 9

Proximal to distal fashion (humerus to distal digits)

Question 10

What is the product of T-box genes

Answer 10

Limb identity (ie pectoral vs thoracic)

Question 11

What do how genes determine?

Answer 11

-where limb buds will form
-what limb will develop at specific location

Question 12

What does the limb field develop

Answer 12

Forms the limb bud

Question 13

What does the limb bud develop

Answer 13

Develops developing limb

MUST START FROM LIMB FIELD

Question 14

What transcription factors act to induce an epithelium-to-mesenchyme transition that induces the formation of the limb bud?

Answer 14

Fibroblast growth factors

T-box transcription factors

Question 15

Describe the early development stages of the limb bud

Answer 15

Originates as ridges in ventrolateral body wall

FOREBUD RIDGE APPEARS FIRST then 1-2 days later, hindbud ridge appears

Question 16

What 3 axes do the limbs develop along?

Answer 16

-proximodistal
-dorsoventral
-craniocaudal

Question 17

How does the proximodistal axes work

Answer 17

Fibroblast growth factors acting at the apical ectodermal ridge (AER)

Question 18

How does the dorsoventral axes work

Answer 18

Dorsal ectoderm - folding initiated by Wnt7a

Question 19

How does the craniocaudal axes work

Answer 19

Sonic hedgehog (Shh) genes acting at the zone of polarizing activity (ZPA)

Question 20

What does the apical ectodermal ridge do

Answer 20

-directs pattern formation on proximal-distal axis

Thickened, specialized epithelium located at the dorsoventral border of the tip of the bud

INDUCES PROLIFERATION OF LIMB BUD MESENCHYME

Question 21

What happens once the limb bud acquires its terminal length

Answer 21

1) terminal portion of each bud becomes flattened to form hand and footplates
2) 2 constrictions divide the portion of each limb bud
-now have 3 main segments!!!
-stylopod, zeugopod, auto pod

Also, mesenchyme condenses into cartilage models - templates of future long bones

Outer shell —> general shape —> fill in details

Question 22

What directs pattern formation on the dorsal-ventral axis

Answer 22

Non-AER ectoderm

Directs dorsal-ventral shape

Question 23

Describe the dorsal-ventral development

Answer 23

-precursor muscle cells from ventrolateral myotome of somite migrate to limb bud
-ventral and dorsal muscle groups visible at base of each limb bud
-ventral and dorsal branches of ventral ramus follow growing muscle and provide motor inner action
-muscles are initially segmented but many fuse
-nerve branches from different segments unite into large nerves that serve the dorsal or ventral limb compartments

Question 24

On the cranial-caudal axis, what directs pattern formation

Answer 24

Zone of polarizing activity

Question 25

Describe the zone of polarizing activity (ZPA)

Answer 25

-area of mesenchyme on caudal border -gives cranial-caudal identity to differentiating cells ZPA cells express retinol acid (RA) and Shh

Question 26

What does the Shh gradient induce on the cranial caudal axis

Answer 26

Induces nested expression of mesenchymal Hox genes

Question 27

What does ZPA and Shh expression require

Answer 27

-input from AER -input from dorsal ectoderm

Question 28

What is the difference between apoptosis and necrosis

Answer 28

Programmed cell death vs traumatic cell death

Question 29

Describe the final limb morphology development

Answer 29

-as limb development progresses, the AER breaks up and only covers the emerging digital rays -interdigital spaces no longer have AER and can now be sculpted by apoptosis

Question 30

What is homology

Answer 30

Existence of shared ancestors between a pair of structures, or genes, in different species

Question 31

Describe the horse digit reduction

Answer 31

Post patterning changes -initial limb segments are all laid in cartilage -digits 1 and 5 are eliminated -digits 2 and 4 reduction is the result of extensive apoptosis

Question 32

Describe the cattle and swine digit reduction

Answer 32

Patterning changes -digit loss and shift of central axis of foot between digits 3 and 4

Question 33

describe the digit reduction of camels

Answer 33

Post-patterning change Similar to horses

Question 34

What is the order of digit loss?

Answer 34

1, 5, 2, 4

Question 35

How do the final position of the limbs form?

Answer 35

Forelimb rotates dorsally Hindlimb rotates ventrally