13_Devo Block 4.1 Limb Development

Front 1

What is the origin of the Limb Mesenchyme?

Back 1

Somatic Mesoderm (in other words, its just an extension of the body wall)

Front 2

What is the limb primordia covered with?

Back 2

Surface ectoderm

Front 3

Label A and B

Back 3

A = Somatic Mesoderm/Somitic Mesoderm
B = Surface Ectoderm

Front 4

How are the fully developed upper and lower limbs attached to the trunk?

Back 4

Via pectoral and pelvic girdles, respectively

Front 5

Establishment of the limb "field" occurs early in week:

Back 5

4

Front 6

Limb Fields arise from this tissue:

Back 6

Somatic Mesoderm

Front 7

Axial positioning is regulated by combinatorial expression of Hox genes. As a result, specific transcription factors are expressed in each LIMB FIELD. This TF if expressed in the upper limb and these TFs are expressed in the lower limb

Back 7

Upper: Tbx5
Lower: Tbx4 and Pitx-1

Front 8

When do LIMB BUDS begin to appear? (in days)

Back 8

Upper limb = 26-27
Lower limb = 28-29

Front 9

Limb buds seem to appear when somatic mesoderm cells begin to produce this grow factor:

Back 9

Fgf-10

Front 10

Budding is an inherent property of this tissue:
It is independent of this tissue:

Back 10

Mesoderm (independent of ectoderm)

Front 11

Other limb precursor cells include: (there are 2; what do these tissues develop into):

Back 11

1. Somitic Mesoderm (somite derived) forms Skeletal Muscles
2. Neural Crest forms Schwann cells and Spinal nn.

Front 12

What is AER and what induces it?

Back 12

Apical Ectoderm Ridge is a thickened ridge of ectoderm that forms along the interface bn the dorsal and ventral surfaces of the limb bud. It is induced by production of Fgf-10 by mesenchyme cells

Front 13

What type of tissue is A pointing to?
What type of tissue is B pointing to and what is it derived from? Note the dense region above the red arrow...what is that and what will it do?

Back 13

A = Surface Ectoderm
B = Mesenchyme (from somatic mesoderm)
It's a myotome of a somite that will migrate into the limb bud. This invasion of Somitic cells into the Mesenchyme will form Skeletal Muscle

Front 14

Will the limb buds form without any signal from the Surface Ectoderm?

Back 14

Yes - this is a unique feature of limb development

Front 15

What are the arrows pointing to?

Back 15

AER - Apical Ectodermal Ridge

Front 16

After establishment of the limb field (phase 1) and budding (phase 2), when does step 3 happen? What is the defining change here?

Back 16

Week 5-9 - elongation of the limb

Front 17

During week 5, this forms and is the earliest indication of a hand/foot

Back 17

A flat paddle-shaped plate develops at the distal end of the elongating limb.

Front 18

By week 6, what can you see in the UL and LL?

Back 18

Three segments and digital rays.

Front 19

By week 8 what's going on with the UL and LL digital rays?

Back 19

They are completely separate

Front 20

What step also begins at step 3?

Back 20

Step 4! Tissue formation and organization.

Front 21

What happens during step 4 - tissue formation and organization.

Back 21

The mesenchyme precursors begin to differentiate into tissues that you find in the limb.

Front 22

In the central part of the limb, what condenses to form what?

Back 22

Somatic MESOderm condenses to form cartilage and ultimately replaced by bone.

Front 23

How does the skeletal muscles form?

Back 23

Somite derived myoblasts migrate into the limb to form skeletal muscles.

Front 24

How do the nerve and vascular precursors develop in the limb?

Back 24

Motor axons and spinal nerves migrate into the limb following guidance cues from cells at the base of the limb bud.

Front 25

Guidance cues for muscle and nerve development are received by:

Back 25

Somatic mesoderm cells at the base of the limb bud.

Front 26

Limb development occurs along three axes - describe what forms in each axis:
1. Proximal-Distal:
2. Anterior-Posterior:
3. Dorsal-Ventral:

Back 26

1. Elongation, segment formation
2. Digit development; cranial-caudal/preaxial-postaxial borders
3. Muscles and neurovascular structure compartments

Front 27

What happens whenever you remove the AER during development?

Back 27

Elongation stops

Front 28

Development along the proximal-distal axis of limb depends on:

Back 28

Inductive interactions between the AER and underlying somatic mesoderm.

Front 29

What happens if an additional AER is grafted onto the limb?

Back 29

Duplication of limb structures occurs beginning from the time the graft is put in place.

Front 30

What happens if mesoderm other than limb bud mesoderm is placed adjacent to the AER? What does this suggest?

Back 30

Limb development is truncated...suggesting that only limb mesoderm is competent to respond to the signals from AER.

Front 31

What can cause this?

Back 31

Duplication of the AER

Front 32

What can cause this?

Back 32

Removal of the AER.

Front 33

If mesoderm from the lower limb is grafted adjacent to AER in the upper limb, what happens? What does this suggest? (see p5)

Back 33

The grafted tissue develops as though it were still in its original location. This suggests that the limb bud mesoderm is programmed as to the axial level at which it is located. For example, a transplanted wing AER will form a wing if placed near a leg.

Front 34

What interaction is responsible for limb elongation? What is the source of the signals inducing elongation?

Back 34

Interaction between Epithelial-Mesenchymal cells (AKA surface ectoderm and Somatic mesoderm, I think)
AER is the source of signals

Front 35

What four conclusions were made from the graft experiments?

Back 35

1. Epi-mesenchymal interactions are responsible for limb elongation
2. AER is source of signals for limb elongation
3. Only limb mesenchyme (from somatic mesoderm) is competent of responding to its inductive signals
4. Axial and segmental patterning is inherent to the limb mesoderm

Front 36

What did studies on embryos with a "limbless" mutation suggest?

Back 36

That somatic mesoderm produces limb bud independent of influence from ectoderm, but cannot sustain limb development further without AER-mesoderm interaction.

Front 37

What TF does somatic mesoderm produce to induce formation of the AER?
What TF does the AER produce to maintain Fgf-10 production in the somatic mesoderm? What does ZPA produce

Back 37

1. Somatic mesoderm makes: Fgf-10
2. AER makes Fgf-8, 4, & 2 to maintain Fgf-10 in SM
3. ZPA produces Shh to maintain Fgf-8 secretion by AER

Front 38

What is the fate of the mesenchyme adjacent to the AER? (2 things)

Back 38

-Forms segment specific skeletal elements
-Forms fibrous CT

Front 39

Summarize Model 1 vs Model 2 of limb development (p6)

Back 39

1: AER signals diffuse along limb as it elongates. Mesenchyme leaving influence first become proximal limb, those 2nd become intermediate segment, those closest to AER become distal segment.
2. Segments along proximal-distal axis is specified early in limb development and AER signals are required for segments to expand and develop.

Front 40

What part of the arm does the limb bud mesoderm form?
1. Bone-wise
2. Fibrous connective tissue-wise

Back 40

1. Limb bones, clavicle, scapula.
2. Fibrous connective tissue that surrounds muscles and subdivides limbs into compartments and becomes the superficial fascia and dermis of the skin.

Front 41

What veins run along the preaxial border in UP and LL?

Back 41

UP = cephalic vein
LL = great saphenous vein

Front 42

What tissue influences patterning along the anterior-posterior axis of the limb? What is this zone called?

Back 42

Somatic mesoderm-derived mesenchymal cells along the posterior border of the limb - known as the Zone of Polarizing Activity.

Front 43

What does the ZPA secrete?

Back 43

Shh and Retinoic Acid

Front 44

What does RA and Shh signaling induce?

Back 44

Induces asymmetric, nested expression of Hox genes that mediates pattering for digit location along the anterior (thumb/big toe)-posterior (pinky/little toe) axis of the limb bud

Front 45

The big digit forms on the ? border and the small digit forms on the ? border

Back 45

Big digit: pre-axial
Small digit: post-axial

Front 46

High concentrations of SHH and RA would result in formation of which digit?

Back 46

Small - in other words, this is closest to the ZPA

Front 47

Describe where the AER remains once apoptosis begins to break down the digital rays

Back 47

AER remains at the tips until the correct length and number of phalanges is achieved.

Front 48

What structures are influenced by patterning along the dorsal-ventral axis?

Back 48

1. Skeletal muscles
2. Neural and vascular structures
3. Limb compartmentalization

Front 49

What are the dorsal signals, where are they expressed, and what/where do they induce expression of? (p8)

Back 49

Wnts and radical fringe (rFng) are expressed in Dorsal Ectoderm. They induce expression of Lmx-1 in dorsal mesenchyme.

Front 50

What is the ventral signal, where is it expressed, and what does it inhibit? (p8)

Back 50

Engrailed-1 (En-1) is the ventral signal expressed in the ventral ectoderm and it inhibits Lmx-1 in the ventral mesenchyme.

Front 51

When do the limb buds begin to rotate?

Back 51

Week 7

Front 52

UL dermatome pattern:
LL dermatome pattern:

Back 52

UL: C3-T2
LL: L2-S3

Front 53

Myoblasts forming the skeletal muscles within the limb bud as well as associated with pectoral and pelvic girdles are derived from:

Back 53

Somitic (somite) mesoderm (ventral part of myotomes)

Front 54

Patterning of limb muscles occur as specific axial levels - what are they?

Back 54

C5-T1 for upper limb
L4-S3 for lower limb

Front 55

What stimulates myoblasts to migrate into the bud?

Back 55

Signals from mesenchyme at the base of the limb bud.

Front 56

Once myoblasts enter the limb bud, they organize into dorsal and ventral premuscle masses with respect to:

Back 56

forming skeletal elements

Front 57

Muscle patterning signals are believed to be derived from:
Splitting of muscles masses into specific muscles may be influenced by:

Back 57

Somatic mesoderm that forms the fibrous connective tissues that will be associated with the various muscle groups. Splitting of muscle masses may be influenced by signals directing blood vessel patterning.

Front 58

When do spinal nerves grow into limb buds?

Back 58

During week 5

Front 59

Where do the guidance cues that spinal nerves follow into the limb bunds originate? (p9)

Back 59

From mesenchyme cells at the base of the limb buds.

Front 60

In what order do sensory and motor fibers enter limb buds? (what about NCC and what do they form?)

Back 60

Motor fibers first then sensory fibers. Note: NCC also follow 2nd to form Schwann cells and melanoblasts.

Front 61

What forms the supporting tissue for neurons and the melatonin in limbs>?

Back 61

Schwann cells and melatonin are formed by Neural crest cells

Front 62

How are the limb buds vascularized? (p9) What about the distal limb segment?

Back 62

Sprouts from intersegmental arteries at specific axial levels enter the limb buds and form a vascular plexus of endothelial-lined tubes. Distally, an avascular zone forms under the ectoderm until all digits have formed.

Front 63

When does limb rotation occur and about which axis does this occur?

Back 63

Week 7 - around the proximal-distal axis

Front 64

What forms dermis vs epidermis of skin?

Back 64

1. Dermis = somatic mesoderm
2. Epidermis = Ectoderm

Front 65

Failure of Formation of Limb Parts is subdivided into two categories:

Back 65

1. Transverse Type (Amelia or Meromelia)
2. Longitudinal Type (Meromelia)

Front 66

Define transverse type malformation:

Back 66

Truncation/amputation anywhere along the proximal-distal axis. Eg missing hand, foot, etc.

Front 67

Define Longitudinal Type malformation:

Back 67

Absense of limb parts occuring along the anterior-posterior axis. Eg. missing radius = anterior absence
Missing ulna, pinky = posterior absence

Front 68

Deformity with too few digits in the middle is called (i.e. missing digits 2, 3, or 4):

Back 68

Oligodactyly (oligo = not enough). AKA "Lobster Claw"

Front 69

What is this called? What is it caused by? Is it longitudinal or transverse anomaly?

Back 69

Phocomelia. Possibly by thalidomide. This is a longitudinal anomaly

Front 70

Define Phocomelia

Back 70

Defect where distal segment of the limb is attached to a more proximal segment with the intervening segment missing or directly attached to the girdle.

Front 71

Anomaly on L? R? Caused by failure of: Specifically, the problem on the R might also be caused by:

Back 71

Syndactyly and Sirenomelia

Caused by interdigital apoptosis. The R might also be caused by abnormal tailbud development.

Front 72

How might this happen>

Back 72

Over activity of Zone of Polarizing Activity.

Front 73

What is the condition on the L and R called?

Back 73

L Achondroplasia
R Gigantism (Marfan Syndrome)

Front 74

What is this condition called? What is wrong?

Back 74

Talipes Equinovarus AKA Clubbed Foot. The talus bone is missing/abnormal (varus = turned in)

Front 75

Define Defect vs Distortion

Back 75

Using clubbed foot as an example: Defect would be if the Talas bone is missing. Distortion would be if the structure is all there but the foot/leg is misshappen bc of pressure throughout development in the womb.

Front 76

Term used to describe a shortened digit due to lack of certain joints?
Termed used to describe long thin digits.

Back 76

1. Brachydactyly
2. Arachnodactyly

Front 77

Developmental Dysplasia of the Hip (congenital dislocation of the hip) is caused by (3):

Back 77

1. Underdevelopment of the femoral head and/or hip socket
2. Maternal hormones that sofent connective tissue
3. Position of the uterus
4. Sometimes there is a hereditary component

Front 78

What is this?

Back 78

Sprengel Deformity - elevated scapula

Front 79

What is this? In mice, they can induce this by creating a heterozygote mouse defective in this gene:

Back 79

Cleidocranial Dysplasia. RunX2 (aka CBFA1)