Limb Development

Question 1

what are the twelve steps of limb development?

Answer 1

1) choose building materials
2) grow
3) pattern the limb
4) establish the individual bones
5) separate the digits
6) form bones
7) form bony eminences
8) create joints
9) add muscle
10) movement and refinement
11) add nerves
12) limb rotation

Question 2

each limb bud has an external layer of ______ and a core of ______

Answer 2

ectoderm, somatic mesoderm

Question 3

muscle and tendon cells of the limbs originate in the:

Answer 3

somite and later migrate into the limb bud

Question 4

when do the limb buds first form?

Answer 4

upper limb buds form in the latter part of the fourth week (28-32 days) and the lower limb buds two days later

Question 5

where do the upper limb buds form?

Answer 5

at the level of the developing vertebrae C5-T1 (and receive innervation from accompanying spinal nerves)

Question 6

where do the lower limb buds form?

Answer 6

at the level of the developing vertebrae L2-S2 (and receive innervation from accompanying spinal nerves)

Question 7

stripes of tissue innervated by a single spinal nerve

Answer 7

dermatomes

Question 8

a distinct ridge of ectoderm at the distal end of the limb bud

Answer 8

the apical ectodermal ridge (AER)

Question 9

what induces formation of the apical ectodermal ridge (AER)?

Answer 9

signals in the underlying somatic mesoderm

Question 10

maintains the cells behind the apical ectodermal ridge (AER) in a proliferative, undifferentiated state

Answer 10

fibroblast growth factor 8 (FGF8)

Question 11

as the apical ectodermal ridge (AER) extends further distally, some cells escape the FGF8 signal and start to:

Answer 11

differentiate

Question 12

the apical ectodermal ridge (AER) drives growth bud along the ______ axis

Answer 12

proximal-distal

Question 13

as limbs grow, they are patterned along 3 axes:

Answer 13

• proximal-distal (ie: shoulder to fingertips)
• cranial-caudal (ie: thumb to pinky)
• dorsal-ventral (ie: palm vs. back of the hand)

Question 14

the upper and lower limbs can be divided into three homologous segments along the proximal-distal axis:

Answer 14

• stylopod (arm and thigh)
• zeugopod (forearm and leg)
• autopod (hand and foot)

Question 15

what evidence proves that the order of differentiation occurs in a proximal –> distal fashion?

Answer 15

• elimination of the AER in chickens early on leads to differentiation of stylopod only
• elimination of the AER in chickens later leads to differentiation of stylopod and zeugopod, but often not autopod

Question 16

the first group of cells to escape the undifferentiated zone becomes:

Answer 16

stylopod

Question 17

the second group of cells to escape the undifferentiated zone becomes:

Answer 17

zeugopod

Question 18

the last group to escape the undifferentiated zone becomes:

Answer 18

autopod

Question 19

each region of limb expresses a different:

Answer 19

Hox gene code, leading to formation of different structures along the proximal-distal axis

Question 20

Hox gene mutations lead to:

Answer 20

missing limb segments

Question 21

what sets up the Hox gene code for proximal-distal patterning?

Answer 21

two models:
- a timing model
- a morphogen model

Question 22

how does the timing model of proximal-distal axis patterning work?

Answer 22

the cells know the order in which they are differentiating (ie: cells that undergo 10 divisions before escaping the AER become stylopod, but cells that undergo 100 divisions become autopod)

Question 23

how does the morphogen model of proximal-distal axis patterning work?

Answer 23

spatial patterning controls cell fate; FGFs in the AER and retinoic acid in the flank form two opposing morphogen gradients along the proximal-distal axis

Question 24

the morphogen model is most involved in patterning ______ along the proximal-distal axis

Answer 24

stylopod/zeugopod

Question 25

the timing model is most involved in patterning ______ along the proximal-distal axis

Answer 25

autopod

Question 26

what mechanism patterns the dorsal-ventral axis of the limb?

Answer 26

morphogens in the overlying ectoderm (ectoderm knows what is dorsal and ventral and communicates that to the internal mesenchyme)

Question 27

what mechanism patterns the cranial-caudal axis of the limb?

Answer 27

establishment of a zone of polarizing activity (ZPA), which expresses the sonic hedgehog (SHH) morphogen and forms a gradient (high caudal - low cranial)

Question 28

what is the effect of transplanting the zone of polarizing activity (ZPA) from a donor onto the cranial margin of the wing bud of a chicken?

Answer 28

induced a mirror duplication of the cranial-caudal axis (essentially giving that chicken two ZPAs)

Question 29

what mechanism establishes individual bones in the limb (especially the autopod)?

Answer 29

a Turing reaction-diffusion mechanism

Question 30

a Turing reaction-diffusion mechanism uses 2 or more interacting molecules that:

Answer 30

sort themselves into a more complex pattern

Question 31

for the Turing reaction-diffusion mechanism to work, you need a(n):

Answer 31

activator (that activates both itself and the inhibitor)

Question 32

which molecules interact in a Turing reaction-diffusion mechanism to create individual digits in the hand/foot?

Answer 32

Wnt, BMP, and Sox9 (reaction creates repeating stripes of Sox9, which will start cartilage formation)

Question 33

how do the digits of the hand/foot become separated?

Answer 33

via apoptosis

Question 34

when forming the limb bones, _______ form from somatic lateral plate mesoderm in the 5th-6th weeks

Answer 34

mesenchymal condensations

Question 35

when forming the limb bones, chondrification (formation of cartilage models) occurs in the:

Answer 35

6th-8th weeks

Question 36

when do ossification centres appear in most of the limb bones?

Answer 36

weeks 7-12 (a few don't appear until after birth)

Question 37

for larger long bones of the limbs, the diaphysis is usually ossified at:

Answer 37

birth

Question 38

in the long bones of the limbs, secondary ossification centers form ______ in each epiphysis and expand ______

Answer 38

postnatally, radially

Question 39

where does most of the lengthening growth of the bone occur in the limbs postnatally?

Answer 39

the epiphyseal cartilage (growth plate) between the epiphysis and diaphysis; growth occurs through proliferation of chondrocytes and replacement by bone

Question 40

bumps for attachment of ligaments and tendons that are patterned during development, but form secondary to the main bone

Answer 40

bony eminences

Question 41

which transcription factors are essential for formation of the bony eminences?

Answer 41

Sox9 and scleraxis (typically expressed in tendons and ligaments)

Question 42

a BMP family member that is essential for joint formation

Answer 42

GDF5

Question 43

a ______ sets up joint spacing by creating repeated, spaced GDF5 expression

Answer 43

Turing reaction-diffusion

Question 44

true or false: to build a joint, you need to both promote and inhibit chondrogenesis

Answer 44

true

Question 45

what happens to the mesenchyme (somatic mesoderm) of the future joint if chondrogenesis is promoted?

Answer 45

becomes articular cartilage

Question 46

what happens to the mesenchyme (somatic mesoderm) of the future joint if chondrogenesis is suppressed?

Answer 46

becomes the synovial cavity, ligaments, and capsule

Question 47

cavitation to form the synovial cavity is achieved by:

Answer 47

secretion of hyaluronic acid, thereby creating tissue spaces

Question 48

in utero movements promote ______ and are required for cavitation

Answer 48

hyaluronic acid synthesis

Question 49

where does limb musculature originate from?

Answer 49

from muscle progenitor cells (MPCs) that delaminate from the ventrolateral edge of the dermamyotome and migrate into the limb bud

Question 50

muscle progenitor cells (MPCs) continue proliferating as the migrate into the limb bud and will differentiate into:

Answer 50

committed myoblasts (in response to cues in the limb bud)

Question 51

how many migratory streams do muscle progenitor cells (MPCs) follow into each limb?

Answer 51

2 (dorsal and ventral)

Question 52

myoblasts form dorsal and ventral masses, which will develop as the:

Answer 52

limb extensor and flexor muscles

Question 53

muscles develop in response to:

Answer 53

signals from the limb mesenchyme (derived from somatic mesoderm)

Question 54

myoblasts continue dividing and eventually differentiate as:

Answer 54

myocytes and start expressing cytoskeletal proteins

Question 55

myocytes fuse together to create:

Answer 55

multinucleated primary myofibres (though some are maintained as satellite stem cells that can repair/grow muscle)

Question 56

true or false: excess muscle fibers are made initially and then undergo apoptosis

Answer 56

true, we make too many and have to trim back

Question 57

how do muscles grow and repair themselves postnatally?

Answer 57

through fusion of progenitor cells with existing muscle fibers

Question 58

are embryonic movements important for development of the musculoskeletal system?

Answer 58

yes! ex: proper attachment of tendons, alignment of bones, growth of muscles, and development of joints

Question 59

spontaneous movements of the embryo/fetus start as early as:

Answer 59

seven weeks

Question 60

limbs are innervated by motor nerves from the ______ and sensory nerves from ______

Answer 60

ventral spinal cord, adjacent dorsal root ganglia

Question 61

at what level of the spine is the brachial plexus?

Answer 61

C5-T1

Question 62

at what level of the spine is the lumbosacral plexus?

Answer 62

L4-S3

Question 63

some motor neuron axons travel into the dorsal side of the limb and innervate the:

Answer 63

extensor muscles

Question 64

some motor neuron axons travel into the ventral part of the limb and innervate the:

Answer 64

flexor muscles

Question 65

motor neurons from the medial motor column (MMC) project to:

Answer 65

the axial and body wall muscles

Question 66

motor neurons of the lateral motor column (LMC) project to:

Answer 66

the limb

Question 67

the lateral pool of the lateral motor column (LMCl) innervates the:

Answer 67

dorsal limb

Question 68

the medial pool of the lateral motor column (LMCm) innervates the:

Answer 68

ventral limb

Question 69

which way do the limb buds point in the 6th week?

Answer 69

laterally and caudally

Question 70

which way do the limb buds point in the 7th week?

Answer 70

they flex ventrally, and the elbow/knee point laterally

Question 71

in the 7th week, the elbow points ______ when flexed

Answer 71

caudally

Question 72

in the 7th week, the knee points ______ when flexed

Answer 72

cranially

Question 73

in the 8th week, the arms rotate 90 degrees ______ so that the ventral/flexor surface faces ______

Answer 73

laterally, anteriorly

Question 74

in the 8th week, the legs rotate 90 degrees ______ so that the dorsal/ventral surface faces ______

Answer 74

medially, anteriorly

Question 75

congenital limb abnormality characterized by an abscence of one or more limbs

Answer 75

amelia

Question 76

congenital limb abnormality characterized by the absence of part of a limb

Answer 76

meromelia

Question 77

congenital limb abnormality characterized by short, underdeveloped limbs

Answer 77

phocomelia

Question 78

congenital limb abnormality characterized by a lack of phalanges

Answer 78

adactyly

Question 79

list four common defects of the limbs

Answer 79

- absence or partial absence of a limb - over or undergrowth of the limb - duplications - failed separation (ex: fused fingers)

Question 80

congenital limb abnormality caused by a Sox9 mutation which results in impaired chondrification

Answer 80

campomelic dysplasia

Question 81

congenital limb abnormality caused by an IHH mutation which results in premature chondrocyte hypertrophy, progenitor depletion, and short bones

Answer 81

brachydactyly type A1

Question 82

congenital limb abnormality caused by an FGFR3 mutation which results in impaired conversion of cartilage to bone and dwarfism

Answer 82

achondroplasia

Question 83

congenital limb abnormality caused by a mutation a collagen gene which results in defective connective tissue and brittle bones

Answer 83

osteogenesis imperfecta

Question 84

what are some environmental causes of limb abnormalities?

Answer 84

anything that compromises blood supply will put the limbs at risk - warfarin - thalidomide - phenytoin - valproic acid - misoprostol - cocaine - sugar (ie: maternal diabetes) - infections

Question 85

a condition where the limbs are caught in bands of tissue spanning the amniotic sac, causing a deformation

Answer 85

amniotic band of constriction

Question 86

a condition where there is low amniotic fluid, which can cause deformation of the limbs

Answer 86

oligohydramnios

Question 87

a condition where the foot is twisted out of shape and/or position due to bone and soft tissue abnormalities

Answer 87

club foot