Development of the Limbs

Question 1

when does development of the limbs begin?

Answer 1

end of 4th week

Question 2

how does development of the limb buds begin?

Answer 2

activation of the mesenchymal cells in the somatic lateral mesoderm

Question 3

what regulates the proximo-distal patterning of the limb buds?

Answer 3

homeobox genes

Question 4

when does the upper limb bud appear?

Answer 4

day 24

Question 5

when does the lower limb bud appear?

Answer 5

day 26

Question 6

what does HOX gene expression result from?

Answer 6

a combination of the expression of SHH, FGF’s and WNT-7a

Question 7

what HOXgenes are expressed in the upper limb?

Answer 7

9-13

Question 8

how do limb buds elongate?

Answer 8

proliferation of mesenchye

Question 9

where do the upper and lower limb buds develop from?

Answer 9

upper - caudal cervical segments

lower - lumbar and sacral segments

Question 10

the apex of each bud the ectoderm thickens to form what?

Answer 10

apical ectodermal ridge

Question 11

what is the AER?

Answer 11

multilayered epithelial structure restricted to the end of a limb bud

Question 12

what does the AER do?

Answer 12

induces proliferation of the limb mesenchyme

Question 13

the mesenchyme adjacent to the AER consists of what?

Answer 13

undifferentiated, rapidly proliferating cells

Question 14

the mesenchyme proximal to the AER differentiated into what?

Answer 14

blod vessels and cartilage bone models

Question 15

where there a progress zone in the limb buds?

Answer 15

behind the AER and in front of the cell differentiation area

Question 16

describe the development of the hand and foot plates

Answer 16

The distal ends of the limb buds flatten into paddle-like hand and foot plates. By the end of the 6th week, mesenchymal tissue in hand plates has condensed to form digital rays. During the 7th week digital rays are formed in the foot plates. Apoptosis (programmed cell death) is responsible for the tissue breakdown in the interdigital regions.

Question 17

what mediates apoptosis in the interdigital regions?

Answer 17

bone morphogenetic proteins

Question 18

describe the final stages of limb development

Answer 18

Cartilaginous precursors of the limb bones develop by chondrification within the mesoderm in the 6th week. Primary centres of ossification appear in weeks 8-12 in response to growth factors. Endochondral ossification for all limb bones

Question 19

what kind of ossification in the clavicle?

Answer 19

membranous

Question 20

describe limb rotation

Answer 20

The upper limb rotates 90° laterally so that the extensors are on the lateral and posterior surfaces. The thumb is now lateral. On the other hand, the lower limb rotates 90° medially so that the extensors are on the anterior surface and the hallux is medial.

Question 21

what are birth defects of the limbs often associated with?

Answer 21

other defects
hereditary
teratogen

Question 22

total absence of the limbs

Answer 22

amelia

Question 23

partial absence of the limbs

Answer 23

meromelia

Question 24

absence of some of the long bones

Answer 24

phocomelia

Question 25

all parts are present but as small (limbs)

Answer 25

micromelia

Question 26

what is the critical period of limb development?

Answer 26

24-36 days

Question 27

describe polydactyly

Answer 27

This is when there is more than 5 digits on either the hands or feet. The extra digit is incompletely formed and lacks muscular development. This can be inherited, or teratogen induced.

Question 28

describe syndactyly

Answer 28

These are common defects of the hand or foot. Cutaneous syndactyly is simple webbing between the digits and is a common limb defect, more common in the foot. Osseous syndactyly, fusion of the bones, occurs when notches between the digital rays fail to develop.

Question 29

describe congenital clubfoot

Answer 29

Clubfoot is also known as talipes equinovarus and is a relatively common birth defect. The sole of the foot is turned medially, and the foot is inverted. The causes can be multifactorial (genetic and environmental factors. It is hypothesised that abnormal positioning or restricted movement of the foetus’s lower limb in utero could cause clubfoot.

Question 30

describe developmental dysplasia of the hip

Answer 30

Abnormal development of the acetabulum causes 15% of infants to have congenital dislocation of the hip. It is common after breech deliveries. Possible breech posture during late part of pregnancy resulting in abnormal development of acetabulum and head of femur. Also associated with congenital dislocation of the hip is generalised joint laxity. Causes of this are multifactorial.

Question 31

describe the effects of thalidomide

Answer 31

Thalidomide was used from 1957-1962 as a sleeping tablet and anti-sickness medication. It caused the absence or deformity of long bones, intestinal atresia and cardiac anomalies. It acts by disrupting cell adhesion and angiogenesis. It is still used today to help cancer, leprosy and AIDS patients.