Embyrology

Question 1

When does limb bud development begin and with what?

Answer 1

4th week with the activation of mesenchymal cells in the somatic lateral mesoderm

Question 2

What is embyrological patterning?

Answer 2

Grouping of cells together in the proper relationship to each other and the tissue surrounding them

Question 3

What genes control embryological patterning of the limbs?

Answer 3

Homeobox genes

Question 4

At what days do the limb buds appear?

Answer 4

Day 24 - upper limb bud appears

Day 25-26 - lower limb bud appears

Question 5

What do HOX genes do?

Answer 5

Control the body plan of an embryo from head to toe

Question 6

Where are HOX genes expressed?

Answer 6

Regionally in limbs

They determine which skeletal element is formed

Question 7

HOX expression results from combination of expression of what 3 other things?

Answer 7

SHH, FGFs, WNT-7a

Question 8

What type of HOX is expressed in the upper limb?

Answer 8

HOX 9-13

Question 9

Removal of what two HOX genes will lead to the radius and ulna not forming?

Answer 9

HOXa11, HOXd11

Question 10

How do limb buds elongate?

Answer 10

By proliferation of mesenchyme (embryonic connective tissue)

Question 11

Where do the upper limb and lower limb buds develop?

Answer 11

Upper limb - opposite caudal cervical segments

Lower limb - opposite lumbar and sacral segments

Question 12

What forms at the apex of each limb bud?

Answer 12

Apical ectodermal ridge (AER)

Question 13

What are the AER?

Answer 13

Multi-layered epithelial structure that is restricted to the end of limb bud

Question 14

What do the AERs do?

Answer 14

Exert an inductive influence on limb mesenchyme

Question 15

What lies adjacent to the AER?

Answer 15

Undifferentiated, rapidly proliferating cells

Question 16

What lies proximal to the AER?

Answer 16

Mesenchymal cells which differentiate into Bas and cartilage bone models

Question 17

What develops at the end of the limb buds?

Answer 17

Flattened hand plates/foot plates

Question 18

By the end of the 6th week, what happens in the hand plates?

Answer 18

Mesenchymal tissue condenses to form digital rays

Question 19

By the end of the 7th week, what happens in the foot plates?

Answer 19

Digital rays form

Question 20

What prevents webbed fingers/toes occurring?

Answer 20

Programmed cell death responsible for tissue breakdown in interdigital regions

Question 21

What mediates the apoptosis in the interdigital regions?

Answer 21

Bone morphogenic proteins

Question 22

Describe the process of bone ossification in the limbs

Answer 22

Cartilaginous precursors develop by chondrofication within mesoderm in the 6th week
Primary centres of ossification appear in wks 8-12 in response to growth factors

(SO all endochondral ossification apart from clavicle)

Question 23

Distinguish between endochondral and intramembranous ossification

Answer 23

Endochondral - cartilage precursor model laid down first (happens in long bones)
Intramembraneous - bone laid down directly into mesenchyme, occurs in flat bones

Question 24

Define ossification

Answer 24

The process of laying new bone down by osteoblasts

Question 25

During limb rotation, in which direction do the limb buds rotate?

Answer 25

Upper limb - 90 degrees laterally | Lower limb - 90 degrees medially

Question 26

What happens, in relation to nerve development in the limb in the 5th week?

Answer 26

In the 5th week, peripheral nerves grow developing limb plexuses (brachial/lumbosacral) into the mesenchyme of the limbs

Question 27

Define amelia

Answer 27

Total absence of limbs

Question 28

Define meromelia

Answer 28

Partial absence of limbs

Question 29

Define phocomelia

Answer 29

Some long bone absence, e.g. radius/fibula

Question 30

Define micromelia

Answer 30

All parts present ut small

Question 31

What is lobster claw foot?

Answer 31

Splitting of the footplate | Can be inherited as an autosomal dominant trait

Question 32

What is the critical period for limb development?

Answer 32

24-36 days after fertilisation

Question 33

Define polydactyly

Answer 33

Presence of more than 5 digits on the hands/feet Extra digit incompletely formed and lacks muscular development

Question 34

What may cause polydactyly?

Answer 34

Alcohol, thalidomide, inherited traits

Question 35

Define syndactyly

Answer 35

Webbing of hands/feet Common birth detect More common in feet as opposed to hands

Question 36

Define cutaneous syndactyly

Answer 36

Simple webbing between digits

Question 37

Define osseous syndactyly

Answer 37

Fusion of bones, notches between digital rays fail to develop

Question 38

What can thalidomide use in early pregnancy lead to?

Answer 38

Amelia/meromelia, intestinal atresia, cardiac abnormalities

Question 39

What is the other name for congenital clubfoot?

Answer 39

Tapiles equinovarus

Question 40

What is club foot?

Answer 40

Sole of foot turned medially and foot inverted

Question 41

What causes club foot?

Answer 41

Multifactorial - genetic and environmental factors | ?Abnormal positioning/restricted movement of foetus's lower limb in utero

Question 42

What may cause abnormal development of the hip?

Answer 42

Common after breech deliveries - ?breech posture during late pregnancy (abnormal development of acetabulum and head of femur) General joint laxity may be attributed to many factors and is associated with congenital dislocation of the hip

Question 43

When is the sensitive period of thalidomide use?

Answer 43

4-8 weeks

Question 44

Why does thalidomide lead to the problems previously mentioned?

Answer 44

It disrupts cell adhesion and angiogenesis in the embryo