Limb Embryology

Question 1

What genes regulate proximodistal patterning?

Answer 1

Homeobox genes

Question 2

When does the upper and lower limb bud appear?

Answer 2

Upper = day 24

Lower = 1-2 days later

Question 3

HOX gene expression results from a combination of the expression of what?

(3 things)

Answer 3

SHH
FGF’s
WNT-7a

Question 4

What HOX genes are expressed by upper limb?

Answer 4

HOXd 9-13

Question 5

Limb buds elongate by the proliferation of what?

Answer 5

Mesenchyme

Question 6

The upper limb buds develop opposite what?

The lower limb buds develop opposite what?

Answer 6

Caudal cervical segments

Lumbar and sacral segments

Question 7

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

Answer 7

An apical ectodermal ridge (AER)

Question 8

What is the apical ectodermal ridge (AER)?

Answer 8

Multilayered epithelial structure

Exerts an inductive influence on the limb mesenchyme

The mesenchyme adjacent to the AER consists of undifferentiated rapidly proliferating cells whereas mesenchymal cells proximal to it differentaite into blood vessels and cartilage bone models

Question 9

How do hand and foot plates develop?

Answer 9

Distal ends of the limb buds flatten into paddle-like hand plates and flipper like foot plates

By the end of the 6th week, mesenchymal tissue has condensed to form digital rays

During the 7th week digital rays are formed in the foot plates

Question 10

Apoptosis is responsible for the tissue breakdown in the interdigital regions.

What is this mediated by?

Answer 10

Bone morphogenetic proteins

Question 11

Cartilaginous precursors of the limb bones develop by chondrification within the mesoderm in what week?

Answer 11

6th week

Question 12

primary centres of ossification appear in which weeks in response to what?

Answer 12

8-12th week

Response to growth factors

Question 13

Endochondral ossification occurs for all bones of the limbs except what?

Answer 13

Clavicle

membranous ossification

Question 14

How do the upper and lower limbs rotate?

Answer 14

Upper limbs rotate 90 degrees laterally so that extensors are on the lateral and posterior surfaces (and the thumb us lateral)

Lower limbs rotate 90 degrees medially so that the extensors are on the anterior surface (and the big toe is medial)

Question 15

Peripheral nerves grow from the developing limb plexus into the mesenchyme of the limbs during which week?

Answer 15

5th

Question 16

What is a total absence of limbs called?

Answer 16

Amelia

Question 17

What is a partial absence of limbs called?

Answer 17

meromelia

Question 18

What is it called when some long bones are absent?

Answer 18

Phocomelia

Question 19

What is it called when all bones and parts are present but they are small?

Answer 19

Micromelia

Question 20

What is the genotype of lobster claw foot?

Answer 20

Autosomal dominant

Question 21

What is polydactyly?

What causes it?

Answer 21

Presence of more than 5 digits on the hands or feet

Extra digit is incompletely formed and lacks muscular development

May be inherited or teratogen indiced

Question 22

What is syndactyly?

What are its two types?

Answer 22

Common birth defect of the hand or foot

Cutaneous syndactyly (simple webbing between the digits) is a common limb defect

More frequent in the foot than in the hand.

Osseous syndactyly (fusion of bones) occurs when notches between the digital rays fail to develop

Question 23

What is club foot also known by?

Answer 23

Talipes Equinovarus

Question 24

What does club foot look like?

Answer 24

Sole of foot is turned medially and the foot is inverted

Question 25

What are the causes of club foot?

Answer 25

Multifactorial (genetic and environmental factors)

?Abnormal positioning or resticted movement of the fetus’s lower limb in the utero

Question 26

What causes congenital dislocation od the hip in 15% of infants?

Answer 26

Abnormal development of acetabulum