Embryology

Question 1

When do the limbs start to develop?

Answer 1

4th week

Question 2

When and where does the upper limb bud appear?

Answer 2

Day 24

cervical region

Question 3

When and where does the lower limb bud appear?

Answer 3

Day 25-26

Lumbar and sacral segments

Question 4

Which genes regulate the patterning of the development of the limbs?

i.e. determine which skeletal elements are formed and making sure the upper/lower limbs are the same on each side

Answer 4

Homeobox (HOX) genes

Question 5

Where do upper limb buds develop?

Answer 5

Opposite the caudal cervical segments

Question 6

Where do lower limb buds develop?

Answer 6

Opposite the lumbar and sacral segments

Question 7

What forms at the apex of each limb bud?

Answer 7

The Apical Ectodermal Ridge (AER) forms from thickening of the ectoderm

Question 8

Apical Ectodermal Ridge (AER)

Answer 8

It’s a multi-layered epithelial structure that forms from thickening of the ectoderm at the distal end of each limb bud and acts as a major signaling center to ensure proper development of a limb.

After the limb bud induces AER formation, the AER and limb mesenchyme—including the zone of polarizing activity (ZPA)—continue to communicate with each other to direct further limb development.

The limb mesenchyme adjacent to the AER consits of undifferentiated, rapidly proliferating cells whereas mesenchymal cells proximal to it differentiate into blood vessels and cartilage bone models

Question 9

What do the distal ends of the limb buds flatten into?

Answer 9

Paddle-like hand plates and flipper like foot plates

Question 10

When do digital rays form in the hands and feet?

Answer 10

Hands - 6th week

Feet - 7th week

Question 11

What process is responsible for the tissue breakdown in the inter-digital regions?

Answer 11

Apoptosis mediated by bone morphogenetic proteins

Question 12

When do cartilaginous precursors/models of the limb bones develop within the mesoderm?

Answer 12

6th week

Question 13

During weeks 8-12 what forms in bones?

Answer 13

The primary centre of ossification

i.e The site where bone begins to form in the shaft of a long bone or the body of an irregular bone

Question 14

All limb bones grow by Endochondral ossification except one. What is it?

Answer 14

The clavicle                     
(intra-membranous ossification)

Question 15

In what direction do upper limbs rotate?

Answer 15

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

Question 16

In what direction do lower limbs rotate?

Answer 16

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

Question 17

During which week do peripheral nerves grow from the developing limb plexus into the mesenchyme of the limbs?

Answer 17

During the 5th week

Question 18

Birth defects of limbs descriptive terms

Answer 18

Amelia - total absence

Meromelia - Partial absence

Phocomelia - some long bone absence

Micromelia - All parts present but small

Question 19

What is the critical period of limb development?

Answer 19

24-36 days after fertilisation

Question 20

What is polydactyly?

Answer 20

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 induced

Question 21

What is syndactyly?

Answer 21

Common birth defect of the hand or foot

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

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

Question 22

How did Thalidomide affect limb development when given to pregnant women in the past?

Answer 22

Babies were born with absent or deformed long bones

Acts by disrupting cell adhesion and angiogenesis

Question 23

Club foot

Answer 23

Relatively common birth defect

Sole of the foot is turned medially and the foot is inverted

Causes:
Multi-factorial (genetic and environmental)
Abnormal positioning or restricted movement of the foetus in the utero