6.5 Embryology of Limb Development

Question 1

When and what is Gastrulation?

Answer 1

• Week 3 of development

- Converts the bilaminar embryo into the trilaminar embryo.

Question 2

What embryonic tissues give rise to the limbs?

Answer 2

The limbs are initially undifferentiated mesenchyme (mesoderm) with an epithelial (ectoderm) covering

Question 3

When do the limb buds appear?

Answer 3

• End of week 4

- Forelimbs appear first followed bt hind limbs 1 to 2 days later.

Question 4

When do the limbs rotate and what direction?

Answer 4

• During the 7th week

- Opposite directions

Question 5

What is the critical period for the limbs?

Answer 5

-Weeks 4 to 8 (4-5 most critical-major anomalies)

Question 6

Define Critical Period

Answer 6

During the critical period (most of the first trimester), the developing embryo is most susceptible to environmental insults (alcohol, infection, radiation, nutritional deficiencies)

Question 7

When does body folding take place?

Answer 7

During the 4th week

Question 8

What two types of folding take place?

Answer 8

1) lateral folding

2) Caniocaudal folding

Question 9

Define lateral folding

Answer 9

Mesoderm splits. As lateral aspects of embryo fold ventrally, the space between the layers of mesoderm forms future body cavities.

Question 10

Define Craniocaudal folding

Answer 10

Head and tail regions bend towards each other and embryo becomes “C” shaped.

Question 11

What forms the gut tube?

Answer 11

Endoderm

Question 12

What becomes the Somite formations?

Answer 12

Paraxial mesoderm organized into segments known as somitomeres

Question 13

What are three types of somites?

Answer 13

1) Sclerotome
2) Dermatome
3) Myotome

Question 14

What does sclerotome become?

Answer 14

Future vertebral segments

Question 15

What does the dermatome become?

Answer 15

Dorsal portion of somite that forms dermis of skin, segmentally supplied by a spinal nerve

Question 16

What does the myotome become?

Answer 16

Mesoderm from somite that will differentiate into myoblasts.

Question 17

What forms the distal border during limb growth?

Answer 17

The AER- Apical ectodermal ridge

Question 18

What happens that causes finger and toe formation?

Answer 18

Cell death of AER gives way to formation

Question 19

When does the limb bud flatten to form hand and footplates?

Answer 19

During the 6th week

Question 20

What direction does each limb rotate during the 7th week?

Answer 20

Upper limbs rotate laterally 90 degrees

Lower limbs rotate medially 90 degrees

Question 21

What is Achondroplasia?

Answer 21

One form of congenital dwarfism resulting from improper develop,met of cartilage at the end of long bones

Question 22

What is Amelia?

Answer 22

Complete absence of one or more limbs

Question 23

What is Phocomelia?

Answer 23

Upper portion of limb is absent or poorly develop

Question 24

What is Syndactyly?

Answer 24

two or more fused fingers or toes

Question 25

What is Macrodactyly?

Answer 25

enlargement of one or more digits

Question 26

What is Polydactyly?

Answer 26

extra digits

Question 27

What is Ectrodactyly?

Answer 27

fewer than normal digits

Question 28

What is Clubfoot?

Answer 28

• common: 1 in 5000 live births
• abnormal position of the foot
• can result from compression of the infant in the uterus

Question 29

What gives rise to the bones of the limbs?

Answer 29

The mesenchyme condenses and cells become chondrocytes

Question 30

During week 6 what substance foreshadows the bones of the extremities?

Answer 30

Hyaline cartilage

Question 31

When does ossification begin in the limbs?

Answer 31

The end of the embryonic period.

• Primary ossification: At birth, the diaphysis of the bone is completely ossified.
• Secondary ossification of the cartilaginous epiphyses begins around the time of birth.

Question 32

What is the Epiphyseal plate?

Answer 32

• A cartilage plate remains between the diaphyseal and the epiphyseal ossification centers.
• Important for growth of long bones
• Once a child has reached the end stages of puberty, the growth plates are essentially closed

Question 33

What nerve supplies the extensor muscles?

Answer 33

• Radial nerve (C5,C6, C7, C8, and T1)

- Formed from dorsal segmental branches

Question 34

What nerve supplies the flexor muscles?

Answer 34

• Ulnar (C8, T1) and median nerves (C6, C7, C8, T1)

- Formed from ventral segmental branches.

Question 35

What happens to muscle formation in limbs during week 7?

Answer 35

Mesenchyme is penetrated by spinal nerves from the same segment derived from.

Question 36

What does somites made from?

Answer 36

paraxial mesoderm