Week 3 - Limb Embryology

Question 0

What does the notochord do first (before the neural tube is there)?

Answer 0

It induces the overlying ectoderm to invaginate, forming the neural groove and tube.

Question 1

What does the neural tube induce?

Answer 1

The formation of vertebrae

Question 2

What is the significance of the notochord?

Answer 2

• Defines axes
• Mechanical role in the folding process
• Signals: formation of neural tube/groove, somites (indirectly), musculoskeletal system (via Shh)
• Contributes to: vertebral bodies & intervertebral discs

Question 3

What happens in the embryo from weeks 3-4?

Answer 3

The notochord induces the formation of the neural tube which then induces the formation of somites!!

Question 4

What is mesenchyme?

Answer 4

Embryonic connective tissue - derived from mesoderm and neural crest- multipotent - turns into myoblasts, chondroblasts, osteoblasts, fibroblasts, etc.

Question 5

What happens at week 4 of development?

Answer 5

1. Neural tube begins to close
2. Cephalo-caudal and lateral folding
3. Appearance of somites/somitomeres
4. Genesis of organ systems
5. Appearance of pharyngeal arches (face and neck)
6. Otic and optic development begins
7. Limb buds appear

Question 6

How many somites are there and what do they form?

Answer 6

There are about 42 pairs of somites with 7 in the head region that stay somitomeres. These 7 contribute mesoderm to the head and neck regions. The lower 35 somite pairs make up the vertebral column and limbs

Question 7

When do primary ossification centers appear?

Answer 7

Week 8 (diaphysis)

Question 8

What is marfan syndrome?

Answer 8

• Problems with collagen everywhere in the body, can cause lens dislocation and early aneurism
• Connective tissue disorder the causes excessive long bone growth
• Fibrillin-1 gene
• Long, thin limbs
• Also can affect heart, vessels, bones, eyes and lungs

Question 9

What is osteogenesis imperfecta?

Answer 9

• Defect in type I collagen gene (most cases)
• Autosomal dominant or recessive
• Extreme bone fragility, thin, curved bones
• Frequent fractures, decreased bone density
• Hypermobile joints

Question 10

What is acrondroplasia?

Answer 10

Most common form of dwarfism

• Affects long bones
• May be autosomal dominant inheritance or spont. mutation in FGFR3 gene
• Brain growth and intellect usually normal

Question 11

What is gigantism?

Answer 11

Excess growth hormone during childhood before growth plates close. Overall large size: height and organs; normal proportions

Question 12

What is acromegaly?

Answer 12

It is excess growth hormone after growth plates closed. Large, heavy bones especially face, hands and feet.

Question 13

What is congenital hip dysplasia/dislocation?

Answer 13

Born with incomplete formation of acetabulum. May self correct or child can also be placed in a harness to correct the syndrome. Symptoms include: leg positions that don’t match, less movement, uneven leg length

Question 14

What happens to sclerotomes during resegmentation of the vertebral column?

Answer 14

Sclerotomes get divided in two and then recombine around the intersegmental artery, leaving room inbetween the new groups for the spinal nerve.

Question 15

How many cervical nerves and vertebrae?

Answer 15

8 nerves, 7 vertebrae

Question 16

What does the notochord become?

Answer 16

The nucleus pulposis of the intervertebral disc

Question 17

What produces the vertebral foramen in vertebrae?

Answer 17

Sclerotome surrounds the notochord and neural tube. Then the sclerotome migrates and ossification centers form and the vertebral foramen, neural arch, etc are formed in the vertebrae

Question 18

What is spina bifida?

Answer 18

It is the mildest of defects in spine. It’s a type of occult spinal dysgraphia, a condition that results from failure of the neural tube to close. Vertebral arch fails to fuse, leaving the spinal cord exposed. Severity varies

Question 19

What is spina bifida occulta?

Answer 19

Least severe form. Usually in lumbrosacral region, often marked by a tuft of hair.

Question 20

What is meningocele?

Answer 20

Meningese protrude through defect in spinal cord or skull.

Question 21

What is myelomeningocele?

Answer 21

It’s where the meninges and spinal cord protrude through defect. Most severe

Question 22

What is congenital dermal sinus?

Answer 22

The neural ectoderm fails to completely separate from the surface ectoderm. The patient presents with a paramedian pit which may or may not communicate with the spinal cord. Most commonly found in lumbar region. The ‘pit’ is very susceptible to infection. Patient can have multiple rounds of sinusitis.

Question 23

What is Klipppel-feil syndrome?

Answer 23

congenital fusion of any cervical vertebrae, scoliosis and shortened neck is often common with this syndrome. Sometimes this condition also presents abnormalities in the heart, kidney, rib, palate abnormalities.

Question 24

What is congenital scoliosis?

Answer 24

It results from hemivertebra. There is only one chondrification center/ossification center that forms. In some cases, you only develop part of one vertebrae and this curves the whole spine.

Question 25

What is thoracic outlet syndrome?

Answer 25

Being borne with one extra rib in the upper cervical area. Neurovascular compression is caused and due to narrowing of passageway between neck and axilla. - Extra rib can crush cervical artery. When this happens in the lumbar region, there usually aren’t as many problems.

Question 26

What is pectus excavatum?

Answer 26

Excessive growth of connective tissue during sternum development. Can cause heart and/or lungs to be compressed. Symptoms include fatigue, rapid pulse, recurrent respiratory infections, heart murmur, chest pain.

Question 27

When is the critical period for limb development?

Answer 27

Weeks 4-8.

Question 28

What controls the location of the development of limb buds?

Answer 28

Hox - homeobox genes

Question 29

What does the zone of polarizing activity do?

Answer 29

It secretes Shh which determines which side of the limb bud will be anterior and which side will be posterior. Limb buds start developing on the front side of the body and grow straight out.

Question 30

When does bone development begin?

Answer 30

During late 3rd week.

Question 31

What are the two types of bone formation?

Answer 31

1. Endochondral

2. Intramembranous

Question 32

What does intramembranous bone formation involve?

Answer 32

Face and skull development

-Mesenchyme cells derived from neural crest cells & paraxial mesoderm of the head give rise directly to osteoblasts