Limb/Embryonic Development Terminology

Question 1

What is a limb?

Answer 1

ventral somatic outgrowth of outer tube. 
Contains bones (made of boney tissue, cartilage, and other tissues), joints, muscles and nerves (w/motor neurons to muscles, sensory neurons to skin proprioreceptors.

Question 2

4th week embryonic segments

Answer 2

segment of neural tube
segmantal nerve
a somite (mesoderm)
overlying ectoderm

Question 3

two divisions of a somite

Answer 3

1. dermomyotome

2. sclerotome

Question 4

dermomyotome

Answer 4

one division of the somite in development

• migrates laterally to form:
• dermatome (dermis of the skin)
• myotome (skeletal muscle)

Question 5

sclerotome

Answer 5

division of the somite in development:

• contains cavity of loose cells;
• migrates medially to surround the notochord & neural tube
• forms axial skeleton

Question 6

4 Main signals and genes in development

Answer 6

Pitx2
FGF-2,4,8
Wnt family
Homeoboc (hox)

Question 7

Pitx2 gene

Answer 7

-left side
protein made from this gene acts as a transcription factor and involved in the development of the eye, tooth, and abdominal organs.

Question 8

FGF-2,4,8

Answer 8

mesoderm

fibroblast growth factors involved in the proliferation and differentiation of cells

Question 9

Wnt family

Answer 9

use cell surface receptors cells to determine cell fate or other differentiation parameters

Question 10

Homeobox (hox) genes

Answer 10

proper placement of embryonic segment structures.

Question 11

Limb Bud development

Answer 11

Limb bud ectoderm to apical ectodermal ridge
FGF8 (mesoderm outgrowth before ridge appears)
FGF2 (cranial; pre-axial)
FGF4 (caudal; post axial)

Question 12

Wnt Signaling

Answer 12

Wnt family is associated with cell-surface and extracellular matrix.
-dorsal-ventral patterning
-wnt-7a signal comes from ectoderm to dorsal mesoderm
En-1 comes from endoderm to ventral mesoderm

Question 13

Hox genes expressed in limb buds

Answer 13

• regionally expressed
• clusters hoxd and hoxa-genes 9-13 in each cluster
• expressed in distal to proximal (13-9) sequence hoxd.

Question 14

how are hoxd genes expressed in definitive limb segments?

Answer 14

they are overlapping expressions in distal to proximal segments hoxd.

Question 15

stages in weeks of limb development

Answer 15

29 weeks

Question 16

week # flexion & rotation of limbs occur between

Answer 16

• 6th and 8th weeks
• pre-axial borders marked by thumb/big toe
• flexion occurs at elbow and knee

Question 17

Rotation of limbs: Adduction of arm

Answer 17

• elbows point caudally
• thumb cranial
• flexors anterior (ventral)

Question 18

Rotation of limbs: medial rotation of hip

Answer 18

• knees point cranially
• big toe medial
• flexors posterior

Question 19

Does the upper limb undergo adduction of lateral rotation?

Answer 19

• simple adduction at the shoulder & flexion at elbow brings upper limb to natural fetal position (thumb cranial; palm facing medially)
• lateral rotation brings upper limb to anatomical position (palm facing anteriorly, thumb lateral)

Question 20

Amelia

Answer 20

absent of whole limb

Question 21

meromelia

Answer 21

absent or rudimentary part of a limb

Question 22

phocomelia

Answer 22

rudimentary, poorly differentiated limb

Question 23

hemimelia

Answer 23

rudimentary distal limb segment

Question 24

adactyly

Answer 24

absence of all fingers

Question 25

ectrodactyly

Answer 25

missing fingers | -many are defects of AER determining proximal-distal patterning

Question 26

radial/tibial reduction defect

Answer 26

absence of radius or tibia +/- absent or abnormal thumb/big toe

Question 27

polydactyly

Answer 27

pre-axial or post-axial - too many fingers (duplication defect) - defect in crania-caudal patterning

Question 28

syndactyly

Answer 28

differentiation defect | -some defect of hoxd-13

Question 29

sirenomelia

Answer 29

"fused lower limb" | defect of caudal mesoderm

Question 30

talipes

Answer 30

- club food | - abnormal structure/position of foot

Question 31

causes of limb defects may be...

Answer 31

1. genetic 2. teratogenic (drug & chemical) 3. mechanical - some cases of polydactyly, syndactyly, brachydactyly, lobster claw hand are autosomal dominant disorders - many limb defects are components of genetic syndromes

Question 32

Thalidomide

Answer 32

used to treat certain skin conditions | teratogen of limb development

Question 33

Dimethadione

Answer 33

an anti-epileptic drug | teratogen of limb development

Question 34

Retinoic acid

Answer 34

used for treatment of acne | teratogen of limb development

Question 35

Cadmium

Answer 35

environmental teratogen

Question 36

Additional teratogen's that can cause limb defects

Answer 36

viruses, radiation, hypothermia and hyperthermia

Question 37

Mechanical causes of limb defects

Answer 37

- Amniotic bands may cause disruption or amputation of part of limb - uterine issues

Question 38

How are limb bones formed?

Answer 38

by endochondral ossification

Question 39

steps of limb bone formation

Answer 39

1. mesenchymal condensation 2. chondrification (bone primordium) 3. bone collar ossification