limb embryology

Question 1

which spinal segments contribute to the brachial plexus?

Answer 1

C5-T1

Question 2

what is the critical period in embryonic development?

Answer 2

day 24-36

Question 3

what germ layer are the limbs derived from?

Answer 3

lateral mesoderm —> somatic layer –> lines body wall

limbs arise from body wall
lateral plate mesoderm forms the connective tissue of limbs

Question 4

what spinal segments contribute to the sacral plexus?

Answer 4

L4-S3

Question 5

lower limb buds form ____ days later than upper limb buds.

Answer 5

2

Question 6

limb bud structure:

Answer 6

-mesenchymal core (loose, undifferentiated mesoderm)
-covered in ectoderm

somatopleure layer

Question 7

what starts the process of limb development?

Answer 7

-activation of mesenchymal cells in the lateral mesoderm

-activation induces the formation of an apical ectodermal ridge (AER)

Question 8

limb development steps:

Answer 8

-activation of mesenchymal cells in lateral mesoderm

-induces apical ectodermal ridge to form (AER). AKA an ectodermal thickening at apex of limb bud.

-mesenchyme condenses and gives rise to skeleton and connective tissue

Question 9

role of AER?

Answer 9

has an inductive influence on the mesenchyme

stimulates mesenchyme proliferation

growth and creation of a proximal-distal axis of the limb

Question 10

when do upper limb buds appear?

Answer 10

day 26-27

Question 11

when does hand/foot plate formation take place?

Answer 11

week 5

Question 12

what is a hand and foot plate?

Answer 12

formed when the limb buds enlarge and flatten at the ends

Question 13

when does digital ray formation take place?

Answer 13

week 6

Question 14

describe digital ray formation process?

Answer 14

mesenchyme/tissue between digital rays condense and undergoes apoptosis

forms notches which are the primordial digits

Question 15

what abnormality results from too many digital rays?

Answer 15

polydactyly

Question 16

during which week do fingers and toes become separated?

Answer 16

week 8

Question 17

what happens if theres an error during week 8?

Answer 17

syndactyly
mesenchyme fails to undergo apoptosis

Question 18

describe limb rotation:

Answer 18

initially, extensors are lateral and flexors are medial

upper limbs 90 deg laterally:
-anterior flexors
-posterior extensors

lower limbs 90 degrees medially:
-anterior extensors
-posterior flexors

Question 19

primary vs secondary ossification centers

Answer 19

primary - shaft, formed during development, allows for osteogenesis (cartilage –> bone) of the long bones during fetal development

secondary - at the epiphyses, formed after birth, at growth plate, ossification of the carpal and tarsal bones (1 year after birth)

Question 20

what germ layer is the limb musculature derived from?

Answer 20

hypaxial division of myotome (paraxial mesoderm)

Question 21

describe the timeline for motor and sensory nerve limb bud innevation?

Answer 21

motor neurons arise and enter limb bud around week 5

sensory neurons enter AFTER because they use the motor neurons as a guide

Question 22

what are the dermatomes created from?

Answer 22

neural crest cells

Question 23

limb bud arterial supply:

Answer 23

intersegmental arteries that arise from the dorsal aorta

the primordial vascular pattern - primary axial artery and its branches

Question 24

what is embryonic band syndrome?

Answer 24

tears in the amnion create “amnionic bands” that can wrap around limbs and digits, causing amputations