Embryology of Back and Limbs

Question 1

Primordium

Answer 1

organ or tissue in earliest recognizable state of development

Question 2

what is the musculoskeletal system derived from?

Answer 2

mesoderm

Question 3

somitomeres

Answer 3

immature segments of paraxial mesoderm

Question 4

Somite development timeline

Answer 4

first pair appears at day 20, 3-4 pairs added each day, process finished by day 30

Question 5

Final result of somite development

Answer 5

37 pairs: 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 3 coccygeal

Question 6

sclerotome

Answer 6

derived from somite, forms vertebrae and ribs

Question 7

dermatome

Answer 7

derived from somite, forms dermis of the back

Question 8

each somite differentiates to form_

Answer 8

sclerotome, dermatome, myotome

Question 9

myotome

Answer 9

forms skeletal muscle for the back, body wall, and limbs

Question 10

medial migration of sclerotome

Answer 10

sclerotome cells around notochord form vertebral body; cells around neural tube form vertebral arch; resegmentation of sclerotome into cranial and caudal half

Question 11

formation of intervertebral discs

Answer 11

sclerotome -> annulus fibrosus, notochord -> nucleus pulposus

Question 12

sclerotome development days 23-25

Answer 12

cells of sclerotome migrate around neural tube (become vertebral arch), around notochord (become vertebral body); ribs develop distally from costal processes of thoracic vertebra

Question 13

why are there seven cervial vertebrae but eight cervical spinal nerves?

Answer 13

resegmentation of sclerotomes: numbers follow original pattern of somite formation; body of C7 vertebra composed of caudal 1/2 of C7 sclerotome and cranial 1/2 of C8 sclerotome, but C8 nerve emerges below

Question 14

myoblasts (derived from and differentiates into what?)

Answer 14

myotome differentiates into myoblasts, which forms skeletal muscles of neck, trunk, limbs

Question 15

Epaxial

Answer 15

dorsal region in further development of myotome; gives rise to intrinsic back muscles

Question 16

What innervates the epaxial myoblasts?

Answer 16

dorsal primary rami of spinal nerves

Question 17

hypaxial

Answer 17

ventral region in further development of myotome; gives rise to muscles of anterior and lateral trunk and limbs

Question 18

What innervates the hypaxial myoblasts?

Answer 18

ventral primary rami of the spinal nerves

Question 19

What happenes to myotomes in later development?

Answer 19

Some remain segmentally arranges (e.g. in the intercostal muscles), but most migrate and form non-segmented muscles; successive myotomes can fuse to form single muscle (erector spinae), or can split into more than one muscle (deltoid, teres minor); muscles can migrate long distance from point of origination (latissimus dorsi)

Question 20

limb bud components

Answer 20

Outer layer of surface ectoderm, underlying core of mesenchyme derived from somatic layer of lateral plate mesoderm

Question 21

when do limb buds become visible?

Answer 21

4th week

Question 22

Axes of developing limb

Answer 22

proximal-distal, preaxial-postaxial, dorsal-ventral

Question 23

apical ectodermal ridge (AER)

Answer 23

thickening of ectoderm at distal border of developing limb that regulates growth of the limb bud

Question 24

AER function

Answer 24

induces proliferation of underlying mesenchyme, causing limb to grow proximal-distal length

Question 25

progress zone model

Answer 25

for proximal-distal limb patterning; cells closest to AER remain undifferentiated, furtherst diff into cartilage models of skeletal structures

Question 26

zone of polarizing activiity (ZPA)

Answer 26

mesenchymal region on postaxial margin of limb bud

Question 27

Hand and foot plate development

Answer 27

terminal portions of limb buds become hand and foot plates. Within plates, mensenchyme condenses into digital rays

Question 28

digital rays

Answer 28

mesenchyme condensation within hand and foot plates; programmed cell death occurs in spaces b/t digital rays -> fingers and toes

Question 29

ventral primary ramus (VPR)

Answer 29

provides innervation for muscles of limbs

Question 30

dorsal muscle mass

Answer 30

once in the limbs, myoblasts organize into dorsal and ventral muscle mass

Question 31

ventral muscle mass

Answer 31

once in the limbs, myoblasts organize into dorsal and ventral muscle mass

Question 32

posterior and anterior divisions

Answer 32

VPR assoc with myoblasts divide into dorsal and ventral branches to supply ventral and dorsal muscle mass respectively

Question 33

Limb flexion and rotation in development

Answer 33

upper limb rotates laterally, lower limb rotates medially