HAE Quiz 2: Embryology of Back and Limbs

Question 1

Primordium

Answer 1

organ or tissue in earliest recognizable stage of development
after gastrulation, before finial product
somites

Question 2

Somitomeres

Answer 2

immature segments of paraxial mesoderm

develop further into somites

Question 3

Somite

Answer 3

give rise to skeletal muscle
each somite associated with spinal nerve from its segment of origin which grows out of neural tube
first appear day 20, 3-4 pairs added each day, process finished by day 30
differentiate to form sclerotome, dermatom andmyotome

Question 4

Somite Pairs

Answer 4

4 occipital
8 cervical
12 thoracic
5 sacral
3 coccygeal

Question 5

Sclerotome

Answer 5

bone (vertebrae, ribs and cartilage)
comprised of loosely organized cells from core of somite- mesenchyme
migrate around neural tube and notochord

Question 6

Dermatome

Answer 6

dorsal dermis
remain in dorsal region, form dermis of the back
cells of dermatome spread out deep to the surface ectoderm dorsally- give rise to dermis of the back

Question 7

Myotome

Answer 7

skeletal muscle of the neck, trunk and limbs
differentiates into myoblasts that migrate into connective tissue derived from parietal layer of lateral plate mesoderm
produce two regions- dorsal epaxial and ventral hypaxial domains
myotomes can fuse to form a single muscle (ex erector spinae)
myostomes can also fuse to form more than one muscle (ex deltoid and teres minor)
can migrate long distances (ex latissimus dorsi)

Question 8

Vertebral Arch

Answer 8

formed by sclerotome cells around the neural tube

Question 9

Vertebral Body

Answer 9

formed by sclerotome cells around the notochord

Question 10

Ribs Development

Answer 10

from sclerotome by growing distally from the costal processes of the thoracic vertebrae

Question 11

Sternum Development

Answer 11

develops from bilateral pair of cartilaginous bars formed in the parietal layer of lateral plate mesoderm that fuse in midline of the ventral body wall

Question 12

Sclerotome Migration

Answer 12

sclerotome cells from both sides migrate centrally around neural tube and notochord
each sclerotome splits into cranial and caudal half
then reform, connecting caudal half of one sclerotome with the cranial half of the adjacent sclerotome to form vertebra
cranial half of first cervical scerotome fuses with caudal half of fourth occipital sclerotome
7 cervical vertebrae develop from 8 cervical somites

Question 13

Failures in Sclerotome Migration

Answer 13

spina bifida occulta- left and right sclerotome faile to fuse- cause gap in vertebral canal
congenital scoliosis- sclerotome from one side fails to divide or form completely

Question 14

Epaxial Myoblasts

Answer 14

give rise to intrinsic back muscles (ex erector spinae) and receive innervation from the dorsal primary rami of the spinal nerves

Question 15

Hypaxial Myoblasts

Answer 15

give rise to muscles of the anterior and lateral trunk (intercostal, abdominal muscles) and limbs, receive innervation from the ventral primary rami of spinal nerves

Question 16

Limb Buds

Answer 16

visible at end of 4th week on ventrolateral body wall of embryo
upper limb appears before lower
consists of outer surface ectoderm and underlying core of mesenchym (derived from somatic layer of lateral plate mesoderm)
has 3 axes determined by relationship to axillary artery: proximal-distal, preaxial- posaxial and dorsal-venral

Question 17

Apical Ectodermal Ridge (AER)

Proximal-Distal Patterning of Limb

Answer 17

at distal tip of limb, thickened surface ectoderm

induces proliferation of underlying mesenchym, causing the limb to grow in proximal-distal length

Question 18

Progress Zone Model

Answer 18

cells closest to AER remain undifferentiated, those farthest differentiate into chondrocytes that form cartilage models of skeletal structures
cells under the influence of AER for shorter periods of time will become more proximal structures (ex scapula and humerus), those under the influence of AER for longer periods of time will become more distal

Question 19

Zone of Polarizing Activity (ZPA)

Preaxial-Postaxial Patterning of Limb

Answer 19

mesynchymal region on the postaxial margin of the limb bud
tissue near ZPA forms postaxial structures (ex ulna and 5th digit)
tissue more distant form preaxial structures (ex radius and pollex)

Question 20

Hand/Foot Plates

Answer 20

terminal portions of the limb bud flatten to form hand and foot plates
separated from the remainder of developing limb by wrist and ankle constrictions

Question 21

Digital Rays

Answer 21

condensed mesenchyme within hand and foot plates

programmed cell death occurs in the spaces between digital rays- forms toes and fingers

Question 22

Dorsal Ventral Patterning of Limb

Answer 22

surface ectoderm covering limb bud specifies the dorsal-ventral axis of limb, dividing limb into posterior and anterior compartments
muscles and nerves of limbs organized into these compartments

Question 23

Innervation of Limbs

Answer 23

Ventral Primary Ramus provides innervation to muscles of the limbs that originate in the hypaxial domain of the myotome
upper limb forms near C5-C7 somites- so VPR of C5-T1 spinal nerves innervate upper limb
lower limb forms near L2-S3 somites- so VPR of these segments innervate lower limb

Question 24

Dorsal and Ventral Muscle Mass

Answer 24

once in the limb, myoblasts organize into these sections on either side of developing bones

Question 25

Posterior Division of VPR

Answer 25

VPR associated with myoblasts in limbs divide into dorsal branch to supply dorsal muscle mass

Question 26

Anterior Division of VPR

Answer 26

VPR associated with myoblasts in limbs divide into ventral branch to supply ventral muscle mass

Question 27

Nerves of Upper Limb

Answer 27

dorsal branches merge to form axillary and radial nerves

venral branches merge to form musculocutaneous, median and ulnar nerves

Question 28

Limb Flexion and Rotation

Answer 28

upper limb rotates laterally- preaxial border (pollex) becomes oriented on lateral aspect of limb, dorsal muscle mass and developing elbow move to posterior location, ventral muscle mass moves to anterior location

lower limb rotates medially- preaxial border (hallux) becomes oriented on medial aspect of limb, dorsal muscle mass and knee joint on anterior aspect of lower limb, ventral muscle mass located posteriorly

Question 29

Minor Defects of Limb Development

Answer 29

oligodactyly, polydactyly, syndactyly- common

Question 30

Major Defects of Limb Development

Answer 30

amelia and meromelia- rare, lack treatment options