HAE Quiz 2: Embryology of Back and Limbs Flashcards
Primordium
organ or tissue in earliest recognizable stage of development
after gastrulation, before finial product
somites
Somitomeres
immature segments of paraxial mesoderm
develop further into somites
Somite
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
Somite Pairs
4 occipital 8 cervical 12 thoracic 5 sacral 3 coccygeal
Sclerotome
bone (vertebrae, ribs and cartilage)
comprised of loosely organized cells from core of somite- mesenchyme
migrate around neural tube and notochord
Dermatome
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
Myotome
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)
Vertebral Arch
formed by sclerotome cells around the neural tube
Vertebral Body
formed by sclerotome cells around the notochord
Ribs Development
from sclerotome by growing distally from the costal processes of the thoracic vertebrae
Sternum Development
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
Sclerotome Migration
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
Failures in Sclerotome Migration
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
Epaxial Myoblasts
give rise to intrinsic back muscles (ex erector spinae) and receive innervation from the dorsal primary rami of the spinal nerves
Hypaxial Myoblasts
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
Limb Buds
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
Apical Ectodermal Ridge (AER)
Proximal-Distal Patterning of Limb
at distal tip of limb, thickened surface ectoderm
induces proliferation of underlying mesenchym, causing the limb to grow in proximal-distal length
Progress Zone Model
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
Zone of Polarizing Activity (ZPA)
Preaxial-Postaxial Patterning of Limb
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)
Hand/Foot Plates
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
Digital Rays
condensed mesenchyme within hand and foot plates
programmed cell death occurs in the spaces between digital rays- forms toes and fingers
Dorsal Ventral Patterning of Limb
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
Innervation of Limbs
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
Dorsal and Ventral Muscle Mass
once in the limb, myoblasts organize into these sections on either side of developing bones
Posterior Division of VPR
VPR associated with myoblasts in limbs divide into dorsal branch to supply dorsal muscle mass
Anterior Division of VPR
VPR associated with myoblasts in limbs divide into ventral branch to supply ventral muscle mass
Nerves of Upper Limb
dorsal branches merge to form axillary and radial nerves
venral branches merge to form musculocutaneous, median and ulnar nerves
Limb Flexion and Rotation
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
Minor Defects of Limb Development
oligodactyly, polydactyly, syndactyly- common
Major Defects of Limb Development
amelia and meromelia- rare, lack treatment options
