Embryology of Back and Limbs Flashcards
Primordium
organ or tissue in earliest recognizable state of development
what is the musculoskeletal system derived from?
mesoderm
somitomeres
immature segments of paraxial mesoderm
Somite development timeline
first pair appears at day 20, 3-4 pairs added each day, process finished by day 30
Final result of somite development
37 pairs: 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 3 coccygeal
sclerotome
derived from somite, forms vertebrae and ribs
dermatome
derived from somite, forms dermis of the back
each somite differentiates to form_
sclerotome, dermatome, myotome
myotome
forms skeletal muscle for the back, body wall, and limbs
medial migration of sclerotome
sclerotome cells around notochord form vertebral body; cells around neural tube form vertebral arch; resegmentation of sclerotome into cranial and caudal half
formation of intervertebral discs
sclerotome -> annulus fibrosus, notochord -> nucleus pulposus
sclerotome development days 23-25
cells of sclerotome migrate around neural tube (become vertebral arch), around notochord (become vertebral body); ribs develop distally from costal processes of thoracic vertebra
why are there seven cervial vertebrae but eight cervical spinal nerves?
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
myoblasts (derived from and differentiates into what?)
myotome differentiates into myoblasts, which forms skeletal muscles of neck, trunk, limbs
Epaxial
dorsal region in further development of myotome; gives rise to intrinsic back muscles
What innervates the epaxial myoblasts?
dorsal primary rami of spinal nerves
hypaxial
ventral region in further development of myotome; gives rise to muscles of anterior and lateral trunk and limbs
What innervates the hypaxial myoblasts?
ventral primary rami of the spinal nerves
What happenes to myotomes in later development?
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)
limb bud components
Outer layer of surface ectoderm, underlying core of mesenchyme derived from somatic layer of lateral plate mesoderm
when do limb buds become visible?
4th week
Axes of developing limb
proximal-distal, preaxial-postaxial, dorsal-ventral
apical ectodermal ridge (AER)
thickening of ectoderm at distal border of developing limb that regulates growth of the limb bud
AER function
induces proliferation of underlying mesenchyme, causing limb to grow proximal-distal length
progress zone model
for proximal-distal limb patterning; cells closest to AER remain undifferentiated, furtherst diff into cartilage models of skeletal structures
zone of polarizing activiity (ZPA)
mesenchymal region on postaxial margin of limb bud
Hand and foot plate development
terminal portions of limb buds become hand and foot plates. Within plates, mensenchyme condenses into digital rays
digital rays
mesenchyme condensation within hand and foot plates; programmed cell death occurs in spaces b/t digital rays -> fingers and toes
ventral primary ramus (VPR)
provides innervation for muscles of limbs
dorsal muscle mass
once in the limbs, myoblasts organize into dorsal and ventral muscle mass
ventral muscle mass
once in the limbs, myoblasts organize into dorsal and ventral muscle mass
posterior and anterior divisions
VPR assoc with myoblasts divide into dorsal and ventral branches to supply ventral and dorsal muscle mass respectively
Limb flexion and rotation in development
upper limb rotates laterally, lower limb rotates medially
