LEC41: Early Limb Development and the Musculoskeletal System Flashcards
where does limb development begin?
upper extremity
when does limb development begin?
end of week 3 - when heart starting to beat
when do upper limb buds appear?
day 26-27
when do lower limb buds appear?
day 28-29
what do limb buds develop from?
proliferation of somatic layer of lateral mesoderm into surface ectoderm
where do upper extremities form?
cervical region of embryo
where do lower extremities form?
lumbar/sacral region of embryo
how does AER form
apical ectodermal ridge, formed by mesenchyme pushing into ectoderm, induces it to thicken at apex
what is AER
dense ridge of tissue that’ll form the limb
function of SHH
sonic hedgehog tissue factor - differentiates limb bud from symmetrical structure to distinct fingers
how does SHH work
gradient across mesenchymal growth area; most dense in zone of polarizing activity which gives hands diff poles
without SHH how would hand appear?
paddle, no fingers
what is zone behind AER called?
progress zone
function of progress zone
proliferates rapidly, causing mechanical growth of AER
what does AER induce
tissue, transcription factors to feed back on mesodermal core (mesenchyme) and become musculoskeletal (bone, muscle) components
when does upper limb > hand develop
finished by end of week 6
how do fingers form
apoptosis in region of upper limb paddle
digital rays
outgrowths from hand/foot paddles that become fingers, toes; each has AER (apical ecotodermal ridge, growth area) at its tip
when do foot plates finish forming
week 7
how do toes for m
apoptosis in region of lower limb paddle
what is endochondral ossification
mesenchyme remodels into cartilage remodels into bone
when does bone ossification occur
begins week 7, finishes after birth
end of week 6, what is make up of limb skeleton?
all cartilage
where is growth plate of long bone?
epiphyseal cartilage plate
where in bone does primary ossification occur?
diathesis
what is direction of bone ossification?
primary ossification in diathesis; bone grows toward secondary ossification centers at ends of bone, the growth plates
how can you visualize ossification centers
ultrasound
when and how does growth plate become obliterated?
at/after puberty; diaphysis-epiphysis fuse, long bone growth stops (or earlier if have something like radiation)
limb muscle muscle type
skeletal, for movement
what are somites derived from
paraxial mesoderm laid down during gastrulation, right next to notochord
what do somites differentiate into?
dermomyotome, sclerotome
what do dermomyotomes become
dermatomes (dermis), myotomes (muscle)
what do sclerotomes become?
vertebrae, ribs
which somite derivative is more dense?
dermomyotome (outer layer) is more dense than sclerotome (inner layer)
what are myoblasts, where are they derived from?
from myotome region of somites; give rise to muscle fibers
what do neural crest cells become?
pigment cells (melanocytes), nerve sheaths, insulators
what migrates to end of limb bud/hand?
myogenic precursors from myotomes & major blood vessels, nerves, muscle fibers (flexors, extensors)
what do myoblasts form from
myogenic precursors
which muscle compartment forms from aggregate of dorsal myloblasts
extensor
which muscle compartment forms from aggregate of ventral myloblasts
flexor
how do muscles elongate
as limb grows outward
what does limb bud mesenchyme give rise to?
loose CT > tendons, ligaments, blood vessels that attach to bone
what controls limb muscles
motor nerve axons, arise from spinal cord; grow into both dorsal and ventral muscle masses
when does motor innervation of limbs occur
week 5
what provides cutaneous innervation of the limbs?
neural crest cells
which nerves mostly supply cutaneous innervation of upper limb?
C6, 7, 8
spinal cord transsection where would allow upper limbs to still work?
C8-T1
what nerve root values mostly involved with lower extremity?
L3, 4, 5
what is root value for dermatomal innervation for anterior aspect for arm?
T1, T2
how do upper limbs rotate, what is the result?
laterally 90o; elbow becomes dorsal
how do lower limbs rotate, what is the result?
medially 90o, knee becomes ventral
when did thalidomide impact embryo
critical period of limb development, days 24-36
process by which vertebral column forms?
endochondral ossification: mesenchymal models of vertebrae made; transform into cartilage; cartilage, w/ osteoplasts & osteoblasts, become ossified, elongated bone
what makes bony encasement of spinal cord, and when?
sclerotomes from somites surround neural tube; embryonic week 4-5
what occurs during mesenchymal stage of vertebral development?
sclerotomal cells “migrate to”
1) notocord
2) neural tube
3) body wall
what is centrome, how does it form?
body of vertebra, neural arches on sides of vertebral bodies; forms when cranial portion of 1 sclerotome joins w/ caudal portion of adjacent sclerotome
action of intersegmental nerves?
grow between 2 portions of sclerotomes, toward myotomes
what does centrum formation create?
space for intervertebral disc
hypaxial dermomyotome becomes
flexor muscles
epaxial dermomyotome becomes
extensor muscles
what happens to notochord during vertebral development?
degenerates within vertebral bodies; forms nucleus pulposis of IV discs
when and where are chondrification centers re: vertebral development?
week 6
formation of cartilagenous vertebral column
when is cartilaginous stage of vertebral development?
week 6
when is bony stage of vertebral development?
week 7
where are ossification centers during bony stage of vertebral development?
3 primary ossification centers in vertebra:
1) body of vertebra
2) 1 in each neural arch, to 2 others
continue to ossify until age 25 - long bones to grow!
what is kyphoscoliosis
when sclerotomes split and nerves grow between, one of them doesn’t split correctly or disproportionate split, etc. - anything to make hemivertebrae not form - get klyphoscholiosis of the spine
what causes kyphoscoliosis
hemivertebra (1 or more)
