Lecture: Butaric - Embryology Flashcards by Alek Ferbet

_______ are responsible for initial division of mesenchymal cells and segmentation (leg vs. thigh elements)

HOX genes

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all lower limb nerves are part of …

lumbosacral plexus

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describe a clubbed foot

congenital talipes equinovarus

foot is inverted, with sole pointed mediall

tendons (including Achilles’ tendon) are shortened

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summary of key points

end week 4 limb bud consists of mesenchyme surrounded by ectoderm
AER and HOX genes promote limb growth
week 7 digital rays appear, week 8 digits separate by apoptosis
bones: from mesenchymal precursors (week 5) follow endochondral ossification (beginning week 7)
muscles: from myogenic cells, organized as myotomes and by extensor/flexor compartments
dermatomes: follow myotome organization and relate to medial 90 rotation of lower limb

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failure of toes to separate

syndactyly

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At what level do the lower limb buds form?

L2-S2

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what are muscle, bone and connective tissue derived from?

mesoderm

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6-7 weeks you see …

digital rays and bone precursors

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8 weeks you see …

differentiated toes and bone

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when is the formation of trilaminar disc?

week 3

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the skin, CNS, and neural crest cells are derived from what?

ectoderm

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when do mesenchymal precursors to cartilage begin?

week 5

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great toe medial dermatome

L4-5 ***

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Describe the outer and inner layers of the lower limb bud?

Outer ectoderm layer w/ apical ectodermal ridge AER

Inne rcore of mesenchyme

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Adult dermatomes
inguinal ligament
anterior knee (patella)
medial side of foot, great toe
posterior thigh, leg
lateral side of foot

L1
L3,4
L4,5
S1,2
S1

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neurogenesis into lower limb begins around…

week 5

Describe toe formation in week 7 and 8.

Early week 7:

distal limb bud flattens into foot plate
5 digital rays appear

Week 8:
- apoptosis of tissue between rays creates separation of toes

development of extra digital ray

polydactyly

cartilage models of future toe bones

condensed mesenchyme

amelia

entire limb missing

cleft foot or hand, caused by failure of middle digital rays to develop

ectrodactyly

4 weeks you see …

lower limb bud

critical period for LL development

24-36 days post-fertilization

myogenic cells differentiate into …

myoblasts

myogenic cells from derm-myotomes migrate into lower limb bud and differentiate into myoblasts / aggregate around developing bones / separate into extensor (dorsal) and flexor (ventral) compartments

which limb rotates medially?

lower limb rotates 90 degrees MEDIALLY upper limb rotates 90 degress LATERALLy at week 8

Removal or non-maintenance of AER results why?

in arrested development of limb

cranial/caudal is the embryo is equivalent to _________ in the adult.

medial/lateral ex: in embryo, the big toe is cranial. In the adult, the big toe is medial.

little toe lateral dermatome

meromelia

part of limb missing

what is the role of AER apical ectodermal ridge?

initiates and controls limb growth with HOX genes

what is the early blood supply to the limb buds?

primary axial/axis artery will mainly disappear and be replaced by external iliac branches

when does ossification begin?

week 7-12 the lg. long bones ossify first, but some tarsal don't even start ossifying until post-natal period *** ossification and fusion of bones continues to adulthood

the inner lining of the gut tube is derived from what embryological structure?

endoderm

which enters the limb bud first motor and sensory axons?

motor axons FIRST arise from spinal cord and enter into developing muscle masses of limb buds sensory axons from neural crest cells follow AFTER motor axons

schwann cells are derived from...

neural crest cells supporting schwann cells myelinate axons

which parts of the precursor cartilages start to chondrify first?

more cranial elements start to chondrify first (distal phalanges last) week 6-8

describe the dermatomes in the embryo compared to an adult

in embryo: follow cranial to caudal pattern in adult: follow superior-ventral down and up to superior-posterior pattern