Lecture: Butaric - Embryology Flashcards
4 weeks you see …
lower limb bud
6-7 weeks you see …
digital rays and bone precursors
8 weeks you see …
differentiated toes and bone
when is the formation of trilaminar disc?
week 3
the skin, CNS, and neural crest cells are derived from what?
ectoderm
what are muscle, bone and connective tissue derived from?
mesoderm
the inner lining of the gut tube is derived from what embryological structure?
endoderm
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.
At what level do the lower limb buds form?
L2-S2
Describe the outer and inner layers of the lower limb bud?
Outer ectoderm layer w/ apical ectodermal ridge AER
Inne rcore of mesenchyme
what is the role of AER apical ectodermal ridge?
initiates and controls limb growth with HOX genes
_______ are responsible for initial division of mesenchymal cells and segmentation (leg vs. thigh elements)
HOX genes
Removal or non-maintenance of AER results why?
in arrested development of limb
amelia
entire limb missing
meromelia
part of limb missing
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
cartilage models of future toe bones
condensed mesenchyme
failure of toes to separate
syndactyly
development of extra digital ray
polydactyly
cleft foot or hand, caused by failure of middle digital rays to develop
ectrodactyly
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 do mesenchymal precursors to cartilage begin?
week 5
which parts of the precursor cartilages start to chondrify first?
more cranial elements start to chondrify first (distal phalanges last)
week 6-8
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
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
neurogenesis into lower limb begins around…
week 5
all lower limb nerves are part of …
lumbosacral plexus
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
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
which limb rotates medially?
lower limb rotates 90 degrees MEDIALLY
upper limb rotates 90 degress LATERALLy
at week 8
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
great toe medial dermatome
L4-5 ***
little toe lateral dermatome
S1
describe a clubbed foot
congenital talipes equinovarus
foot is inverted, with sole pointed mediall
tendons (including Achilles’ tendon) are shortened
critical period for LL development
24-36 days post-fertilization
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
