Embryology Flashcards
When does limb bud development begin?
End of week 4
Specifically the upper limb bud forms around day 24 and the lower 1-2 days later
What embryonic cells give rise to the limb buds?
Lateral plate somatic mesoderm
What regulates limb development?
Homeobox (HOX) genes
They determine what skeletal element is formed
What happens to the apex of each limb bud?
Surface ectoderm thickens to form a the Apical Ectodermal Ridge (AER)
What is the function of the AER?
It induces the mesenchyme to proliferate
What happens to the mesenchyme of the limb bud as it elongateS?
Mesenchyme adjacent to AER is mainly undifferentiated rapidly proliferating cells.
But more proximal mesenchyme differentiates into vessels and cartilage bone models
How do the hands/feet first form?
The distal limb buds flatten into paddle-like hand and foot plates
What happens after hand/foor plates are formed?
Mesenchyme condenses to form digital rays
- > Then notches appear between them
- > Webbed fingers/toes
- > Seperate digits
When do digital rays form?
end of week 6 for hand plates and week 7 for foot plates
When are separate digits formed?
by the end of week 8
How is the tissue between individual digits removed?
Apoptosis mediated by Bone Morphogenetic Proteins
When do cartilage bone-precursors start to form and how?
in week 6 by chondrification
When does cartilage ossify into bone?
weeks 8-12 in response to growth factors
What type of ossification do limb bones undergo?
Endochondral
Except for the clavicle which ossifies by membranous ossification
How do limbs rotate and what is the result of this?
Upper limbs rotate 90* laterally
Lower Limbs rotate 90* medially
Hence the upper limb flexors are anterior and the lower are posterior
At what point do peripheral nerves start to grow into the limb buds?
They develop from the brachial/lumbosacral plexuses during wk 5
Whens the critical period for MSK birth defects?
24-36 days
What do we call absent bones?
Total - Amelia
Partial - Meromelia
Long bones - Phocomelia
What do we call it when the bones are small?
Micromelia
What do we call it if you have too many digits?
Polydactyly
The extra digit usually lacks muscle and is incomplete
What causes polydactyly?
Its inherited or teratogen induced
What do we call webbed fingers?
Cutaneous Syndactyly
What is the other type of syndactyly?
Osseous Syndactyly
Where the actual digit bones are fused
Much more common in feet
What causes osseous syndactyly?
Failure of notches between digital rays to develop
