Limbs Flashcards
Limbs elongate with this signaling:
AER (apical ectodermal ridge)
Duplication of AER means the fetus may end up with this physical abnormality
More than 1 limb
AER induces a zone called
ZPA - the zone of polarizing activity
If the ZPA is duplicated a fetus might end up with these physical anomalies
Duplicated thumbs or mirrored hands
Orientation of the limb buds in the beginning: elbows and knees face
Laterally
Orientation of the limb buds in the beginning: palms and soles face
the trunk
The preaxial border is
Cranial (1st digit of hand or foot)
Post axial border will face
Caudally (5th digit side of hand or foot)
During development, the upper limbs will rotate
Laterally
During development, the lower limbs will rotate
Medially
During development, upper limb extensors will eventually be here ____ while lower limb extensors will be here ______
Upper limb extensors lie on lateral and posterior aspect;
Lower limb extensors lie on anterior aspect
Amelia
Absence of limbs
Why might Amelia happen?
AER failure: suppression of limb bud development early in week 4
An earlier (slightly after week 4 development starts) failure of AER may result in
Short limbs
Meromelia
Absence of part of a limb
Why meromelia?
AER failure: suppression of limb bud formation in week 5
Polydactyly
Supernumerary digits
Why polydactyly?
ZPA failure
Extra digits often formed completely without musculature; extra fingers are usually medial or lateral, not central. Extra toes are usually lateral.
Syndactyly
Webbing between digits (cutaneous) or fusion of phalanges in adjacent digits (osseous)
Brachydactyly
Short digits
Why brachydactyly?
Growth plate defect; autosomal dominant genetics that are variably expressed
Why syndactyly?
Cutaneous — lack of apoptosis between digits
Osseous — often seen when notches fail to form between developing digital rays
