QuestionSBA
QuestionChoice
SHH is produced in the zone of polarising activity in the caudal aspect of the distal limb bud. This results in the differential development of the digits into the digitus minimus to pollux morphologies.

Injection of SHH or transplantation of the zone of polarising activity to another part of the limb bud in chicken embryos is sufficient to alter the development of the autopod into a different orientation.

Enhanced SHH expression in vivo leads to polydactyly in which extra digits are produced.
10001061
a
Polydactyly
QuestionChoice
Chondrodysplasia Grebe type is a micromelia (small, deformed limb) congenital malformation of the limbs. It results from autosomal recessive mutations in GDF5, a growth and differentiation factor which enhances the condensation of the mesenchyme to promote bone thickening.
10001062
b
Grebe-type chondrodysplasia
QuestionChoice
Phocomelia describes the lack of formation of the stylopod (forms the adult humerus) and sometimes zeugopod (forms the adult radius and ulna - lost in type 1 phocomelia, retained in type 2) whilst the development of the autopod (forms the most distal features including the carpals, metacarpals and phalanges) is retained.

In development, the more distal features of the embryonic limb bud are induced to become the autopod and zeugopod based on the concentration of FGF received from the apical ectodermal ridge.

Phocomelia was a common thalidomide-induced birth defect as the drug impaired the proliferation of cells in the progress zone of the developing limb bud while not affecting FGF secretion.
10001064
d
Phocomelia
QuestionChoice
Split hand-foot malformation (SHFM) occurs due to multi-factorial genetic and environmental factors causing the failure of the apical ectodermal ridge, resulting in aplasia and hypoplasia of some phalanges and metacarpals/metatarsals. SHH can be implicated in SHFM but due to deficiency, not excess.

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10001065
e
Split hand-foot malformation
QuestionChoice
Pituitary dwarfism occurs when the growth of an individual is restricted uniformly across the entire body due to a deficiency in growth hormone. This can occur due to factors at birth (genetic disease, congenital pituitary agenesis) or acquired factors in early life (severe brain injury, pituitary adenoma compressing the somatotropic anterior pituitary cells).
10001063
c
Pituitary dwarfism
Concept
Chapter
2693
3876
Development of the limb buds
Topic
171
Embryology
12759
During limb development, excessive Sonic Hedgehog (SHH) release can lead to which condition?
a