Transcript Limb Development

Development of the Limbs
Biology 208. Developmental Biology.
Carlson, 4th ed. Chapter 10.
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Web Lecture from Temple University
• http://isc.temple.edu/marino/embryology/lim
bs97/ppframe.htm
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Limb Bud Formation
• The distal tip of the limb bud is covered
with a transient structure known as the
apical ectodermal ridge (AER). Limb
outgrowth is dependent upon
interactions between the AER and the
underlying mesoderm.
• If the AER is removed, the mesoderm stops
dividing.
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Limb Development
• If a supernumerary AER is grafted adjacent to a developing
limb, a supernumerary limb results.
• Prospective limb mesoderm grafted under flank ectoderm
will promote formation of an AER and a supernumerary
limb.
• If limb bud mesoderm is removed from an early limb
bud, the AER regresses and the mesoderm ceases
proliferation.
• Therefore, limb outgrowth involves reciprocal
interactions between the AER and underlying
mesoderm.
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Axes of Development
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What Controls Establishment of Limb Axes?
• The limb has three axes: Proximo-distal; Anterior-posterior
and Dorsal-ventral.
• Proximo-distal
Removal of the AER at progressively later stages results in a
truncated limb with progressively more distal elements.
Thus, these elements are laid down in a proximal-to-distal
direction.
The AER does not regulate P-D polarity. Fates of cells are
determined by the length of time they spend in the
progress zone. Those residing there the longest become
the most distal elements.
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Anterior-Posterior Axis
This axis is determined by the zone of polarizing
activity (ZPA), which is located at the junction
between the limb bud and the body wall.
Grafts of the ZPA to the anterior margin of a host
limb bud causes duplication of digits in mirrorimage symmetry.
A diffusible morphogen (retinoic acid?) is released
from the ZPA.
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Which axis forms first?
• 1. anterior/posterior (1st to 5th digits)
• 2. ventral/dorsal (palm/ dorsum of hand)
• 3. proximal/distal (base of limb / digits)
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Turtle skeleton
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Ambystoma maculatum
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Tuatara
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Tuataras
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Limbs of the
chicken
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Mammalian Limbs
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Mammalian Limbs
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Primate Limbs
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Primate Limbs: Gorilla
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Human Anatomy
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How has the orientation of the
limbs changed?
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Differentiation and Determination
of the Limb Bud
• Myogenic regulatory factors (MRFs) are required
for their differentiation
• Differentiation of myogenic cells in culture is
arrested by growth factors, such as FGF and TGFß
• These results have led to the hypothesis that
differentiation of myogenic precursors occurs in
response to low growth factor levels (Konigsberg,
1971; Yaffe, 1971).
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Limb Development: Hox Genes
• Homeobox (Hox) Genes: Master
Developmental Genes
• 1st identified in Drosophila
• Regulate development of body segments
• Homologous genes are present from
Arthropods to Vertebrates, highly conserved
• Crucial to development, but functions are
poorly understood
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Human HOX Genes
• 39 different Hox genes distributed in 4 linkage
groups
• Homeobox domain is highly conserved
• Homeobox domain encodes a small protein
called homeodomain protein segment
• This protein binds DNA and regulates gene
activity (i.e., it is a transcription factor)
• Hox genes are expressed in sequences that
correlate with development of specific regions
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Homeobox Regulation of Gene
Expression
From Human Development, by Professor Danton O’Day.
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www.utm.utoronto.ca/%7Ew3bio380/lecture20.htm
Hox Gene Expression in the
Mammalian Limb
• The HoxD gene complex is expressed in a
specific pattern in the developing mouse
forelimb. The pattern of gene expression
correlates with the linear arrangement of
the genes in the genome.
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This figure shows a summary of the expression of the
Hox genes relative to the final developed limb
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Hox-9 expressed earliest;
Hox-13 expressed only in final stages of
digit formation
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Retinoic Acid : A Developmental
Morphogen
• Retinoic acid is the active form of Vitamin A
• Both excess retinoic acid and vitamin A
deficiency each affect left-right symmetry
(e.g., abnormal formation of the mammalian
heart) through alterations in gene expression.
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Cell Death and Digit Formation
Without apoptosis, the fingers would be webbed.: syndactyly
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• www.ucidevcell.org/faculty/susan-bryant/
– For information on salamander limb development
and regeneration
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Gallus gallus (Chick) Limb Development-Retinoic Acid
and Sonic Hedgehog (SHH)
Normal chick wing
A bead soaked in Retinoic Acid was
implanted into the anterior margin of
the early wing limb-bud. Note the
mirror-image duplication of the digits.
IV
III
Digit IV represents a posterior limb structure. The ectopic
II
release of Retinoic Acid from the bead leads to ectopic
___
II
III
IV
expression of Sonic Hedgehog (SHH), forming a
secondary ZPA.
www.luc.edu/faculty/wwasser/dev/chicklmb.htm
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• www.acs.ucalgary.ca/~browder/limb_dev.html
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