BIOL 370 – Developmental Biology
Topic #16
Development of the Tetrapod Limb
Lange
Pattern formation – study of the tangible (statistically), orderly
outcomes of self-organization and the common principles behind
similar patterns in nature.
In developmental biology, pattern formation refers to the
generation of complex organizations of cell fates in space and
time. Pattern formation is controlled by genes.
Periodicities of limb pattern formation:
Stylopod  humerus & femur
Zeygopod  ulna & radius /
tibia & fibula
Autopod  carpals, metacarpals, &
phalanges / tarsals, metatarsals, &
phalanges
Figure 13.1 Skeletal pattern of the chick wing
Figure 13.2 Prospective forelimb fields of the salamander
Figure 13.4 Multilimbed Pacific tree frog (Hyla regilla), the result of infestation of the tadpole-stage
developing limb buds by trematode cysts
These cysts drive
additional limb
development
when they feed
on the limb bud
leading to
splitting of the
bud into multiple
growing regions.
Figure 13.5 Fgf10 expression and action in the developing chick limb
Fgf10 – fibroblast growth factor 10… proteins encoded by this gene are of the
fibroblast growth factor (FGF) family. FGF family genes all possess broad based
mitogenic and cell survival actions, including embryonic development.
Figure 13.6 Molecular model for initiation of the limb bud in the chick between 48 and 54 hours of
gestation
Watch the process
involving the Fgf10
gene.
Figure 13.7 Specification of limb type in the chick by Tbx4 and Tbx5 (Part 1)
Tbx4 – T-box 4 gene… hind limb
development,
Tbx5 – T-box 5 gene… forelimb
development.
Figure 13.7 Specification of limb type in the chick by Tbx4 and Tbx5 (Part 2)
In the study in (B) we see a study in which an
FGF bead is inserted in the area between the
action of Tbx4 and Tbx5 resulting in an
additional limb that is chimeric.
Figure 13.9 Summary of experiments demonstrating the effect of the apical ectodermal ridge
(AER) on the underlying mesenchyme
Apical Ectodermal
Ridge - a structure that
forms from the
ectodermal cells at the
distal end of each limb
bud and acts as a major
signaling center to
ensure proper
development of a limb.
After the limb bud
induces AER formation,
the AER and limb
mesenchyme—
including the zone of
polarizing activity
(ZPA)—continue to
communicate with each
other to direct further
limb development
Figure 13.10 Fgf8 in the apical ectodermal ridge
Fgf8 – fibroblast growth factor 8… important within AER action
Figure 13.11 The AER is necessary for wing development
Figure 13.12 Control of proximal-distal specification by the progress zone mesenchyme
(A) Transplantation of
an early wing bud
progress zone to a
late wing bud
zone.
(B) Transplant of a
late wing bud
progress zone to
an early wing bud
zone
Extra radius & ulna
No extra development
Figure 13.15 When a ZPA is grafted to anterior limb bud mesoderm, duplicated digits emerge as a
mirror image of the normal digits
Figure 13.16 Sonic hedgehog protein is expressed in the ZPA
ZPA - Zone of Polarizing
Activity… an area of
mesenchyme that releases
signals instructing the
developing limb bud to form
along its anterior/posterior
axis.
Figure 13.17 Ectopic expression of mouse sonic hedgehog in the anterior limb causes extra digit
formation
The mutant form in (B)
is called the Hx mutation
(hemimelic extratoes).
Shh
expression
that is
ectopic
(abnormal)
can result in
polydactyl
digits in
these mice.
Figure 13.18 Deletion of limb bone elements by the deletion of paralogous Hox genes
Notice how the differential
expression of Hox prologues
may shape overall
development, but that there
are still many similarities.
Examples of human
polydactly.
Figure 13.18 Deletion of limb bone elements by the deletion of paralogous Hox genes (Part 4)
Human Synpolydactyly (the term means simply,
“many fingers joined
together”)… a result of
homozygosity at the HoxD13 loci.
In people with this
syndrome, there is also often
malformations of the
urogenital system, which
also expresses this gene.
Ernest Hemingway’s home (now a
museum) in Key West Florida houses
descendants of his own pet polydactyl
cats. In cat breeding circles, the
polydactyl cats are sometimes called
Hemmingway cats.
Figure 13.22 Regulation of digit identity by BMP concentrations in the interdigital space anterior to
the digit and by Gli3
Gli3 ( producing Zinc finger protein Gli3) represses dHand and
Gremlin, which are involved in developing digits.
This experiment shows a potential mechanism for
“webbing” of digits.
Figure 13.26 Patterns of cell death in leg primordia of (A) duck and (B) chick embryos
Result is a
webbed foot.
Result is a
non-webbed
foot.
Figure 13.27 Autopods of chicken feet and duck feet are shown at similar stages
The expression of Gremlin in the duck (arrows) is thought to prevent the full
apoptotic death of the webbing in the duck.
Figure 13.28 Inhibition of cell death by inhibiting BMPs
Gremlin protein
soaked beads placed
in the mesodermal
webbing of (B)
show persistence of
the webbing.
Figure 13.29 Possible involvement of BMPs in stabilizing cartilage and apoptosis
BMP and
noggin
expression may
be the cause of
the cartilage
defects showin
in (C).
Figure 13.30 Tiktaalik, a fish with wrists and fingers, lived in shallow waters ~375 million years ago
Tiktaalik - a monospecific genus of extinct
sarcopterygian (lobe-finned) fish from the
late Devonian period, with many features
akin to those of tetrapods.
Originally these were first discovered in
northern Canada on Ellesmere Island.
End.