digit 4 progenitors
digit 5 progenitors
-Cells that make digit 4 and 5 (and part of
digit 3) are direct descendants of Shh
secreting cells.
-Digit 5 cells spend more time exposed to
Shh (autocrine) than cells in digit 4
-Digit 3 is specified by both time and Shh
concentration (paracrine)
-Digit 2 is specified by Shh concentration
(paracrine)
- Digit 1 is Shh-independent
Shh effects are mediated by a gradient of BMP

Shh (ZPA) dual role in A/P limb
patterning
1.
Long range
Controls digit number: define the regions in the bud that are
competent
to form digits.
2.
Short range
Establish and maintains a
BMP2
morphogen gradient
that
specifies digit identity.
Adding BMP inhibitors (Noggin) to limbs treated with Shh:
-
extra digits still form but
they are all the same type

Regulation of digit identity by BMP in the
interdigital space
Noggin beads
•
Removal of interdigital region 2 transforms digit 2 into digit 1
•
Removal of interdigital region 3 transforms digit 3 into digit 2
•
Placing beads soaked with noggin (BMP inhibitor) into the interdigital spaces causes the same effect
•
BMP from Interdigital tissue specifies the identity of the digit anterior to it

Signaling in forelimb development
Positive feedback loop between AER and ZPA
Ectoderm
AER
ZPA
AER

Dorso-Ventral limb axis
•
Determined by the
ectodermal
tissue that surrounds the limb bud
(
non-AER ectoderm
). Not by mesoderm.
•
If ectoderm is rotated 180
o
the dorsal-ventral polarity of the digits is
reversed.
•
Signaling molecules are expressed asymmetrically in the limb
ectoderm:
–
Wnt7a
(dorsal) induces activation of
Lmx1
(homeodomain)
–
Engrailed
(ventral)
•
Lack of Wnt7a or LMx1 “
ventralizes
” limbs and digits (no nails, no
knee caps).
•
The boundary where dorsal and ventral ectoderm meet is essential
for AER placement: dorsalized limbs do not form AER.

Summary of signaling interactions in
limb axes specification
A. AER induction
•
Ffg10
from mesenchyme (lateral plate mesoderm)
activates
Wnt3
in ectoderm.
•
Wnt3
activates
Fgf8
that in turns activates
Fgf10
(positive feedback loop).
•
BMP
from ventral ectoderm activates
Engrailed
(En1) which represses
Wnt7
.
B. AER maintenance
As limb grows:
•
Shh
from ZPA induces A-P polarity (
BMP
gradient)
•
Shh
induces gremlin (
Grem1
) an inhibitor of
BMP
which allows continued
Fgf8
synthesis by the AER.

.

Homework
Mutations (loss of function) of Gli3 in both
mice and humans causes polydactyly.
Why?

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- Spring '14
- Transcription, Developmental Biology, limb development, AER