Development of the Tetrapod Limb WEL COME TO LOVYANSH LIFESCIENCE I HAVE EXPLAIN THIS TOPIC IN DETAILS ON YOUTUBE MY YOUTUBE CHANNEL LINK https://youtu.be/o1h68DJ8vCg PLEASE SHARE , LIKE & COMMENTS FOR MORE SUCH VIDEOS

1. Presented by :- lovyansh lifescience

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 The vertebrate limb is an extremely complex organ with
an asymmetrical arrangement of parts. The positional
information needed to construct a limb has to function in
a three-dimensional* coordinate system:
 The first dimension is the proximal-distal axis ("close-far";
shoulder-finger or hip-toe).
 The second dimension is the anterior-posterior axis
(thumb-pinkie).
 • Finally, limbs have a dorsal-ventral' axis: our palms
(ventral) are readily distinguishable froru our knuckles
(dorsal).
Three-dimensional
coordinate system

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 Emergence of the limb bud.
 Proliferation of mesenchymal cells (arrows) from the
somatic region of the lateral plate mesoderm causes
the limb bud in the amphibian embryo to bulge
outward. These cells generate the skeletal elements
of the limb.
 Contributions of myoblasts from the lateral
myotome provide the limb's musculature
Formation of the Limb
Bud

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When mesenchyme cells enter the limb field, they
secrete Fgf10that induces the overlying ectoderm to
form a structure called the apical ectodermal ridge, or
AER The AER runs along the distal margin of the limb
bud and will become a major signaling center for the
developing limb
The apical ectodermal
ridge

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 (1) maintaining the mesenchyme beneath it in a
plastic, proliferating state that enables the linear
(proximal-distal, shoulder-finger) growth of the
limb;
 (2) maintaining the expression of those molecules
that generate the anterior-posterior (thumb-pinkie)
axis.
 (3) interacting with the proteins specifying the
anterior-posterior and dorsal-ventral (knuckle-palm)
axes so that each cell is given instructions on how to
differentiate.
Its roles of The apical
ectodermal ridge

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 (A) Fgf10 expression is first seen throughout the lateral
plate mesoderm. It becomes stabilized by Wnt8c to the
area where hind limbs form.
 (B) Fgf10 synthesis is stabilized by the actions of Wntlb to
the region where the forelimbs will form.
 (C) Fgf10 from these two regions of lateral plate
mesoderm induces Fgf8 in the apical ectodermal ridge
(AER). This induction is accomplished through a pathway
involving Wnt3a in the responding ectoderm. Fgf8
secreted from the AER induces the continued mesodermal
expression of Fgf10. The roles of Fgf8 in the intermediate
mesoderm in inducing or maintaining Wnt expression are
uncertain.
Molecular model for
initiation of the limb bud
in the chick

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 The mesodermal cells that give rise to a vertebrate
limb can be identified by
 (1) removing certain groups of cells and observing
that a limb does not develop in their absence
 (2) transplanting groups of cells to a new location
and observing that they form a limb in this new
place
 (3) marking groups of cells with dyes or radioactive
precursors and observing that their descendants
partake in limb development.
Specification of the limb
fields

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 . (A) In situ-hybridizations show that during normal chick development, Tbx5 is
found in the anterior lateral plate mesoderm, whileTbx4 is found in the posterior
lateral plate mesoderm.
 Tbx5-containing limb buds produce wings/ while Tbx4-containing limb buds
generate legs.
 (B) If a new limb bud is induced with an FGF-sccreting bead, the type of limb formed
depends on which Tbx gene is expressed in the limb bud. If placed between the
regions of Tbx4 and Tbx5 expression, the bead will induce the expression of Tbx4
posteriorly and Tbx5 anteriorly. The resulting limb bud will also express Tbx5
anteriorly and Tbx4 posteriorly and will generate a chimeric limb.
 (C) Expression of Tbx5 in the forelimb (w, wing) buds and in the anterior portion of a
limb bud induced by an FGF-secreting bead. (The somite level can be determined by
staining for Mrf4 mRNA, which is localized to the myotomes.)
 (D) Expression of Tbx4 in the hindlimb buds and in the posterior portion of an FGF-
incluced limb bud.
 (E,F) A chimeric limb induced by an FGF bead contains anterior wing structures and
posterior leg structures.
 (F) is at a later developmental stage, after feathers form.
Specification of limb type
in the chick by Tbx4 and
Tbx5.

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 The proximal-distal growth and differentiation of the limb bud
are made possible by a series of interactions between the AER
and the limb bud mesenchyme directly beneath it. This distal
mesenchyme is called the progress zone (PZ) mesenchyme (and
sometimes the undifferentiated zone), since its proliferative
activity extends the limb bud.
 the structures of the extra set of digits are mirror images of the
normally produced structures. The polarity has been
maintained, but the information is now coming from both an
anterior and a posterior direction. Thus this region of the
mesoderm has been called the zone of polarizing activity
(ZPA). SONIC HEDGEHOG DEFINES THE ZPA
The zone of polarizing
activity

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 When a ZPA is graphted 10 anterior limb bud
mesoderm, duplicated digits emerge as a mirror image
of the normal digits

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 Deletion of limb bone elements by the deletion of
paralogous Hox genes. Hox gene patterning of the
forelimb.
 Hox9 and Hox 10 paralogues specify the humerus, and
Hox l0 paralogues are expressed to a lesser extent in the
zeugopod.
 Hox11 paralogues are chiefly responsible for patterning
the zeugopod
 Hox 12 paralogues function in the wrist, with a little
patterning of the autopod; and
 the Hox 13 paralogue group functions in the autopod.
Hox gene specification

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 In addition to the interdigital necrotic zone, divided
the limb digits .
 three other regions of the limb are "sculpted" by cell
death.
 The ulna and radius are separated from each other
by an interior necrotic zone, and
 two other regions, the anterior and posterior
necrotic zones, further shape the end of the limb &
seprated of anterior & posterior part of limb .
Necrotic zone followed by
cell death

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 The signal for apoptosis in the autopod is probably provided by
the BMP proteins.
 BMP2, BMP4, and BMP7 are each expressed in the interdigital
mesenchyme, and blocking BMP signaling (by infecting
progress zone cells with retroviruses carrying dominant
negative BMP receptors) prevents interdigital apoptosis;
 Since these BMPs are expressed throughout the progress zone
mesenchyme, it is thought that cell death would be the "default"
state unless there were active suppression of the BMPs. This
suppression may come from the Noggin protein, which is made
in the developing cartilage of the digits and in the
perichondrial cells surrounding it
 If Noggin is expressed throughout the limb bud, no apoptosis is
seen.
Bmp signal for apoptosis

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Possible involvement of
BMPs in stabilizing
cartilage and apoptosis