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Limb devlopment
M.Sc. 2012
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Human embryo -32 days
Placodes sensory placodes, lens pit, otocyst, nasal placode,primary/secondary vesicles, fourth ventricle of brain Mesoderm continuedsegmentation of paraxial mesoderm (somite pairs), heart prominence
Head 1st, 2nd and 3rd pharyngeal arch, forebrain, site of lens placode, siteof otic placode, stomodeum
Body - heart, liver, umbilical cord, mesonephric ridge visible externally as
bulges. Limb upper and lower limb buds growing
Neural first appearance of the future cerebral hemispheres. Cerebellar platedifferentiated to an intermediate layer, and future rhombic lip identifiable
Ventricular System Subarachnoid space initially as irregular spaces on theventral surface of the spinal cord.
Liverhepatic gland and its vascular channels enlarge, hematopoieticfunction appears
Eye - Lens the lens placode is indented by the lens pit
http://php.med.unsw.edu.au/embryology/index.php?title=Placodeshttp://php.med.unsw.edu.au/embryology/index.php?title=Mesodermhttp://php.med.unsw.edu.au/embryology/index.php?title=Head_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Gastrointestinal_Tract_-_Liver_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Musculoskeletal_System_-_Limb_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Neural_System_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Neural_-_Ventricular_System_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Gastrointestinal_Tract_-_Liver_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Vision_-_Lens_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Vision_-_Lens_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Vision_-_Lens_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Vision_-_Lens_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Vision_-_Lens_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Vision_-_Lens_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Gastrointestinal_Tract_-_Liver_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Gastrointestinal_Tract_-_Liver_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Neural_-_Ventricular_System_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Neural_-_Ventricular_System_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Neural_System_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Neural_System_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Musculoskeletal_System_-_Limb_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Musculoskeletal_System_-_Limb_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Gastrointestinal_Tract_-_Liver_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Head_Developmenthttp://php.med.unsw.edu.au/embryology/index.php?title=Mesodermhttp://php.med.unsw.edu.au/embryology/index.php?title=Placodes7/30/2019 2012 Limb Devlopment
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LIMB DEVELOPMENT Each limb results from a developmental field. The
developmental fields are determined during gastrulation. These limb fields are established by the expression of
HOX genes. The expression ofTbx-4 causes the limb todevelop into a forelimb and expression of the factorTbx-5 causes the limb to develop into a hind limb.
Beginning from the fourth week from fertilization, overa period of 25 days, a complex of genetic signals controlthe intricate pathways that result in a limb with thecorrect orientation, size, and number of digits.
Limb development is a continuous process divided into
four stages:1. bud stage (initial outgrowth),
2. paddle stage (dorsoventral flattening),
3. the plate stage (relative expansion of the distal end),
4. rotation stage (rotation around the proximodistal axis).
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1. LIMB BUD FORMATION
34 day old human embryo(5mm)
34 pairs of somites
Forelimb (lower left) started to
develop Hindlimb just beginning (rightside)
By day 37,In upper limb bud :
1. nerves: median nerve, radial
nerve and ulnar nerve enteredinto hand plate,
2. myoblasts: spindle shapedand oriented parallel to limbbud axis
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Limb Bud formation(contd.)
The limbs of the embryo develop from buds that protrudefrom the side of the main body axis.
Limb buds arise on the lateral body at the level ofsclerotomes as ectoderm and mesoderm (somite)proliferations.
Each limb bud consists of a mesenchymal core ofmesoderm covered by an ectodermic cap.
Limb buds will become the early arms and legs of theembryo. The upper limbs appear before the lower limbs
that are delayed about two days in respect to the upperlimbs.
At the early stages of embryonic development, theforelimb and hind limb buds look like paddles on eitherside of the embryo and are indistinguishable from one
another.
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2. LIMB PADDLE FORMATION
The limb buds continue their formation by the migrationand proliferation of the differentiating mesenchymaltissues.
The ectoderm at the tip of the bud thickens to form aspecialized structure, called the apical ectodermal ridge.This structure is the signaling center that allows propergrowth along the proximodistal axis (shoulders to digits).
Along with it , the limb becomes flattened along thedorsoventral axis (back of hand to palm) and asymmetricalong the anteroposterior axis (thumb to little finger).
Proximodistal, dorsoventral, and anteroposterior axesrepresent the routes of the normal limb growth.
The most proximal structure (stylopod) begins todifferentiate first, followed by the progressivedifferentiation of more distal structures (zeugopod andautopod).
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Limb paddle (contd.)
This outgrowth and patterning depends on theestablishment and maintenance of other signalingcenters within the limb bud, named the zone of polarizingactivity located in the mesenchyme at the posteriormargin of the bud, and the non ridge ectoderm of the
bud. These developmental components areinterdependent and, through a series of reciprocalsignals and feedback systems, yield the correct tissuepatterning and growth.
Each bud develops to form a complex of interconnected
limb elements comprised of bone, muscle, and tendoncharacteristic of either the fore limb or hind limb. Theactual trigger for limb bud initiation is still unknown,although likely candidates have been identified asFibroblast Growth Factor 8 and 10.
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3.PLATE STAGE
The plate stage is characterized by the formation of flattened plate-like areas on the distal ends of the limbs called the hand plates andfoot plates
They are flattened along the dorsoventral axis. Within these distalplates, some structure is noticeable.
There are radially arranged thickenings called digital rays(precursors of the digits). Between the digital rays are thin areaswhere cells begin to undergo apoptosis (programmed cell death)that allow the separating of the digits. The thin areas are calledinterdigital grooves. This arrangement gives rise to free digits.
A constriction on the limb just proximal to the hand and footplates,called primary constrictions of the limb, is evident in this stage.
These constrictions will develop into wrists and ankles. At approximately seven weeks, the longitudinal axes of the upper
and lower limb buds are parallel.
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4. ROTATION
In the rotation stage, the position of the limb buds relative to thetrunk change in a predetermined manner not related to muscleactivity or inherent osseous torsion.
During this stage, the rotation of the limbs creates a threedimensional structure. Because of the differential growth of thecartilage model that continue to elongate the limb, different parts
grow at different rates. This causes a twisting or rotation of eachlimb around its proximodistal axis.
Upper limbs rotate one way (laterally or externally), while lowerlimbs rotate the other way (medially or internally) bringing the greattoe to the midline from its initial postaxial position. This creates thecharacteristic positions of the limbs with the point of the elbowfacing caudally and dorsally, and the knee facing cranially andventrally.
Consequently, the equivalent bones and muscles of the upper andlower limbs are oriented 180 degrees apart. This means that in thestructural organization of the upper and lower limbs, their flexorsand extensors are positioned on opposite sides and themovements at equivalentjoints are in opposite directions
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Carnegie stage 12 to 23 Human Forelimb Development:
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Summary
In the 5th week hand and footplates appear at the ends oflimb buds and ridges form digital rays. Cells between the
digital rays are removed by programmed cell death
(apoptosis).
Late in Carnegie stage embryogenesis (Stage 20-23, 8th
week) limb rotation occurs. Forelimbs and hindlimbs rotate
in different directions, upper limb rotates dorsally, lower
limb rotates ventrally, thumb and toe rostral, knee and
elbow face outward.
Bones within the limb form by endochondrial ossification(begins Carnegie stage 18) throughout embryo. This
process is the replacement of cartilage with bone (week 5-
12).
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Timeline limb development
By day 44,LimbBone forms by endochondrialossification and throughout embryo replacementof cartilage with bone (week 5-12).
By day 50, upper limbs begin to rotate ventrally
By day 53, fingers and toes lengthen By day 56, upper limbs longer and bent at
elbow, hands and feet turned inward, foot withseparated digits, wrist, hand with separated
digits By day 64, fingernails appear
By week 14,toenails appear
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Molecular control of limbformation
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Limb bud
Limb bud: Mesoderm &
Epithelial Ectoderm
Ectoderm over
mesoderm
Ectoderm thickened as
Apical Ectodermal Ridge
(AER)
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Cells that Contribute to Mesoderm
of the Limb Bud Limb mesoderm (mesenchyme) comes
from the somite and lateral platemesoderm
The Lateral Plate Mesoderm contributesto the skeleton, blood vessels &connective tissue
The Somite Mesoderm contributes tothe Musculature
Nerve cells & Neural crest cells migratein as well
Motor Axons from spinal cord willinnervate limb
Neural Crest gives rise to sensorynerves, schwann cells, pigment cells
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Apical ectodermal ridge (AER).
Secretes fibroblast growth factor (FGF)
proteins. Required for limb growth and patterning along
the proximal-distal axis.
Required for
pattern formationalong the
dorsal-ventral
axis.
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Early limb dvelopment
Limb grows & develops proximo-distallyZone of Cell Division: Region of actively dividing cells
Zone of Differentiation: Region of cell specialization
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Organizer
regions.
Zone of polarizing activity (ZPA).
Secretes Sonic hedgehog, a protein growth factor.Required for pattern formation of the limb along the
anterior-posterior axis.
Homeobox-containing (Hox) genes play a role in
specifying the identity of regions of the limb, as well as the
body as a whole.
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The Organization and Polarity of the Developing Limb Bud
The limb bud has a strict pattern and polarity. Development is organized around the A-P, D-V and P-V
axes. The tissues of the limb will differentiate in a specific pattern that is defined in part by the existing
embryonic regions: the Apical Ectodermal Ridge (AER), the Zone of Polarizing Activity (ZPA) and the
Progress Zone (PZ).
The AER acts as the organizing region for the proximodistal axis of the limb. The ZPA organizes the limb
along the A-P axis. It does this in part through the expression ofSonic hedgehog resulting in theproduction of the soluble sonic hedgehog protein. Sonic hedgehog mediates many developmental events.
In the limb it not only meditates A-P Axis formation but also the maintenance of the AER.
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EPITHELIAL-MESENCHYMAL
INTERACTIONS DURING LIMB
DEVELOPMENTExperiments originally done in chickens
Modified here to show how results might apply to
human limb
Removal of AER stops limb development
Addition of AER causes formation of 2nd limbSplitting AER leads to 2nd limb
Inferences :AER controls limb developmentLimb mesoderm dictates limb development;
almost any epithelial ectoderm can replace
normal limb epithelium
Type of limb depends on type of mesoderm
Not species specific: Inter-specific grafts show
same induction
Inducer may be universal
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Pattern Formation in theVertebrate Limb is a chainof events involving cellsignaling anddifferentiation. .
Induction plays a majorrole in pattern formation.
Positional
information, suppliedby molecular cues,tells a cell where it isrelative to the animalsbody axes
e.g.Limb development in chicks as
a model of pattern formation.Wings and legs begin as limb buds.Each component of the limb is orientedwith regard to three axes:Proximal-distalAnterior-posteriorDorsal-ventral.
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Transcription factor code for developmental
identities (particular region)
HOM
HOX
Hox: homeotic selector-fly mutant: transform one part of the body into another
Homeodomain-bind to DNA-TF( regulate a large set of downstream genes)
Structure and action-
conserved
limb
Rostral-caudal axis
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Homeotic transformation of the wing and
haltere
Homeotic genes
mutated into homeosis transformation
Bithorax-haltere into wing
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Mutation in HoxD13
synpolydactylyExtra digits & interphalangeal webbing (hetero)
Similar but more severe & bony malformation
of hands, wrists (Homo)
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Organizers in animal embryos: Spemannorganizerand ZPA
Spemann organizer
Transplantation experiment
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Action of morphogen (paracrine signal)
Human Sonic headehog
(notochord secreted to induce brain
and spinal cord development)
High conc.-neural tube
Low conc.motor neurons
In limb (asymmetrical pattern of
Digits)-zone of polarizing activity
HedgehogDrosophila mutant (alter epidermal bristles)
Different concentrations to different fates
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Signal Transduction & Limb Formation
During limb development the limb bud grows away from the body in a proximo-
(close) distal (away from) fashion.
Developmentally, as the limb bud lengthens and limb components are
specified and start to differentiate, what were once distal regions become
proximal as new distal regions form. This continues until the limb is fully
developed and the final relationship of limb components is defined.
For exampe , in early embryogenesis, the humerus as it forms is initially the most
distal component but once the radius and ulna and subsequent components form, it
becomes proximal to them.PROGRESS-ZONE MODEL OF LIMB DEVELOPMENT,
1. The AER secretes FGF that influences the closest cells (those in the progress zone)
to develop into distal structures. FGF is a distalizing factor in limb development.
Those cells that are not within range of the AERs influence remain proximal in nature.
2. As the AER extends out due to the continued division of cells in the progress zone, it
continues to affect the closest cells by causing them to be specified as distal structurecells.
3. Those that fall out of the range of influence of the AER are no longer influenced by
the effects of FGF and retain their previously defined status (i.e., are now proximal
components not influenced by the distalizing effect of FGF).
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Events of Signal Transduction & Limb
Formation
1. Fibroblast Growth Factor (FGF) family of factors islinked to the initiation of bud formation, maintaining budoutgrowth, and the induction of a regeneration
2. FGFs are secreted primarily by AER
3. Tyrosine kinase FGF receptor is expressed on the
surface mesenchyme cells4. FGF Released by AER binds to FGF Receptor (areceptor tyosine kinase or RTK) & activates It
5. RTK then phosphorylates critical proteins
6. This causes the mesenchyme cells to release retinoic
acid (RA)7. RA induces Hox Gene Expression in target cells
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FGFs
FGFs produced in the AER serve at least two major functions.
1. to stimulate the proliferation of cells in the progress zone due to their mitogenic
activities for limb bud mesenchyme and thus produce the new cells required for limb
outgrowth.
2. to maintain Shh expression in the ZPA.
FGF 4 although not required to induce Shh expression is largely responsible for
maintenance of its expression as the limb elongates. The regulatory interaction
between FGF4 and Shh could be reciprocal as Shh produced in the ZPA induces and
maintains FGF 4 expression in the AER. This positive feedback loop between FGF 4
and Shh could be one of the mechanisms by which outgrowth and patterning of limb
would be coordinately regulated, although additional molecules such as Wnt7a are
likely to play a role in regulating Shh expression.
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FGFs
One of the target for FGF signaling from the AER is FGF 10 which is expressed in the
distal limb bud mesenchyme. This factor is able to interact with FGF 8 and there
might be a positive feed-back loop between FGF 10 and FGF 8. This reciprocal
regulation is likely to be mediated by two isoforms of FGFR 2, FGFR 2b (that binds
FGF 10 exclusively) and FGFR 2c (that binds FGF 8).
A recent model has been proposed in which FGF 10 made in the mesenchyme of the
limb field diffuses in the ectoderm where it binds FGFR 2b and induces FGF 8 in theectoderm. The FGF 8 in turn diffuses into the mesoderm and activates FGFR 2c
which causes the upregulation of FGF 10. The FGF 10 then continues the loop and
results in limb bud induction.
Hence FGFR 2 appears to be essential for limb bud initiation whereas FGFR 1 seems
to play an essential role at several stages of limb development. This assertion is
based on the study of mouse models and expression patterns which have revealedan important function of FGFR 1 in specification of P-D axis formation.
FGFR 1-mediated signals are required for maintaining ZPA and progress zone
activities.
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Thalidomide-induced embryopathy
Teratology-teratogens
Thalidomidedamaging tissue within the proliferating center
Proximal-distal axis
PZ: progress zone, AER: apical ectodermal ridge, FGF: fibroblast growth factor
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Role of HOX ,BMP
Some of the FGF in conjonction with Shh can affect expression of the bone
morphogenetic protein (Bmp-2 and 7) and Hox genes, mostly Hoxd-12 and
Hoxd-13. These latter genes are members of the Hoxdcomplex and are
expressed within the distal wrist (Hoxd 12) and within the hand and fingers
(Hoxd 12 and 13).
The role of the Hoxd 13 gene in the proximodistal differentiation of limbsegments has been illustrated by the demonstration that mutations in the
human gene transforms the metacarpals to carpals and metatarsals to
tarsals. Likewise, overexpression of Hoxd13 in chick limb bud resulted in
the transcriptional repression in the proximal part of the limb ofMeis, the
vertebrate ortholog of an homeo-box containing gene in drosophila called
homothorax(hth) that is required for proximal leg development.
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BMP antagonists in signaling networks.
(a) Gremlin in limb-bud development. The predicted interactions between Gremlin, BMP4, FGF4
and Shh : Gremlin maintains FGF/Shh positive-feedback signaling during limb outgrowth by
preventing BMP4 inhibition of this loop. Maintenance of this loop is essential for correct limb-bud
development.
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skeleton of the limb
The skeleton of the limb arises from somatic mesodermby means of endochondral ossification.
Formation of the intermediate segment (forearm)involves programmed apoptosis to separate a single
mesenchymal condensation into two cartilage models(one for the radius and one for the ulna).
In addition, separation of the digits depends onapoptosis within the interdigital grooves.
Cartilage breaks down to form the joints in specific
points. Periosteum, ligaments, tendons, andintramuscular connective tissues form from the non-condensed mesenchyme.
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Skeleton of limb
In addition to somatic mesoderm, there are cells that migrate
into the limb bud from the body wall. These cells are identified
into three groups:
(1) somitic components (somitic myotomes in particular) that
migrate into the limb buds and give rise to all of the
musculature of the limb
(2) spinal nerves from the brachial plexus that go to the upper
limb and from the lumbosacral plexus that go to the lower limb,
and
(3) blood vessel precursors going into the limb to provide the
vasculature.
By the end of the eighth week, the limb is perfectly formed. From
there on out, the only remaining development is growth that is
synchronized with that of the fetal body.
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Skeleton formation
The skeletal elements of the limb developfrom a column-like mesodermal
condensation that appears along the long
axis of the limb bud during the fifth week of
gestation in human.
With the exception of clavicle, the bones
of the limbs form by ossification of a
cartilaginous precursor according to aprocess called endochondral
ossification.
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Skeleton formation
Mesenchymal cells from the lateral plate condense to form prechondrogenic
elements in the proximal region of the limb, giving rise to the anlagen of the humerus
(or femur). Distal extension of the process results in the formation of the ulna and
radius (or fibula and tibia) which further branches and segments to form the posterior
proximal carpal (tarsal) element as well as the digital rays of digits IV-II.
Prechondrocytes in the prechondrogenic condensations differentiate into
chondrocytes in response to growth factors and secrete molecules characteristic ofthe extracellular matrix such as collagen type II and aggrecan (a large
proteoglycan). The initial phase of chondrification results in the formation of a
cartilaginous envelope, the perichondrium.
This perichondrium in which bone morphogenetic protein 2, 4 and 7 (BMP 2, 4 and 7)
and parathyroid hormone/ parathyroid hormone-related peptide receptor
(PTH/PTHrPR) are expressed, inhibits chondrocyte proliferation and maturationthereby helping to control the growth and differentiation of the forming cartilage
elements.
As the cartilage elements grow different zones can be distinguished that demarcate
the progressive differentiation of the chondrocytes. Cells at the ends of the elements
are immature and undergo rapid proliferation.
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Skeleton formation
Adjacent to the proliferation zone are the larger and more sparsely distributed pre-
hypertrophic chondrocytes that express Indian hedgehog(Ihh), PTH/PTHrPR,
BMP 6 and BMP receptor IA (BMPRIA). The terminally differentiated hypertrophic
cells express a unique form of collagen, type X collagen, and eventually undergo
programmed cell death and are replaced by osteoblasts.
Defective cartilage growth occurs in a wide spectrum of disorders called
chondrodysplasias that usually result in dwarfisms of variable severity. The mostcommon of these disorders is achondroplasia , a dominant genetic disease caused
by a recurrent activating mutation in the transmembrane domain of FGFR3 affecting
chondrocyte proliferation and differentiation.
The process ofbone ossification begins in a region called the primary ossification
center. Mesenchymal cells in the perichondrium differentiate into osteoblasts that
secrete the calcium salt matrix of mineralized bone and form a primary bone collararound the bone which thickens as osteoblasts differentiate. In addition to
chondrocytes and osteoblasts, a third cell type of hematopoietic origin, the
osteoclasts contribute to skeletal remodeling throughout development. Indeed, the
function of osteoblasts and osteoclasts is intimately linked since osteoblast
synthesize and secrete molecules that control osteoclast differentiation.
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(C) Schematic showing the
contribution of the neural
crest, lateral plate
mesoderm, paraxial
mesoderm, and notochord tothe three major parts of the
skeleton. (D) Mid-sagittal
sections through thenotochord of mouse
embryos at the gestation
days 12.5 (E12.5, top) and
E15.5 (bottom). The E12.5
notochord is a rod-like
structure that becomes
surrounded by themesenchymal cell
condensations of theprospective vertebral bodies
(VB) and IVD. E15.5 VB are
cartilaginous and notochord
cells have migrated into the
intervertebral disc spaces,
where they have formed NP.
Sections are stained withnuclear fast red and with
Alcian blue, which is specific
of the notochord and
cartilage extracellular matrix.
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Fate and molecular control of skeletogenic mesenchymal cells.
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hondrocyte early differentiation and development of cartilage primordia. (A)
Alcian blue staining of a mouse embryo at E14.5 demonstrates that
chondrocyte differentiation of skeletogenic cells leads to the formation of a
primary skeleton that is entirely cartilaginous. (B) Sections through the
developing paws of mouse embryos illustrate the major steps of early
chondrogenesis. At E10.5, the limb bud is filled with skeletogenic cells. By
E12.5, some of these cells have formed precartilaginous condensations that
prefigure the future digits. By E14.5, condensed prechondrocytes have
undergone chondrocyte early differentiation.
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Chondrocyte maturation and development of cartilage growth plates.(A) Sections through a mouse embryo tibia (T) illustrate the development of growth plates andendochondral bone. At E13.5, early chondrocytes in the center of cartilage primordia undergo
prehypertrophic and hypertrophic maturation. They reach terminal maturation and are replaced by
endochondral bone by E15.5. Later on, growth plates maintain themselves and elongate developing bones.
Chondrocytes keep proliferating and give rise, layer by layer, to maturing chondrocytes. These cells, which
eventually die, are replaced by bone. The sections are stained with Alcian blue and nuclear fast red. (B)
Schematic of the molecular control of GP chondrocytes.
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Osteoblast differentiation
and intramembranous and
endochondral ossification.(A) Sections through an endochondral
bone in a newborn mouse show the
replacement of cartilage by bone. The left
section is stained with Alcian blue and the
right one with the von Kossa reagent,
which leaves a brown precipitate on the
mineralized bone matrix. (B) Schematic
showing how GP chondrocytes and bone-
forming cells interact with each other toachieve endochondral ossification. (C)
Coronal sections of a newborn mouse
head. In the suture linking the two frontal
bones (top panel), osteoblast precursors
are surrounded by an abundant
collagenous matrix. Further away (bottom
panel), osteoblasts mature and deposit a
mineralized bone matrix. This matrix is
stained with the von Kossa reagent. (D)
Schematic of the molecular control of
osteoblast differentiation.
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Synovial joint development.(A) Sections through the mouse knee joint at various stages of development. At E12, the presumptive joint region
(arrow) is not distinguishable from the femur (F) and T precartilaginous condensations. At E13.5, this region becomes
distinguishable as surrounding cartilage primordia are overtly developing. At E16.5, joint morphogenesis is well
advanced. The joint cavity has formed between the patella (P) and F. Cruciate ligaments and FP, lined with synovial
tissue, are developed. At the postnatal day 19, the knee joint is mature. The AC is separated from the epiphyseal GP
by a secondary center of ossification. The sections are stained with Alcian blue and nuclear fast red. (B) Schematic of
the molecular control of synovial joint cell differentiation.
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Models for the development of sexually
dimorphic digit proportions
AR (blue circles) and ER (pink circles) are
present in the digit condensations of
male and female embryos, with higher
levels found in 4D
. (A) In males, digits are exposed to high
levels of circulating androgen and lowlevels of circulating estrogen, which
results in preferential binding and
activation of AR
(ARA represents the androgen bound to
the AR). High AR activity and low ER
activity (ARA/er) in males leads to
differential gene expression profiles in4D relative to 2D (green indicates genes
higher in 4D, and red indicates genes
higher in 2D). In turn, chondrocyteproliferation is increased in the proximal
phalanx of 4D, which results in elongation
of 4D relative to 2D,
leading to a lower 2D:4D ratio.
(B) In females, digits are exposed to high
levels of estrogen and low levels of
androgen, leading to preferential binding
and activation of ER (ERE). Low AR
activity and high ER activity (ar/ ERE)induces an opposite shift in the
skeletogenic gene expression profile
of 4D relative to 2D (indicated by gene
names in green and red, as above).Higher levels of activated ER cause
decreased chondrocyte proliferation in the
middle phalanx of 4D, which reduces its
growth relative to 2D and results in a
higher 2D:4D ratio.
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Summary molecular control
Limb Initiation
FGF
FGF10 , FGF8 (lateral plate intermediate mesoderm)
prior to bud formation
FGF8 (limb ectoderm) FGFR2
FGF can respecify Hox gene expression (Hox9- limb position)
Hox could activate FGF expression Limb Specification (Fore- Hind-)
regulated by T-box genes (transcription factor)
Tbx5- forelimb
Tbx4 leg
Limb Axes
Limb Patterning- Axes Signals give positional information which is interpreted by Hox gene expression establishing
programs of differentiation.
Proximodistal Axis
Dorsoventral Axis
Anteroposterior Axis
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Summary
Limb Patterning- Axes :Wing has been used as Model of limb development as chick wingeasy to manipulate: removal, grafting, additional ARER, ZPA etc
Limb Patterning- Axes
Proximodistal Axis
AER formed by Wnt7a
then AER secretes FGF2, 4, 8
stimulates proliferation and outgrowth Dorsoventral Axis
somite provides dorsal signal to mesenchyme
which dorsalizes ectoderm
ectoderm then signals back (Wnt7a) to mesenchyme to pattern limb
Anteroposterior Axis
ZPA zone of polarizing activity
mesenchymal posterior region of limb
addition of extra ZPA duplicated digits
signal is Shh