发育生物学：VI 胚层发育与器官系统发生（II）

1、Developmental Biology,Chapter 6: Organogenesis development of the vertebrate limb,The embryonic chick wing,The vertebrate limb development is a particularly good system in which to study pattern formation during organogenesis (I),The vertebrate limb is a complex organ with three developmental axes.

2、The limb development is a particularly good system in which to study pattern formation The proximo-distal axis running from the base of the limb to the tip The antero-posterior axis running parallel with the body AP axis The dorso-ventral axis running from the back of the hand to the palm Since the

3、limbs, unlike heart or brain, are not essential for embryonic life. Thus, the developing limbs have good advantages in studying pattern formation in organogenesis: One can experimentally remove or transplant parts of the developing limb orgenerate limb-specific mutants without interfering with the v

4、ital processes of the organism,The vertebrate limb development is a particularly good system in which to study pattern formation during organogenesis (II),The limb is a good model for studying cell induction within a structure containing a large number of cells, and for elucidating the role of inter

5、cellular signaling in development. Recently, particular signal proteins have been identified to function in patterning along each of three limb axes: FGF family in Pr-D axial patterning Sonic hedgehog (Shh) protein in A-P axial patterning Wnt7a in D-V axial patterning,The vertebrate limb develops fr

6、om a limb bud,AER: apical ectodermal ridge,3 The limb development 3.1 Specification and formation of the limb bud 3.2 The limb outgrowth and generation of the proximal-distal axis 3.3 The anterior-posterior axial patterning 3.4 Patterning the dorsal-ventral axis 3.5 Coordinating the three axes 3.6 D

7、evelopment of the digits,Development of the vertebrate limb,3 The limb development 3.1 Specification and formation of the limb bud 3.2 The limb outgrowth and generation of the proximal-distal axis 3.3 The anterior-posterior axial patterning 3.4 Patterning the dorsal-ventral axis 3.5 Coordinating the

8、 three axes 3.6 Development of the digits,Development of the vertebrate limb,The vertebrate limb develops from a limb bud,AER: apical ectodermal ridge,Proliferation of mesodermal cells from the lateral plate mesoderm causes the limb bud to bulge outward,Limb bud Ectodermal tissue: the outer layer Ce

9、lls from the lateral plate mesoderm: skeletal elements Cells from the somites: the limbs musculature,FGF10 secreted from the lateral plate mesoderm cells is the signal for limb bud induction,A: FGF10 becomes expressed in the lateral plate mesoderm of the limb bud-forming region B: Ectopic expression

10、 of FGF10 beneath the flank ectoderm induces extra limb,FGF10,Ectopic limb,Knock-out of FGF10 in the mouse embryo results in embryos lacking the limb bud,FGF10 initiates the limb bud-forming interactions between the ectoderm and the mesoderm via dual signaling induction,A: FGF10 is originally expres

11、sed throughout the lateral plate mesoderm B: FGF10 becomes restricted to the limb bud-forming regions, controlled by Wnt 2b and Wnt 8c respectively (via modulating FGF10 protein stability) C: Dual induction between FGF10 in the mesoderm and FGF8 in the apical ectoderm ridge to maintain the mesenchym

12、e cells in the limb bud proliferating.,Molecular model for the initiation of the limb bud in the chick between 48 and 54 hours of gestation,Limb bud formation: the mesenchyme cells from lateral plate mesoderm proliferate to accumulate under the ectodermal tissue for forming a circular bulge,3 The li

13、mb development 3.1 Specification and formation of the limb bud 3.2 The limb outgrowth and generation of the proximal-distal axis 3.3 The anterior-posterior axial patterning 3.4 Patterning the dorsal-ventral axis 3.5 Coordinating the three axes 3.6 Development of the digits,Development of the vertebr

14、ate limb,AER: Apical ectodermal ridge PZ: Progress zone,The AER is required for both the outgrowth of limb and the correct proximo-distal patterning,FGF8 is expressed in the apical ectodermal ridge (AER),FGF8 can functionally substitute for the AER,The AER: a major signaling center for the developin

15、g limb, and maintains the mesenchyme in a proliferating phase that enables the Pr-D growth of the limb The mesenchyme cells: induce and sustain the AER, and determine the type of limb to be formed.,The proximal-distal growth of the limb bud involves interactions between the AER and the underlying me

16、senchyme,Role of the AER or the PZ (progress zone) in specifying the proximal-distal axis? The mechamisms underlying the PD axis specification and patterning?,How is development of the proximal-distal axis accomplished during the limb outgrowth ?,An early wing-bud progress zone was transplanted to a

17、 later wing bud A late wing-bud progress zone was transplanted to an early wing bud,Extra proximal structure,Lack of intermediate structures (digits emerge from the humerus without an intervening ulna/radius),Specification of proximal-distal axis is controlled by the progress zone mesenchyme, rather

18、 the AER (I),The following two transplantation experiments indicate that the PZ controls the specification of the proxiomal-distal axis of the developing limbs,Specification of proximal-distal axis is controlled by the progress zone mesenchyme, rather the AER (I),Older AER combined with younger meso

19、derm generates normal limbs, rather than producing limbs with deletions in the middle. Younger AER combined with older mesoderm generates normal limbs, rather than producing duplications of structures.,The following two transplantation experiments do not support the notion that the AER specifies the

20、 proxiomal-distal axis of the developing limbs,Two proposed models for the mesodermal specification of the Pr-D axis of the limb,a: the timing mechanism, the cell fate being determined by how long they remain in the progress zone b: predetermination in early bud,Hox genes specify the Pr-D axis of th

21、e limb,Hox genes specify the Pr-D axis of the limb,Hox genes specify the Pr-D axis of the limb,Loss of function mutations in Hoxa-11 and Hoxd-11 lead to lacking the ulna and radius of the forelimbs,Human synpolydactyly (many fingers joined together) syndrome results from homozygosity for a mutation

22、at the Hoxd-13 loci,A: Wild-type mouse forelimb B: Forelimb of mouse doubly mutant for Hoxa-11 and Hoxd-11,3 The limb development 3.1 Specification and formation of the limb bud 3.2 The limb outgrowth and generation of the proximal-distal axis 3.3 The anterior-posterior axial patterning 3.4 Patterni

23、ng the dorsal-ventral axis 3.5 Coordinating the three axes 3.6 Development of the digits,Development of the vertebrate limb,Zone of polarizing activity (ZPA) can specify the limb pattern along the anterior-posterior axis,The zone of polarizing activity (ZPA) in the posterior limb bud has organizing

24、properties that are almost as striking as those of the Spemann organizer in amphibians,Sonic hedgehog (Shh) is expressed in the ZPA,Shh,Shh,Sonic hedgehog (Shh) defines the signaling activity of the ZPA for development of the AP axis,Sonic hedgehog (Shh) forms a morphogen (signaling) gradient that s

25、pecifies pattern along the limbs anterior-posterior axis,The molecular basis for specification of ZPA,FGF8 in the AER induces the mesenchyme expressing the transcription factors Hoxb-8 and dHAND in the posterior region of limb bud to express Shh, thereby forming the ZPA,3 The limb development 3.1 Sp

26、ecification and formation of the limb bud 3.2 The limb outgrowth and generation of the proximal-distal axis 3.3 The anterior-posterior axial patterning 3.4 Patterning the dorsal-ventral axis 3.5 Coordinating the three axes 3.6 Development of the digits,Development of the vertebrate limb,The dorsal-v

27、entral polarity of the limb bud is determined by the overlying ectoderm,If the ectoderm is rotated 180 with respect to the limb bud mesenchyme, the dorsal-ventral axis is partially reversed The Wnt7a gene is specifically expressed in the dorsal ectoderm (but not ventral ) of chick and mouse limb bud

28、s, suggesting a role of Wnt7a in specifying dorsal-ventral polarity,Loss of Wnt7a function leads to disruption of the dorsal-ventral patterning of the limb,Vt: the ventral tendons（肌腱）,Vp: the ventral footpad（脚垫）,A: Wild type embryonic mouse forelimb paw B: The mice deficient for Wnt7a gene,dorsal,ve

29、ntral,The molecular mechanisms underlying the Wnt7a-mediated DV patterning of the limb,Wnt7a in the dorsal ectoderm,Lmx1 (a transcription factor) in the dorsal mesoderm,Specify the dorsal cell fates in the limb,3 The limb development 3.1 Specification and formation of the limb bud 3.2 The limb outgr

30、owth and generation of the proximal-distal axis 3.3 The anterior-posterior axial patterning 3.4 Patterning the dorsal-ventral axis 3.5 Coordinating the three axes 3.6 Development of the digits,Development of the vertebrate limb,Molecular interactions that coordinate limb growth and patterning along the three limb axes,In many Wnt 7a mutant mice, posterior digits are lacking, suggesting that Wnt 7a is also required for normal antero-posterior patterning,3 The limb de