Chapter 16- Tetrapod limb development “It is one thing to differentiate the chondrocytes and osteocytes that synthesize the cartilage and bone matrices; it is another thing to produce those cells in a temporal- spacial orientation that generates a functional bone.” “How is it that the fingers form at one end of the limb and nowhere else?” “How is it that the little finger forms at one edge of the limb and nowhere else?” We will use limb development to examine the larger question of morphogenesis

5 weeks 11 weeks 5 weeks 7 weeks11weeks Feet Hands Human development

Morphogenic rules These rules appear to be the same in all tetrapods Example- Grafted mammalian limb bud will direct formation of chick limbs These rules must follow a _______________ coordinate system Dimension 1. ________________ (hip-toe) 2. _________________ (thumb-pinky) 3. _________________ (knuckle-palm) Proteins families involved _______________ _________

Fig Hox genes are organized in a _______________ that concurs with _________________ structures This is referred to as the ____________ Recall from Chapter 11- Hox genes dictate _______________ axis development

Also recall that ____________ has a profound effect on development by activating _____gene expression Structure of retinoic acid (not in textbook) Fig Recall amphibian development (Ch. 10) RA Retinoic acid activates mammalian ____genes Wild-type mouse embryo RA-treated mouse embryo Lacks all distal _______ Hox gene Retinoic acid bind a _______, then the complex binds promoter of a ________ RA receptor RA

Another unusual observation- Experiment- Amputate tadpole tail, then soak the stump in ___ Limb development initiates from __________ of _____________ of _____________ mesoderm and from ___________ Fig mesoderm lineages Somatic layer of ___________ mesoderm _______ How does a _____develop? Limb bud Result- Several _____ develop

What dictates forelimb from hindlimb? Transcription factor ____ is expressed in mouse ________ Transcription factor ____ is expressed in mouse _________ Loss of one copy of _____ gene in humans results in __________________ If Place FGF-soaked bead near forelimb bud, generate a _____ ______ If Place FGF-soaked bead near hindlimb bud, generate a __________________ Observations- Thus, FGF _______ to make a limb, but tbx5 and tbx4 instruct ______ of limb If Place FGF-soaked bead between hindlimb and forelimb buds, generate a ___________ Normal FGF bead inserted Fig. 16.5

Fig. 16.5F -Chimera limb from FGF bead placed between limb buds- Fig. 16.5E Extra wing from FGF bead placed near forelimb bud The apical ectodermal ridge (AER) is a critical structure

The _______ ____________ (AER) is a critical structure AER removed AER duplicated AER replaced with leg mesoderm AER replaced with other mesoderm AER is required for: 1)______________, 2)_______________ axis formation, 3)______ axes specification Forelimb AER experiments Fig Generating the ______________ axis Limb devel. ceases Duplicate wing chimera Limb devel. ceases AER replaced by FGF bead normal

Hox genes specify two things- 1. Where the limb _______ (recall chapter 11) 2. Whether a cell will become a ________ (humerus), ________ (Ulna and radius) or an __________ (metacarpels and digits) Fig Example- If KO Hoxa-11 and Hoxd–11 in mouse, no _________________ Fig Example- Humans with defective HOX-13 display ________________ Generating the _____________________ axis Ulna, Radius Humerous Scalpula Metacarpels Digits

What dictates thumb-pinky orientation?? 1. A group of cell located near ____________ region of limb bud- called __________________________ (ZPA) directs polarity Specifying the anterior-posterior limb axis the proximal-distal axis 2. These cells in the ZPA express __________________ (Shh) If place Shh bead in anterior limb bud… …then observe second set of __________ Extra set Normal 3. Sonic hedgehog (Shh) activates __________ genes Hoxd genes specify _____________ orientation Fig Evidence

Specifying the _____________ limb axis The surrounding ectoderm signal dorsal-ventral axis _____ activates ______expression in mesenchyme _____ is required to specify ______cell fate Lmx1 mutant humans have loss of _________, ________________ May also have “____________” All three axes are ________________ Ectoderm secretes ____…..which maintains ____ expression in ___....which activates ___ genes in limb bud....which specifies _____________ limb axis.