Introduction to the development and evolution of limbs in tetrapod vertebrates : continuation and conclusion

In this second lesson, after a brief review of the mechanisms leading to limb positioning and initiation of growth, the phenomenon of sustained growth and extension of limb buds and the molecular basis of their morphogenetic organization into three major segments are discussed, in particular the existence of retroactive regulatory loops that ensure both correct limb sizing and termination of growth. Next, the genetic bases of the different polarities (proximo-distal, antero-posterior and dorso-ventral) are described, each time emphasizing the key role of certain genes. Finally, the essential question of the often significant differences between forelimbs and hindlimbs is discussed, from a developmental and evolutionary perspective, and the important function of the PITX1 gene in this context is evoked. The lecture concludes with a reminder of the notions of "sequence enhancer" and "pleiotropy", two essential elements to be integrated for the rest of the lecture.