Dear Mark

Thanks for your answer and the reference you mentioned. However, as you said similarity in vertebrate morphogenesis is well known for us all but we can not ignore the differences specially if it comes to the mechanism during organogenesis. Yes there are similarities and also yes there are differences. I have evidence that apoptosis is clearly involved in limb morphogenesis of birds but not at all in its amphibian counterpart. The question that we should think about is: what both similarities and differences mean? Thereafter, we should reflect accordingly. 

Cheers,

Gamal

In all non-amphibian tetrapods the limb skeletal elements are laid out temporally in a proximosdistal order during development. In some (but not most) amphibians, however, some distal elements emerge before proximal ones.

Excellent difference, Stuart. Let us give it the number: 1

hope to see more differences.

Cheers

Nevertheless Gamel, there is a new paper (J. Raspopovic, L. Marcon, L. Russo, J. Sharpe. Digit patterning is controlled by a BMP-Sox9-Wnt Turing network modulated by morphogen gradients. Science Vol. 345, Iss. 6196, 566-70, August 2014), saying that the mouse limb is  n o t  developed in a proximo-distal fashion. The paper shows lots of photographs. I also have prepared a paper showing a computer simulation, also proposing that the mouse autopod (digit bones) is built synchronously. Fotos using SOX9 marker confirm a synchronous development. A definite difference between some amphibians (salamander) and land tetrapods is the capability of limb regeneration of the first. Kind regards Axel  

Thanks Axel

Limb regeneration is a different story. The fact that the mechanism during normal limb development is different in case of amphibians in comparison with other tetrapods has a very important implication with regard to the so-called Evolution. If this is added to the fact that the embryonic origin of fish paired fins is also different from its counterpart of tetrapod limbs then the whole matter called for more investigation, more thinking and perhaps more rightness.

Sounds thrilling Gamal.  Concerning vertebrate limb development please see my article "Extremitätenentwicklung" (=limb development) in the German Wikipedia. There you will find the whole bunch of key papers of the last 50 years. Some of the authors have also been focussing on amphibians, for ex. Brian Hall (ed.): "Fins into Limbs - Evolution, Development and Tranformation". University of Chicago Press, Chicago/ London 2007. Good luck Axel   

Indeed Axel, it sounds thrilling. Many thanks for the references you mentioned and of course including yours. However, it is my belief that fins transformation into limbs can merely be accepted as "Science  fiction" otherwise where are the evidences for such transformation.

Cheers

Those were great conversations. I believe that the main difference between limb development in both tetrapods ad amphibians is the mechanism of digit formation. In tetrapods it seems that apoptosis is the main process responsible for digit separation, however, in amphibians, mainly Anurans, differential growth is the dominating mechanism in digit formation with no sign of interdigital apoptosis at all. However, in some urodeles apoptosis was found to take part in this process along with differential growth. 

Be aware: Apoptosis is a late process in limb development. It is not responsible for bone patterning itself or for early digit formation. When digit separation occurs, digit bones are already perfectly built. Mammals also show apoptosis, but in mouse embryo this occurs  after  E12.0. 

Apoptosis during limb morphogenesis is well documented in mammals and birds.  The timing of occurring either apoptosis or differential growth is subsidiary object. we are discussing either the happening (or the absence) of  one or both processes in different vertebrate classes. Then we are looking to the implication of that.

Cheers

I completely agree with you Axel. I meant to say soft tissue digit separation occurs via apoptosis in tetrapods.