Embryonic Origin and Patterning of Cranial Muscle in the Mexican Axolotl ( Ambystoma mexicanum )

Abstract

Cranial muscles play critical roles in numerous and diverse functions, including feeding, respiration, facial expression and (in some groups) vocalization. Despite the conservation of their basic structure in vertebrate evolution, cranial muscles have also played a significant role in evolutionary transitions among species. Here, we employ embryonic transplantations, using GFP-transgenic axolotls as donors, to determine the mesodermal contribution to cranial bone, cartilage and muscle. Expression patterns of cranial mesoderm markers are also analyzed within developing muscles. We define the axial level where unsegmented mesoderm can contribute to head muscles in the axolotl. Analysis of the posterior limit of unsegmented myogenic mesoderm indicates close overlap with the head-trunk boundary in the somites. This may help explain some of the differences in fate maps for muscles of birds and amphibians in the posterior head. Finally, gene expression analysis reveals markers for muscle groups important to the evolution of early tetrapods, including the gill levators.