Development of fore‐ and hindlimb muscles in frogs: Morphogenesis, homeotic transformations, digit reduction, and the forelimb–hindlimb enigma

Abstract

Here we provide the first detailed description, based on immunohistochemistry and dissections, of the limb muscle development in the direct developing frog Eleutherodactylus coqui. We compare E. coqui with other tetrapods and discuss our results in a broad evolutionary and developmental context to address some major questions concerning the origin, evolution, and ontogeny of the tetrapod limbs. Our observations and comparisons: (1) support the "in-out" developmental mechanism of the appendicular pectoral muscles; (2) show that the protractor pectoralis and its amniote derivatives trapezius and sternocleidomastoideus clearly develop, anatomically, from the branchial muscles; (3) corroborate that the similarity between the forearm/hand and the leg/foot muscles in tetrapods is due to derived homoplasic events that occurred during the fins-limbs transition and not due to serial homology; (4) lend some support for the hypothesis that the morphological transformation of the anuran tibiale and fibulare represents a distal shift in the zeugo-autopodial border; (5) provide evidence that the identity of the tetrapod hand and foot muscles is mainly related to the topological position of the digits to which they attach; and (6) for the first time, show that apart from a proximo-distal morphogenetic gradient there is also an ulno-radial/fibulo-tibial gradient within the development of the fore- and hindlimb muscles and a dorsoventral gradient within the ontogeny of the hindlimb (but not forelimb) muscles of the frog E. coqui; the two latter gradients are seen in the ontogeny of amniotes such as chickens but are markedly different to those seen in axolotl regeneration and ontogeny.

Development of fore‐ and hindlimb muscles in GFP‐transgenic axolotls: Morphogenesis, the tetrapod bauplan, and new insights on the Forelimb‐Hindlimb Enigma

“…As explained in the Results section, the fact that the axial pectoral muscles are often deep girdle muscles probably explains why the levator scapulae and serratus anterior could not be seen in the pictures obtained by us of the developing muscles of axolotls from stages 46 to 54. As our ontogenetic study of frog limbs (Diogo and Ziermann, ), the present study of axolotls also supports Valasek et al's () “in–out” mechanism because at earlier ontogenetic stages the appendicular pectoral muscles start to develop far from the midline, at the level of the proximal region of the arm; only later in development they extend medially to cover a substantial part of the ventral and dorsal surfaces of the thoracic region (compare, e.g., Fig. with Fig.…”

“…However, it should be noted that a dorso‐ventral gradient was reported in Kardon's () study of the ontogeny of the hindlimb muscles of chickens. Moreover, our recent ontogenetic study of the frog Eleutherodactylus coqui (Diogo and Ziermann, ) has shown that both limbs display a proximo‐distal muscle morphogenetic gradient and that the hindlimb displays a dorso‐ventral gradient, as reported in chickens by Kardon (). Also, instead of a radio‐ulnar/tibio‐fibular gradient as seen in the regeneration and ontogeny of the musculature of both limbs of axolotls, there is mainly a ulno‐radial/fibulo‐tibial gradient in the ontogeny of the musculature of the fore‐ and hindlimbs of this frog.…”

“…In the text provided below we will explicitly follow a similar order, and discuss some of the same issues, that were discussed in our recent paper on muscle limb development in frogs (Diogo and Ziermann, ). This will allow us to directly compare the results obtained in urodeles and anurans and to point out if the data obtained in the present study supports, or not, the major evolutionary and developmental hypotheses supported by the data obtained from the study of frogs.…”

“…The main goals of the present publication are therefore: (1) to set the basis for future regenerative, developmental, and morphogenetic studies on the limbs of axolotls and other tetrapods; (2) to analyze the topological and morphogenetic sequence of axolotl limb muscle ontogeny, that is, which muscles develop first and how are they related to each other three‐dimensionally (e.g., is there a tibio‐fibular, a proximo‐distal, and/or a ventro‐dorsal gradient? ); (3) to discuss, based on comparisons with the data provided in the literature and obtained in our recent studies of axolotl limb regeneration (Diogo et al, , submitted for publication) and frog limb development (Diogo and Ziermann, ), broad developmental and evolutionary topics such as the evolution and morphogenesis of tetrapod limbs and the supposed serial homology between the fore‐ and hindlimbs.…”

Characteristic tetrapod musculoskeletal limb phenotype emerged more than 400 MYA in basal lobe-finned fishes

“…Regarding the muscular anatomy of lobe-finned fishes, the major novel contributions of this work are 1) description of new muscles; 2) re-appraisal of evolutionary origin (e.g., from ventral/abductor vs. dorsal/adductor masses) and identity of previously described muscles; and 3) first comprehensive comparisons of pelvic and pectoral appendages (PELA, PECA) among these and other fish and in tetrapods, leading to proposal of new names, evolutionary origins and one-to-one homology hypotheses for all muscles of these taxa (see SI for more details, in particular between the differences between our work and previous studies). We discuss our results in the context of the anatomical, developmental, and paleontological literature, including recent papers on the soft tissues of adult dipnoans1920 and coelacanths14, of phylogenetically basal extant bony fishes such as Polypterus 21, and works on appendicular muscle development in most gnathostome clades111222232425262728293031323334.…”