|Embryonic origin and further development of some anterior viscerocranial structures in the Anura
Department of Zoology, Faculty of Natural Sciences, Charles University, Prague
Viscerocranium is that part of the skull which evolved from serially homologous branchial arches of primitive jawless vertebrates. As evidenced by paleontological record, each branchial arch was originally consisting of several elements (branchials). At the same time, each branchial arch was fixed to the neurocranium by its dorsal elements (supra- and infrapharyngobranchials), whereas ventrally the arches were fixed with each other by means the unpaired basibranchial located along the midline. The main part of the arch (on each side) was represented by the epibranchial, ceratobranchial, and the hypobranchial which, together with its counterpart from the opposite side, was articulated with the basibranchial. The regular serial arrangement of the branchial arches was lost when jaws evolved from two of them – the upper jaw (palatoquadrate) in primitive Devonian fishes is a derivate of two epibranchials, the epimandibular and epipraemandibular, whereas the lower jaw (Meckel’s element) was the ceratomandibular. The branchials in those arches which were located anterior to jaws lost their original function (to support the branchial slot) and supposedly fused to surrounding structures, or were reduced or lost. This condition is preserved in the most primitive contemporary gnathostomes, for instance the Chondrichthyes.
It was discovered long time ago that these viscerocranial elements develop from the neural crest, i.e., they are derivates of neuroectoderm. This is a striking difference if compared with the neurocranial braincase which, like the vertebrae, takes its origin from sclerotomic material; hence, the vertebrate skull is of dual origin. The neurocranial braincase (but not the capsules) develops from two pairs of rod-like structures, a pair of the parachordals, and pair of cranial trabecles (trabeculae cranii). These structures may be followed in the early postembryonic development when they appear as condensations of mesenchyme which later chondrify.
In amphibians, the viscerocranium underwent further modifications associated with their transition to the life on dry land, however, these changes occurred mostly in the postmandibular series of branchial arches. There is no reason to assume that the transition to dry-land dwelling affected substantially the praemandibular viscerocranium which was already modified earlier, in connection with origin of jaws. This is why the early development of the anterior part of the amphibian viscerocranium may reflect changes which occurred during that period of vertebrate evolution which preceded origin of jaws.
In the early postembryonic development of the Anura, the very first branchial arches (i.e., NC derivates) appear simultaneously with the trabecles (i.e. sclerotomic derivates). The mandibular arch is represented by a continuous, vertical strip of mesenchyme, located perpendicular to the trabecles. This is a condition similar to other vertebrates. From this point onwards, however, anurans display strikingly different cranial development which results in their special larval jaw apparatus which cannot be found in any other vertebrates, incl. amphibians.
The anuran palatoquadrate rotates anteriorly, so it soon attains the position nearly parallel to the trabecles. Anterior to the trabecles, two rod-like structures develop (each continuous with the corresponding trabecle), called the trabecular horns (cornua trabecularum). The details of their early development remain unknown, however, it is known that they take their origin from the neural crest and, consequently, they are supposedly part of the viscerocranium. The larval jaw apparatus consists of the upper labial cartilages (suprarostrals) which are movable structures supported by the tips of the cornua, and the lower labial cartilages (infrarostrals) which are supported by Meckel’s cartilage which, in the meantime, became movably separated from the anterior (formerly lower) end of the palatoquadrate. Besides these movable elements, there were discovered also some others which later contribute to the structure of the postnasal wall (partition between the nasal capsules and orbit).
During metamorphosis of the anuran larva, the palatoquadrate rotates posteriorwards keeping, however, its connection with the postnasal wall by a developmentally complex slender rod (consisting mainly of the larval commissura quadratocranialis anterior) delimiting the orbit laterally. Accordingly, Meckel’s cartilage becomes also elongated because the jaw joint is shifted underneath the otic capsule in adult, as in other vertebrates. Larval jaws, as well as mentioned free elements either fuse with neighbouring structures or diminish and ultimately disappear.
This development is characteristic for non-pipid anurans. In pipids, one may observe some developmental differences. For instance, instead of the trabecular horns there is an internasal plate (planum internasale), the anterior margin of which serves as the upper larval jaw. The lower jaw is represented by Meckel’s elements, as in adults. The palatoquadrate does not rotate posteriorly during the metamorphosis. The overall condition in pipids corresponds to that in the Triassic temnospondyl amphibians.
It is obvious that early postembryonic cranial development of the Anura raises some questions: (1) The larval jaws (i.e., suprarostrals and infrarostrals) presumably correspond to branchials of the premandibular arches. Is it possible to identify their homologues within the original, unmodified viscerocranium of pre-gnathostomes? (2) Similarly, is it possible to determine the identity of the free elements located within the region of the adult postnasal wall (i.e., of the lamina orbitonasalis, adrostral, processus maxillaris posterior? (3) Does the commissura quadratocranialis anterior belong to the viscerocranium (does it take its origin from the NC cells)? (4) What is the posterior extent and identity of the trabecular horns (do they correspond to the branchials and if so, of which branchial arch)? (5) Are the trabecular horns homologous with the internasal plate of pipids and if so, can one be derived from the other? What was then the original condition, the plate or the horns?
It is assumed that these and other related problems may be solved principally by investigation of earlier, pre-cartilaginous stages because cartilaginous structures are already deviated from the original condition. It is therefore essential for answering the above questions whether it is possible to investigate the flow of the NC cells in the anterior, premandibular region, at the stage when they reach their ultimate location and condensate into morphologically defineable structures. It is expected that it is in these stages that one can observe the original structure of the viscerocranium of early gnathostomes. This can also help to decide to what degree the ethmoidal endocranium in the Triassic temnospondyl amphibians (which are considered immediate anuran ancestors) is derived from the original, pre-gnathostome or early-gnathostome condition.
If the principal aim of the thesis should be briefly outlined, then it should clarify the early, precartilaginous development of the premandibular anuran viscerocranium.