Chondrification and Character Identification in the Skull Exemplified for the Basicranial Anatomy of Early Squamate Embryos

Abstract

The neurocranium of vertebrates is mainly derived from early cartilaginous anlagen, the so-called chondrocranium, the base of the future skull. Two initial bar-shaped and paired chondrifications flank the notochord, the rostral trabecles and the caudal parachordals. In most reptiles, there is an additional component, the transverse acrochordal, which is placed between trabecles and parachordals. All these elements compose the base of the future chondrocranium. There are several drastically different hypotheses concerning the development and interrelationship of these elements. We reexamined the basicranial development in four squamates and found that all species show very similar conditions of early chondrocranial development. The anterior part of the notochord is not embedded into the basal plate as it was previously reported. It remains free. The medial edges of the parachordals form the lateral walls of the basicranial fenestra. Only the posterior portions of the parachordals fuse and form the basal plate. The space in-between the parachordals is filled with a thin layer of cells, which never chondrify. The anterior tips of the parachordals later fuse with the posterior edge of the acrochordal, which ultimately delimitates, as crista sellaris, the basicranial fenestra anteriorly. We consider the observed processes a common development at least in lizards and review a variety of methodological approaches and differences in data interpretation as reasons for the anatomical differences reported in the literature. Moreover, based on our data we argue that the acrochordal is of mesodermal origin, which coincides with results of fate map experimental studies.

Tracing the developmental origin of a lizard skull: Chondrocranial architecture, heterochrony, and variation in lacertids

“…The posterior ends of the trabecles chondrify and become completely fused with the cartilaginous acrochordal (= crista sellaris). Thus, the triangular pituitary fenestra is formed and is limited by the trabecles anterolaterally and by the acrochordal posteriorly (Yaryhin & Werneburg, ). Where trabecles and acrochodral fuse, a small notch is formed medially and through which the internal carotid artery enters the intracranial space.…”

“…De Beer () and Kamal and Abdeen () used different methodological approaches, the limitations of each were already discussed in our previous paper (Yaryhin & Werneburg, ) and include (a) different section thicknesses, (b) different staining methods, (c) different stage resolutions of developmental series, and (d) the use of diverging terminology. All of them resulted in major differences in the interpretation of chondrocranial element identity.…”

“…The first detailed study tracing chondrocranial development in L. agilis was published by De Beer (), and aside from numerous controversies, he provided a strong anatomical background and influenced craniogenetic studies for the next 50 years (see Bellairs & Kamal, ; Evans, ). Many researchers working on chondrocranial development in the past, including De Beer (), used different methodological approaches to study embryos, which finally resulted in obvious misinterpretations of several structures, particularly in the early developing skull (Yaryhin & Werneburg, ). For this reason, we decided to restudy the development of the skull of L. agilis in great detail using traceable methodologies for different periods of development.…”

Variation in the formation of crista sellaris and basisphenoid in the skull of the grass snake Natrix natrix embryos (Serpentes, Colubridae)

“…It led the researchers to the statement about resorption of the acrochordal cartilage in the middle of embryogenesis, and its later reformation. Yaryhin and Werneburg () discussed this result by pointing out the drawbacks of used methods. The cases, described in L. agilis and P. bicolor , as well as our data on the intraspecific variation in N. natrix , suggests that variation is also possible in the formation of this structure in lizard chondrocranium.…”

“…It is curious how de Beer claimed that the transverse bar (acrochordal cartilage) in L. agilis appeared as a result of the merging of the processes deriving from the places of the cranial trabeculae and parachordals confluence. The similarity of the patterns of the acrochordal cartilage formation in the studied lacertid lizard (Yaryhin & Werneburg, ) led to the conclusion that de Beer's description was considered a mistake, due to the imperfection in the methods of research and other reasons (Yaryhin & Werneburg, ). However, it is well possible that de Beer was not mistaken.…”