The morphology of the rostral notochord in embryos of Ichthyophis kohtaoensis (Amphibia, Gymnophiona) is comparable to that of higher vertebrates

Abstract.

The rostral notochord plays an important role in the head development of vertebrates. Yet, in contrast to trunk notochord, the course and behavior of the head notochord shows species-specific variations. To analyze normal and abnormal development in the head region, knowledge of the normal behavior of the rostral notochord is a prerequisite. Therefore, we studied the rostral notochord of Ichthyophis kohtaoensis, not only in anatomical view a relatively unknown order of Gymnophiona, in embryos of the stages B, C, D, E according to Himstedt (1996) or stages 21, 22, 26, 31 according to Dünker et al. (2000). We described the course, form, and structure of this part of the notochord and compared it with morphological features and variations of the notochord in existing studies of higher vertebrates (Barteczko and Jacob 1999). We found that the rostral notochord of this oviparous Gymnophiona from Thailand is quite similar to that of higher vertebrates: its tendency to elongate directly in a rostral direction is prevented by the adenohypophysis as a barrier; at stage B the neurohypophysis/infundibulum takes the role of a hindrance. Light microscopic results, laser scanning micrographs, and plastic reconstructions based on serial sections showed unequivocally in stages B–E the effects resulting from this hindrance: buckling, variations in volume, screw-like torsions, undulations, deviations, and splintering. The notochord was found to lie dorsal to the developing cartilaginous basicranium. The tendency to press the dura towards the brain was conspicuous. The formation of a bursa pharyngea was likewise observed; the predisposition of several such structures exist. We suggest that the vertebrate phenotype with the appearance of preaxial structures, in contrast to the chordata phenotype, correlate with the formation of a hypophysis. Experiments have yet to prove this hypothesis.