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.

A new scenario of hypothalamic organization: rationale of new hypotheses introduced in the updated prosomeric model

“…Nevertheless, several credible images reported on such later stages show that the tip of the notochord usually contacts or approaches the mamillary pouch (e.g., Romanoff, 1960 ; Figures 85, 105, 335; diverse images in Kuhlenbeck, 1973 ; e.g., his Figure 48C). Consistently with this result, most workers describing the earliest topography of the notochord relative to the forebrain underlined a rostral end at or under the prospective mamillary pouch (e.g., His, 1893a , b ; Von Kupffer, 1894 , 1906 ; Jurand, 1974 ; Morris-Kay and Tuckett, 1987 ; Saucedo and Schoenwolf, 1994 ; Sulik et al, 1994 ; Puelles, 1995 ; Alvarez-Bolado and Swanson, 1996 ; Barteczko and Jacob, 2002 ; Bardet, 2007 ; Sanchez-Arrones et al, 2009 ). This agrees with observations of Johnston ( 1923 ) on the existence of a modified floor plate rostral to the isthmus, all the way to the mamillary area, a result which we reproduced with whole-mount histochemical labeling of an AChE-positive epichordal floor-plate strip ending at the mamillary area (Puelles et al, 1987a ).…”

Is There a Prechordal Region and an Acroterminal Domain in Amphioxus?

“…Interestingly, early comparative work on the notochord primordium had made it clear that the vertebrate notochord has its rostralmost tip under the hypothalamic mamillary area [Dohrn, 1883;Lwoff, 1894;von Kupffer, 1894von Kupffer, , 1906Sterzi, 1907;Johnston, 1909Johnston, , 1910. Modern studies have also corroborated this conclusion [Jurand 1974;Morriss-Kay and Tuckett, 1987;Sausedo and Schoenwolf, 1994;Sulik et al, 1994;Barteczko and Jacob, 2002;Puelles et al, 2012a, b;Puelles and Rubenstein, 2015]. The notochord proper is thus strictly coextensive with the neural floor plate, this being a causal relationship, since floor plate specification and differentiation is selectively induced by notochordal morphogens [Marti et al, 1995;Placzek, 1995;Ericson et al, 1996;Harland and Gerhart, 1997;Dodd et al, 1998;Placzek et al, 2000;Stemple, 2005;Sanchez-Arrones et al, 2009, 2012.…”

Imaging of nasopharyngeal cysts and bursae

“…The notochord course between the basiocciput and the basisphenoid comes into contact with the endoderm [5] at the level of the primitive pharyngeal roof (Fig. 2a).…”