Neurulation in the Mexican salamander (Ambystoma mexicanum) : a drug study and cell shape analysis of the epidermis and the neural plate

Abstract

We analysed the neurulation movements in the Mexican salamander Ambystoma mexicanum. Embryos were exposed to colchicine or nocodazole prior to neural fold formation. Exposure to these drugs prevented the anterior neural folds from closing. Neurulation however proceeded normally in the posterior regions of the embryo. We were unable to find apically constricted cells in the neural plate of colchicine-blocked neurulae. Only rounded-up neural plate cells were present (semithin sections). This situation was typical in embryos exposed to colchicine prior to neural fold formation. Concentrations of colchicine up to 2·5 × 10−3 were not capable of blocking neurulation once the neural folds were formed. The wedge-shaped cells were present in similar numbers to those found in controls. We quantified the cell shape changes in the neural plate and in the epidermis in both controls and drug-arrested embryos. The comparison of these to classes of data shows that epidermal spreading is prevented by colchicine but only slightly affected by nocodazole. Embryos blocked in late neurulation by exposure to these drugs can resume neurulation following neural plate excision in nocodazole but not in colchicine. We conclude from this observation that the epidermis contributes to raising and closing of the neural folds. The presence of neural folds in absence of wedge-shaped cells in the neural plate is also taken as evidence that neurulation is not exclusively driven by forces generated in or acting on the neural plate. Our view on the concerted interplay of various embryonic components is illustrated in a summarizing diagram (Fig. 11).

Multi-scale finite element modeling allows the mechanics of amphibian neurulation to be elucidated

“…(3) When these cell actions occur in a coordinated way, reshaping occurs at the tissue level (figure 1(D)) (Armstrong 1989, Clausi and Brodland 1993, Veldhuis et al 2005, Keller 2006). ( 4) When these tissue movements are suitably orchestrated, morphogenetic movements arise at the embryo level (figure 1(E)) (Burnside and Jacobson 1968, Brodland et al 1996, Zamir et al 2005, and (5) Specific phenotypes are produced (Brun and Garson 1983). ( 6) Feedback to the genes and to various signaling pathways is produced by various patterns of mechanical deformation, a process described by the term 'mechanobiology' (Stoltz and Wang 2002).…”