Lymphocytes of the toad Xenopus laevis have the gene set for promoting tadpole development.

Nuclear transplantation experiments show that differentiated cells, such as lymphocytes, from the adult frog can express the genes necessary for tadpole development. The transplanted cells were proven to be lymphocytes by immunological methods. The origin of the tadpoles that developed after lymphocyte nuclei injections was ascertained by a karyotypic marker.

The eyeless mutant Mexican salamander (Ambystoma mexicanum): evidence for an imbalanced anteroposterior morphogenetic system.

Prospective anterolateral neural fold was grafted from normal axolotls into the posterior neural fold region (statocyst area) of eyeless mutant hosts. These unilateral anteroposterior grafts stimulated bilateral eye formation in the eyeless mutant at a rate of 79%. Replacing the statocyst area of mutants with the statocyst area from normals stimulated bilateral eye formation in 49% of the cases. Grafting of prospective anterolateral neural fold between normals and mutants or excising the statocyst region of mutants, had no effect. The results are interpreted on the basis of a hypothetical anteroposterior morphogenetic system that might be out of balance in the mutant.

Neural fold and neural crest movement in the Mexican salamander Ambystoma mexicanum.

In studies of amphibian neurulation, the terms "neural ridge," "neural fold," and "neural crest" are sometimes used as synonyms. This has occasionally led to the misconception that grafting of the neural crest is equivalent to grafting of the neural fold. The neural fold, however, is composed of three parts: the neural crest, prospective neural tube tissue, and epidermis. In order to investigate how these neural fold components move during neurulation, time-lapse photography, electron microscopy, and grafting were performed. Ambystoma mexicanum embryos were photographed during neurulation at regular intervals. The photographs were analyzed to find the position of those cells at beginning of neurulation that end up on the line of fusion as the neural folds close. Posteriorly, these cells are already on the emerging neural fold. In the anterior neural folds, however, these cells are located in the lateral epidermis. Electron microscopy of the neural folds confirms the presence of epidermis. To follow the movement of the cells differentiating into melanophores (neural crest), neural fold parts were grafted into albino hosts. The crest cells differentiating into melanophores following ectopic grafting are located in the flank of the neural fold that is in contact with the neural plate. In grafts from the outside (distal) flank, no melanophores developed. Semithin sections show that the third part of the neural fold consists of apically constricted cells known to differentiate into neural tissue. Because the neural folds consist of epidermis, neural tissue, and neural crest, neural fold and neural crest cannot be used as synonyms.

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

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 x 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).