The Presence of Identified Neurons Allows Analysis of Neural Differentiation at the Single Cell Level

In addition to genetic studies, there are other advantages to using the zebrafish for studies of early development. Principal among these is the presence in the developing nervous system of identified neurons, cells that differentiate in the same place and at the same time in every embryo (24). The identified neurons that have been most heavily studied, in terms of their time of origin, the mechanisms leading to their determination, and subsequent differentiation are the primary motor neurons of the spinal cord (25,26), and the reticulospinal neurons of the hindbrain (27-29). The presence of identified neurons within the central nervous system allows analysis at the level of the single cell and provides a level of

Fig. 2. (opposite page) Stages in the development of the zebrafish. This figure is adapted, with permission of the authors, from the definitive zebrafish staging series published by Kimmel et al. (32). (A) Two-cell staged embryo showing the large yolk cell as an incomplete cleavage. (B) The blastula stage illustrated with an embryo at 3.3 h of development. The ever-smaller blastomeres are piled up on the yolk cell. (C) Gas-

Fig. 2. (opposite page) Stages in the development of the zebrafish. This figure is adapted, with permission of the authors, from the definitive zebrafish staging series published by Kimmel et al. (32). (A) Two-cell staged embryo showing the large yolk cell as an incomplete cleavage. (B) The blastula stage illustrated with an embryo at 3.3 h of development. The ever-smaller blastomeres are piled up on the yolk cell. (C) Gas-

analysis not possible with higher vertebrates. An example of such a study is the identification of rhombomere respecification in the hindbrain following retinoic acid treatment of the gastrula (30). In this study, the reticulospinal complex, which is illustrated in Fig. 1, (shown on p. 433) was used to identify respecifi-cation of the Mauthner cell, normally present in rhombomere 4, in rhombomere 2 (30).

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