INTRODUCTION

Modified from: Kimmel et al., 1995. Developmental Dynamics 203:253-310. Copyright © 1995 Wiley-Liss, Inc. Reprinted only by permission of Wiley-Liss, a subsidiary of John Wiley & Sons, Inc.

A staging series is a tool that provides accuracy in developmental studies. This is because different embryos, even together within a single clutch, develop at slightly different rates. We have seen asynchrony appearing in the development of zebrafish, Danio (Brachydanio) rerio, embryos fertilized simultaneously in vitro (C. Walker & G. Streisinger, in Westerfield, 1994) and incubated at an optimal temperature without crowding (28.5C, 5-10 embryos/ml). Asynchrony arises at the earliest stages, and it becomes more pronounced as time passes. Comparisons reveal more of this variability among embryos from different clutches than from within a single clutch. Genetic uniformity may alleviate but does not eliminate this problem; even embryos of a clonal strain of zebrafish (Streisinger et al., 1981) develop asynchronously.

As for other kinds of embryos, staging by morphological criteria partly resolves this problem. For example, primary trigeminal sensory neurons in the head (Metcalfe et al., 1990) and primary motoneurons in the trunk (Eisen et al., 1986) both initiate axogenesis during stages when somites appear successively along the body axis. Staging by somite number more accurately predicts where these neurons will be in their development than does staging by elapsed time after fertilization (personal communications from W. K. Metcalfe and J. S. Eisen). Recording one's experiments with reference to a staging series provides a good way to ensure reproducibility, and to allow subsequent incorporation of new observations and details. A series based on morphology can also facilitate communication; the descriptor "18-somite embryo", has more meaning than "18-hour old embryo", particularly in cross species comparisons.

An earlier staging series for zebrafish, although less complete than the present one, fairly accurately portrays the first third (or first day) of embryonic development, and includes useful sets of photographs (Hisaoka & Battle, 1958; Hisaoka & Firlit, 1960). Warga & Kimmel (1990) briefly described stages of the blastula and gastrula. Preliminary versions of this series were circulated and published in The Zebrafish Book (Westerfield, 1994). The present version substantially revises, corrects, and expands the earlier ones. Hard copies of the figures can be obtained from Bonnie Ullmann: ullmann@uoneuro.uoregon.edu.