COMPARISON OF STAGING SYSTEMS FOR THE GASTRULATION AND EARLY NEURULATION PERIOD IN RODENTS - A PROPOSED NEW SYSTEM

Because there is no standard developmental staging system for the early postimplantation period of rodent embryos, investigators must now choose between a variety of systems that differ significantly. We have reviewed many of these staging systems and have summarized the ambiguities within them and the inconsistencies among them. In order to compare systems, we first obtained a consensus of the order of developmental events from the literature, and then attempted to fit existing systems into this order taking into account inconsistencies in terminology and blurred borderlines between stages. We were able to do this for most systems but not all because some were too divergent. We found that inconsistencies in definition of some terms, such as "primitive streak stage" and those used to describe the early neurulation process (neural plate, neural groove, neural folds, and head fold) cause much confusion. In order to develop an unambiguous system which can be used by all investigators, we propose to modify Theiler's system, which is one of the most commonly used systems but is not defined precisely during the early postimplantation period. We suggest making subdivisions of the original stages as follows: 1) stage 8 into 8a and 8b, by the degree of extension of the proamniotic cavity into the extraembryonic region; 2) stage 10 into 10a and 10b, by the completion of amnion formation; 3) stage 11 into 11a, 11b, and 11c, by the appearance of neural folds and foregut pocket. After Stage 12, the number of somite pairs can be used to precisely stage embryos. These modifications to Theiler's system allow much more precise staging of early postimplantation embryos, allow it to be used as a standard by all investigators.