Changing the axis changes the perspective

Among the studies reported in this issue are ones by Lane and Sheets (2002) and by Kumano and Smith (2002) that concern the reassignment of dorsoventral and anteroposterior axes on the fate map of the Xenopus embryo. The two studies complement one another, with Lane and Sheets (2002) emphasizing the assignment of the anteroposterior axis to what had previously been called the dorsoventral axis and with Kumano and Smith (2002) emphasizing the assignment of the dorsoventral axis to a dimension not previously assigned, although aligned with the animal–vegetal axis. They dwell mostly on axis designations for the mesoderm rather than for ectoderm or endoderm, on the grounds that mesoderm determines the fates and axes of the other two germ layers. This is true in two ways. (1) Within the mesoderm is Spemann’s organizer, which is composed of a head organizer (the prospective prechordal plate/head mesoderm territory) and a trunk–tail organizer (the prospective notochord territory). During gastrulation and neurulation, the ectoderm is induced by the organizer to form the central nervous system. Recent work on Bmps, Bmp-antagonists, and the default pathway for neural development has illuminated the neural–epidermal bipotentiality of the ectoderm and the essentiality of the organizer for neural fates, their orientation, and their placement in the embryo. Later, the organizer induces the floor plate of the neural tube, which is a source of signals for the dorsoventral patterning of the tube. Mesoderm itself is induced by the organizer to form somites running the length of the body and to form the heart and tail bud. Later, the organizer induces the sclerotome portion of the somites. Endoderm is induced to form anterior gut regions and the hypocord. Most aspects of the dorsoventral and anteroposterior axis of the embryo depend on the organizer’s inductions and, therefore, its location. And (2) the organizer mesoderm has a major role in axis formation by way of its morphogenetic activities, particularly the convergent extension morphogenesis by cells of the prospective notochord territory, and the migration of cells of the head mesoderm territory. Thus, if researchers want to emphasize the developmental mechanisms of axis formation, they should focus on the mesoderm, the germ layer most involved, and they should name its axes as accurately as possible. I support the efforts of Lane and Sheets (2002) and Kumano and Smith (2002) to make the designations useful and accurate rather than historical. It will take many of us, myself included, a while to speak of “anterior and posterior” marginal zones rather than “dorsal and ventral” marginal zones, or to speak of cortical rotation and beta-catenin accumulation establishing the anteroposterior axis rather than dorsoventral axis, or to designate various treated embryos as “posteriorized” rather than “ventralized,” and “anteriorized” rather than “dorsalized,” or to rename Xvent genes as Xpo genes. The old dorsoventral axis designation may retain some usefulness in nonmechanistic circumstances. For example, if you mark the egg surface with a spot of Nile blue dye halfway from the animal pole to the equator on the same side as the dark sperm entry point, the mark will generally show up in the ventral epidermis of the tadpole. If you apply the spot at the same level in the animal hemisphere on the side opposite the sperm entry point, the mark will in general show up as a streak in the centerline of the neural plate. Because the neural tube is dorsal and the belly epidermis ventral, this makes a dorsoventral axis for the ectoderm. Fate maps of the ectoderm (not mesoderm) confirm this assignment. We and others have used such convenient marking of fertilized eggs for years to predict where the dorsal midline will be in the tadpole. Much of the old work with marking was of this gross predictive sort. But making convenient predictions about the embryo’s final orientation is a long way from making analytical statements about mechanisms of axis formation. Lane and Sheets (2002) review the designations of axes in the old literature. The old descriptive predictive language, which was suitable for ectoderm, did not change as mechanistic interpretations moved to mesoderm formation and the inductive-morphogenetic activities of mesoderm. The fate map of mesoderm is oriented differently from that of ectoderm, and so a different set of axes is needed. Figure 1B is meant to help the reader visualize the new axes in the mesoderm of the Xenopus late blastula