Antagonism within and around the organizer: BMP inhibitors in vertebrate body patterning.

The vertebrate organizer has been of intense interest since its discovery in amphibia by Spemann and Mangold in the early 1920s (Ref. 1). The organizer was operationally defined as a piece of tissue that can induce a duplicated dorsal body axis and head when transplanted into the ventral side of a host embryo. The original Spemann and Mangold organizer experiment provided the first convincing demonstration that intercellular, or inductive, interactions affect cell fates in vertebrate embryos. Organizer mesodemL in addition to forming part of the dorsal axis and head, exerts two key inductive influences on adjacent tissues: the organizer redirects the adjacent ventrolateral mesoderm to form tissues with more dorsal characteristics (dorsalization), and the organizer induces the nervous system in adjacent ectoderm. Organizers have been described in virtually all classes of vertebrate embryos. In general, organizers are located at the site where gastrulation movements begin, and in nonamphibian species organizers correspond to the Koller's sickle in birds (from which Henson's node is derived), the embryonic shield in fish and the node in mammals. The patterning actions of organizers are also well conserved among vertebrates, and this conservation is also underscored at the molecular level, as illustrated by the consela:ed expression of an array of organizer-specific genes among vertebrates. The functions of vertebrate organizers have been reviewed recently 2-~. Here, I focus on recent data from the amphibian Xenopus, which reveal a rather unexpected twist on the mechanism whereby several organizer-specific factors mediate dorsalization and neural specification.