Boundary formation and compartition in the avian diencephalon.

The diencephalon comprises three functionally distinct regions: synencephalon, dorsal thalamus, and ventral thalamus. Patterning of the diencephalon has been proposed to involve subdivision of its anteroposterior axis into segments, neuromeres or prosomeres (Bergquist and Kallen, 1954; Vaage, 1969; Figdor and Stern, 1993; Rubenstein et al., 1994; Redies et al., 2000; Yoon et al., 2000). However, the number and sequence of diencephalic neuromeres, or even their existence, are uncertain. We have examined the proposed subdivisions by morphology, gene expression, acquisition of boundary-specific phenotypes, and cell lineage restriction. We find that at stage 16 in chick the diencephalon is divided into synencephalon and parencephalon. The synencephalon exhibits neuromeric morphology, expresses Prox, and acquires neuromere boundary properties at its interface with both the midbrain and the parencephalon. Although the mesencephalic/synencephalic boundary restricts cell mixing, the synencephalic/parencephalic boundary does not. Similarly, there is no lineage restriction between the parencephalon and the more rostral forebrain (secondary prosencephalon). Subdivision of the parencephalon into ventral and dorsal thalamus involves the formation of a narrow intraparencephalic territory, the zona limitans intrathalamica (zli). This is correlated with the acquisition of cell lineage restriction at both anterior and posterior borders of the zli, the appearance of boundary-specific properties, and Gbx2 and Dlx2 expression in dorsal thalamic and ventral thalamic territories, respectively. At stage 22, the synencephalon is divided into two domains, distinguished by differential gene expression and tissue morphology, but associated with neither a boundary phenotype nor cell lineage restriction. Our results suggest that the diencephalon does not have an overt segmental pattern.