Transient appearance of the epithelial invagination in the olfactory pit of chick embryos.

In this study, immunohistochemical analysis has been performed using neuronal markers (GAP43, NCAM and PGP 9.5) to characterize the epithelial invagination in the medial wall of the olfactory pit in the chick embryos. At stages 26-27, the epithelial invagination was primarily composed of characteristic round-shaped cells, which were negative for neuronal markers. These cells were also found in the medial wall of the olfactory pit at stage 24, whereas the epithelial invagination was not observed at any stages other than stages 26-27. The possible relationship between the round-shaped cells and the migratory cells is discussed.

Differential expression of GAP-43 protein in the rostral brain neurons of early chick embryos.

Brain development is composed of several processes, which are chronologically and mechanistically overlapping each other. However, the process of the earliest neural circuit formation in the rostral brain is less understood compared with other processes in brain development, in part because of the lack of appropriate molecular markers. Accordingly, the identification of molecular markers for nerve cells may accelerate the detailed analysis of neural development. Growth associated protein 43 (GAP-43) is a major growth cone protein that regulates F-actin dynamics, and it has been often used as a marker for developing neurons. To test whether GAP-43 can be used as a general marker for developing neurons in chick early embryos, we analyzed the expression pattern of GAP-43 protein in the brain. While the majority of the neurons were GAP-43 positive, the earliest neurons in the dorsal mesencephalon (future tectum) were GAP-43 negative. However, a subset of the GAP-43 negative neurons became positive at later stages. Such a difference in the expression of GAP-43 protein may contribute to the precise patterning of the neural circuits in the mesencephalon in the subsequent development. The earliest neurons in the telencephalon, which belong to the terminal nerve (TN), were also GAP-43 positive. Since the development of TN is poorly understood compared to other cranial nerves, GAP-43 could help the detailed analysis of the development of TN as the marker.

Possible role of Hes5 for the rostrocaudal polarity formation of the tectum.

The alar plate of the mesencephalon differentiates into the optic tectum. Retinal fibers project to the tectum topographically in a retinotopic manner. Engrailed (En) is responsible for the tectum polarity formation and regionalization. Former study indicated the presence of the molecule whose expression is repressed by En and that represses the isthmus-related gene expression. To isolate such molecules, we constructed a subtracted library between cDNA population of the normal rostral mesencephalon and of the rostral mesencephalon that misexpresses En2. From the library, we isolated cHes5, a chicken homolog of Drosophila hairy/Enhancer of split. cHes5 begins to be expressed in the rostral part of the E2 mesencephalon, and spreads to caudal mesencephalon by E3. To our expectation, cHes5 expression was repressed by En2. Furthermore, misexpression of cHes5 in the mesencephalon inhibited expression of ephrinA2, a marker of caudal mesencephalon. An active repressor form of Hes5, which is a chimeric molecule of Hes5 and repressor domain of En2, showed a similar but more severe phenotype. The results indicate that Hes5 is regulated by En and is responsible for rostral identity of mesencephalon by repressing ephrinA2.

Predominant expression of a cytoskeletal actin gene in mesenchyme cells during embryogenesis of the ascidian Halocynthia roretzi.

During ascidian embryogenesis mesenchymal cells proliferate and have no known function until after metamorphosis, when they give rise to various mesodermal tissues of the adult. Despite their simple lineage, specification mechanisms of the embryonic mesenchyme cells remain to be investigated; this is mainly due to lack of specific molecular markers for this cell type. Here we report that, in Halocynthia roretzi, zygotic expression of a cytoskeletal actin gene (HrCA1) begins at the late gastrula stage and that transcripts were predominantly distributed in embryonic mesenchyme cells, although some expression was observed in special cells of the central nervous system as well as in notochord cells. When HrCA1 expression was examined in cleavage-arrested embryos, it was found only in mesenchyme-lineage blastomeres of arrested 8-cell and later stages, consistent with the predominant expression of HrCA1 being in mesenchyme cells. To examine whether cell-cell contact until the 8-cell stage is required for mesenchyme cell specification, blastomeres were continuously dissociated during the period of 2-8-cell stages and division of the blastomeres was then arrested. Results showed that two of eight dissociated and division-arrested blastomeres from a single fertilized egg (presumably those corresponding to B4.1 cells) expressed HrCA1, suggesting that specification of embryonic mesenchyme cells does not require cell-cell contact until the 8-cell stage.