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.