Figure 5.

SV2 immunoreactive profiles at different ages of the owl embryo appear as dark puncta in photomicrographs of rostral and caudal sections through NL. At E17, in rostromedial regions, NL cells were arranged in the typical embryonic laminar structure (note neutral red-stained cell bodies, arrowhead), and darkly stained SV2 profiles were confined to the external edges of the nucleus. In the caudolateral region, the nuclei (NM and NL) have not become fully differentiated, and SV2 profiles were seen on the outside of the column. Cell bodies in the mass were labeled with neutral red (compare Fig. 2, E17, Caudal). By E21, NL has begun to undergo secondary morphogenesis at its more rostromedial extent, and SV2 profiles have migrated from their location on the distal dendrites into the neuropil of NL. In more caudal areas in the same embryo, the laminar structure is still compact, and SV2 profiles remain concentrated mainly toward the outside edges of the nucleus, in the distal dendrites (arrowheads). By E24, neurons in the rostral regions of NL show a distribution similar to that observed in the adult and lack the dendritic polarization that characterized them at earlier ages. SV2 profiles are rather evenly distributed throughout the dorsoventral dimension of the nucleus. More caudolateral regions of the nucleus at this age continue to exhibit more immature morphology, and SV2 profiles are denser in the outside edges (top, dorsal; left, lateral).