Clones derived from individually cultured mesectodermal midline progenitors. (A) Schematic cross-section showing one side of the ventral half of an embryo at stage 7 (early gastrula stage; midline to the left, dorsal to the top). The row of mesectodermal midline progenitors (me; marked in red) is located at the ventral furrow between the neuroectoderm (n; grey) and the invaginated mesoderm (m; white). (B) Three frames from a time-lapse recording (real time is indicated) showing morphologically symmetrical division of an isolated midline progenitor. (C-I) Individually cultured midline progenitors give rise to clones that, based on morphological criteria, consist of neuronal (C,D) or glial cells (E,F) or a mixture of both (G-I). In (C,D) the entire lineage stains positive for the neuronal marker anti-horse radish peroxidase (HRP), and cells in (E) for the midline glia marker AA142-lacZ (nuclear staining). In (F) only one of the two cells is labelled by the midline glia marker anti-Tramtrack (TTK). The clone in (G) comprises two anti-HRP-positive (brown; arrowheads) and two AA142-positive cells (blue nuclei; arrows)). (H,I) Lineages including cells that co-express glial and neuronal markers. The clone in (H) consists of four TTK (green) expressing cells, two of which (yellow; arrows) co-express HRP (red); the clone in (I) consists of three cells in which AA142-lacZ (red) and HRP (green) are co-expressed (yellow). Markers applied in (C-I) are indicated. The size of midline progenitor cells is approximately 12 μm and of progeny cells 5 to 8 μm.