Fig. 1.

Tracheoles and undifferentiated cells are spatially segregated within the larval brain. (A) The central brain (left part of the hemisphere shown in this image) contains most tracheoles (shown in A and B with chitin-derived autofluorescence) and synapses (B, stained with anti-Bruchpilot), but the optic lobe (right part of the hemisphere in A and B, see also C) has very few tracheoles and synapses (n=6). The larval optic lobe is enriched in proliferative progenitors and immature cells while most terminally differentiated neurons are found in the central brain (schematically illustrated in C). At the border between the central brain and the optic lobe a boundary can be observed between differentiated neurons and progenitors (C, dashed square). Transmission electron microscopy of a region similar to that squared in C reveals the boundary (arrowheads in D) between central brain differentiated neurons with a low nucleus/cytoplasm ratio (left in D, pseudo-coloured in magenta) and undifferentiated cells in the optic lobe, with much less cytoplasm (right in D, pseudo-coloured in yellow) (n=5). Differentiated neurons are larger and contain more a well-differentiated cytoplasm, enriched in organelles like mitochondria (E, pseudo-coloured in orange), rough endoplasmic reticulum cisternae (pseudo-coloured in light green), Golgi cisternae (pseudo-coloured in cyan) and other organelles. Immature neurons in the optic lobe (F) are less differentiated than central brain neurons and show a thin ring of cytoplasm around the nucleus. CB, central brain; OL, optic lobe; OPC, outer proliferation centre; IPC, inner proliferation centre; Nu, nucleus; Cy, cytoplasm; sNB, secondary neuroblasts; pNr, primary neurons; GMC, ganglion mother cells; sNr, secondary neurons; NE, neuroepithelium; NB, neuroblasts; uNr, undifferentiated neurons. Scale bars: A,B: 50 µm; D: 5 µm; E,F: 500 nm.