Fig. 6. Modes of brain NB formation differ between mitotic domains. (A) In mitotic domain B, NBs form by basally orientated delamination from the neuroectoderm (NE). The scheme on the left highlights, in a lateral view, the arrangement of mitotic domains (at stage 7) that contribute to the embryonic brain. (B) NE cells in domain 9 move apically (red arrow in Ba; see also D) and subsequentially reintegrate into the NE layer to delaminate as NBs (Ba1) or remain in the ectoderm to develop as epidermoblasts (EB in Ba2). Other cells in domain 9 divide perpendicular to the ectodermal surface (as indicated by the mitotic spindle; Bb); one daughter cell moves apically but later reintegrates into the NE as an epidermoblast, the other is deposited basally to become a NB. (C) In domains 1 and 5, NE cells divide parallel to the ectodermal surface; thereafter one of the two daughter cells usually stays in the peripheral ectoderm as an epidermoblast, the second delaminates as a NB. (D) Timelapse sequence focusing on part of the NE layer in mitotic domain 9 a few minutes after cephalic furrow formation. Sequence of pictures shows the apically directed movement of NE cells. First, the yellow NE cell leaves and reintegrates into the layer, followed by the red cell; each cell needs about 2 minutes to achieve this movement (time is depicted in pictures). Note the changing shape of apically moving cells (compare with Ba).