Annexin A7 changes its subcellular localization in the embryonic brain at E16
Next we analysed the cellular and subcellular distribution of Annexin A7 in the developing mouse brain (Fig. 3). At E13–E16 in transverse sections of the embryonic brain, most of the immunoreactive cells are present in the ventricular germinative zone surrounding the lateral ventricle (Fig. 3A, overview E16; Fig. 3H, overview E13). A closer inspection revealed that Annexin A7 is mainly localized in the cytosol at E13 (Fig. 3B,E). During further development the immature cells have rounded up two days later, but they retain Annexin A7 in the cytosol at E15 (Fig. 3C,F). At E16 we observed the first prominent nuclear staining of Annexin A7 (Fig. 3D,G) in cells of the intermediate zone (Fig. 3I, oval), which contains neurons radially migrating towards the growing neopallium cortex [13,14] and also glial cells forming the white matter of the adult cortex. Cells located marginally in the neopallial cortex also exhibit a nuclear stain. Labelling in the ventricular germinative zone is less prominent and the cytosol is no longer more strongly stained than the nucleus.
Figure 3  Subcellular localization of Annexin A7 in embryos E13, E15 and E16. Paraffin sections of embryonic brain were stained with purified mAb 203–217. Annexin A7 was visualized with Alexa Fluor 488-conjugated secondary antibody. (A) Overview, at E16 the immature GFAP-negative cells of the forming cerebral neocortex (b) surrounding the lateral ventricle (a) are strongly stained; square, a higher magnification of this area is given in (I). (B) Higher magnification of the earlier stage E13 (H) shows, that the cells are stained in the cytosol (arrowhead). (C) Two days later at E15 the cells have rounded up, and Annexin A7 stays in the cytosol. (D) At E16 the first nuclear staining becomes apparent (arrowhead) in cells of the intermediate zone located between ventricular germinative zone and marginal neopallial cortex as seen in (I), oval. (E-G) Confocal microscopy confirms the results in B-D. (H) Overview, at E13 the immature GFAP-negative cells are strongly stained; a, lateral ventricle. (I) Higher magnification of a cortical section of (A, square) demonstrating ventricular, intermediate, and marginal zones; oval, a higher magnification of this area is given in (D,G).