Hi Pilou, it looks like ZBrush decided to undo the welding for that top pole point.

Some programs might decide to do this on import if they are not set up internally to have different UV texture coordinates for several polygon faces that share one same 3d vertex point.

You need to get that top point welded inside of ZBrush to get the kind of subdivision you expect, you have to have just one point shared in common by all those polygons. Probably there is some kind of "Weld" or "Merge vertices" function for this in ZBrush somewhere, but I'm not familiar enough with it to know where.

You also might try exporting to .3ds with welding turned on - this will actually avoid writing UV texture coordinates to the .3ds file and that might actually help you out in this particular situation since it may avoid the automatic unwelding that ZBrush appears to be doing.

- Michael