Photoactivation of PAGFP in a single cell in a living chick embryo. (A) A global view of a 16-somite chick embryo (5× magnification) showing post-photoactivation of PAGFP in a single cell (green-colored cell, within the box region) surrounded by a subpopulation of host cells (red). (A¢) A magnified view of the box region in A shows the photoactivated cell (green-colored). (B) The photoactivation process in a single cell within the chick embryo is shown as a sequential series of confocal excitation scans. The top row represents the 405-nm excitation scans of the cell (blue) at various time points of a typical sequence. The second row shows the resulting increase in GFP-fluorescence after each 405-nm excitation scan is followed by a 488-nm excitation scan. (C,D) As an example of the ability to observe individual cells within the chick embryo, two individual migrating cells (labeled as 1 and 2) were photoactivated separately at a time when the cells were just starting to emerge and migrate into the surrounding unlabeled tissue. The subpopulation of cells (red) within the chick neural tube provide a background view of other adjacent migrating and non-migrating cells. After a 13-hr reincubation of the whole embryo explant in culture, the final image (D) shows the location of the original cells that have migrated further laterally away from the neural tube. The cells (1 and 2) appear to have divided. The scale bar is 50 µm. The embryo in (A) is ~2 mm in length. The individual cell in B has a diameter of ~15 µm. The labels refer to the midbrain (m) and rhombomeres 1 to 4 (r1, r2, r3, r4).