Multi-position photoactivation and multi-time acquisition for large-scale cell tracing in avian embryos

Vertebrate development is a complex orchestration of cell and tissue movements. Tracing individual cell positions can rapidly become a large-scale problem because cell numbers often grow exponentially in the early embryo. A typical approach consists of fluorescently marking small numbers of cells within a large number of embryos, followed by comprehensive three-dimensional static or time-lapse imaging to map cell positions. However, for large-scale cell tracing, such as during organogenesis, the time, effort, and expense of this approach can be limiting. The multi-position acquisition method can be used to capture more than one location on a microscope stage and allow for multi-specimen imaging. When combined with photoactivation cell labeling, a tool for selective cell marking using laser excitation, multi-position imaging offers a powerful tool for rapid data acquisition. This protocol describes the technique and demonstrates its use to map cell movements in the chick spinal cord, using slice culture explants. The details of multiple slice culture preparation, multi-position photoactivation, and multi-time acquisition are described. These events are coordinated by setting up blocks of microscope instructions that execute sequentially. This method significantly decreases the time, effort, microscopy, and embryo costs by a factor of the number of specimens imaged per session, typically six.