Overview: Orchestrating Life?s Processes The development of a fertilized egg into an adult requires a precisely regulated program of gene expression Understanding this program has progressed mainly by studying model organisms Stem cells are key to the developmental process Orchestrating proper gene expression by all cells is crucial for life Figure 16.1 Concept 16.1: A program of differential gene expression leads to the different cell types in a multicellular organism A fertilized egg gives rise to many different cell types Cell types are organized successively into tissues, organs, organ systems, and the whole organism Gene expression orchestrates the developmental programs of animals A Genetic Program for Embryonic Development