As cell layers migrate in the embryo to different regions, genes are switched on or off in response to neighbouring cells and environmental cues.
Cell appearance and function is determined by active genes.
Active genes produce proteins – proteins are the building blocks of the cell structures.
Approximately 200 types of cells in humans.
Specialisation comes at a price – cells depend on each other for their survival e.g. nerve cell depends on red blood cell for oxygen and will die if separated.