If we also consider cell chemistry, many more different types can be recognized. The nervous system and the immune system are tops in complexity. Different types of cells are specialized for different functions. Understanding how cells become differentiated is the major goal of modern developmental biology. We now know that there are three distinct components to cell differentiation and the formation of different tissues.
Differentiation begins with the first cell division that leads to a new animal.
The fertilized egg is loaded with maternal RNAs and maternal proteins. These maternal contributions are distributed unevenly during the first cell divisions. The cells that result are then different, and will produce different sets of proteins. Drosophila melanogaster provides an excellent example of how animal embryos develop. The mRNAs cannot move freely to fill the entire egg because they are attached to the cytoskeleton of the egg.