Every cell in a multicellular organism contains the same identical genome - yet in a human being there are eye cells, bone cells, and brain cells - which all serve different purposes yet genetically are identical. How can this be?
There are two key differences between cells that we call differentiated.
- Cells differentiate through a complex system that involves the establishment of positional information and cell differentiation through paracrine signaling and other processes.
- The genome interacts with proteins which enable or disable certain genes. Due to the initial developmental processes, a different set of proteins is maintained in a given cell, allowing for an identical genome but with entirely different functions, and proteins.
When development begins, positional information must be established.
Once positional information is established, cells begin to change due to differences in gene transcription among the developing zygote. Some cells become intermediates - cells that would never be found in an adult organism but nonetheless act as a starting point for a number of other cells. These are known as pluripotent cells.
There are 204 different cell types in Human beings - so not every cell can be "unique". When development occurs, pluripotent cells divide and through signalling prescribe a cell fate to a set of cells, known as a developmental field. The end result is a large number of grouped cells all develop identically, into a muscle for example, while a set somewhere else will develop into something else altogether.