At what stage does cell differentiation start?

Cell differentiation begins approximately three weeks following fertilization. At this stage differentiation begins in the embryo through two main processes. One process is cytoplasmic localization, which involves the concentration of specific molecules in different areas of the cell. The second process is via cytoplasmic induction, which is triggered by external signals present in the stem cell’s surroundings that cause alterations in gene expression. Differentiation occurs only in genes which are expressed, and doesn’t occur in the other genes that remain unexpressed. Through this mechanism, the development of distinct cell types with structures specialized for specific functions are achieved.