Virtually all cells in the human body have the same DNA. What's make some undifferentiated cells differentiated into brain cells and other undifferentiated cells specialize as bone cell

Changes in gene expression

Explanation:

This phenomenon is the result of changes in gene expression. That means, how the information in the DNA is used by the cell. The genes that are active in the cells of the brain will be very different from the genes that are active in the cells of the bone marrow.

These patterns of gene expression are different for each cell, and dictate the identity of that cell. Gene expression patterns are controlled by a variety of factors in the cell that allow tissue-specific expression, such a transcription factors.

This can also be facilitated by another layer of regulation called epigenetics, which literally means "on top of" genetics, and refers to modifications of DNA (and the proteins around it), that can reflect and influence the activity of the genes within.