In an embryo, one cell becomes a brain cell and another a skin cell through the process called cell differentiation, which involves changes in gene expression. The genome stays the same in each specialized cell, while each cell has a different structure and functions.
A human being begins as a fertilized egg, which is called a zygote. This single cell divides many times until it contains hundreds of cells, which are called totipotent. They can become either an embryo or the placenta supporting it. Once the embryo becomes distinct from placenta, all cells within the embryo's body can become a cell of any tissue type. These are called pluripotent cells, or embryonic stem cells.
The embryo is defined as fetus from the 8th week of gestation. While in the embryonic stage of development, the human pluripotent cells first become multipotent stem cells that have the ability to become particular tissue cells: skin cells or brain cells. Tissues in the human body consist of different types of cells. Each cell completes its process of differentiation and becomes a particular type of cell, such as a hair cell or a neuron.
Each cell keeps the complete set of genes relevant to the whole organism. However, only a specific set of genes is activated in each to tell it which shape and role to take in the organism. The mechanism of this gene expression is not fully understood by science.