Animals are all multicellular organisms, which means they are made up of many cells. To get to this stage, the original fertilized egg must undergo cleavage, which is the rapid mitotic divisions of the zygote during the three days post-fertilization that produce cells called blastomeres, and specialization. Cleavage is the early cell divisions that change the diploid zygote into a larger collection of generic, undifferentiated cells. Through the activation of certain genes, each cell in the growing mass of cells becomes determined and develops a particular purpose or cell type. Cell determination is the process of identifying how a cell's function or role is set.
In an early embryo, the differences among cells and the differentiation of cells are determined by genes, which produce the blueprints needed for cells to turn into one type or another. Regulatory mechanisms in the cell's cytoplasm turn genes on and off. The cytoplasm has mitochondrial DNA and RNA that carry the mother's genome. The maternal influence of early cell differentiation is called a cytoplasmic determinant. Once the egg is fertilized, the cytoplasm of the zygote is distributed into separate cells so that the developing embryo, which consists of multiple cells, may come into contact with cytoplasm from other cells. This exposure helps to regulate the genes that get expressed.
Another very important factor that helps to determine what kind of cell an undifferentiated cell will become is the type of cells and environment that surround it. This is especially important as the embryo starts to divide quickly and repeatedly, becoming a large mass of individual cells. Cell-surface molecules receive signals from neighboring cells that tell a target cell what to do. These signals work through induction, which is the process of one cell or group of cells causing modifications to the development of another cell or group of cells through close proximity.
Early in embryo development, there are changes that occur at a molecular level that lead to cell specialization. There is a change in the expression patterns of genes for specific proteins, which will give rise to particular traits, such as differentiating an immature cell into a nerve cell or a blood cell. It is believed that embryonic cells undergo determination, which means they reach a point in their development where they can no longer be modified and remain as a specific cell type. The specific proteins that are expressed are found only in those particular cells that have them, affording those cells their characteristic structures. Differentiation of cells is regulated, especially during transcription, the formation of mRNA from the template DNA strand used to build proteins. Examples of specialized cells that have been differentiated because of regulation of transcription include liver cells that make albumin, nerve cells that carry stimuli and motor signals, skeletal muscle cells that move bones, and immune cells that digest pathogens in the blood.