Differentiation produces specialized cells and the overall form of the organism is the result of morphogenesis , due to pattern formation. The fate of cells will be determined at different points during development of embryos of different species. In some species, determination occurs relatively early in cleavage, in other species it occurs later. Figure 15.2 illustrates how tissue from early frog embryos has not been determined. Once determination has occurred, the fate of a tissue and its cells are sealed.
Determination precedes differentiation.
Differential Gene Expression Differential gene expression is the means by which differentiation comes about. The zygote is totipotent and is capable of forming a complete adult. Depending on the species, the cells resulting from cleavage may be totipotent at earlier or later stages in development of the early embryo. An early theory concerning differentiation suggested that the cell nucleus undergoes permanent change during development. Hence, differentiation should be irreversible.