Cloning experiments have shown that cell nuclei do not undergo irreversible changes early in development. All cells retain the complete genetic code from which an entire organism can be produced. Still, not all cells behave the same way during development. Cells eventually differentiate to become the diverse tissues and structures of the body. Generalizing, we may say that differentiation results from differential gene expression—that is, from the differential regulation of transcription, posttranscriptional events such as mRNA splicing, and translation in different cell types.
Some differences in gene expression patterns are the result of cytoplasmic differences between cells. One such cytoplasmic difference is the emergence of distinct "top" and "bottom" ends of an organism or structure; such a difference is called polarity. Polarity is established early in development, and reflects differences between one end of an organism and the other—the head versus the tail, for example.
Although each cell of the developing embryo contains the same genetic material, differences in the contents of the cytoplasm lead to changes in the embryo. Unequal distribution of cytoplasmic determinants in the egg, zygote, or embryo can lead to cell determination. One such change is the establishment of polarity. The embryo has two poles, the animal pole and the vegetal pole, and each pole contains a different combination of factors that result in the development of different body parts. If the 8-cell sea urchin embryo is divided in half so that the animal pole is separated from the vegetal pole, the cells derived from the animal pole do not develop or differentiate. Those derived from the vegetal pole develop into an abnormal larva. However, if the 8-cell embryo is bisected so that each half contains both animal and vegetal cells, both halves develop into small, but otherwise normal, larvae.
Polarity comes about because of unequal distribution of cytoplasmic materials in the egg. Some factors essential for development concentrate in one half of the cytoplasm, and when the cell divides, this unequal distribution is retained. The relative concentration of cytoplasmic factors therefore influences the fate of the cell during development.
Textbook Reference: Concept 14.2 Changes in Gene Expression Underlie Cell Fate Determination and Differentiation