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.
Experiments with single cells taken from an early embryo have shown that these cells are not irreversibly differentiated. In some species, the two separated cells are each capable of developing into a complete organism. Yet, as the embryo develops from a single cell to a multicellular entity, the cells do not remain identical.
Different parts of the embryo develop into diverse structures in the adult organism. Given that each cell contains the same genetic information, how do the cells of the embryo know how to form these diverse structures? The answer can be found in the cytoplasm.
One key developmental event is the establishment of polarity—the difference between one end of the organism and the other. These differences can be obvious, like the anatomical distinctions between our head and our feet.
Polarity can be established quite early in development, even in the egg itself, when yolk and other factors may be distributed unevenly. Based on these differences, the top of the egg is referred to as the animal pole, whereas the bottom is called the vegetal pole.
After cell division, any factor that is unequally distributed in the cytoplasm ends up in some daughter cells or regions of cells, but not others. Following fertilization by a sperm cell, the newly formed zygote divides by mitosis to form two identical cells, with no increase in total size. These two cells split into four, then eight.
Experiments with sea urchin embryos reveal the effects of cytoplasmic segregation on development. The 8-cell embryo can be bisected either vertically or horizontally, in each case producing two groups of four cells.
When the 8-cell embryo is bisected horizontally, the animal pole cells are separated from the vegetal pole cells. The top half gives rise only to a hollow ball of ciliated cells. The bottom half develops into a small, slightly abnormal larva.
However, bisecting the 8-cell embryo vertically leaves each half with both animal and vegetal cells. Both halves develop into small, but otherwise normal, larvae. This experiment demonstrated that the animal and vegetal pole halves of the embryo differ in their developmental potential.
There must be at least one factor essential for development that is segregated in the bottom half of the egg, so that the cells of the vegetal pole have it but not those of the animal pole. Clearly, the unequal distribution of cytoplasmic determinants can have a profound effect on the subsequent development of an organism.
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.