MOLECULAR BASIS OF DIFFERENTIATION
Whenever growth occurs, increase in mass, weight etc. takes place. Number of cells increases due to cell division. Sometimes, cells grow in size and they do not divide and lead to growth. The simple multiplication of cells would produce masses of cells but not an organism. Knowledge of developmental processes as growth and differentiation is essential for understanding the events that lead to the formation of an embryo or foetus. When the zygote divides, the embryonic cells generally remain totipotent. In other words, every embryonic cell is capable of giving rise to embryo and forming a new adult organism. However, gradually the cells lose this capability and no longer remain totipotent. By watching and observing, we can get a reasonable idea of how one cell produces an entire individual.
It is the dogma of modem biology and genetics that nucleus of most somatic cells of higher organisms, no matter how differentiated the cell, contains copies of all nuclear genes of an individual. Differentiating cells start new sets of proteins or lose the ability to form a set of proteins.
It is considered that differentiation of cells in different types during development of an organism involves regulation of the expression of genes, rather than some other mechanism such as mutation.
Few of late embryonic change are observed under microscope. But changes at molecular level start are very earlier stage- Such changes occur much before appearance of morphological changes. Differentiation involves processes like mitosis, cell fusion, cell migration or intercellular interactions. These all processes work independently of each other. A proper coordination and approach is required to know about definite pattern of development.
It can be concluded that:
- Differentiation and development does not require massive permanent changes in nuclear DNA.
- Process involves self reinforcing changes in cytoplasm and selective gene transcription.
Plants as opposed to animals are usually capable of forming a new plant from a vegetative part. However, now it is a established fact that partially differentiated animal somatic cells are like plant somatic cells in this regard. Robert Briggs and Thomas King have shown that nuclei from the cells of blastula and gastrula stages of frog (Rana pipiens) embryos when transplanted into enucleated eggs can produce a complete embryo. The story of nuclear transplantation is useful in determining when the nucleus of a tissue loses its capacity for generating a complete development for the formation of an adult.
Briggs and King used grass frog, Rana pipiens and African frog Xenopus for sophisticated experiments. They removed/destroyed the nuclei of frog or toad cells and transplanted fresh nuclei from embryonic and tadpole cell into enucleated eggs. Many embryos with freshly planted nuclei develop into normal tadpoles . But if nuclei were from differentiated intestinal cells, tadpoles did not develop. It was found that nuclei from the early cleavage stages (upto 64 cells) could readily be transplanted into enucleated eggs and they would normally develop into tadpoles and frogs.
But nuclei from latter stages usually cause the embryos to abort. No fully differentiated adult frog tissue can be used for making clonal frogs. It clearly shows that nuclei undergo some changes during differentiation. Process of differentiation is reversible in early embryonic stages. However, in plants, even mature cells can undergo differentiation to form callus to raise the whole plant successfully. Main process of differentiation is due to altered gene activity. This change in gene activity is mainly brought about by its interaction with environment. The immediate environment can be cytoplasm. Cytoplasm is further affected by many parameters like temperature, humidity, light, cell-cell interaction etc. Modulation of gene and its cytoplasm is independent.