RESUMO
1. If a few exceptions are disregarded, the several somatic cell types of a differentiated organism all have an identical genome. They all differ in their plasma membrane-cytoskeletal complex. 2. Differences in plasma membrane properties usually result in differences in ionic concentrations/activities in the cytoplasm and nucleoplasm. A basic question therefore is whether there exists a causal relationship between the ionic environment of the nucleus and differential gene expression/protein synthesis. 3. Development is switched on by a "Ca2+ explosion", often accompanied by pH changes and plasma membrane depolarisation. The penetration of the spermatozoon in the plasma membrane acts as a trigger. 4. All animal species develop from a blastula. At this stage they organise themselves as an epithelium enclosing an inner (fluid) compartment. This suggests that epithelium formation is absolutely essential in animal development. 5. As development proceeds, more and more compartments, lined by different epithelia, are formed. Differentiated organisms largely consist of folded epithelia. Some cells leave their original epithelial environment and become free floating (e.g. blood cells) or engage in other types of organisation. 6. Epithelial cells have the ability to segregate some membrane proteins, e.g. receptors, ion pumps, ion channels etc., so as to make selective transcellular transport possible. The cytoskeleton plays an important role in this segregation and in the interconnection of epithelial cells. 7. Transembryonic electric currents which have been measured by the vibrating probe technique, are due to electrogenic ion transport by epithelia. 8. Segregation of membrane proteins is not an exclusive property of epithelial cells but it is probably a property of all animal cell types, single cells inclusive; asymmetry is the rule, symmetry--if it exists at all--the exception. 9. Differences in several plasma membrane proteins (receptors, ion transporting molecules, cell adhesion molecules and signal transducing systems) are not only causally related to differential transcellular transport but also indirectly to differential protein synthesis and hence to differentiation. There are already a few well documented examples of "electrical" control of gene expression. 10. The major "strategy" which applies in differentiation seems to be to keep the genome constant but to change over and over its ionic and macromolecular environment, both acting in a complementary way. The first one may be considered as the coarse tuning mechanism of gene expression-protein synthesis, the second as the fine one. In our opinion this might be a principle universal to differentiation processes in all animal species.
