Totipotency and Pluripotency
In the starting we said that the fertilized egg cell (zygote) has the capacity or potentiality to give rise to all kinds of cell types, like a blood cell or a bone cell or a muscle cell etc. In embryological terminology such a cell is said to be totipotent. The zygote cleaves and makes a large number of blastomeres. The blastomeres may as well remain totipotent up to a certain stage and have the capacity to form every cell type of the organism. Though, as development proceeds the various blastomeres lose totipotency but may still be pluripotent. By pluripotent it is meant that they are able to make several but not all of the various cell types or tissues. Ultimately, with further development, even the pluripotency is lost and different groups of cells of the embryo are now capable of forming only one particular cell type or tissue.
For instance, up to a certain stage of embryonic development prospective ectodermal cells can make either epidermis or neural tissue, i.e. they are pluripotent. But at a later stage cells of one region of ectoderm can make only epidermal tissue and those of another region can distinguish only into neural tissue. What may be the cause of progressive restriction of potency in diverse embryonic cells although they are all descendents of the same single cell, zygote? Each one of these embryonic cells contains a nucleus which is also the descendent of similar zygotic nucleus. Does the restriction in potency in several embryonic cells result from differential distribution of specific cytoplasmic substances between different blastomeres during cleavage of zygote and subsequent divisions? We may ask a identical question with regards to the nucleus which contains the genes that direct the differentiation into specific tissues and cells types.