Cell Differentiation

Cell Differentiation

A special characteristic of cell growth and cell division is cell differentiation, which refers to changes in physical and functional properties of cells as they proliferate in the embryo to form the different bodily structures and organs. The description of an especially interesting experiment that helps explain these pro-cesses follows.

When the nucleus from an intestinal mucosal cell of a frog is surgically implanted into a frog ovum from which the original ovum nucleus was removed, the result is often the formation of a normal frog. This demonstrates that even the intestinal mucosal cell, which is a well-differentiated cell, carries all the necessary genetic information for development of all structures required in the frog’s body.

Therefore, it has become clear that differentiation results not from loss of genes but from selective repression of different genetic operons. In fact, elec-tron micrographs suggest that some segments of DNA helixes wound around histone cores become so con-densed that they no longer uncoil to form RNA mol-ecules. One explanation for this is as follows: It has been supposed that the cellular genome begins at a certain stage of cell differentiation to produce a regu-latory protein that forever after represses a select group of genes. Therefore, the repressed genes never function again. Regardless of the mechanism, mature human cells produce a maximum of about 8000 to 10,000 proteins rather than the potential 30,000 or more if all genes were active.

Embryological experiments show that certain cells in an embryo control differentiation of adjacent cells. For instance, the primordial chorda-mesoderm is called the primary organizer of the embryo because it forms a focus around which the rest of the embryo develops. It differentiates into a mesodermal axis that contains segmentally arranged somites and, as a result of induc-tions in the surrounding tissues, causes formation ofessentially all the organs of the body.

Another instance of induction occurs when the developing eye vesicles come in contact with the ecto-derm of the head and cause the ectoderm to thicken into a lens plate that folds inward to form the lens of the eye. Therefore, a large share of the embryo devel-ops as a result of such inductions, one part of the body affecting another part, and this part affecting still other parts.

Thus, although our understanding of cell differenti-ation is still hazy, we know many control mechanisms by which differentiation could occur.