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
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
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.