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While these steps may seem confusing and full of unusual words,
they are crucial to understanding both the intrigue and debate
concerning stem cells. Humans, along with other living things,
do not begin life as miniature, complete beings. Instead, they
progress through a series of steps that can only be labeled
de novo (from new) synthesis. A single cell becomes 2 and then
4 and then 8…….and then 10 trillion cells! How does
this happen and how can a single cell give rise to the multitude
of complex and differentiated systems found in a mature organism?
A cardiocyte muscle cell is vastly different than a red blood
cell or a neuron, yet within a given organism, they are all
descendents of a common ancestor (the original zygote). The
answers to these questions are found in stem cell biology and
further caveats of the process of development.
At
the morula stage, all cells are considered totipotent: they
are indistinguishable from one another and any individual cell
is capable of directing the development of a complete organism
including all organ and tissue systems as well as extra-organism
support membranes and structures (i.e. the placenta). The fantastic
ability of these cells to develop into quite literally any part
of an organism is referred to as plasticity. However, as development
continues, the plasticity of the growing cells begins to diminish.
At the blastocyst stage, the inner cell mass (essentially the
developing embryo) has been reduced from totipotent to pluripotent.
A
pluripotent cell can still become any part of the developing
organism, but it can no longer completely sustain development
as the ability to direct the formation of the placenta and other
extra-organismal membranes has been lost. As development continues
through gastrulation and beyond, the pluripotent cells begin
to differentiate and become developmentally restricted to specific
organ or tissue systems. The stem cells that remain are now
considered multipotent; they can develop into any part of a
specific system, but unlike their pluripotent or totipotent
predecessors, the system in question has already been chosen.
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