Stem cells: science, policy, and ethics

Abstract

Human embryonic stem cells offer the promise of a new regenerative medicine in which damaged adult cells can be replaced with new cells. Research is needed to determine the most viable stem cell lines and reliable ways to promote the differentiation of pluripotent stem cells into specific cell types (neurons, muscle cells, etc). To create new cell lines, it is necessary to destroy preimplantation blastocysts. This has led to an intense debate that threatens to limit embryonic stem cell research. The profound ethical issues raised call for informed, dispassionate debate.

Figure 1

From zygote to blastula: the early stages of human development. Shortly after fertilization, the zygote repeatedly divides to form a solid mass of cells known as the morula. Two to three days after fertilization, the morula enters into the uterine cavity and forms a hollow sphere: the blastocyst. The surface cells form the trophoblast and give rise to extraembryonic tissues, while the inner cell mass is the source of embryonic stem cells and ultimately gives rise to the embryo, following implantation in the uterine wall.

Figure 2

Pluripotent stem cells, isolated from the ICM in the blastocyst, have the ability to give rise to all types of cells in the human body, but not the placenta and other supporting tissues.

Figure 3

Integration of transplanted mouse embryonic cell–derived motor neurons into the spinal cord in vivo. Transverse section through the lumbar region of the spinal cord reveals that enhanced GFP⁺ axons exit the spinal cord via the ventral root and project along nerve branches that supply dorsal and ventral limb muscles. The pathway of axons is detected by neurofilament (NF) expression. Reprinted with permission from Cell Press (5).

Figure 4

Normal development versus development during reproductive cloning and therapeutic cloning. During normal development (A), after fertilization, a diploid zygote is formed, which then undergoes cleavage to form a blastocyst that may be implanted in the uterus and result in a live birth. During reproductive cloning (B), the diploid nucleus of an adult donor cell is introduced into the enucleated oocyte. Following artificial activation, division results in a cloned blastocyst. Upon transfer into a surrogate mother, a small number of cloned blastocysts give rise to a clone. Therapeutic cloning (C) requires the explantation of cloned blastocysts in culture to yield an ESC line able to differentiate in vitro into any type of cell for therapeutic purposes. Figure modified with permission from the New England Journal of Medicine (31).