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Recent scientific achievements in cell and developmental biology have provided unprecedented opportunities for advances in biomedical research. The demonstration that fully differentiated cells can reverse their gene expression profile to that of a pluripotent cell, and the successful derivation and culture of human embryonic stem cells (ESCs) have fuelled hopes for applications in regenerative medicine. These advances have been put to public scrutiny raising legal, moral and ethical issues which have resulted in different levels of acceptance. Ethical issues concerning the use of cloned human embryos for the derivation of stem cells have stimulated the search for alternative methods for reversing differentiated cells into multi/pluripotent cells. In this article, we will review the present state of these reprogramming technologies and discuss their relative success. We also overview reprogramming events after somatic cell nuclear transfer (SCNT), as they may further instruct ex ovo strategies for cellular manipulation.
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The germ line and soma together maintain genetic lineages from generation to generation: the germ line passes genetic information between generations; the soma is the vehicle for germ line transmission, and is shaped by natural selection. The germ line and somatic lineages arise simultaneously in early embryos, but how their development is related depends on how primordial germ cells (PGC) are specified. PGCs are specified by one of two means. Epigenesis describes the induction of PGCs from pluripotent cells by signals from surrounding somatic tissues. In contrast, PGCs in many species are specified cell-autonomously by maternally derived molecules, known as germ plasm, and this is called preformation. Germ plasm inhibits signaling to PGCs; thus, they are specified cell-autonomously. Germ plasm evolved independently in many animal lineages, suggesting convergent evolution, and therefore it would be expected to convey a selective advantage. But, what this is remains unknown. We propose that the selective advantage that drives the emergence of germ plasm in vertebrates is the disengagement of germ line specification from somatic influences. This liberates the evolution of gene regulatory networks (GRNs) that govern somatic development, and thereby enhances species evolvability, a well-recognized selective advantage. We cite recent evidence showing that frog embryos, which contain germ plasm, have modified GRNs that are not conserved in axolotls, which represent more basal amphibians and employ epigenesis. We also present the correlation of preformation with enhanced species radiations, and we discuss the mutually exclusive trajectories influenced by germ plasm or pluripotency, which shaped chordate evolution.
