Mapping the human embryome: 1 to 10e13 and all the cells in between.

We live in an age of extraordinary medical vision. Buoyed by the flood of new information on gene regulatory networks in mammalian development, and the emerging field of human embryonic stem cell (hESC) technology, it is now possible to imagine a host of new cell-based modalities for the treatment of disease. The question suddenly becomes not so much, ‘How?’ but, ‘When?’. While the starting material for these cell-based therapies may have been discovered, there still remains a number of challenges before they can become part of everyday routine clinical practice. To quote directly from the US Department of Health and Human Services: ‘If regenerative medicine researchers and clinicians are able to gain a detailed understanding of how cells interact with each other and how to mass produce, preserve, catalog and build these cells, they can then apply this knowledge towards developing tissue and organ-based therapies’ [101]. Many column inches have been written discussing the challenges of cell–cell interaction, mass production and preservation, but very little on what is arguably the greatest immediate hurdle for the field: how do we demonstrate that we have isolated a pure population of a particular kind of cell from hESCs when there is no consensus on the unique molecular markers for any of the human embryonic progenitor lineages? While the scientific literature contains scattered references to markers of particular fully differentiated cell types, the molecular markers of the embryonic progenitors to these cell types are scant and vague at best, and there is no authoritative standardized reference for such markers. Why is this? The answer lies in the shear enormity of the task. Characterizing all the cells that make up both the developing and mature human bodies is undoubtedly a backbreaking endeavor. It encompasses going from the single-cell stage (fertilized egg) through to the adult human, estimated to be composed of 10,000 billion cells (i.e., 1 to 10e13) and all the embryonic progenitor cell types in between [102]. While the challenge may appear overwhelming, such a database may eventually be viewed as an essential component of regenerative medicine. The construction of this internationally critiqued and consensus database should begin now such that the expected rapid growth of research on hESC differentiation in the coming decade may benefit from an accurate understanding of the best signposts on the road to making pure populations of life-saving products.
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