Developmental biology and the redirection or replacement of cells

The aim of developmental biology is to understand how an egg converts itsel f into a complete organism through the processes of cell differentiation, m orphogenesis and size regulation. The principles that have emerged over rec ent decades include the constancy of the genome in nearly all cells of an i ndividual, the existence of stem cells in many organs and the overwhelming importance of signalling between cells for the determination of their fate. These and other characteristics of development are discussed here in relat ion to the prospect of achieving cell and tissue correction or replacement with thr help of nuclear transplantation and signalling factors. Nuclear tr ansplantation offers a one-step procedure for generating multipotent embryo cells from the cells of an adult tissue such as skin. It should be possibl e to proliferate the resulting cells as carl be done for mouse embryonic st em cells. Embryo cells can be made to differentiate in many directions by e xposing them to various agents or to different concentrations of a single f actor such as the transforming growth factor beta class signalling molecule activin. The possibility of a cancerous condition being acquired during th ese experimental manipulations can be guarded against by transfecting cells with a conditional suicide gene. Thus it may be possible to generate repla cement cells or tissues from an adult human for trans plantation back to th e original donor, without the disadvantage of any genetic incompatibility.