Harveian Oration 2014: Stem cells and cell replacement prospects 

Xenopus egg. The egg is surrounded by a thick layer of jelly that is impenetrable by needles.

Microinjection needles (A) before microforging (B) after microforging.

Intestinal tract of feeding tadpole (GFP marked).

Serial nuclear transplantation. Wakayama et al (2013)⁴ demonstrated 25 generations of serial nuclear transfer, with 500 cell divisions from the first donor nucleus. No cancer incidence was observed.

Mechanisms of cell differentiation. The first mechanism is asymmetric distribution of determinants in the egg, giving rise to an asymmetric distribution of cells in the embryo. Subsequently, signalling between cells (red arrows) causes further differentiation to give the ectoderm, mesoderm and endoderm.

Formation of specialised cells from stem cells. Gene activation molecules accumulate in the stem cell and move to one side in response to an external signal. Asymmetric cell division gives rise to a specialised skin cell and depends on the concentration of transcription factors (red spots).

Cells specialise in ways determined by the concentration of morphogen molecules (pink circles) that come from the source and reach distant cells by diffusion.