Despite the breathless coverage in the general media last week suggesting that research on frog embryos could apply to cloned humans grown for organ transplantation, the studies done in frogs are not directly applicable to mammals. Embryo culture in mammals also is extremely difficult, and other - less ethically questionable - approaches to organ transplantation via tissue engineering are more likely to be successful.

The African clawed toad, Xenopus laevis, has provided perhaps the best vertebrate model system for understanding the cell-cell signaling events that give pattern to a developing embryo. Much of Xenopus research has focused on the requirements to generate dorsal tissue, which gives rise to the head and notochord (required for central nervous system patterning and the spinal cord precursor).