IT IS INCREASINGLY difficult to distinguish between hype and hope, with each new headline promising amazing progress in genetics and cell biology. We will soon be able, they proclaim, to replace any organ - growing a human heart in the lab is just a decade or two away. Advances in handling cells taken from human embryos means they can be engineered to create any body structure required, so providing a means of curing illnesses like Parkinson's.

And matters just get worse when such claims are coupled with genetic sensationalism which frightens and titillates with Frankenstein-like fantasies. Take, for example the widely shown picture of a mouse with a human ear growing on its back. It was recently shown on BBC's Tomorrow's World, which should know better as it isn't a human ear at all but a bit of artificial material shaped like a human ear that has been placed under the skin of the mouse for no good reason, other than publicity. These claims may get the venture capitalists to put their money into the biotech firms that promote these stories. But they may also, in the long run, lead to a serious public mistrust of scientists.

So how realistic are these claims? A group from the University of Wisconsin is the first to proclaim success in culturing (growing in the laboratory) human embryonic stem cells, which are obtained from early human embryos. The human body develops from just a small number of special cells - probably less than 30 - in the early embryo that are set aside for that purpose. The rest of the cells at that early stage go on to form the placenta and other structures that protect and nourish the developing embryo. These 30 or so special cells give rise to every cell in our body, from our skin to the neurons in our brain, but at this early stage their fate is not yet determined. They could develop into a muscle or a neuron, and all possibilities are open to them. The identity they acquire will mainly be the result of signals received from neighbouring cells.

The recent excitement is due to the Wisconsin scientists' ability to obtain large numbers of these multi-potent human cells in the laboratory, and to manipulate their development. Given the right nutrients, the cells happily multiply and seem to be immortal - they go on multiplying without any sign of ageing. If the cells in the dish are exposed to particular chemical treatments then their behaviour is altered and they can give rise to several cell types including muscle, neuron and cartilage. So these cells could be transplanted into individuals suffering from diseases that result from the absence of certain cells: Parkinson's patients, for example, are missing certain neurons in the brain, and those with juvenile diabetes lack the cells that make insulin. It may also be possible to repair the damage from heart attacks by introducing heart muscle cells.

At this stage, very little is known about the signals required to get the embryonic stem cells to do what the scientists want them to. Another major problem is the rejection by the immune system of implanted cells. In the future, it may be possible to modify the cells' genetic constitution so that they are not treated as foreign. Unless this is done, it would be necessary to make a cloned embryo for every individual from which to obtain the stem cells - but where would the human eggs come from?

Growing whole organs like kidneys or hearts from these cells is currently no more than a dream - the technical problems are enormous. Nevertheless, there are promising attempts at tissue engineering - growing bone or cartilage cells on artificial scaffolds which can then be used for the repair of bones, the scaffold eventually dissolving away. And sheets of cultured skin cells have long been used for burns.

I do hope that all this optimism does not turn out to be the "cold fusion" of biology.