Immaculate conception- modern day style.

Hamburg - A team of scientists in Germany have "manufactured" human sperm from bone marrow, raising the prospect of mass-producing sperm that can be used in IVF treatment or to restore fertility to men made sterile by cancer therapy.

Such applications are still many years away. But scientists hope to grow fully formed sperm cells in as little as three years.

The research, conducted in Germany, has been published in the journal Reproduction: Gamete Biology.

The team led by Professor Karim Nayernia, from the University of Goettingen, first took bone marrow from male volunteers. From the samples, they isolated mesenchymal stem cells, which have previously been shown to grow into body tissues such as muscle.

Stem cells are immature cells that can be made to follow different functional pathways.

Using a form of vitamin A, the scientists coaxed the mesenchymal cells to become primordial germ cells (PGCs) - the first stage in the genesis of sperm.

Specific genetic markers showed that some PGCs had further developed into more advanced spermatagonial stem cells.

Under normal circumstances, these cells eventually turn into mature, functional sperm that can fertilise an egg.

About three percent of the original bone marrow stem cells were able to reach the point of being spermatagonial cells. In earlier work, Professor Nayernia derived sperm stem cells from mouse bone marrow and transplanted them into the animals' tests.

Here they multiplied and began to differentiate further, without actually becoming fully formed sperm.

Nayernia said: "We are very excited about this discovery, particularly as our earlier work in mice suggests that we could develop the work even further.

"Our next goal is to see if we can get the spermatagonial stem cells to progress to mature sperm in the laboratory, and this should take around three to five years of experiments."

A priority will be to investigate why the cells fail to enter meiosis, the final stage of cell division before they become fully formed sperm.

One possibility is that they need to be accompanied by sertoli cells - special "nursing" cells found in the testes that have the job of nurturing growing sperm.

Producing sertoli cells from bone marrow, as well as sperm, is another avenue being explored.

Nayernia hopes in future the research will lead to new male infertility treatments. In particular he wants to look at the feasibility of restoring fertility to young men who have undergone chemotherapy for cancer.

This could involve culturing early-stage sperm cells in the laboratory from samples of bone marrow, or testes tissue. They would then be implanted back in the testes.

"We now have the first step, but the cells don't enter meiosis," said Nayernia. "If we could solve this problem, and I think it's scientifically possible, the next stage will be to produce functional sperm." - Sapa-DPA

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