A team of scientists have created primitive human sperm in the laboratory by using bone-marrow tissue from male volunteers.
The German team from the University of Göttingen was led by Professor Karim Nayernia, now at the Northeast England Stem Cell Institute (NESCI), at the Centre for Life in Newcastle upon Tyne.
Professor Nayernia and colleagues isolated human "mesenchymal" stem cells from the bone marrow and stimulated them by using a type of vitamin A to encourage them to develop into male reproductive cells or "germ cells".
The genetic markers in the cells showed that the germ cells also contained "spermatagonial stem cells", an early stage version of male sperm cells which eventually become fully mature sperm cells.
The team were only able to take the cells to the pre-meiosis, or cell division stage, a special kind of cell division that sperm and egg cells go through.
In humans it takes several stages to both divide and at the same time reduce the number of chromosomes from 46 to 23 so that when the sperm from one human fertilizes the egg of another, the recombined chromosome count of is 46 again.
Sperms and eggs are known as gametes, because they only have 23 chromosomes, whereas the fertilized egg is a zygote, because it is a viable future organism with all its requisite chromosomes present and correct.
In an earlier study Professor Nayernia and a team of scientists had previously made spermatagonial cells from the bone marrow of mice, inserted them in the testes of live mice and saw them divide and develop into mature sperm cells.
Nayernia and his team also revealed last year that they had successfully bred seven live mice made from mouse eggs fertilized with sperm cells developed from embryonic mouse stem cells.
Nayernia says the latest experiment has produced very exciting results and suggests the work could be developed even further.
He says the next step will be to get the spermatagonial stem cells to develop into fully mature sperm in the laboratory, a process which could take 3 to 5 years because the nature of such research demands the development be within an ethical and social framework due to the potential applications for fertility treatment in humans.
Nayernia says one of their initial goals will be to find out why the cells stopped at the meiosis stage, the final phase before becoming mature sperm.
They suspect when this stage occurs in the male testes, another group of cells called "sertoli cells" help to "nurse" the sperm cells through division and maturation and are considering the possibility that they may be able to also produce sertoli cells from stem cells taken from bone marrow.
The researchers believe their work could one day lead to new treatments to help infertile men.
The research has been commended by other scientists who say however that there is still a long way to go before sperm cells produced in this way can be shown to fertilize a human egg.
The research also presents a plethora of dilemmas before such human fertility applications can be considered, regarding the possibility of producing female only sperm cells, where a woman could make a baby with another woman and eggs using male genes which could allow homosexual couples to have their own genetic children.
The research also raises questions about the long term genetic effects of creating sperm in this way which could have implications for the health and development of embryos and the children and adults they eventually become.
However the general consensus amongst scientists is that this is unlikely because embryos generally require genetic material from both a male and a female to develop normally.
The British Government apparently intends to outlaw artificial gametes in fertility treatment, and this could affect Professor Nayernia's plans for clinical trials.
The findings are published in the journal Reproduction: Gamete Biology.