Where are sex cells formed. Sexual reproduction.



Where are sex cells formed

Where are sex cells formed

See more Both of these sorts of animals have gone on to breed successfully. Moreover, in principle—though he has not yet done so in practice—he could fertilise his somatic-cell-derived eggs with his somatic-cell-derived sperm to create an entirely somatic-cell-derived adult animal.

He might even, if he so chose, be able to derive sperm and eggs from the same animal, for the processes do not require that the eggs be made from female cells and the sperm from those of males. That would create a mouse which had only one parent, yet was not a true clone of that parent because the sex cells which united to form it would both have undergone the internal genetic mixing that biologists call meiosis. One step back, two steps forward Dr Hayashi and his colleagues do not create their eggs and sperm directly from somatic cells.

First, those cells have to undergo an alchemical transformation to rejuvenate them into an ancestral form known as a pluripotent stem cell. Mature body cells eggs and sperm included derive from progenitors, known as stem cells, that have the power to divide, proliferate and eventually to turn into particular cellular components of a particular tissue.

Pluripotent stem cells are, in turn, the ancestors of these tissue-forming stem cells. In nature, pluripotent cells are restricted to embryos. Such harvesting is no longer necessary.

Instead, for mice, men and many other species, pluripotent cells can be made to order by taking an ordinary body cell and adding to it active copies of the four genes which encode the genetic switches that cause pluripotency. As he had discovered in his experiments creating sperm, judicious application of a molecule called bone morphogenetic protein 4 turns pluripotent cells into primordial germ cells—the type of stem cell ancestral to both sperm and eggs.

Which of these a primordial germ cell goes on to become depends on the sex of the tissue it finds itself in. In those earlier experiments Dr Hayashi injected them into the testes of newly born mice, thus persuading them to become sperm when they underwent meiosis.

This time he used ovarian tissue extracted from mouse fetuses to induce egg-forming meiosis. To keep track of this process, and to avoid confusion, Dr Hayashi took the cells used to make the eggs from a dark-eyed mouse. He then fertilised the eggs he had created in vitro with sperm from a pink-eyed male, and also implanted the resulting embryos into pink-eyed females. Furthermore, as had happened before with the somatic-cell-derived sperm, these pups developed normally into adults and were themselves able to reproduce.

All this is a long way from enabling scientists to perform the same trick with people. First and foremost, using human embryonic tissue in any part of the process is out of the question for ethical reasons. That means someone needs to work out exactly which of the chemicals in testes and ovaries tell primordial germ cells whether to become eggs or sperm. Second, at the moment the process is extremely inefficient.

Third, though mice have proved useful models for examining many questions of human medicine, mere models is all they are. A lot more research will be needed before anyone or, at least, anyone with any ethical sensibility tries something similar on a human being. If and when that day comes, though, the unwillingly childless around the world will be watching with great interest.

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Sperm and Eggs Cells



Where are sex cells formed

See more Both of these sorts of animals have gone on to breed successfully. Moreover, in principle—though he has not yet done so in practice—he could fertilise his somatic-cell-derived eggs with his somatic-cell-derived sperm to create an entirely somatic-cell-derived adult animal. He might even, if he so chose, be able to derive sperm and eggs from the same animal, for the processes do not require that the eggs be made from female cells and the sperm from those of males.

That would create a mouse which had only one parent, yet was not a true clone of that parent because the sex cells which united to form it would both have undergone the internal genetic mixing that biologists call meiosis. One step back, two steps forward Dr Hayashi and his colleagues do not create their eggs and sperm directly from somatic cells. First, those cells have to undergo an alchemical transformation to rejuvenate them into an ancestral form known as a pluripotent stem cell.

Mature body cells eggs and sperm included derive from progenitors, known as stem cells, that have the power to divide, proliferate and eventually to turn into particular cellular components of a particular tissue. Pluripotent stem cells are, in turn, the ancestors of these tissue-forming stem cells. In nature, pluripotent cells are restricted to embryos. Such harvesting is no longer necessary. Instead, for mice, men and many other species, pluripotent cells can be made to order by taking an ordinary body cell and adding to it active copies of the four genes which encode the genetic switches that cause pluripotency.

As he had discovered in his experiments creating sperm, judicious application of a molecule called bone morphogenetic protein 4 turns pluripotent cells into primordial germ cells—the type of stem cell ancestral to both sperm and eggs. Which of these a primordial germ cell goes on to become depends on the sex of the tissue it finds itself in.

In those earlier experiments Dr Hayashi injected them into the testes of newly born mice, thus persuading them to become sperm when they underwent meiosis. This time he used ovarian tissue extracted from mouse fetuses to induce egg-forming meiosis. To keep track of this process, and to avoid confusion, Dr Hayashi took the cells used to make the eggs from a dark-eyed mouse. He then fertilised the eggs he had created in vitro with sperm from a pink-eyed male, and also implanted the resulting embryos into pink-eyed females.

Furthermore, as had happened before with the somatic-cell-derived sperm, these pups developed normally into adults and were themselves able to reproduce.

All this is a long way from enabling scientists to perform the same trick with people. First and foremost, using human embryonic tissue in any part of the process is out of the question for ethical reasons. That means someone needs to work out exactly which of the chemicals in testes and ovaries tell primordial germ cells whether to become eggs or sperm.

Second, at the moment the process is extremely inefficient. Third, though mice have proved useful models for examining many questions of human medicine, mere models is all they are. A lot more research will be needed before anyone or, at least, anyone with any ethical sensibility tries something similar on a human being.

If and when that day comes, though, the unwillingly childless around the world will be watching with great interest.

Where are sex cells formed

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