Female sperm can be either:
- A sperm that contains an X chromosome, produced in the usual way by a male, referring to the occurrence of such a sperm fertilizing an egg and giving birth to a female.
- Sperm containing genetic material from a female.
This article focuses on the second definition.
Since the late 1980s, scientists have explored how to produce sperm where all of the chromosomes come from a female donor. In the late 1990s, this concept became a partial reality when scientists in Japan developed chicken female sperm by injecting bone marrow stem cells from a female chicken into a rooster's testicles. This technique proved to fall below expectations, however, and has not yet been successfully adapted for use on humans.
Female sperm production
Creating female sperm was first raised as a possibility in a patent filed in 1991 by injecting a woman's cells into a man's testicles, though the patent focused mostly on injecting altered male cells into a man's testes (to correct genetic diseases). In 1997, Japanese scientists partially confirmed such techniques by creating chicken female sperm in a similar manner. "However, the ratio of produced W chromosome-bearing (W-bearing) spermatozoa fell substantially below expectations. It is therefore concluded that most of the W-bearing PGC could not differentiate into spermatozoa because of restricted spermatogenesis." These simple transplantation methods follow from earlier observations by developmental biologists that germ stem cells are autonomous in the sense that they can begin the processes to become both sperm and eggs.
One potential roadblock to injecting a woman's cells into a man's testicles is that the man's immune system might attack and destroy the woman's cells. In usual circumstances, when foreign cells (such as cells/organs from other people, or infectious bacteria) are injected into the human body, the immune system will reject such cells. However, a special property of a man's testicles is that they are immune-privileged, that is, a man's immune system will not attack foreign cells (such as a woman's cells) injected into the sperm-producing part of the testicles. Thus, a woman's cells will remain in the man's testicles long enough to be converted into sperm.
However, there are more serious roadblocks. Biologists have well established that male sperm production relies on certain genes on the Y chromosome, which, when missing or defective, lead to such men producing little to no sperm in their testicles. An analogy, then, is that a cell from a woman suffers from complete Y chromosome deficiency. While many genes on the Y chromosome have backups (homologues) on other chromosomes, a few genes such as RBMY on the Y chromosome do not have such backups, and their effects are needed to be compensated for to convert cells from a woman into sperm. In 2007, a patent application was filed on methods for creating human female sperm using artificial or natural Y chromosomes and testicular transplantation. Key to successful creation of female sperm (and male eggs) will be inducing male epigenetic markings for female cells that initially have female markings, with techniques for doing so disclosed in the patent application.
Bone marrow stem cell conversion
Scientists from the University of Newcastle upon Tyne led by biologist Karim Nayernia discovered a method of creating partly developed sperm cells, otherwise known as "spermatogonial" stem cells, from the bone marrow of both sexes, entirely in-vitro (outside the human body), and is seeking funding to see whether such techniques can be used to make female sperm.
If created, a "female sperm" cell could fertilize an egg cell, a procedure that, among other potential applications, might enable female same-sex couples to produce a child that would be the biological offspring of its two mothers. It is also claimed that production of female sperm may stimulate a female to be both the mother and father (similar to asexual reproduction) of an offspring produced by her own sperm even though many queries both ethical as well as moral may arise on these arguments.
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