When does human life begin?

The story of when human life begins has a checkered past. Centuries before people knew of sperm and eggs, Aristotle argued that the fusion creating a new person did not exist until “quickening,” the first noticeable movements in a woman’s womb. He reckoned quickening occurred 40 days into pregnancy (18-20 weeks is the actual time). The 40-day rule was picked up by Jewish and Muslim religions. In 1591, Pope Gregory XIV supported this view of delayed animation and ensoulment.

The Catholic Church did not reach its current conclusion that life begins at fertilization until 1896, when Pope Pius IX condemned abortion at any age after the moment of conception. Many Jewish theologians now argue that life begins seven days into pregnancy, with implantation of the embryo.

Gene transcription of male-contributed genes starts even later (well after stem cells are harvested), and many scientists feel human individuality cannot be said to begin until then, when the embryo starts to actually use the genes contributed by fertilization.

The United States Supreme Court takes the position that human life begins much later, when the fetus becomes capable of independent life if separated from the mother—roughly the third trimester.

Early this month researchers at the University of Pennsylvania reported suprising findings that could profoundly alter our views of when human life begins. Indeed, if these results, obtained in cultures of mouse cells, can be repeated in human cells, theologians may have to reconsider their ideas about the very nature of life.

The researchers were raising dense cultures of mouse embryonic stem cells for research. Scientists are particularly interested in embryonic stem cells because each one of them has the potential to develop into any cell of an adult individual. In a routine screening, the researchers treated the mouse cells with a fluorescent “marker” protein they had developed to test for early-stage eggs and sperm. The marker protein sticks specifically to the surfaces of eggs and sperm, but not to any other kind of cell. Because these stem cell cultures were not eggs or sperm, the screening should have had negative results.

Surprisingly, in a culture of stem cells that had been maintained 8 days, the marker protein did stick. Over 40% of the cells glowed green, showing the fluorescent protein marker had stuck to them.

Were these egg cells? Studying them more closely, the researchers learned that groups of the cells had begun to produce estradiol, a hormone that converts to the female hormone estrogen. After 16 days, the oocyte-like cells began producing tell-tail proteins typical of meiosis, the special form of cell division that sperm and egg cells undergo. The observations were tantalizing: many of the embryonic stem cells seem to have spontaneously developed over a few weeks many of the characteristics of egg cells (oocytes).

Are they really egg cells? To be sure, the researchers will need to demonstrate that the “oocyte-like” cells can be fertilized and produce viable offspring. Until those experiments are completed, the case that they are eggs is incomplete, although strong.

The researchers continued to observe the aging cell culture, and their perseverance was rewarded with a result that may fundamentally alter how we look at stem cell research. After 40 days, the groups of oocyte-like cells in the aging culture formed what appear to be early embryos!

Examining these early embryos, the researchers found that their cells were manufacturing the sorts of proteins one would expect of normal 16-cell embryos. A few of the embryos in the culture went on to form complex balls of cells resembling blastocysts, the early-stage embryos from which embryonic stem cells are harvested.

What is going on here? These egg cells were never fertilized — how could they develop into embryos?

Biologists have long recognized that in insects, many fish and some reptiles, adults develop from unfertilized eggs, a process called parthogenesis. That seems to be what is going on here. Normal mouse egg cells can be induced to form embryos parthenogenetically, but despite many attempts to implant them in a womb, none have ever survived to birth.

While there is no guarantee that what happens in mice will happen in humans, there seems a very good chance that similar results will be obtained in human embryonic stem cell cultures, which leads to a very interesting question. What is the ethical status of human embryos created from embryonic stem cells without fertilization?

The very possibility of human embryos produced without fertilization must have some theologians reconsidering their ideas about the nature of life as something that starts at conception with the union of egg and sperm. One thing seems sure. The controversies raised by stem cell research will continue.

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