As the dramatic abortion debate plays out in courtrooms and legislatures across the nation, a basic and vitally important question arises ever more insistently--when does human life begin?
The Roman Catholic Church claims conception is life's starting point, whereas most current abortion laws are keyed to when the fetus might survive if born prematurely (at the end of the second trimester).
But armed with the latest findings on fetal development, some scientists argue that "brain birth" in the fetus should mark the beginning of human life, just as "brain death" already is used to define the end of life.
"What we take ourselves to be as persons," said developmental biologist Michael Flower of Lewis and Clark College in Portland, Ore., "are capacities made possible by our brains." In a recent paper, Flower outlined the stages of brain development in the embryo and fetus and their implications for the abortion debate.
"Prenatal life is full of changes, by nature, and any of those changes may be of moral significance," Flower added.
The first brain cells do not appear in a human embryo until close to the third week after conception, and studies indicate that the thinking and sensing portions of the brain where awareness resides do not click on until after the 28th week. (All ages of embryos or fetuses in this article are days or weeks since conception, not gestational age.)
When an embryo reaches the seventh week, the first fully developed nerve cells, called neurons, can be found topping the spinal cord and forming the brain stem, according to Ronan O'Rahilly, director of the Carnegie Laboratories of Embryology at UC Davis. This portion of the brain--once a person is born--regulates breathing, blood pressure, heartbeat and primitive reflexes such as swallowing and the pupils' response to light.
A new study by O'Rahilly has shown that by eight weeks the brain stem of the embryo looks "extraordinarily similar to that of a newborn baby." About this point, the 1 1/4-inch embryo has a face, hands and feet that also look, at first glance, like those of a human baby.
The development of the brain stem probably is responsible for most of the embryo's movements, which begin about six weeks after conception and include a general fluttering of the body, known as the startle reflex.
At this point, the embryo does not have the upper portions of the brain that allow a person to intentionally move a part of his body, according to Michael Bennett, neuroscience chairman at Albert Einstein Medical School in the Bronx, N.Y.
Some ethicists say a human life begins when the brain stem is born. But Bennett counters that "the first appearance of a group of recognizable neurons or of reflexes doesn't make us human, because those are seen in lower animals."
What makes us distinctly human, according to Bennett and many other neuroscientists, is the outer layer of brain, called the neocortex. This critical portion of the brain is the seat of consciousness and complex thought. It enables a person to be aware and respond to what surrounds him.
"The neocortex allows us to recognize one another, speak and make plans," Flower said. "Without our neocortex," Bennett added, "we wouldn't be much better than a reptile."
The first neocortical cells appear about a month after conception, but three-quarters of the neocortex is not formed until the fetus is close to 6 months old, according to Dartmouth College's Miguel Marin-Padilla, an expert on human fetal brain development. Although the fetal neocortex is nearly developed at this time, the bulk of it will not be working for several more weeks--"the phones are in place, but there are no wires connecting them," according to Flower.
In order for the neocortex to function, its nerve cells must establish a chain of communication by sprouting interconnecting fibers. A few isolated connections between nerves in the neocortex and the muscles they control can be detected in a fetus as young as 15 weeks, and by 22 weeks the most primitive part of the cortex that governs movement of the fetus's limbs has matured to the point of being functional, according to Marin-Padilla.
But at this point, there still is not enough circuitry for intentional movements. "If I pinch a child that is born at 22 weeks after conception," Marin-Padilla said, "he's going to move his arm away because of his reflexes. He's not thinking, 'He pinched me, therefore I'm going to move my arm away from him.' "
Starting about 28 weeks, there is a burst of connections made between the neurons in all parts of the neocortex by cells appropriately called interneurons. "The bulk of what we do with our brains," Marin-Padilla said, "is done by these interneurons. These little guys allow you to write, play tennis and carry out a variety of complex functions."
