device ever constructed is on its way. And it starts out a fraction of the size of the period at the end of this sentence.
I have been studying this stuff for more than 20 years, and I still find it amazing. As scientist Lewis Thomas put it in The Medusa and the Snail: “The mere existence of such a cell should be one of the great astonishments of the earth. People ought to be walking around all day, all through their waking hours, calling to each other in endless wonderment, talking of nothing except that cell.” Go ahead, call your neighbor; I’ll wait.
The miracle continues. If you could see it in action, this embryo floating in seawater, you would notice that the inner cell mass is actually swarming with cells, scurrying around like busy short-order cooks at the county fair. The cells arrange themselves into three living layers, looking for all the world like a cheeseburger. The bottom bun, called the endoderm, will form most of the cell systems that line your baby’s organs and vessels. The burger layer, the mesoderm, forms your baby’s bones, muscles, blood, and various connective tissues, amongst other systems. The top bun is the ectoderm. It will create your baby’s skin, hair, nails, and nervous systems. It is within the ectoderm that the miraculous little pre-brain cell resides.
Looking closer, you would see the tiniest line of cells forming atop the bun’s center. Below, a log-shaped cylinder begins to form and elongate itself, using the overhead line as a guide. This cylinder is the neural tube. It will give rise to the spinal column—the far end of the log becoming your baby’s butt, the near end becoming baby’s brain.
When something goes wrong
It’s vital that this neural tube develop properly. If it doesn’t, the baby could have a protruding spinal cord or a tumor near his lower back, a condition known as spina bifida. Or the baby could grow without a complete head, a rare condition known as anencephaly. This is why every pregnancy book strongly recommends taking the B-complex vitamin folic acid: It helps shape the proper neural tube—both the near and far ends. Women who take folic acid around conception and during the first few weeks of pregnancy are 76 percent less likely to create a fetus with neural tube defects than those who don’t take the supplement. It is the first thing you can do to aid brain development.
Parents-to-be throughout history have worried about whether their babies are developing properly. In 1573, French surgeon Ambroise Paré catalogued the events to which prudent young pregnant couples should be alerted to avoid a child with birth defects. “There are several things that cause monsters,” he wrote in On Monsters and Marvels. “The first is the glory of God. The second, His wrath. The third, too great a quantity of seed [sperm]. The fourth, too little a quantity.” Paré hypothesized that a birth defect could be caused by indecent posturing of mom (she sat too long with her legs crossed). Or it could be due to the narrowness of the uterus, demons and devils, or the wicked spittle of beggars.
We can perhaps forgive Paré’s pre-scientific misunderstanding of in utero brain development. Even to the modern mind, it is scary, hopelessly complex, and mostly mysterious. Researchers today are at a complete loss to explain nearly two-thirds of all birth defects. Indeed, only a quarter of all known birth defects have been tied to an isolable DNA problem. One of the reasons we know so little is that mom’s body appears to have a fail-safe mechanism. If something goes wrong during development, her body often senses trouble and deliberately induces a miscarriage. About 20 percent of pregnancies end in spontaneous abortion. Known environmental toxins, things you can actually monitor, account for only 10 percent of the birth defects observed in the lab.
A delicate web of cells, crackling with electricity
Fortunately, most babies’ brains form just fine. The brain end of the neural tube continues its construction project by creating bulges of cells that look like complex coral formations. These eventually form the large structures of the brain. Before the first month, the baby’s tiny pre-brain cell has grown into a hefty army, millions of cells strong.
The brain does not develop in isolation, of course. The early embryo temporarily displays gill arches around the fourth week, for example, much like the ones fish have. These soon convert into face muscles and the throat structures that will allow your baby to speak. Your embryo next gets the stub of a tail but soon reverses course and resorbs the structure. There are strong evolutionary roots to our development, and we share this miracle with every other mammal on the planet. Except for one thing.
Those bulges at the end of your embryo’s neural tube will turn into a great big, fat, super-smart brain—about the heaviest brain-per-body mass that exists on the planet. This massive structure is composed of a delicate spider web of cells, crackling with tiny bolts of electricity. Two types of cells are important here. The first type, glial cells, make up 90 percent of the brain cells inside your child’s head. They give the brain its structure and help the neurons correctly process information. It’s a good name; glial is a Greek word for glue. The second type of cell is the familiar neuron. Though they do a lot of your child’s thinking, neurons make up only about 10 percent of the total number of brain cells. That’s probably where we get the myth that you use only 10 percent of your brain.
One neuron, 15,000 connections
So how do cells turn into brains? Embryonic cells are manufactured into neurons in a process called neurogenesis. This is when the baby would like to be left alone, in the first half of pregnancy. Then, in the second half of pregnancy, the neurons migrate to the region they eventually will call home and start wiring together. This is called synaptogenesis.
Cell migration reminds me of when tracking bloodhounds are suddenly loosed from the sheriff’s truck to pick up the scent of a criminal. Neurons bolt out of their ectodermal cages, crawling over one another, sniffing out molecular cues, pausing, trying out different pathways, slithering helter-skelter throughout the developing brain. Eventually they stop, having arrived at a destination that may be pre-programmed into their cellular heads. They look around their new cellular digs and try to hook up with the neighbors. When they do, tiny, lively gaps between neural cells are created, called synapses (hence, the term synaptogenesis). Electrical signals jump between the naked spaces to allow neural communication. This final step is the real business of brain development.
Synaptogenesis is a prolonged process, for an easily understood reason: It is ridiculously complex. A single neuron has to make an average of 15,000 connections with the locals before its wiring job is done. Some neurons have to make more than 100,000 connections. That means your baby’s brain has to lash together an astonishing 1.8 million new connections per second to make a complete brain. Many of the neurons never complete the process. Like post-sex salmon, they simply die off.
Even given this incredible speed, baby brains never make the birth deadline. About 83 percent of synaptogenesis continues after birth. Surprisingly, your baby girl’s brain will not completely finish its wiring until she is in her early 20s. Boys’ brains may take even longer. In humans, the brain is the last organ to finish developing.
When can baby hear you, smell you?
The purpose of that furiously fast (then frustratingly slow) production is to build a functioning brain, one that can receive and respond to inputs. So the questions for prying parents become: What do fetuses know, and when do they know it? When is your baby capable of sensing, say, taps on your belly?
The developmental principle to remember is this: The brain spends the first half of pregnancy setting up its neuroanatomical shop, blissfully ignoring most parental involvement. (I am referring to well-intentioned interference. Drugs, including alcohol and nicotine, clearly can damage a baby’s brain during pregnancy.) The second half of pregnancy is a different story. As brain development moves from mostly neurogenesis to mostly synaptogenesis, the fetus begins to exhibit much greater sensitivity to the outer world. The wiring of cells is much more subject to outside influences—including you—than the
