Brain Development
QUICK LOOK AT THE CHAPTER AHEAD
At about the beginning of the third week of pregnancy, the first signs of the fetus’s nervous system appear.
Neural induction causes a portion of the ectoderm to become the neural plate, which leads to the formation of the nervous system.
The neural plate forms a groove and begins to fold in on itself, leading to the creation of the neural tube.
Developmental biologists believe that neurons know which part of the brain they will travel to and which type of nerve cell they will become (e.g., motor, vision, hearing).
The speed of nervous system development is stunning. To produce the 100 billion neurons of the adult brain, nerve cells are produced at the rate of 250,000 per minute during pregnancy.
TERMS
Brain lateralization
Cell migration
Frontal lobe
Glia
Neural induction
Neural tube
Neurogenesis
Neurons
Occipital lobe
Parietal lobe
Temporal lobe
Any aspect of development—physical, emotional, cognitive, or psychosocial—depends on the intricacy of the brain for its optimal development. In spite of today’s sophisticated technology, just how many neurons develop in the course of a lifetime is not known. Nevertheless, the rapid development of the nervous system is reflected in the following numbers:
200 billion brain cells develop in the fetus’s brain by the fifth month.
100 billion neurons (brain cells) survive in a newborn baby’s brain.
1 trillion glia (support) cells appear in a baby’s brain.
1000 trillion connections are formed in a baby’s brain.
What causes the fetus to lose 100 billion brain cells in the space of the 4 remaining months of pregnancy? The answer lies in nature’s overproduction of brain cells or neurons. Those neurons that do not make connections simply die. This startling fact recalls Darwin’s famous belief in the survival of the fittest: The fittest of our neurons are those that make connections and survive.
What causes the fetus to lose 100 billion brain cells in the space of the 4 remaining months of pregnancy? The answer lies in nature’s overproduction of brain cells or neurons.These amazing figures testify to an infant’s potential for processing information. For example, the appearance of new cognitive abilities is correlated with rapid brain growth, both of which are major features of infancy.
BEGINNINGS OF BRAIN DEVELOPMENTThe beginning of brain development (about the 3rd prenatal week) lies in a process called neural induction, when a chemical signal from the mesoderm to the ectoderm triggers the onset of nervous system development (Figure 25-1). Nerve cells proliferate rapidly in the neural tube, but quickly leave this area and commence a sometimes lengthy, tortuous, and even perilous journey to the region of the brain where they will become functional. This phase, called cell migration, begins during the 7th prenatal week. During their passage, the nerve cells may double back, twist, and turn—some will even die. In their migration, the nerve cells continue to grow and develop and acquire their familiar neuronal shape. Those destined for survival reach their point of destination and become functional.
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