last updated:

March 13, 2021

[Brain Image]    

PSY 340 Brain and Behavior

Class 15: Development of the Brain


    [fetus image]    [Aguillera-Hellweg] 

"Prenatal Surgery May Be Preferable for Spina Bifida" (USN&WR Feb 10, 2011)
"The results also show that 42 percent of children who had undergone surgery in the womb were able to walk unassisted at age 3 compared with 21 percent of those who received the surgery postnatally. The two groups didnít show marked differences in mental development. The in utero surgery was done between 19 and 26 weeks of gestation."

I. Maturation of the Vertebrate Brain

Week 1: Conception -> migration of emerging organism to uterus

Week 2: Implantation of organism (termed a "blastocyst" [III] into the uterine endometrium (wall of uterus; IV)

Week 3: Differentiation of embryonic nervous system begins (see below)

Early Embryonic Development

[Ectoderm]    [Embryonic Development]
  • CNS begins to develop at the beginning of Week 3. The ectoderm thickens to form the neural plate which in term folds and forms the neural tube around a fluid-filled cavity. The upper portion of the neural tube becomes the brain while the lower portion becomes the spinal cord. The cavity becomes the central canal of the spinal cord and the ventricles of the brain. In diagram above on the right note how the folds of the neural tube at Day 22 begin closing like a zipper (on Day 23 and following).
  • At birth, the human brain weighs ca. 350 grams (vs. 1000 grams at the end of year 1 and 1200-1400 grams for the adult brain)

A. Growth and Development of Neurons

1. Proliferation = production of new cells

2. Migration (Movement)

3. Differentiation = develops dendrites and an axon

4. Myelination = glial cells produce fatty sheaths to insulate (myelinate) axons

5. Synaptogenesis = formation of synapses

B. New Neurons (Neurogenesis) in Later Life?

II. Pathfinding by Neurons

A. Chemical Pathfinding by Axons

              Experiment]How do axons know where to go in linking up to target organs? They use chemical trails or pathways.

B. Competition Among Axons as a General Principle

III. Determinants of Neuronal Survival

[Rita Levi-Montalcini]1. Nerve Growth Factor (NGF)

The Nobel-winning Italian scientist, Rita Levi-Montalcini (d. 2012), discovered that muscles produce a chemical, nerve growth factor (NGF), which is released at the synapse with axons. NGF promotes the survival and growth of these axons which are part of the sympathetic nervous system.

All neurons are born with a "suicide" program, i.e., instructions to die automatically if they do not make the right synaptic connections. Programmed cell death is called apoptosis. Some neurons must receive NGF or they will die.

2. Neurotrophins = Chemicals like NGF or BDNF (brain-derived neurotrophic factor) that promote survival & activity of neurons. We do not entirely understand what controls the survival of neurons in the cortex and other places of the brain. But, research suggests that NGF and BDNF lead to:

  • Prevention of apoptosis
  • Increased axonal branches
  • Increased regrowth of axons after brain damage

3. Surplus Neuron Production

  • The developing brain of the fetus produces more neurons than it will eventually need. For example, between Weeks 10 and 25 of gestation, the number of motor neurons in the ventral spinal cord decreases from 175,000 to about 120,000.
  • The brain produces 2 to 3 times as many neurons as needed in adulthood
    • There is a decrease in both the njmber of neurons and the number of synapses in the first two-three years after birth,
    • Adolescent brain shows decrease in number of neurons but growth of white matter in parietal & temporal cortex, i.e., more connections & myelination

IV. The Vulnerable Developing Brain

The pattern of brain development in most animal species is directed by a set of genes known as homeobox genes. These control how other genes express themselves and, thus, how anatomical development unfolds.

"Homeobox genes are a large family of similar genes that direct the formation of many body structures during early embryonic development. A homeobox is a DNA sequence found within genes that are involved in the regulation of patterns of anatomical development morphogenesis in animals, fungi, and plants. In humans, the homeobox gene family contains an estimated 235 functional genes and 65 pseudogenes, which are structurally similar genes that do not provide instructions for making proteins. Homeobox genes are present on every human chromosome, and they often appear in clusters." (Structural Biochemistry, Wikibooks)

During prenatal development, the nervous system is highly vulnerable to a variety of threats (compared to what would happen later in life). These include

      Toxic Chemicals

[FAS]Alcohol Use in Pregnancy --> Fetal Alcohol Spectrum Disorders (FASD) which includes Fetal Alcohol Syndrome (FAS)

"As suggested by the American Academy of Pediatrics, the message about alcohol use during pregnancy to the public should be clear and consistent: there is no safe amount, time, or type of alcohol to drink during pregnancy or when trying to get pregnant."(Lange, Rehm, & Popova, 2018, p. 448)

[Cigarette smoke contents]Cigarette Smoking: prenatal exposure to maternal cigarette smoking may lead to a variety of disorders

(Omit: Differentiation of the Cortex)

V. Fine-Tuning by Experience

Our nervous system can be "fine-tuned" or altered (within limits) by the effects of experience

[Dendritic Branching]1. Experience & Dendritic Branching

2. Effects of Special Experiences

[Braille Alphabet]Blindness

Musical Training

Focal hand dystonia: fingers of musicians become clumsy & have difficulty playing correctly. Appears to be related to reorganization of the thalamus and somatosentory cortex devoted to the fingers: areas for individual fingers begin to overlap.

VI. Brain Development and Behavioral Development

1. Adolescence

Teen Peer
            Driving Risk

2. Old Age


Albert, D., Chein, J.,  Steinberg, L. (2013). Peer influences on adolescent decision making. Current Directions in Psychological Science, 22(2), 114-120. doi: 10.1177/0963721412471347

Chein, J., Albert, D., O'Brien, L., Uckert, K., & Steinberg, L. (2011). Peers increase adolescent risk taking by enhancing activity in the brain's reward circuitry. Developmental Science, 14(2), F1-F10. doi: 10.1111/j.1467-7687.2010.01035.x. 

Duque, A. & Spector, R. (2019). A balanced evaluation of the evidence for adult neurogenesis in humans: implication for neuropsychiatric disorders. Brain Structure and Function. https://doi.org/10.1007/s00429-019-01917-6

Lange, S. Rehm, J., & Popova, S. (2018). Implications of higher than expected prevalence of fetal alcohol spectrum disorders. JAMA, 319(5), 448-449. doi:10.1001/jama.2017.21895

May, P. A., Chambers, C. D., & Kalberg, W. O., et al. (2018). Prevalence of fetal alcohol spectrum disorders in 4 US communities. JAMA, 319(5), 474-482. doi:10.1001/jama.2017.21896

This page was first posted February 16, 2005.