Neurodevelopment: From Embryo to Adult Brain

By Deliana InfanteReviewed by Danielle Ellis, B.Sc.

Introduction to neurodevelopment
Early stages: From embryo to fetus
Fetal brain development
Neonatal and infant brain development
Childhood and adolescent brain development
Adult brain: Continuing development and plasticity
Factors influencing neurodevelopment
Neurodevelopmental disorders
References
Further reading

Neurodevelopment involves the brain's formation of systems responsible for learning, memory, social skills, and overall function. Critical neurodevelopmental processes such as neurogenesis, synaptic pruning, and neuroplasticity guide this journey, which is strongly influenced by the interplay of genetic and environmental factors.

Introduction to neurodevelopment

The basic definition of neurodevelopment refers to the brain's development of systems or networks responsible for learning, memory, social skills, and overall brain function.¹

The brain's development initiates shortly after conception, with critical phases like cell differentiation, migration, synapse formation, and myelination unfolding rapidly in early life, continuing postnatally with synaptic pruning and neurogenesis in specific regions like the hippocampus.²

This process is complemented by synaptic plasticity, defined as the brain's ability to modify connections in response to experience, facilitating learning and memory that persist throughout life.²

Early stages: From embryo to fetus

Early embryonic brain development starts with the formation of the neural plate, a cumulus of nerve cells in the dorsal part of the embryo, which later forms the neural tube, eventually turning into the brain and the spinal cord.²

These nerve cells divide repeatedly and migrate to the different parts of the brain (the forebrain, midbrain, and hindbrain) formed by the neural tube, where they will have specific functions as neurons or glia.²

The forebrain is composed of the cerebral hemispheres, while the midbrain functions as a bridge containing significant routes to and from the forebrain.² Ultimately, the hindbrain contains the brainstem and cerebellum.² The remaining parts of the neural tube that do not form the principal brain structures become peripheral nerves and certain endocrine glands.²

Fetal brain development

In the second trimester, the fetal brain, specifically the cerebellum in the hindbrain, begins to direct specific movements³; the compression of the chest muscles and movement of the diaphragm, commonly called "practice breaths," are controlled by the brainstem.⁴

This period represents a critical step in the fetal developmental process. The fetus's prefrontal cortex experiences a burst of synaptic development approximately 4-5 months after conception.⁴ Furthermore, the developing brain wires up the special structures that allow people to think abstractly, speak, and engage in sophisticated social relationships within this tangled network of connections.⁴

Myelination is another key process in brain development, which is essential for efficient neural communication.²^,⁵ This process begins prenatally in the third trimester and continues into the postnatal period through adolescence.²^,⁵

During these periods, the fetus is extremely sensitive to harmful exposures and the availability of folate and omega-3 fatty acids in the mother's diet.⁴ Additionally, retinoic acid, a derivative of vitamin A, has been shown to be an indispensable molecule that regulates the synaptic burst in the second trimester.⁶

Neonatal and infant brain development

After birth, the brain undergoes significant synaptic changes. In the visual cortex, synaptic overproduction peaks around the middle of the first year of life, followed by pruning until preschool age, when synapse numbers reach adult levels.² Similar patterns are observed in brain areas related to audition and language.²

On the other hand, the prefrontal cortex, responsible for complex cognitive behaviors, shows a different developmental timeline.² Synaptic overproduction peaks around one year of age, but adult levels of synapses are not achieved until late adolescence.²

Neuroplasticity is at its peak during early childhood, allowing the brain to capture experiences more efficiently, shaping neural connections based on interactions with the environment.⁷⁻⁸ This highlights the importance of positive early experiences and stimulation, as they are indispensable for proper cognitive, emotional, and social development.⁷

Childhood and adolescent brain development

In the period between adolescence and adulthood, children develop brain connections through everyday experiences and interactions with parents and caregivers.⁷ It has been shown that children who experience more positive interactions in their early years tend to be healthier and more successful in life.⁷ Early stimulation and experiences have a lasting impact on a child's chances for achievement, success, and overall happiness.⁷

During this time, there is continuous brain remodeling⁷⁻⁸, as well as neurogenesis⁹, the process of generating new nerve cells. Both processes are vital for memory and learning.²

However, children are especially vulnerable to neurodevelopmental disorders (NDDs).¹⁰ These conditions typically have an onset in early childhood and can persist into adulthood, affecting various aspects of life.¹⁰

Adult brain: Continuing development and plasticity

In adulthood, the brain may heal from injuries, adjust to new situations, and acquire new abilities due to synaptic plasticity.¹¹ This ongoing process is also influenced by aging.¹¹

Aging leads to a decline in synaptic connections, reduced white matter, and decreased neurotransmitter activity, contributing to slower cognitive processing and reduced cognitive functions.¹¹

Adult neurogenesis also seems to occur, but only in certain brain regions, such as the hippocampal dentate gyrus.²^,⁸ Nonetheless, there is still controversy about this concept.¹²⁻¹³

An Overview of Brain Development

Factors influencing neurodevelopment

Several factors can influence brain development, such as inherited diseases, genetic predispositions, and their interplay with environmental factors, as well as maternal nutrition, stress, immune activation, and exposure to teratogens during pregnancy.¹⁴⁻¹⁵

Early life experiences¹⁶ and socioeconomic status¹⁷ during a child's growth to adulthood can influence the neurodevelopmental process.

Neurodevelopmental disorders

NDDs, such as autism spectrum disorder (ASD) or attention-deficit/hyperactivity disorder (ADHD), are highly heritable and associated with common and rare genetic variants.¹⁰

A wide range of environmental factors, including prenatal exposures (e.g., maternal anemia, advanced parental age, toxic chemicals, maternal diabetes) and postnatal factors (e.g., low birth weight, neonatal problems), have also been linked to NDDs.¹⁰

Early signs of NDDs are social interaction difficulties, communication deficits, restricted and repetitive behaviors, and sensory processing abnormalities. These can be diagnosed using the DSM-5 criteria and diagnostic tools such as the First Year Inventory (FYI) and the Quantitative-Checklist for Autism in Toddler (Q-CHAT).¹⁸

Ultimately, NDDs treatments involve behavioral interventions and medication management, although current trends are directed to the use of neuromodulation (e.g., TMS and tDCS).¹⁸

References

1. Chakraborty, R., Kumar, M. V., & Clement, J. P. (2021). Critical aspects of neurodevelopment. Neurobiology of Learning and Memory, 180, 107415. https://doi.org/10.1016/j.nlm.2021.107415
2. From Neurons to Neighborhoods. (2000). In National Academies Press eBooks. https://doi.org/10.17226/9824
3. Lindberg, S. (2020). When Does a Fetus Develop a Brain? Healthline. [Online] https://www.healthline.com/health/when-does-a-fetus-develop-a-brain#second-trimester
4. Critical Periods of Development. (2023). Mother to Baby | Fact Sheets - NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK582659/
5. Olivieri B, et al (2021). Myelination may be impaired in neonates following birth asphyxia. NeuroImage. Clinical, 31, 102678. https://doi.org/10.1016/j.nicl.2021.102678
6. Shibata M, et al. (2021). Regulation of prefrontal patterning and connectivity by retinoic acid. Nature, 598(7881), 483–488. https://doi.org/10.1038/s41586-021-03953-x
7. Baby's Brain Begins Now: Conception to Age 3. (n.d.). Urban Child Institute. [Online] http://www.urbanchildinstitute.org/why-0-3/baby-and-brain#:~:text=The%20brain%E2%80%99s%20ability%20to%20shape%20itself%20%E2%80%93%20called%20plasticity%20%E2%80%93%20lets%20humans%20adapt%20more%20readily%20and%20more%20quickly%20than%20we%20could%20if%20genes%20alone%20determined%20our%20wiring
8. Mowery, T. M., & Garraghty, P. E. (2023). Adult neuroplasticity employs developmental mechanisms. Frontiers in Systems Neuroscience, 16. https://doi.org/10.3389/fnsys.2022.1086680
9. Eriksson P. S, et al(1998). Neurogenesis in the adult human hippocampus. Nature Medicine, 4(11), 1313–1317. https://doi.org/10.1038/3305
10. Carlsson T, et al. (2020). Early environmental risk factors for neurodevelopmental disorders – a systematic review of twin and sibling studies. Development and Psychopathology, 33(4), 1448–1495. https://doi.org/10.1017/s0954579420000620
11. Nichols, H. (2023). What happens to the brain as we age?.  [Online] https://www.medicalnewstoday.com/articles/319185#Therapies-to-help-slow-brain-aging
12. Alshebib Y, et al (2023). Adult human neurogenesis: A view from two schools of thought. IBRO Neuroscience Reports, 15, 342–347. https://doi.org/10.1016/j.ibneur.2023.07.004
13. Lupo, G. (2023). Adult neurogenesis and aging mechanisms: a collection of insights. Scientific Reports, 13(1). https://doi.org/10.1038/s41598-023-45452-1
14. Workalemahu, T, et al. (2018). Genetic and Environmental Influences on Fetal Growth Vary during Sensitive Periods in Pregnancy. Scientific Reports, 8(1). https://doi.org/10.1038/s41598-018-25706-z
15. Doi, M., Usui, N., & Shimada, S. (2022). Prenatal Environment and Neurodevelopmental Disorders. Frontiers in Endocrinology, 13. https://doi.org/10.3389/fendo.2022.860110
16. Malave, L., Van Dijk, M. T., & Anacker, C. (2022). Early life adversity shapes neural circuit function during sensitive postnatal developmental periods. Translational Psychiatry, 12(1). https://doi.org/10.1038/s41398-022-02092-9
17. Tooley, U. A., Bassett, D. S., & Mackey, A. P. (2021). Environmental influences on the pace of brain development. Nature Reviews. Neuroscience, 22(6), 372–384. https://doi.org/10.1038/s41583-021-00457-5
18. Qin, L., Wang, H., Ning, W., Cui, M., & Wang, Q. (2024). New advances in the diagnosis and treatment of autism spectrum disorders. European Journal of Medical Research, 29(1). https://doi.org/10.1186/s40001-024-01916-2

Further Reading

• All Neuroscience Content
• What is Neurology?
• What is the Difference between Neurology and Neuroscience?
• What is Neuroscience?
• Implicit vs. Explicit Memories