Can Adults Grow New Brain Cells? Neurogenesis Explained

Introduction

The human brain is an adaptive organ whose cells — neurons — and their associated connections continue to change throughout life. These continuous changes enable people to acquire new skills, learn and remember, and even recuperate from brain injury. A significant percentage of brain cells are formed while in the womb, but some parts of the brain continue to create new brain cells during infancy. Traditional wisdom suggests that adults cannot grow new brain cells because of various aspects of aging. However, this issue has attracted considerable attention in the fields of psychology and neuroscience and has been the subject of numerous recent studies seeking to determine whether the brain continues to develop new cells throughout an individual's lifespan. These studies have shown that the conventional view — that adults cannot grow new brain cells — is a myth. The adult brain is characterized by processes and factors that induce morphological changes in brain areas, resulting in the creation of new brain cells, or neurons.

Overview of Neurogenesis

According to Cherry (2020), a significant percentage of brain cells are developed while we are in the womb, though some parts of the brain continue to create neurons during infancy. Neurogenesis — the creation of new brain cells — has long been characterized by the assumption that the brain has a limited capacity to regenerate. This limited-capacity view has contributed to the belief that the creation of new brain cells ceases after infancy. Consequently, scientists have continued to debate whether brains change with age but no new brain cells are developed past infancy.

Kumar et al. (2019) state that neurogenesis in adults remains a controversial issue among neuroscientists and psychologists alike. On one hand, some neuroscientists contend that a sharp decline in the creation of new brain cells occurs as the human brain ages. On the other hand, others contend that neurogenesis persists into old age, especially in the dentate gyrus (DG) of the hippocampus. Despite these differing opinions, neurogenesis remains a critical component of brain development. Given the controversies surrounding it, developing an evidence-based understanding of neurogenesis is important, since its presence or absence can affect the foundations of learning and memory in relation to aging.

Adult neurogenesis remains the most compelling phenomenon of neuroplasticity, as it takes place in the developing central nervous system. The brain has traditionally been considered a non-renewable organ — unlike most cells of the body, such as skin, gut, or blood cells, which are continuously renewed (Fuchs & Flügge, 2014). However, since some illnesses like multiple sclerosis occur in adulthood, there are legitimate concerns about whether adult brains are capable of replacing lost neurons.

Debunking the Myth

Adult neurogenesis continues to attract significant attention among neuroscientists and researchers. As noted above, conventional wisdom has long suggested that adults cannot grow new brain cells, a view fueled in part by the fact that certain cognitive functions and motor abilities decline with age (Cherry, 2020). However, the idea that no new brain cells are developed during adulthood has been challenged for more than five decades. Some scientists have carried out studies to debunk the notion, partly motivated by evidence that even in mammals, parts of the adult central nervous system retain the ability to replace neurons. Existing evidence has shown that adult neurogenesis occurs in mammals, birds, reptiles, amphibians, and bony fishes (Fuchs & Flügge, 2014), forming the foundation for research into adult neurogenesis in humans.

This paper argues that adults can, in fact, grow new brain cells. As recent studies demonstrate, neurogenesis is a process that continues throughout an individual's lifespan. Two core questions guide the argument:

Evidence Refuting the 'No New Neurons' Dogma

The controversy over whether adults can grow new brain cells has generated numerous arguments and counterarguments. Some studies have argued that adult neurogenesis does not occur because the human brain peaks in development by age 13. According to Scutti (2018), a study published in the journal Nature found that the development of new brain cells in the hippocampus ends before an individual reaches adulthood. For new brain cells to be created, the newly formed cell must migrate to the appropriate location — a highly complex process given the increasingly dense structure of the adult brain. The new cell must also create space to grow and connect with existing neurons in order to make functional contributions to the neural circuit. New brain cells are produced by progenitor cells, also known as stem or precursor cells, and are considered important because they possess properties distinct from those of mature brain cells. This study provided strong evidence that the human brain's ability to produce new cells in the hippocampus is limited as a person ages.

In addition, experiments involving lab mice, rats, and non-human primates have been used to argue that neurogenesis declines with age. According to Mandal (2018), these experimental studies found that the creation of new brain cells decreases as the animals grow older. Some scientists further argue that certain studies have found no evidence of young or new neurons in adult tissue samples, and they question claims that antidepressant drugs and exercise can stimulate the creation of new brain cells in the hippocampus.

While these scientists have used their findings to argue against adult neurogenesis, a growing body of recent evidence points in the opposite direction. Neurogenesis does appear to continue through adulthood, as demonstrated by research into both brain processes and the brain's capacity to replace lost neurons.

Brain Processes and Factors That Drive Adult Neurogenesis

Fuchs & Flügge (2014) note that the scientific view of the mature vertebrate brain has changed significantly over the past four decades. Prior to this shift, the adult brain was long considered unchanging and stable — subject only to the inevitable decline that accompanies aging. However, clear evidence of structural plasticity occurring throughout an individual's lifespan has shown that the adult brain is far from fixed (Gage, 2004). These structural changes encompass the creation of new neurons and other brain cells as well as the formation of new connections between them. Such changes are necessary to give individuals the additional capacity to manage the complexities of everyday life, and they provide the basis for adaptability — what neuroscientists call plasticity. Structural plasticity in the brain, therefore, supports the creation of new neurons as individuals cope with challenges throughout their lives.

Gage (2004) further explains that the structural plasticity of the adult brain provides the mechanism by which the brain repairs itself. Generally, each body organ — including the brain — has the ability to repair itself once it has matured. The brain's capacity for self-repair into adulthood is driven by processes and factors that induce the development of new neurons. Changes that occur in the adult brain are influenced by both the behaviors the individual engages in and his or her environment. Environmental factors such as environmental stimulation, learning, certain drugs, stress, growth factors, neurotransmitters, and adrenal and gonadal hormones alter neuronal structures and functions (Fuchs & Flügge, 2014). These factors generate changes in brain processes, including morphological alterations in brain areas, changes in neuron morphology, and network alterations. Together, they produce changes in neuronal activity, neurobiochemical shifts, and the creation of new neurons. Structural plasticity of the adult brain is therefore driven by environmental factors and individual behaviors that, in turn, influence brain processes resulting in the creation of new brain cells.

The evidence of structural plasticity implies that the notion that adults cannot grow new brain cells is a myth. Because of structural plasticity, scientists have discovered that neurogenesis continues throughout the lifespan and is influenced by environmental factors and brain processes. Scientists have also observed neurogenesis in the adult nervous system, leading to the discovery of stem cells in the adult brain that contribute to the creation of new neurons. These multipotent stem or precursor cells divide periodically, producing daughter stem cells that can develop into functional cells. Gage (2004) further notes that adult neurogenesis does not occur spontaneously throughout all brain regions but is concentrated in two areas: the zone of cell genesis and the region of white cell division.

Given this evidence, brain processes and environmental factors clearly influence brain development during adulthood. As structural changes occur, individuals develop greater capacity to cope with the complexities of daily life — a process that generates neural activity contributing to the development of new brain cells. There is, therefore, a strong link between brain development and adult neurogenesis, implying that adults do develop new brain cells as they adapt to challenges across their lifespans. Notably, while there are differences between the brains of young people and older people, a similar number of immature neurons and progenitor cells have been found in human brains regardless of age (Netburn, 2018). This suggests that the human brain continues to develop hundreds of new neurons on a daily basis.

Conclusion

This paper examined whether adults can grow new brain cells — a question that has sparked enduring debate in neuroscience and psychology. Conventional wisdom and some studies have suggested that the human brain does not generate new brain cells in adulthood after reaching a developmental peak around age 13. However, a growing body of scientific evidence challenges this view on two key grounds: the role of brain processes and environmental factors in driving structural plasticity, and the brain's ability to replace neurons lost to diseases such as Parkinson's disease and multiple sclerosis. Existing evidence demonstrates that the idea of a fixed, unchanging adult brain is a myth. Structural plasticity and the documented replacement of lost neurons indicate that the brain undergoes morphological changes that induce the creation of new brain cells throughout the lifespan.

References

Cherry, K. (2020, April 10). How brain cells regenerate. Retrieved September 25, 2020, from https://www.verywellmind.com/adult-neurogenesis-can-we-grow-new-brain-cells-2794885

Fuchs, E., & Flügge, G. (2014, May 4). Adult neuroplasticity: More than 40 years of research. Neural Plasticity, 2014(5), 1–10.

Gage, F. H. (2004, June). Structural plasticity of the adult brain. Dialogues in Clinical Neuroscience, 6(2), 135–141.

Kumar, A., Pareek, V., Faiq, M. A., Ghosh, S. K., & Kumari, C. (2019, May 1). Adult neurogenesis in humans: A review of basic concepts, history, current research, and clinical implications. Innovations in Clinical Neuroscience, 16(5–6), 30–37.

Mandal, A. (2018, April 6). New brain cells are added in elderly adult brains too. Retrieved September 25, 2020, from https://www.news-medical.net/news/20180406/New-brain-cells-are-added-in-elderly-adult-brains-too.aspx

Netburn, D. (2018, April 5). Surprise! Scientists find signs of new brain cells in adults as old as 79. Los Angeles Times. Retrieved September 25, 2020, from https://www.latimes.com/science/sciencenow/la-sci-sn-new-brain-cells-20180405-story.html

Scutti, S. (2018, March 8). Your brain stopped making new cells at age 13, study claims. CNN. Retrieved September 25, 2020, from https://edition.cnn.com/2018/03/07/health/new-brain-cells-adulthood-study/index.html