Neurogenesis is the process by which the brain produces new neurons from neural stem cells. For over a century, the scientific consensus, influenced by Santiago Ramón y Cajal, maintained that the adult brain was a static organ incapable of generating new nerve cells after early development. This paradigm suggested that the number of neurons in the central nervous system was fixed, meaning any lost cells were gone permanently. Decades of research have since overturned this belief, confirming that neurogenesis is a continuous process that persists into adulthood in certain brain regions. This continual renewal of neurons plays a fundamental part in maintaining cognitive function and emotional well-being.
Where New Neurons Are Formed
The capacity for new neuron formation in the adult human brain occurs primarily within two specific areas known as neurogenic niches. The most intensely studied is the subgranular zone (SGZ), a thin layer located within the dentate gyrus of the hippocampus. The hippocampus is recognized for its function in memory formation and emotional regulation, making the new neurons generated here relevant to these processes.
The second niche is the subventricular zone (SVZ), which lines the lateral ventricles, fluid-filled spaces deep within the brain. In many mammals, cells generated in the SVZ migrate to the olfactory bulb to become interneurons involved in the sense of smell. In the adult human brain, however, the extent of this migration and the number of new neurons integrating into the olfactory bulb are considerably lower. Both the SGZ and the SVZ contain specialized neural stem cells that serve as the precursor pool for adult-born neurons.
The Three Stages of Neurogenesis
The generation of a new functional neuron begins with a neural stem cell and culminates in an integrated nerve cell. The first stage is proliferation, where the radial glia-like stem cells in the SGZ begin to divide. This division generates transient amplifying cells, which rapidly multiply before committing to a neuronal fate.
The second stage involves survival and differentiation, where the newly generated cells, called neuroblasts, begin to mature. They migrate into the granule cell layer of the hippocampus and start to express specific markers that confirm their neuronal lineage. Many of these new cells will die during this phase unless they receive sufficient support signals, such as growth factors.
The final stage is integration, in which the neurons extend axons and dendrites to form synaptic connections with the existing neural circuitry. This process transforms the young cells into mature, functional granule neurons. Successful integration means the newly formed neuron becomes a contributing member of the hippocampal network.
Connecting Neurogenesis to Learning and Mood
The continuous addition of new neurons to the hippocampus supports two major brain functions: memory and emotional processing. Adult-born neurons are important for pattern separation, the ability to distinguish between two highly similar contexts or experiences. These young cells help the brain encode the distinct differences between similar places, preventing memories from merging into a single, confusing representation.
Neurogenesis is also linked to mood regulation and the brain’s response to stress. Newly formed neurons help modulate the activity of the hypothalamic-pituitary-adrenal (HPA) axis, the body’s primary stress response system. A reduction in the generation and survival of new neurons is often associated with symptoms of anxiety and depression. This connection suggests that the neurogenic process helps the brain adapt to environmental challenges.
Modulating Neurogenesis Through Lifestyle
The rate at which new neurons are generated is responsive to environmental and behavioral factors, influencing brain health. Aerobic exercise promotes neurogenesis, significantly boosting the production of brain-derived neurotrophic factor (BDNF), a protein that supports the growth and survival of new neurons. Activities like running or brisk walking stimulate the proliferation stage by increasing blood flow and growth factors in the neurogenic niche.
Cognitive activities that demand complex thought, such as learning a new language or skill, also enhance the survival and integration of new neurons. Dietary modifications, including calorie restriction and consuming foods rich in omega-3 fatty acids and flavonoids, support neurogenesis. These nutritional factors often reduce oxidative stress and inflammation, creating a healthier microenvironment for cell growth.
Conversely, certain lifestyle factors inhibit new neuron formation. Chronic stress reduces neurogenesis by elevating levels of glucocorticoids, or stress hormones, which suppress the proliferation of precursor cells. Similarly, sleep deprivation and excessive consumption of alcohol or diets high in saturated fat and sugar can negatively affect the survival and maturation of new neurons. These negative inputs disrupt the neurogenic niche, leading to a decrease in the brain’s capacity for renewal.
Neurogenesis and Neurological Conditions
The discovery of adult neurogenesis has impacted the understanding of several neurological and psychiatric disorders, suggesting that a breakdown in this process contributes to pathology. Major depressive disorder (MDD), for example, is consistently linked to a reduction in hippocampal neurogenesis and a decrease in hippocampal volume. This observation forms the basis of the neurogenesis hypothesis of depression, which posits that impaired neuron formation contributes to the cognitive and emotional symptoms of the illness.
Many effective antidepressant medications increase the rate of neurogenesis, suggesting that stimulating the birth of new neurons may be how these drugs exert their therapeutic effects. Similarly, neurodegenerative conditions like Alzheimer’s disease are associated with impaired neurogenesis. Research indicates that the ability of the brain to generate and integrate new neurons declines in these patients. This disruption is considered a potential mechanism linking cognitive decline and mood disorders. Current therapeutic research is exploring ways to harness neurogenesis to repair damaged circuits and restore function in various conditions.