For a significant portion of the twentieth century, the dominant view in neuroanatomy texts was that the adult human brain was incapable of producing new neurons, a belief that was largely shaped by Santiago Ramón y Cajal’s perspectives in the early 1900s. This notion was transformed in 1998 when a pivotal study spearheaded by Peter Eriksson and Fred Gage documented neurogenesis in the adult human hippocampus. The research used brain tissue from cancer patients who received BrdU, a DNA synthesis tracer, uncovering evidence of new neurons identified by both BrdU and mature neuron markers in the dentate gyrus.
Despite this revelation, doubt lingered due to the surprising nature of adult neurogenesis in mammals, which had long been deemed impossible. Earlier findings from Joseph Altman and Fernando Nottebohm had indicated neurogenesis in animals, but these reports were often ignored or dismissed, especially concerning humans. Pasko Rakic contended against the possibility due to the stability necessary in human neural circuits, although Eriksson’s results rekindled the discussion.
The employment of BrdU as a tracer faced criticism for its constraints, including its capacity to label non-dividing cells, complicating the assertion that new neurons were genuinely being generated. The research team endeavored to tackle these concerns through double-staining techniques; however, their work represented only an initial observation rather than a conclusive account of human neurogenesis.
Over the following two decades, these findings were supported by additional studies utilizing alternative techniques like carbon-14 dating from nuclear tests and advanced imaging methods. Nevertheless, this consensus faced a challenge in 2018 when researchers at UCSF announced their inability to detect new neurons in adult human hippocampus tissue. This discord underscored a methodological deadlock regarding tissue processing methods and the interpretation of markers.
More recently, evidence has shifted back in favor of adult neurogenesis. A 2026 study employing single-nucleus RNA sequencing revealed immature neuron signatures in older adults, implying ongoing neurogenesis but raising questions about their functional integration into neural circuits. These findings indicate that while adult neurogenesis is occurring, the extent and implications continue to be subjects of exploration, leaning towards modest neurogenesis with age-related decline and unclear biological significance.
Eriksson’s publication sparked a crucial conversation in neuroscience, bolstering the idea that although humans may undergo limited hippocampal neurogenesis, broader claims concerning lifestyle-induced cognitive regeneration lack support. Nonetheless, the discovery altered the foundational belief that the adult brain was incapable of adaptation or self-repair, challenging Cajal’s long-standing principle of neural immutability.