Comprehensive review on adult neurogenesis through immunological studies in the brain regions of vertebrate classes.

Adult neurogenesis is the process in which new neurons are produced, mature, and integrated into the neural circuits of adult vertebrate brains- fish, amphibians, reptiles, aves, and mammals. All vertebrate groups exhibit a great capacity for neurogenesis, which is dependent upon the number of neurogenic zones in the brain. The fish brain is known to have the highest neurogenic rate because they acquire the maximum number of neurogenic zones, such as the olfactory bulb, telencephalon, preoptic area, cerebellum, thalamus, hypothalamus, optic tectum, and medulla oblongata. The amphibian brain is also known for its high number of proliferative zones similar to fish, whereas reptilian, avian, and mammalian brains show adult neurogenesis primarily in the telencephalon. The process is affected by aging and hormones but is highly sensitive to environmental factors, such as stress, seasonal changes, and temperature, which subsequently control (increase or decrease) the neurogenesis rate. However, aging is the most common factor known to suppress neurogenesis. Various stages of adult neurogenesis were distinguished by a number of molecular markers, like 5-Bromo-2'-deoxyuridine, Doublecortin, nestin, glial fibrillary acidic protein, etc. The study of neurogenesis in various organisms aids in the better understanding of the capacity of the brain to modify itself according to the changing environment, to form new synaptic connections by replacing old neurons with new ones. The new synaptic connections help with memory and cognition.