Development of the cerebral cortex: VII. Growth factors: II.

Scientists have been surprised by their find ings when they attempt to link intellectual ability among people of normal intelligence to [he number of neurons in [he brain. Contrary to popular belief, there is no observable correlation between the number of neurons and one's intelligence or ability. Yet it is remarkable that most of the idiopathic cases of mental retardation are associated with smaller head size as well as smaller brain size compared to ageand sex-matched normal subjects. It is likely that some of these cases of idiopathic mental retardation arise through disturbances in the birth of neurons and their subsequent differentiation and migration. However, it is also important to consider not only [he total number of neurons that are born but also how they are connecred and [he ability of these neurons to communicate with each other and process information through neural networks. In previous columns, we reviewed [he birth of neurons and their migration to their final cortical destinations. A later marurational event is [he development of interneuronal synaptic connections. In the final stage of development, large groups of neurons are eliminated by programmed cell death. Growth factors have been impl icated in all of these phases . In this column, we discuss how growth factors mediate these effects. Certain growth factors have been shown to increase [he number of progenitors if added to cells in tissue culture.