Evolution of neuronal numerical density in the developing and aging human visual cortex.

Neuronal numerical density in area 17 of the human cerebral cortex was estimated over a wide range of ages from mid-gestation to old age. The density at 21 weeks of gestation is very high (over one million/mm3) and decreases steeply but regularly until birth, when it is around 90,000 neurons/mm3. It continues to decrease until about 4 months postnatally, when it reaches values of about 40,000 neurons/mm3. Thereafter neuronal density remains more or less stable and the mean value for young adults is about 35,000 neurons/mm3. Values for supragranular, granular and infragranular layers follow a similar temporal sequence and also stabilize a few months postnatally. However, until 31 weeks of gestation, the granular layer IVc has a slightly lower neuronal density than that of the supragranular zone including layers II, III, IVa and b. Around birth, the neuronal density of layer IVc is about double that in supragranular layers and remains relatively high in the adult. With aging the cortical neuronal density again increases slightly, reaching a mean of 44,000 neurons/mm3 for five cases aged over 80 years. The number of neurons under one square millimeter of cerebral cortex also undergoes a very steep decrease until birth, stabilizes a few months after birth and seems to remain stable in adult and old age. We find no conclusive evidence for neuronal loss with aging.