Development of the indusium griseum. II. A semithin light microscopic and electron microscopic study.

Neurogenesis and gliogenesis in the indusium griseum were studied in semithin sections and with the electron microscope. Immature neurons were present at birth, but differentiated rapidly until, by 15 days, they appeared to be fully differentiated. At birth 38% of glia were glioblasts, 47% were immature astrocytes and 15% were mature microglia. Astrocyte differentiation appeared to occur earlier than in adjacent white matter. In the adult, 57% of glia were astrocytes, 21% were oligodendrocytes, and 22% were microglia. The volume density of neuronal nuclei did not change with age, but the volume density of neuronal perikaryon decreased from 34% at birth to 13% at 15 days, while the neuropil increased from 24% to 58%. Very few immature synapses were present at birth, but by 5 days a number of axodendritic synapses were present, and these increased with age.