Neuropeptide Y-containing neurons in the human infant hippocampus.

Using immunohistochemistry, high concentrations and widespread distribution of neuropeptide Y-immunoreactive (NPY-IR) neurons were found and examined in each region of the hippocampal formation from birth to 42 years. NPY interneurons are particularly numerous in the stratum oriens of the CA1 subfield, in the deep layers of the subicular complex and entorhinal cortex. They are multipolar round, ovoid or triangular or bipolar and fusiform. There is a dense network of NPY-IR nerve fibers in the subicular complex and the entorhinal cortex. In addition, numerous NPY-IR nerve cell bodies and fibers are observed in the angular bundle and the adjacent white matter and this contrasts with the absence of NPY immunoreactivity in the fiber tracts of the alveus. These NPY-IR neurons which correspond to the interstitial neurons of the white matter, have the morphology and the size of the interneurons detected in the cortex. During the postnatal brain growth spurt which corresponds to the phase of rapid myelination, there is no decline in total number of NPY-IR neurons but there is a decrease in density. They have been spread apart by the growth of the rest of the tissue. So in humans, the total number of NPY nerve cell bodies in the hippocampal system, firmly established at birth, is not modified during consequent brain growth which continues until ages 3-4 years and stays stable at least until age 42 years.