Prenatal development of parvalbumin immunoreactivity in the human striate cortex.

In human primary visual cortex, parvalbumin (PV) is expressed by Cajal-Retzius cells in layer I by 20 weeks of gestation (20W), but its immunoreactivity is mostly lost by term. PV immunoreactivity in layers II-VI mainly develops later, from 26 to 34W, following an approximately 'inside-outside' sequence in a series of bands. PR-positive perikarya appear in layer V by 20W, but only in small numbers. They increase in number and staining intensity by 26W. By 30W a band of densely labelled somata and neuropil occupies layers IVC-VI. By 34W a second, less dense, band of cell bodies and neuropil appears in IVB and IVCalpha, separated from the deep band by IVCbeta which is cell-sparse and almost fibre-free. Between 38 and 40W, a third minor band consisting mainly of fibres is seen in layer IVA. Reactive cell bodies form clusters, and the neuropil staining is mosaic-like. PV-positive neurons are of two main types: large with wide dendritic arbor, and smaller with simpler dendrites. However, a few have characteristics of pyramidal cells, and a few others resemble glial cells. The laminar pattern of PV-immunoreactive somata in human striate cortex is established by term, rather than postnatally as in most mammals, implying that PV may be involved in neuronal development in prenatal human striate cortex.