Postnatal development of zinc-containing cells and neuropil in the visual cortex of the mouse

The postnatal development of zinc-containing synaptic boutons and their cells of origin in the visual cortex of a pigmented mouse is described. Two phases can be distinguished. During the early phase zinc-containing neuropil is first apparent by postnatal day 3. By day 7 a light, but distinct neuropil staining sketches the primary and secondary visual cortices. The primary visual area contains light precipitate in layers V and VI as well as the monocular portion of layer II/III. The secondary visual areas contain slightly denser precipitate in layers II/III through VI. The transition to the second phase is marked by a large increase in precipitate density by day 11. Thereafter, the intensity of the neuropil staining increases to day 28, first in layer II/III and then in layer V, as the adult pattern of neuropil staining gradually develops. In the primary visual cortex precipitate is dense in layers II/III and V, moderate in layer VI, and sparse in layers I and IV. In the secondary visual areas the precipitate is dense in layers II/III and V and moderate in the lower portion of layer I and in layers IV and VI. Cells of origin of zinc-containing boutons are visible by the end of the second postnatal week in layer II/III of the secondary visual cortex. By 21 days of age the pattern of staining in the mature mouse is established, and cells in layers II/III and VI are labeled in both the primary and secondary visual cortices. The developmental sequence of zinc-containing cells and neuropil does not preclude an involvement of zinc in the postnatal regulation of NMDA receptor function.