Development of neocortical circuitry: histochemical localization of acetylcholinesterase in relation to the cell layers of rat somatosensory cortex.

Acetylcholinesterase (AchE) was histochemically localized in neocortex cerebri of newborn to 1-week-old rats. At birth AchE-dependent staining is limited to scattered somata (Cajal-Retzius cells) in the marginal zone and a few fibers and somata in primordial layer VI. By the end of the first week, neuronal elements are relatively well stained in particular cell laminae, giving the appearance of three horizontal AchE-rich "bands" which alternate with AchE-poor laminae. The subpial band (layer I) is a narrow tangential zone of intensely staining fibers and scattered somata. The mid-cortical band contains an AchE-positive fiber plexus (primordial layer IV) and numerous stained somata (pyramids of primordial layer V). In layer IV of the SmI region, intermittent foci of staining are noted which overlap the barrels' distribution in the barrel field. The deep cortical band (bottom of primordial layer VI) consists of numerous stained somata (Martinotti cells). It is concluded that there is a laminar pattern of acetylcholinesterase-dependent staining in postnatal rat somatosensory cortex, and that the laminar pattern bears a consistent spatial relation both to the cell layers and to the depths of high synapse density in immature cortex.