The Ontogenesis of Cortical Circuitry: The Spatial Distribution of Synapses in Somesthetic Cortex of Newborn Dog
The ontogenesis of cortical circuitry is the subject of the investigation reported in this paper!. Strategie elements of cortical circuitry are synaptic contacts: sites of neuronal interaction. If we first determine the location of early formed synap ses
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M. E. Molliver and H. Van der Loos:
(Fig.3A). These sections are inspected with phase contrast to judge whether the plane of sectioning is parallel to the predominant orientation of cells and cell streams; corrections are made by angular adjustments of the block axis. The block is then trimmed so that a sliver parallel to the block face is removed from the deep part of the cortex; the sliver is about 0.1-0.2 mm thick (Fig.3B). Several thick sections are then cut and mounted on slides, and adjacent thin sections are cut and mounted on grids. These sections contain superficial cortex and include the pial surface. After approximately ten thin sections have been retrieved, thick sections are cut repeatedly until a section passes through the entire block face. This last section is saved (Fig. 3C) and is later used for the spatial representation of our data. A shallow cut (0.2 mm deep) is made into the block face, perpendicular to the plane of section (Fig. 3D) and about 1 mm above the deep edge of the cortex. Then a few thick sections are cut. Two sections come off with each pass of the knife and these are saved in pairs (Fig. 3D). They are later to be superimposed on the section which goes through the entire block face (Fig. 3 Cl, and the cut edge is used to determine the location of the ad-pial border of the deep section. The portion of the block face containing superficial cortex is then removed by undercutting (Fig. 3E), and several thick sections (of deep cortex) are cut and saved. Then aseries of approximately ten thin sections is cut and mounted on grids. These sections containing deep cortex are adjacent to thick section E. Note that the cut in step D is made such that the superficial and deep sections overlap. A corner is always cut off so that the orientation of sections can be easily maintained in subsequent procedures. Synapses observed in both superficial and deep thin sections are located by reference to adjacent thick sections 3B and 3E, respectively. The synaptic locations in these sections are, in turn, transposed to section 3C in order to combine both sets of data (i.e., deep and superficial) on a single scale of cortical depth. Examples of this method will be shown below. (Note that the full thickness sections shown in Figs.17-19 correspond to section C in Fig. 3). Mounting 0/ Thin Sections. Thin sections for EM examination are mounted on single hole grids covered with a formvar film. Our adoption of this (tedious) method is based on the following considerations. The analysis of a tissue, such as cortex, which has a distinct geometric organization is facilitated by having the entire section available for inspection, unobstructed by grid bars. Our sampling method, described below, requires an unobstructed field. The determination of section orientation and the identification of large structures, e. g., cell bodies and pial surface, are facilitated by the use of single-hole grids. Copper grids with a single round hole, I mm in diameter, are preferred. Grids with an oval I X 3 mm hole were tried and
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