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Figure 3. Criteria for assigning dendritic spines to one of four shape classes. Spines were judged thin if serial viewing revealed the length to be greater than the neck diameter, and the diameters of the head and neck to be similar. Spines were judged mushroomif the diameter of the head was much greater than the diameter of the neck. Spines were judged stubby if the diameter of the neck was similar to the total length of the spine, and branchedspinehasd more than one head. Occasionally, spines in the mushroom category overlapped the thin and stubby shape categories, especially at PND 15. This nomenclature follows that of Peters and Kaiserman-Abramhof (1970); similar spine shapes were described by Jones and Powell (1969) as “sessile” for the stubby spines and “pedunculated” for the mushroom or thin spines in neocortex. L, length; d,, diameter of the spine neck; d,,, diameter of the spine head; h = head.

Table: LONG = longitudinally sectioned dendrites; X/OB, combined values of crossand obliquely sectioned dendrites that did not differ significantly from one another at either age. The diameter of each dendrite was measured on the serial section that had the maximal central diameter in the plane of section. The mean LONG diameters are underestimated because a chord is measured instead of the true diameter when these are sectioned near to their tangential edge on the reference section. The diameters of five randomly selected LONG dendrites were measured through serial sections at a designated location to obtain an average underestimate of 15% for the LONG dendrites. A process without spines or shaft synapses was classified as nondendritic. In the PND 15 sample fields, 3 18 processes, and in the adult sample fields, 220 processes, were indistinguishable from small dendrites on the reference section, but when viewed across serial sections they were identified as axons, glia, or still not identifiable. NS, no significant differences between PND15 and adults.