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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.