Dendritic morphology of CA1 pyramidal neurones from the rat hippocampus: II. Spine distributions.

The numbers and distributions of dendritic spines were estimated for six adult and three juvenile biocytin-injected neurones from the CA1 region of the hippocampus of the albino rat. For each cell, a sample of long dendritic segments that lay favourably in the plane of focus was drawn at high magnification and the visible spines counted. Correction was made for spines obscured by dendritic shafts. Within individual cells, dendrites of similar type and diameter had similar spine densities. For adults, long basal segments averaged 2.4 spines/microns and obliques averaged 3.2 spines/microns. In juveniles, basals averaged 2.3 spines/microns and obliques, 2.5 spines/microns. Apical tuft segments were less spiny, averaging 1.4 spines/microns in adult cells and 1.8 spines/microns in juveniles. There was a positive correlation between spine density and dendrite diameter. Values from this sample were used to assign spine densities to the other segments, and so the total number of spines was estimated for each cell. Adult cells averaged 30,500 +/- 3,900 (S.D.) spines and juveniles, 23,800 +/- 7,100 spines. Adult cells had roughly 50% of their spines in stratum radiatum, 40% in s. oriens, and 10% in s. lacunosum-moleculare. Juvenile cells had a rather higher proportion (20%) in s. lacunosum-moleculare. In general, some 50% of all spines were located within a path length of 200 microns from the soma. These total numbers of spines were much higher than earlier values from Golgi-impregnated cells but align well with estimates of the numbers of axonal boutons supplied to CA1 by CA3 pyramidal cells.