Body mass increases 16-fold between 1 and 22 months of age in the rat (Figure ). The increase is progressive, with a significant correlation with age (Spearman correlation ρ = 0.7463, *p* < 0.0001). Brain mass also increases in the period, although only by 1.45-fold (Figure ; ρ = 0.7188, *p* < 0.0001). A significant progressive increase in mass is observed between 1 and 22 months of age for each of the structures analyzed (Figure : Cx, 24% increase in mass, ρ = 0.6217, *p* = 0.0001; Hp, 60% increase, ρ = 0.4193, *p* = 0.0169; Cb, 36% increase, ρ = 0.7877, *p* < 0.0001; RoB, 84% increase, ρ = 0.6894, *p* < 0.0001), except for the OB (Spearman correlation, *p* = 0.9841). No structure shows a tendency toward decreased mass with age.

In the same period, all brain structures undergo an initial increase followed by a decrease in their numbers of neurons (ANOVA, all F-ratios >5, *p* < 0.01; Figure ). Numbers of neurons increase significantly between 1 and 2 months in all brain structures, more than doubling in some (Figure ; Mann-Whitney, Cx, 78.7% increase, *p* = 0.0143; Hp, 145.0% increase, *p* = 0.0253; Cb, 52.9% increase, *p* = 0.0066; olfactory bulb, 84.8% increase, *p* = 0.0066; RoB, 124.5% increase, *p* = 0.0066). This represents, in the space of 1 month, an addition of 11.8 million neurons to a Cx that initially had 14.9 million neurons; 2.5 million neurons to a Hp that had 1.7 million neurons; 46.0 million neurons to a Cb that had 86.9 million neurons; 4.5 million neurons to an OB that had 5.3 million neurons; and 11.4 million neurons to the RoB, which had 9.1 million neurons at age 1 month (Figure ). The total number of brain neurons increases 64.7% between 1 and 2 months of age, from 111.9 ± 8.2 to 184.3 ± 11.3 million neurons (Figure ).

At 3 months, the total number of brain neurons is even higher compared to 1 month (195.0 million neurons, 74.2% higher than at 1 month, Mann-Whitney *p* = 0.0201), but is not significantly higher from that at age 2 months (*p* = 0.1441). The same pattern is found for each brain structure individually, where numbers of neurons are not significantly different between 2 and 3 months of age (Mann-Whitney, all values of *p* > 0.1), even though all structures (except the Hp) exhibit on average larger numbers of neurons at 3 than at 2 months (Figure ).

From 3 months on, in contrast, numbers of neurons in each brain structure decrease progressively until 22 months, with a significant negative correlation with age (Spearman correlations: Cx, ρ = −0.5191, *p* = 0.0111; Hp, ρ = −0.6563, *p* = 0.0009; Cb, ρ = −0.7025, *p* = 0.0002; olfactory bulb, ρ = −0.5332, *p* = 0.0073; RoB, ρ = −0.4525, *p* = 0.0264). By 22 months, the Cx and Cb have 30% fewer neurons than at age 3 months (Mann-Whitney, *p* = 0.0389 and *p* = 0.0143 respectively); the Hp and OB have on average 33% and 17% fewer neurons at 22 months than at age 3 months, and although these differences do not reach significance (*p* = 0.0707 and *p* = 0.2207), the two structures have significantly fewer neurons at age 12 months than 3 months (reductions of 32% and 23%, *p* = 0.0131 and 0.0169 respectively). Only in the RoB does the number of neurons not reach significance between age 3 and ages 12 or 22 months (*p* = 0.1962 and *p* = 0.1025 respectively), despite the significant negative correlation between age and number of neurons shown above.

Numbers of other, non-neuronal cells are not significantly different between ages 1 and 3 months in the Cx, Hp, and Cb (Mann-Whitney, *p* = 0.4555, 0.1556, and 0.1066, respectively; Figure ), but are higher by 90.8% in the OB and by 58.1% in the RoB by age 3 months compared to 1 month (Mann-Whitney *p* = 0.0085 in both; Figures ). As a result, the total number of other cells in the brain is significantly increased by 22.1% at 3 months compared to 1 month (*p* = 0.0282; Figure ).

There is no significant difference between numbers of other cells in the brain structures analyzed between 3 and 22 months (Mann-Whitney, all values of *p* > 0.3; Figure ). However, that might be due to large variance at age 22 months; indeed, again the OB and RoB, but not the other brain structures, exhibit numbers of other cells significantly decreased by 34.5% and 24.8% respectively at age 12 months compared to 3 months (Mann-Whitney, *p* = 0.0055 for both).

Corroborating this decrease in numbers of other cells with age in the OB and RoB, we find a significant trend toward decreased numbers of other cells with age in the two structures between 3 and 22 months (Spearman correlation, ρ = −0.5232, *p* = 0.0087 and ρ = −0.5779, *p* = 0.0031 respectively; Figures ), but not in the Cx, Hp, and Cb (Spearman correlation, all values of *p* > 0.3; Figures ).

Inspection of the graphs in Figure shows that there is a fair range of variation in numbers of neurons in each structure in the younger rats, and large variations in numbers of neurons in the older rats, at ages 12 and 22 months. These variations raise the alternate possibilities that (1) in each animal, some structures lose neurons with age while others do not, in which case neuronal loss should not be correlated across structures when all animals are pooled together; and (2) neuronal loss happens in all structures coordinately, but some animals are more prone than others to lose neurons with age, in which case neuronal loss should be correlated across structures when all animals are pooled together.

To distinguish between these alternatives, we performed maximal likelihood factor analysis on the numbers of neurons in each brain structure between 3 and 22 months. We find that while the null hypothesis of no common factor accounting for the variations in numbers of other cells across the structures cannot be safely rejected (*p* = 0.0528), for variations in numbers of neurons the null hypothesis can be rejected at *p* < 0.0001. Two factors account for 60.5% of the variance: factor 1 (rotated loading: Cx, 0.959; olfactory bulb, 0.638; RoB, 0.409) accounts for 31.5% of the variance, and factor 2 (rotated loading: Cb, 0.769, Hp, 0.751, RoB, 0.447) accounts for 29.0% of the variance. These factors are well aligned with the strongest correlations that we find in how numbers of neurons vary between 3 and 22 months across the structures (Figure ): variations in numbers of neurons in the Cx are correlated with variations in the OB (ρ = 0.6896, *p* = 0.0004) and RoB (ρ =0.4637, *p* = 0.0297), and variations in the Cb are correlated with variations in the Hp (ρ = 0.6421, *p* = 0.0023) and also RoB (ρ = 0.4532, *p* = 0.0299). Together with the factor analysis, these correlations suggest that numbers of brain neurons do not necessarily decrease with age in all animals, but when they do occur in an animal, they tend to happen coordinately across structures. Indeed, across the six animals aged 12 months, we find one animal (rat 49) always among the two animals with the smallest numbers of neurons in the five brain structures; another animal (rat 34) appears among the three animals with the smallest numbers of neurons in three out of five structures; and two other animals (rats 48 and 50) appear among the four animals with the smallest numbers of neurons in four out of five structures (not shown).