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Quantitative analysis of the total number and distribution of dendritic spines along the apical shafts of layer V cerebral cortical pyramids has been performed on aging rats (90-120 to 1,135 days old) and on rats during the period of early and late development (10-80 days). As expected from previous work, present results show that the total number of dendritic spines along the shafts increase from 10 to 80 days, after which it starts to gradually decrease until the last age studied (1,135 days). The quantitative analysis of the effect of aging on the relative decrease of dendritic spines shows that this decrease starts being homogeneous along the whole length of the apical shafts and that from a certain age onwards, estimated according to present results in 400 days, this effect is significantly more pronounced in layers IV and III-II than in deep layers. Furthermore, the comparison made between the distribution of dendritic spines along the apical shafts of pyramidal neurons of old and young animals has shown that aging produces a regression of this distribution to juvenile conditions.

A cytoarchitectural study was made of the supraoptic nucleus (SON) of the hedgehog with special attention to the quantitative comparison of its main neuronal types. The main purposes were (1) to relate the characteristics of this nucleus in the hedgehog (a primitive mammalian insectivorous brain) with those in the SONs of more evolutionarily advanced species; (2) to identify quantitatively the dendritic fields of the main neuronal types in the hedgehog SON and to study their synaptic connectivity. From a descriptive standpoint, 3 neuronal types were found with respect to the number of dendritic stems arising from the neuronal soma: bipolar neurons (48%), multipolar neurons (45.5%) and monopolar neurons (6.5%). Within the multipolar type 2 subtypes could be distinguished, taking into account the number of dendritic spines: (a) with few spines (93%) and (b) very spiny (7%). These results indicate that the hedgehog SON is similar to that in other species except for the very spiny neurons, the significance of which is discussed. In order to characterise the main types more satisfactorily (bipolar and multipolars with few spines) we undertook a quantitative Golgi study of their dendritic fields. Although the patterns of the dendritic field are similar in both neuronal types, the differences in the location of their connectivity can reflect functional changes and alterations in relation to the synaptic afferences.

We have previously shown that changes occur in pyramids of area 17 of the rat visual cortex both after thyroidectomy (T) at 10 and at 40 days of age. To assess the effects of thyroxine treatment, instituted at different ages after T, two series of experiments were performed. A : rats were T at 10 days of age and either left untreated, or injected once daily with 1.5 micrograms thyroxine (T4)/100 g body weight. Treatment was started at 12, 15, 20, 30 or 40 days of age. Groups of untreated and T4-treated T rats and of age-paired intact controls were killed at different ages, ranging from 40 to 80 days. B : rats were T at 40 days of age, a group being treated with the same T4 dose starting 30 days after T. These animals, solvent-treated T rats and intact age-paired controls were killed at 90 days of age. The number and distribution of spines along the shaft of Golgi-stained pyramidal cells of the visual cortex were measured and fitted by a mathematical model developed previously. Body weights, pituitary growth hormone contents, plasma thyrotropic hormone, thyroxine and triiodothyronine levels were measured to assess the degree of hypothyroidism. It was found that treatment with T4 of rats T at 10 days of age prevented the alterations of pyramidal cells, provided treatment was started by 12 days of age and euthyroidism was maintained. In rats T at 40 days of age, treatment with T4 had an ameliorating effect despite a delay in onset of treatment of 30 days after T. Whatever the mechanisms which are involved, the present results stress once more the importance of very early treatment of hypothyroid newborns, if permanent cortical brain damage is to be prevented.

In order to study the influence of hypothyroidism on the maturation of the visual cortex, rats were thyroidectomized (T) surgically at 10 days of age and injected with 131I within 2 days after the operation. These T rats, and age-paired controls (C), were killed at 20, 25, 30, 40 and 80 days of life. The groups of T rats were subdivided into 'severely' hypothyroid and hypothyroid animals, on the basis, respectively, of complete or incomplete stasis of body growth and depletion of pituitary growth hormone stores. The pyramidal cells of the visual coretx of 'severely' hypothyroid rats were studied separately from those of hypothyroid animals. It was found that the number of spines along the apical shaft of such cells was markedly and equally reduced in all of the T rats, as compared to C rats, irrespective of the degrees of hypothyroidism attained, and stopped increasing in number by 30 days of age. From this age onwards, the distribution of the spines could no longer be fitted by a mathematical model31 which, however, adequately described for normal rats the developmental changes which take place with increasing age in the distribution of spines along the shaft of pyramidal cells from the visual cortex. This lack of fit was only found for the 'severely' hypothyroid animals; data from the other hypothyroid animals could still be fitted by the model. Such data show that the above-mentioned model31 may be a useful tool for the study of the intensity of the effects of hypothyroidism on the maturation of cortical neurons.