The cerebellum, the deep cerebellar nuclei, and the inferior olivary nucleus of the heterozygote Lurcher mutant mouse have been compared with the same structures in normal littermates. The comparison was made using light and electron microscopic methods for qualitative observations and light microscopic methods for quantitative observations. The study included the newborn period from 4 days of age up to 730 days, which is old age for a mouse. The cerebellum of the normal mouse is similar to that of many other species though apparently minor structural differences are seen. Amongst these was the similarity between the mouse climbing fibre and mossy fibre glomeruli which contrasts with the rat where they can be distinguished by the high density of synaptic vesicles and central cluster of mitochondria in the climbing fibres. In Golgi stained material the inferior olivary nucleus of the normal mouse showed cells with highly ramified dendrites and cells with simple dendrite patterns. In the adult Lurcher mouse the cerebellum is much smaller than is normal. There are no Purkinje cells and the internal granule cell layer is reduced in thickness and density. Examination of younger animals shows that Purkinje cells are present and that they undergo degeneration. In Golgi stained material from younger animals Purkinje cells often show more than one primary dendrite, sometimes as many as five, and somatic spines persist well beyond the first week of life. Cytoplasmic organelles often have a random orientation and the mitochondria are rounded rather like those seen in the nervous mutant. Granule cells in the adult Lurcher mutant are reduced in number and during the developmental period degenerative changes are seen. The Golgi cells and stellate cells are relatively normal and some cells, identified as basket cells, are seen. The inferior olivary nucleus is found with ease in the Lurcher mutant and is as extensive as in the normal mouse. However, in Golgi stained material only cells with highly ramified dendrites are seen. In addition the total number of neurons is reduced. It is possible that the neurons with a simple dendrite pattern have climbing fibres which pass only to the Purkinje cells. The deep cerebellar nuclei in the normal mouse cannot be separated easily into their three subdivisions, lateral, interpositus and medial. In the Lurcher mutant the neurons are of similar size to those of the normal mouse but they are crowded more closely together than is normal. In the Lurcher mutant as in the normal adult the neuronal cell bodies are covered with synapses and not with glial cells. Estimates of total cell numbers were made in order to obtain evidence about the time course of the development of the changes in structure and to make a detailed comparison between the normal mouse and the Lurcher mutant with respect to Purkinje cells, granule cells, olive neurons, and deep cerebellar nuclei neurons. In the normal mouse the mean number of Purkinje cells between 10 and 730 days was 177 000, s.d. ± 11600,
n
= 12. The number of granule cells probably reached a peak at about 17 days. At 26 days post-natal the number estimated was 27 million and at 730 days 28 million. The mean number of olive neurons between 14 and 730 days post-natal was 32700, s.d. ± 1900, 9; the mean number of deep cerebellar neurons counted at three adult ages was 17 600, s.d. ± 1800. In the adult the ratio of Purkinje cells to olive cells is
ca
. 5.4:1, of granule cells to Purkinje cells is
ca.
170:1, of Purkinje cells to deep cerebellar nuclei neurons is 10:1, and of olive neurons to deep cerebellar nuclei neurons is 1.85:1. This last would chiefly be of interest if there are olive neurons projecting solely to deep cerebellar neurons. In the Lurcher mutant the number of Purkinje cells falls below normal from 8 days post-natally, reaches 10% of normal at 26 days and probably falls to zero at around 90 days. At this point such are the changes in the overall structure that confusion of Purkinje cells with Golgi cells may occur. At 4 days post-natal age the number of granule cells is smaller than normal by 25 % and this difference increases with age to a reduction of ca. 90 %. The number of olive cells is close to normal until 8 days of age, is only 60 % of normal at 15 days when the highest number is reached, and is 25 % of normal at 121 days. The deep cerebellar nuclei neuron numbers were the same as those in the normal. Included in the discussion is a detailed critical comparison of these results from the normal mouse with all previous estimates of cell numbers in the cerebellum. The lesion in Lurcher is compared with that found in the other mouse cerebellar mutants and with experimentally evoked lesions of the cerebellum. For the Lurcher mutant the tentative conclusion is that the primary lesion may arise in the Purkinje cells.
Efficient generation of reciprocal signals by inhibition