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The dopaminergic (DA) neurons in the midbrain play a role in cognition, affect and movement. The purpose of the present study was to map and quantify the number of DA neurons in the midbrain, within the nuclei that constitute cell groups A8, A9 and A10, in the mouse. Two strains of mice were used; the C57BL/6 strain was chosen because it is commonly used in neurobiological studies, and the FVB/N strain was chosen because it is used frequently in transgenic studies. DA neurons were identified, in every fifth 20-microns-thick coronal section, using an antibody against tyrosine hydroxylase. Cell locations were entered into a computer imaging system. The FVB/N strain has 42% more midbrain DA neurons than the C57BL/6 strain; on one side of the brain there were 15,135 +/- 356 neurons (mean +/- S.E.M.) in the FVB/N strain, and 10,645 +/- 315 neurons in the C57BL/6 strain. In both strains, approximately 11% of the neurons were located in nucleus A8 (the DA neurons in the retrorubral field), 38% in nucleus A9 (the DA neurons in the substantia nigra pars compacta, pars reticulata, and pars lateralis), and 51% in nucleus A10 (the DA neurons in midline regions such as the ventral tegmental area, central linear nucleus, and interfascicular nucleus). The number of midbrain DA cells, and their distribution within the three nuclear groups, is discussed with respect to findings in other species.

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During studies on methods for the isolation of Brucdla from blood by Pickett and Nelson (1, 2), it was noted that in broth media, inoculated with lysed blood and incubated at 37°C., a turbidity consistently developed which had the appearance of bacterial growth, but from which bacteria could not be isolated. Microscopic examination of the turbid broth revealed many organized, microbial like elements surrounded by some poorly defined materials. It was impossible, on the basis of microscopic appearance alone, to distinguish the organized elements observed from L forms of bacteria. Because L forms, on occasion, were isolated from blood cultures, it was imperative to distinguish between bacterial L forms and those microscopically similar elements of non-bacterial origin. L forms were considered present in blood cultures only if typical L colonies, reacting specifically with appropriate antisera, could be isolated and transferred on soft agar.Experiments were designed to identify the materials and conditions responsible for the non-bacterial turbidity seen in blood cultures. The results indicate that hemoglobin can be organized into particulate elements when incubated at 37°C. in the presence of certain compounds present in peptones used for bacteriologic media. The particulate elements, in turn, account for the visible turbidity. Normal serum in relatively low concentrations inhibits the in vitro development of particles. This fact explains the apparently random occurrence of turbidity in cultures of whole blood (3). EXPERIMENTALInocula.--The development of turbidity in broth occurred when either lysed or intact red cells were used as inocu]a. However, the period of incubation required for turbidity to develop differed according to the type of erythrocyte preparation employed and was associated with the time required for red cell autolysis. An inoculum of from 0.1 to 1.0 ml. was used with