In order to see more of the details of the individual cells, we must use high magnification light microscopy, as in this preparation from the spinal cord of a cat. The most prominent structures are the cell bodies of two large neurons, called motoneurons, which innervate muscle. Each consists of a large clear nucleus containing a prominent dot, the nucleolus. The nucleus is surrounded by the cytoplasm of the cell body, which contains large clumps of stained material called Nissl bodies after the German neurologist Franz Nissl, who first applied aniline-dye stains to the nervous system. Note that the outline of the cell is not uniform. The elongated extensions are the commencements of some of the rather long processes called dendrites, which we shall consider in detail later.
The most common stain for the demonstration of neurons is named after Nissl. The Nissl stain can be any one of a number of basic aniline dyes. In this case, the dye is thionin. Cresyl violet also is widely used. The basic nature of the dye gives it an affinity for concentrations of acid radicals in the nerve cell, of which the greatest are the masses of ribose nucleoprotein in the nucleolus and in (ribosomes) rough endoplasmic reticulum in cytoplasm hence the staining of the nucleolus and the Nissl bodies in the cytoplasm. You might ask why the nucleoprotein of the nucleus does not stain. That is because the neuron is one of the most actively synthesizing cells of the body, continually transcribing a large part of its genome. Thus the chromatin material of the nucleus is dispersed and does not stain intensely. Around and between the two large motoneurons lie the tiny nuclei of the supporting cells or glia. They are not much larger than the nucleoli of the motoneurons, and normally they are less active metabolically than the neurons. Hence their chromatin is less dispersed and stains intensely.
Note that the supporting cells, or neuroglia, as well as being small, have very little detectable cytoplasm around the nucleus. They are also present in large numbers. They far outnumber neurons and may account for as much as 50% of the weight of the brain.
The other things to observe in this important early slide are the four apparent holes in the section. These are capillaries. In three of them the flattened nucleus of an endothelial cell can be seen. Blood vessels of this type will be apparent in many ensuing slides. Red blood cells are not observed in their lumens because the brain has been perfused with saline and then a formalin solution.