The lack of neuron staining techniques prior to the Nissl stain and the Golgi stain. Though Nissl’s technique came first, it only revealed neurons’ endoplasmic reticulum and parts of the cell body. It was only until Camillo Golgi developed his staining method, first known as the “black reaction”, that neurons as a whole were revealed. This was a groundbreaking technique that led to decade long debates about the nature of the nervous system, especially as to whether neurons were contiguous or separate. Since then, staining techniques have evolved to allow imaging of anything from axonal networks to individual neuropeptides. The Weigert-Weil stain enables us to visualise the myelin sheath of axons, giving us the opportunity to observe how connections are established throughout the brain. Modern techniques like in-situ hybridization (ISH) allow us to see which genes are expressed in a sample of neurons. The first step in ISH involves binding a labelled mRNA strand complementary to the mRNA produced by the gene of interest. The second step involves the introduction of a primary antibody with a variable region keyed to the mRNA label binds to the mRNA strand. The final step is injection of fluorescently labelled secondary antibodies with variable regions that recognize the species-specific heavy chain of the primary antibodies, and then imaging the cells with the appropriate wavelength of light. This technique allows scientists to know which neurons are expressing the gene of interest. Another modern staining method called immunohistochemistry (IHC) is also an antibody stain that uses fluorescently labelled antibodies to visualise certain molecules. However, as opposed to ISH, IHC reveals the presence of proteins such as neuropeptides, which can also be indicative of neuronal function.
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