Finding a Partner in a Crowd Neuronal Diversity and Synaptogenesis

amacrine cells (using a new method they developed, termed " photofilling "), MacNeil and Masland were able to describe at least 26 different types of this neuron, including the four major types previously described (Figure 1). They found that most of the cell types " tiled " the retina very efficiently; namely, the amacrine cells of a given class populated the retina at a density such that Over 125 years ago, Camillo Golgi published a technique their dendritic arbors filled the retina with little overlap for silver-staining neurons that retains his name and is between adjacent arbors. This indicates that the authors still used to this day. Because only relatively few, widely very likely did not " invent " cell types and that the de-separated neurons in a sample of cell-dense neural tis-scription they present is likely to be a very good estimate sue react, the Golgi technique was important in enabling of the actual diversity present. biologists to discover an incredible morphological diver-Tiling of a brain region by a given neuronal type makes sity of neurons. Many types of neurons were described sense teleologically: One assumes that each particular using the technique, including the sole output neurons type of neuron contributes to information processing in of the cerebellar cortex, Purkinje cells. Santiago Ramó n a distinct way, and therefore all other neuronal types y Cajal, studying the neuronal input to these Purkinje should have similar access to at least one member of cells at the turn of the century, discovered that early in each distinct class. Tiling would accomplish this by fill-development, these neurons were innervated primarily ing a brain region with minimal redundancy. Stevens on their cell bodies by hindbrain-derived axons. He (1998) has argued that, assuming tiling by individual found that later in development, these axons break these neuronal types is a general organizational principle, an synaptic connections and " climb " the dendritic tree to upper limit on the possible number of neuronal types form new connections elsewhere on the same Purkinje within a brain region such as the neocortex can be calcu-cell. From this reconfiguration of connectivity, Ramó n y lated. Given the density of neurons in the cortex (10 5 Cajal concluded that synapses in the nervous system neurons/mm 2), the average dendritic spread of each were not made at random; instead, synaptic connec-neuron (0.05 mm 2), and a generous estimate of 10-fold tions …