This structure may be especially suited to convey the direction of movement of tactile stimuli across the skin. One particularly interesting class of labeled neuron possesses axons with C-shaped endings that only partly encircle hair follicles, and that terminate on a consistent side of the follicles arrayed across a large skin area. One class of axon terminal may be consistent with fibers conveying itch sensation, but most of the endings conveying pain, pleasant and unpleasant temperatures, and chemical irritants are probably not revealed by the labeling method used. They acknowledge that this must be a preliminary system, because only a subset of sensory neurons is sampled. The Johns Hopkins researchers attempt to systematically categorize, for the first time, the complex axons of many individual sensory neurons with respect to their morphology, the number and density of their endings, and also their relationship to hair follicles, where most of the fibers terminate. By trial-and-error titration of the tamoxifen dose administered to pregnant mice bearing transgenic litters, it was possible to label a small number of discrete sensory neurons. A key feature of the experiments was the use of a form of the Cre enzyme that is translocated to the nucleus only in the presence of an estrogen-like drug, tamoxifen, so the probability that the marker gene is expressed should be related to tamoxifen concentration. Wu, Williams and Nathans used this method to excise a signal sequence blocking the expression of a histochemical marker gene which had been previously engineered into the chromosomal location of a transcription factor (Brn3a) that is important in the development of the sensory nervous system. This latest work was made possible by the Cre-Lox system-a widely-used approach in which a Cre recombinase enzyme is used to remove chromosomal DNA flanked by two genetically engineered loxP recognition sequences. A combination of genetic and histochemical techniques were used to record the image from which the trace is taken ( Wu et al., 2012). The trace on the right shows nerve endings in the skin of a mouse. The cells were stained with potassium dichromate and silver nitrate. The drawing on the left was made by Santiago Ramón Y Cajal in 1899 and shows Purkinje cells in the cerebellum of a pigeon. Thus we are reminded that our knowledge of even well-studied experimental systems is still very fragmentary.ĭrawings of neurons and nerve endings made more than a century apart. The results of the Johns Hopkins experiment are largely descriptive in nature, and we might assume that the results reported are already buried somewhere in the literature, but they are not. Now, writing in eLife, Hao Wu, John Williams and Jeremy Nathans of Johns Hopkins University report the results of experiments that involve some very modern transgenic tricks, yet evoke the studies of neuronal morphology in the early 20 th century-using methods introduced by Golgi and perfected by Ramón Y Cajal-that first revealed the complex architecture of single neurons (see Figure 1). It has long been recognized that nerve endings in the skin display a diverse range of forms, but prior studies have generally used histological methods in tissue sections that do not reveal the complete morphology of each neuronal axon. Nociceptors and mechanoreceptors terminate in the skin, whereas proprioceptors terminate in muscles and tendons. Traditionally, somatic sensory neurons have been divided into three broad subtypes: nociceptors (for sensing pain), mechanoreceptors (for sensing touch), and proprioceptors (which sense body position). These neurons reside in discrete ganglia that lie peripheral to the brainstem and spinal cord, including the trigeminal ganglia that receive signals from the face and head, and the dorsal root ganglia that serve the trunk and limbs. However the sense of touch, which is conveyed by general somatic sensory neurons, is much less well defined. Four of the five traditional senses, the ‘special senses’ of vision, scent, hearing and taste, are conveyed by discrete sense organs that contain a few types of highly specialized signal transducing cells, such as rods and cones in the retina, or cochlear hair cells in the ear. Sensory neurons are the brain's portal to the external world.
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