Morphological Aspects of Neurons as Secretory Cells

The concept of a neurosecretory system consisting of a secretory center (production of bioactive substances in neuronal soma), a transport pathway (transport by axonal flow) and a neurohemal organ (storage and release at nerve endings) has been established through a series of histological studies on the hypothalamic magnocellular nuclei by two pioneers, SCHARRER and BARGMANN, together with their coworkers. In the early stages of the investigation, these actively secreting neurons were considered as exceptional neurons, separated from ordinary ones. However, since the application of modern techniques, such as electron microscopy, immunohistochemistry, and in situ hybridization, have made progress in tracing the cytological course from the gene expression to the release of bioactive product, it has become obvious that the classical definition of neurosecretion must be extended to include a variety of neuroepithelial derivatives, such as aminergic and peptidergic neurons and also paraneurons. At present, all neurotransmitters and neuromodulators can be regarded as secretory substances produced in neurons. Newly developed precise techniques, e. g., a tracing method with a computer graphic system for the demonstration of axonal arborizations, have led to a dramatic change in our understanding of the fine details of cytological features, offering much more complicated structures than images presented by the classical impregnation technique. Immunohistochemically demonstrated serotoninergic neurons in the brain have revealed enormous reticular extensions and anastomoses of beaded processes which were previously unknown. Neurons composing the nervous system show greatly varied shapes and structures. This morphological heterogeneity makes possible the diversity of the brain function. This review stresses that advances in studies in the field will be promoted by a dual strategy: one to investigate neurons in their general features especially the Secretory aspects, and the other to pay attention to the special features of each variety of neurons.