SEGMENTAL DIFFERENTIATION IN THE NERVOUS SYSTEM

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One of the fundamental yet largely unanswered questions about the development of the nervous system is how one region becomes morphologically and functionally different from another during neurogenesis. The long-range goal of this project is to discover the mechanisms that give rise to this regional specialization in the leech nervous system. Regional specialization could derive from factors intrinsic to the neuronal precursors for each region, or from factors introduced by interactions with other neural or non-neural tissues. We have chosen to carry out our observations and experiments on leech segmental ganglia because of their relative simplicity and accessibility to experimental manipulation. We have previously determined that the ganglia in body segments five and six, which innervate the sexual organs, contain a few hundred more neurons than adjacent ganglia. The first objective of this project is to characterize in detail these extra neurons, both in development and in the adult. This will entail describing when these neurons arise, what their birthdates, are, which cells are their precursors, when they innervate the sexual organs, and what their functions are. We will do this by using a variety of techniques, including computer-aided techniques to do cell counts and cell reconstructions, [3H]-thymidine labeling to obtain birthdates and to identify precursors, and cell recording, dye injection and histology to determine their functions. In addition we will use monoclonal antibodies to identify particular cells during development. On the basis of the detailed information we obtain, we will refine our hypotheses about the mechanisms that underlie segmental specialization in the leech. The second objective of this project is to begin testing these hypotheses experimentally by performing various manipulations on the developing ganlia, such as isolating the ganglia in the animal by cutting connecting nerves, placing ganglia in culture with or without other tissues, transplanting ganglia into hosts of different ages, and removing individual ganglia. The effects of these manipulations will be studied with the techniques mentioned above. Detailed study of segmental specialization in the leech, with its relatively simple nervous system, can yield valuable insights applicable to studies of this fundamental problem in more complex organisms.