The normal processes of neural development in the central nervous system (CNS) depend upon precisely-timed expression of various cellular and molecular factors. Delayed or precocial onset of these factors can affect the outcome of cell origin, migration, axonal and dendritic outgrowth, synaptogenesis, and cell death in a number of brain regions. The consequences can be devastating to normal neural function. The thrust of this proposal is to study, in detail, a brain region critically dependent upon timely development of afferent innervation for its growth and maturation. In the African clawed frog, Xenopus laevis, we will examine the development of the olfactory bulb in the normal embryo, in animals deafferented early in development, and in animals receiving augmented afferentation during embryonic development. Experimental manipulations to be used are (1) removal of the olfactory placode before receptor axons contact the olfactory bulb, (2) removal of the placode after some axons penetrate and stimulate some differentiation of the bulb, and (3) transplantation of an extra placode to the rostral cranium of normal embryos. By comparing the development of the olfactory bulbs in normal animals to that of the bulbs from the experimentally manipulated embryos, we hope to identify those cellular factors induced or influenced by the olfactory axons during development. These comparisons will be made with a variety of experimental methods. 3H-thymidine autoradiography will be used to study the time of origin of neurons in the olfactory bulb and determine the number of neurons generated at each stage. Neurons will be counted at various stages to see if cell death normally occurs in the bulb and whether this process is affected by the experimental manipulations. Golgi impregnation studies will serve to examine dendritic development in the mitral cells, which are the major output cells of the bulb. With immunocytochemistry, we will study onset of neurotransmitter expression, and with electron microscopy, synaptogenesis in the glomerular and external plexiform layers. All studies will include quantitative analyses. The underlying purpose of my work is to understand the cellular and molecular factors that control the development of sensory systems, with a hope of preventing or alleviating problems associated with CNS birth defects and disorders.
