Development of synaptic organization in the tangential vestibular nucleus: a quantitative electron microscope study.

The objective of the present study is to quantify the developmental changes in the total synaptic pattern of one part, the soma, of one particular cell type, the principal cells of the tangential vestibular nucleus. The term "synaptic space" is defined and quantified. Intermediate stages in the development of synaptic organization are compared. The findings show that the synaptic space available to the full complement of afferents is constant throughout development, while specific terminals in the afferent population change their synaptic space allotments. The synaptic-junction covering is invariant for small terminals at intermediate stages of development with set proportions between "active" and "non-active" zones of the synaptic surfaces. However, the spoon endings and the postsynaptic target cells are covered by synaptic junctions in variable amounts. The findings are important to the fields of neuroembryology and neural plasticity, for the system provides a useful basis to measure the influence of factors in the local environment and the role of formation of synaptic connections in the competition for synaptic space. This study will assist investigators to probe the mechanisms operating in the selection of competing afferents for the limited amount of surface area available under the changing conditions of maturation and aging in the central nervous system.