EMBRYONIC-DEVELOPMENT OF THE PARS INTERCEREBRALIS CENTRAL COMPLEX OF THE GRASSHOPPER

We have studied the embryonic development of the pars intercerebralis/ central complex in the brain of the grasshopper using immunocytochemic al arid histochemical techniques. Expression of the cell-surface antig en lachesin reveals that the neuroblasts of the pars intercerebralis f irst differentiate from the neuroectoderm at around 26% of embryogenes is. Differentiation of medial and lateral neuroblasts occurs first. By the 28% stage a more or less uniform sheet of 20 neuroblasts has form ed. As a result of both cell proliferation and cell translocation, the pars intercerebralis proliferative cluster in each hemisphere expands so that at 30% the most medial neuroblasts lie apposed at the midline . We followed the further development of the pars intercerebralis of e ach brain hemisphere using bromo-deoxy-uridine incorporation and osmiu m-ethyl-gallate staining. Within the pars intercerebralis itself, the neuroblasts redistribute into discrete subsets. The neuroblasts of eac h subset generate clusters of progeny which extend in a stereotypic, s ubset-specific direction in the brain. We have used this feature, to i dentify one subset of four neuroblasts as being the likely progenitor cells for four clusters of embryonic neurons (W, X, Y, Z) which develo p at around 55% of embryo-genesis. We show that these progeny project axons via four discrete fascicles (w, x, y, z) into the embryonic cent ral complex. At the single cell level, Golgi impregnation reveals that the axons from these neighbouring cell clusters remain discrete, and those from the same cluster tightly fasciculated, as they project into the central complex, consistent with a modular organization for this brain region.