Morphology and pigment-dispersing hormone immunocytochemistry of the accessory medulla, the presumptive circadian pacemaker of the cockroach Leucophaea maderae: A light- and electron-microscopic study

To provide a framework for a cellular analysis of the accessory medulla, which is the presumptive circadian pacemaker of hemimetabolous insects, we have studied this neuropil and its associated neuronal structures by light- and electron-microscopy in the cockroach Leucophaea maderae. The accessory medulla is situated at the ventromedial edge of the medulla and shows no evidence of a retinotopical organization. Instead, it is composed of dense noduli that are embedded in internodular neuropil. The distal fiber tract, which appears to originate from the lamina and the medulla, provides a possible visual input to the accessory medulla. Electron-microscopic studies show that the accessory medulla contains accumulations of at least four types of dense-core vesicles, viz., granular, small, medium-sized, and large. Granular vesicles occur almost exclusively in processes of the nodular neuropil and are apparently confined to local neurons; the other three types are restricted to the internodular and loose neuropil that surrounds the accessory medulla. Immunostaining for light- and electron-microscopy has demonstrated that arborizations of presumptive pacemaker neurons, viz., neurons immunoreactive for the pigment-dispersing hormone, are restricted to the internodular and the loose neuropil of the accessory medulla. Immunoreactivity for pigment-dispersing hormone is found only in subpopulations of terminals containing medium-sized or large dense-core vesicles, in association with the vesicles.