The ultrastructure of the corpus cardiacum of Aedes sollicitans and the histology of the cerebral neurosecretory system of mosquitoes.

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Abstract The anatomy of the cerebral neurosecretory system is essentially the same in adult Aedes sollicilans, A. aegypti. A. taeniorhynchus, A. triserialus , and Culex pipiens . Four paired groups of neurosecrotory cells, medial. lateral, anterior, and ventral, are located in the brain. The axons of the medial and lateral neurosecretory cells (nervi corporis cardiaci I and II) emerge from the ventral surface of the brain. enter the aortal wall, and form the corpus cardiacum. Electron micrographs of the corpus cardiacum of A. sollicilans reveal that the ganglion cells and neuropile are not concentrically arranged as in other insect ganglia. Instead, the corpus cardiacum is a linearly arranged ganglion in which the ganglion cells are centrally located between an anterior and posterior neuropile. Thus the corpus cardiacum consists of three regions: an anterior neuropile, a central region of ganglion cells, and a posterior region containing neuropile but mainly composed of neurosecretory terminals of the lacunar neurohemal site. The corpus cardiacum is a neurohemal organ located in the head of mosquitoes in contrast to other Diptera, in which it is found in the neek and thorax. Two neurohemal sites were found in this corpus eardiacum; an aortic neurohemal site in which neurosecretory axons extend between the muscle fibers of the aorta and terminate in the lunen, and a lacunar neurohemal site in the posterior region of the corpus cardiacum. In the lacunar site the neurosecretory axons terminate in extracellular channels which are confluent with the hemocoel. The axons of the lacunar site are associated with a type of glial cell restricted to this region. Five different types of neurosecretory axons were noted in the corpus cardiacum based upon relative size and density of the granules and density of the axoplasm. Two different types of axons were usually found paired in the lacunar release site, encapsulated by a glial cell process. The cephalic aorta associated with the corpus cardiacum is composed of two parallel layers of muscle, the internal and external aortal muscle. Although closely apposed, the corpus cardiacum and hypocerebral ganglion are two distinet ganglia, separated by muscle or glial sheaths. The hypocerebral ganglion terminates posteriorly in an interganglionic nerve which connects the two ganglia. A pair of neurosecretory nerves extend caudally from the corpus cardiacum. Each bifurcates; the dorsal branch becomes the nervus corporis allati, the ventral branch is the nervus ocsophagci. Each esophageal nerve is joined by a nonneurosecretory stomatogastric nerve, thereby forming the two ventricular nerves that continue to the ventricular ganglia in the thorax. Large secretory cells are located adjacent to the anterior and occasionally the posterior margins of the corpora allata. These cells contain numerous electron dense and lucent granules. Although an axonlike process appears to extend from these cells, they have no glial association and their soma are directly apposed; hence their identity as a neurosecretory cell or an endocrine cell requires further investigation, and the name X-cells has been retained until the function of these cells can be determined.