The neurosecretory system in the aphid Megoura viciae, with reference to unusual features associated with long distance transport of neurosecretion

Abstract

Cytological evidence of neurosecretory activity in the ganglia of the central and stomatogastric nervous systems of the aphid, Megoura viciae, has been investigated with a view to analysis of the significance of identified neurosecretory cells (NSC) as sources of hormones and their possible rôle in the control of polymorphism. NSC defined by cytological criteria are found in all ganglia of the central nervous system. The cells display an unusual spatial segregation of the functions of synthesis, storage and transport of stainable neurosecretory material (NSM). NSM does not normally accumulate near its site of synthesis in the cell body but moves out into swollen axonal “reservoirs” in the adjacent neuropile where it appears to undergo cytological modification. It also accumulates in them in much larger quantities than occurs either in the cell bodies or in known neurohemal sites; storage of NSM takes place primarily in the axons of these cells. The large quantity of stainable axonal NSM enables the axonal projections of the cells to be traced for long distances. Only a few NSC project to neurohemal organs such as the corpora cardiaca. By contrast, the axons of many NSC run throughout the length of the central nervous system and into the ventral nerve cord which supplies branches to the abdominal organs. Branches of some axons appear to end blindly in areas of neuropile; these are interpreted as collaterals through which the NSC would receive presynaptic input. The medial NSC group in Megoura appears to be divided into two anatomically separate groups of cells. One of these sends axons directly to the abdomen; no branches to the corpora cardiaca were seen. The long distance axonal transport of NSM is discussed in relation to the “directed delivery” and “neurohemal” concepts of neurosecretion.