The organization of the corpora cardiaca and of the nerve tracts linking these with the brain of the stick insect has been examined in the electron microscope. Particular attention has been paid to the appearance of neurosecretory products within the axons of the nervi corporis cardiaci, and within the cell processes of the intrinsic secretory neurons of the corpus cardiacum; these observations are considered to be an essential preliminary to a consideration of the mechanism of release of secreted products from the organ into the ambient haemolymph.
The corpus cardiacum contains cells of three main categories: axons of neurons located in the brain, intrinsic secretory neurons, and glial cells. Within the organ the cellular components form a loosely knit tissue incorporating ramifying extracellular spaces confluent with the haemolymph via the peripheral basement-membrane sheath of the organ. The present electron-microscopic account relates particularly to the mode of release of neurosecretory products into the extracellular milieu. This evidence indicates that the corpus cardiacum is the site of release of both extrinsic and intrinsic secretions, but does not support the suggestion that intrinsic neurosecretory materials synthesized within the corpus cardiacum are discharged via axon terminals at sites outside the corpus cardiacum. It is suggested that release of both extrinsic and intrinsic material takes place by fusion of the membrane surrounding the neurosecretory droplet with the plasma membrane of the nerve cell in regions where the glial sheath is absent.
The evidence on the mechanism of extrusion of materials initially secreted and sequestered within nerve cells and their processes is described and discussed in terms of present concepts of the passage of neural and other materials across plasma membrane boundaries.
It is stressed that, in this functionally versatile organ, the electron microscope may reveal the mechanism of release of active neurohumoral materials, but is at present unable to differentiate between the rich variety of these pharmacologically active products which affect numerous target organs throughout the body.