Presence of T-bars, intramembranous particle arrays and exocytotic profiles in neuroendocrine terminals of an insect.

Definitive evidence is presented to show that arthropod neurohaemal terminals contain electron-dense T-bar structures with clustered microvesicles similar to those present in neuropilar and neuromuscular terminals. In terminal membranes of the locus corpus cardiacum, studied by freeze-fracture, intramembranous particle arrays, considered to correlate with the dense bars, are seen. However, there does not appear to be a spatial association between the arrays and the exocytotic profiles seen following exposure to stimulants for hormone release. The presence of the densities in both neuroendocrine and conventional terminals is discussed in the light of current theories for mechanisms of release of neurotransmitters and neurohormones in arthropods and vertebrates.