Ultrastructural study on the innervation of prothoracic glands in Galleria mellonella

Abstract

Nerve bundles which contain axons, tracheoles, and glial elements penetrate the prothoracic glands of Galleria mellonella larvae. The axons contain electron dense neurosecretory granules (120–150 nm in diameter), rarely electron lucent vesicles (50 nm in diameter), and, exceptionally, dense core vesicles (100–150 nm in diameter). Nerve fibers are generally enclosed by a thick extracellular stroma but in rare cases glia- and basal lamina-free neurosecretory axons can be detected closely adhering to the endocrine cells; the distance between axolemma and the plasma membrane of the glandular cell is reduced to 20–30 nm. In addition to these close contacts, which may be regarded as neuroglandular junctions, exocytosis of neurosecretory granules into extracellular spaces between the gland cells was observed. This suggests that neurosecretory axons affect the glandular function via a peptidergic “messenger” released in proximity to endocrine cells. No synaptoid junctions between axons and the endocrine cells were found.