Contribution � l'�tude infrastructurale du syst�me neuros�cr�teur r�troc�r�bral chez Locusta migratoria migratorioides (R. et F.): I. Les corpora cardiaca

Cell biology of the adipokinetic hormone-producing neurosecretory cells in the locust corpus cardiacum

The adipokinetic cells are neuron-like unipolar cells, the cell bodies and cell processes of which are intermingled within the glandular part of the corpus cardiacum. In Schistocerca gregaria, they produce two adipokinetic hormones, AKH-I and -II, whereas in Locusta migratoria an additional hormone, AKH-III, is present. The three AKHs are produced by the same cells and are co-localized in secretory granules. The biosynthesis and processing of the AKH prohormones to the bioactive hormones, which has been elucidated in detail for AKH-I and -II in S. gregaria, takes less than 75 min and goes on continuously. In older locusts in particular, the adipokinetic cells contain intracisternal granules, widely dilated cisternae of the rough endoplasmic reticulum, which function as stores of prohormones of AKH-I and -II, not of AKH-III. The adipokinetic cells are subjected to regulation by a number of neural and humoral substances, neural influences coming from secretomotor cells in the lateral part of the protocerebrum. Flight activity is the only natural stimulus unequivocally shown to induce the release of AKHs, which in L. migratoria results in parallel secretion of all three AKHs. During secretory stimulation, young secretory granules containing newly synthesized hormones are preferentially released over older granules. Secretory stimulation is not accompanied by a clear increase in the levels of the AKH mRNAs and the AKH prohormones and in the rate of synthesis of the (pro-)AKHs. Apparently, a coupling between release and biosynthesis of the AKHs in the adipokinetic cells is very loose or does not even exist.

Ultrastructural comparison of the perisympathetic organs in three Coleoptera: Chrysocarabus auronitens F., Oryctes rhinoceros L. and Tenebrio molitor L

Ultrastructural comparison of different types of perisympathetic organs (POs) in three species of Coleoptera (Chrysocarabus auronitens, Oryctes rhinoceros, and Tenebrio molitor) showed that the structure of these organs was not related to their morphological types but to their topography. Two kinds of PO structure may be distinguished: compact median and diffuse lateral. They were similar in that both were surrounded by thin neural lamellae and exhibited numerous glial cells originating in the perineurium (type I perineurial cells) as well as abundant neurosecretory endings. They were different in as much as in median POs, the neurosecretory endings were generally surrounded by perineurial processes but in transverse POs, these endings were sheathless. Only one type of neurosecretory axon was distinguished in the median organs but three or four in the transverse. The nature of the processes by which neurosecretory granules are released may depend on the type of neurosecretory axon. For instance, exocytosis always occurred for dense spherical granules, and granule fragmentation was visualized for granules of smaller size.