Neuroendocrine control of moulting cycle in mealworms: Histology and autoradiography of cephalic components

Fluctuations in the activities of the cephalic neuroendocrine system of larval mealworms (Tenebrio molitor) have been investigated by autoradiographic and histological techniques. Shortly after ecdysis, the proteinaceous granules in the cytoplasm of the ‘A’ type medial neurosecretory cells of the brain undergo a marked increase in numbers per cell and in chromophilia; both numbers and stain density reach a maximum about two-thirds of the way through interval between ecdyses and then both decline precipitously. The cyclic histological changes are nicely correlated with overall patterns of protein synthesis, as demonstrated by incorporation of ³H-amino acids into the medial cells. Paradoxically no cyclicity was observed downstream: no fluctuations in numbers or chromophilia of stainable inclusions were detected in the axons of the medial neurosecretory cells, in the corresponding efferent nerves to the corpora cardiaca, or within the cardiaca themselves. Mechanisms are proposed to account for this apparent paradox, and the patterns of protein synthesis within the ‘A’ type medial neurosecretory cells are correlated with previously determined fluctuations in moulting hormone activity.