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Structural Aspects of Neurosecretory and Corpus Allatum Activity in the Adult Colorado Beetle, Leptinotarsa Decemlineata Say, as a Function of Daylength

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image of Netherlands Journal of Zoology
For more content, see Archives Néerlandaises de Zoologie (Vol 1-17) and Animal Biology (Vol 53 and onwards).

In this paper the gross anatomy of the neurosecretory system and the structure of neurosecretory cells (NSC), corpora cardiaca (CC), and corpora allata (CA) of the female Colorado beetle has been described in detail, and an analysis of the patterns of secretory activity throughout adult life and under short-day and long-day breeding conditions has been presented. In the protocerebrum seven types of cells have been distinguished which differ from ordinary neurons by the cyclic changes in stainability and the dimensions of cellbodies and nuclei. These cells are therefore taken to be neurosecretory. For five of them, i.e., the so-called A-, A1-, B-, C-, and L-NSC it could be established that their neurosecretory material (NSM) is transported to, and released from, the CC; for two of them, i.e., the D- and E-NSC this could not be ascertained. NSM of the A-, A1-, and C-NSC should be called Gomori-positive, but with regard to the different shades of purple revealed by these kinds of NSM, and the apparent differences in the content of sulphydryl groups, they differ in chemical respects. NSM of the Gomori-positive NSC in the suboesophageal ganglion (SOG-A-NSC) very much resembles that of the A-NSC in the brain. The other types of NSC distinguished take up one or more components of the counterstain. The five above-mentioned kinds of NSM produced in the pars intercerebralis are temporarily stored within the axon-terminals in the CC. In addition, the CC contain materials that are elaborated by intrinsic and extrinsic secretory cells. Most of the swellings of the cell processes are clearly facing the haemocoel, either at the periphery of the CC, or at the numerous crevices that deeply penetrate the gland, and all "secretory" materials are eventually released. These observations demonstrate the multiple role of the CC as a neurohaemal organ. Within the CC, some of the axons originating in the brain disentangle from the major axon bundle and run straight to the CA, where they penetrate between the cells. In all physiological stages axons of the A-NSC are discernible. It remains to be investigated whether other materials as well normally enter the CA; occasionally the material from the exterior secretory cells of the CC was found to accumulate in the CA. In CA of ovipositing females which are known to be actively secreting juvenile hormone, cells are relatively large and nuclei have extremely irregular shapes. Many of the cells located centrally possess extensions reaching the periphery. The occurrence of a temporary overshoot in CA volume in stages of beginning activation-during the pre-oviposition periods-has been explained on the assumption that these cell processes have not yet fully developed; hence the release of the secretion is hampered, resulting in a passive hypertrophy of the CA. It seems unlikely that the cessation of oviposition in aged females is primarily caused by a cessation of juvenile hormone production. During diapause, the CA are inactive; there are some indications, however, that after five months of diapause they have been activated to some extent. In animals of varying age and bred under long-day and short-day conditions the pattern of activity of several types of (neuro)secretory cells has been investigated. To that purpose, various parameters of glandular activity had to be taken into account. The pattern of secretory activity of the A-NSC has been unravelled in three different ways. 1. From the study of accumulation of A-NSM in perikarya and axon-terminals it could be concluded that during the first few days after ecdysis, in long-day as well as in short-day bred animals, much of the NSM that had accumulated in the late pupal stage, is released. 2. From measurements of nuclear diameters it followed that the synthetic activity is high during oviposition and low during diapause. 3. Incorporation experiments (involving the use of 35S-labeled 1-cysteine as a hormone precursor, and the detection of this isotope in the NSC by means of the stripping-film technique) showed that in ovipositing females the uptake of the isotope took place at a much higher rate than in diapausing females. In an attempt to "translate" the patterns of labeling into more or less exact relative rates of NSM synthesis in both experimental groups, in addition to silver grain density the rate of decrease of the 35S-concentration in the blood has been taken into account. As a preliminary result, it has been stated that the rate of NSM synthesis in ovipositing females is about seven times as high as in diapausing females. The results indicate that in diapausing females the A-NSC are by no means completely inactive and that in ovipositing animals within one hour an amount of NSM is passed to, and released from the CC, that corresponds with the amount of NSM visible within the perikarya at any given moment. In considering these figures it has been concluded that the amount of NSM temporarily accumulated in the CC bears no information as to rates of release, in general. On the other hand, the measure in which elementary neurosecretory granules aggregate in the perikarya, may be taken as a rough indication for the secretory activity of the NSC concerned: the relatively small dark inclusions occur in actively ovipositing females only, the vast hazy aggregations in diapausing females. By making an analysis of accumulation phenomena in individuals, evidence has been obtained for the assumption that the amount of NSM in perikarya and axon-terminals is primarily controlled by the rate of axonal transport, rather than by the mere rate of release from the axon-terminals. Considering the apparent synchronization of separate cells it does not seem impossible that a nervous stimulation of NSC is involved. At any event, the structural organization of the A-NSC (i.e., the presence of axon-collaterals and "dendrites") should enable the cell to be stimulated in the neuropile regions. The knowledge of the other types of NSC distinguished is rather fragmentary. Attention has mainly been focussed on the appearance or disappearance of NSM accumulations (i.e., for A1-, C-, L-, SOG-A-NSC and extrinsic secretory cells of the CC); in some cases (i.e., for E-NSC and also for C- and L-NSC) the changes in nuclear dimensions have been taken into account. Following these direct, respectively indirect, signs of secretory activity, the patterns of activity of these types of NSC have also been resolved. All data have been brought together in a diagram given on page 218. All types of NSC and the CA are more active during oviposition than during diapause, but between the patterns differences exist. Some of the physiological implications have been discussed in connection with the results from pure physiological experiments on Leptinotarsa and other insects. Possibly, the A1-NSC produce a factor which in the presence of juvenile hormone is responsible for the synthesis of the sex-specific "female-protein". The E-NSC may be concerned with the tanning of the young adult cuticle, and the extrinsic secretory cells of the CC with the (inhibitory) control of corpus allatum activity.

Affiliations: 1: Department of Entomology, Agricultural University, Wageningen, The Netherlands

10.1163/002829670X00015
/content/journals/10.1163/002829670x00015
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/content/journals/10.1163/002829670x00015
1969-01-01
2016-12-04

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