The Corpora Allata of Insects

Transcriptomic analysis of Bombyx mori corpora allata with comparison to prothoracic glands in the final instar larvae

The corpus allatum (CA) is an endocrine organ of insects that synthesizes juvenile hormone (JH). Yet little is known regarding the global gene expression profile for the CA, although JH signaling pathway has been well-studied in insects. Here, we report the availability of the transcriptome resource of the isolated CA from the final (fifth) instar larvae of the silkworm, Bombyx mori when the JH titer is low. We also compare it with prothoracic gland (PG) that produces the precursor of 20-hydroxyecdysone (20E), to find some common features in the JH and 20E related genes between the two organs. A total of 17,262 genes were generated using a combination of genome-guided assembly and annotation, in which 10,878 unigenes were enriched in 58 Gene Ontology terms, representing almost all expressed genes in the CA of the 5th instar larvae of B. mori. Transcriptome analysis confirmed that gene for Torso, the receptor of prothoracicotropic hormone (PTTH), is present in the PG but not in the CA. Transcriptome comparison and quantitative real time-PCR indicated that 11 genes related to JH biosynthesis and regulation and six genes for 20E are expressed in both the CA and PG, suggesting that the two organs may cross talk with each other through these genes. The temporal expression profiles of the two genes for the multifunctional neurohormonal factor sericotropin precursor and the uncharacterized protein LOC114249572, the most abundant in the CA and PG transcriptomes respectively, suggested that they might play important roles in the JH and 20E biosynthesis. The present work provides new insights into the CA and PG.

Homology of insect corpora allata and vertebrate adenohypophysis?

Animal species of various phyla possess neuroendocrine glands whose hormonal products regulate developmental and physiological mechanisms and directly impact behavior. Two examples, the corpora allata of insects and the vertebrate adenohypophysis have previously been regarded as analogous tissues that evolved independently from diffuse epidermal nerve nets of early metazoans. More recent developmental and functional studies accumulated evidence suggesting that the bilaterian nervous systems including its modern parts (e.g. pallium or cortex and mushroom bodies) and its neuroendocrine appendages (that are considered to be more ancient structures) possess a single evolutionary origin. The corpora allata of insects and the vertebrate adenohypophysis share a number of characteristics in respect of morphology, control of hormone release by RFamides, metabolites produced by closely related cytochrome P450 enzymes and gene expression during embryonic development. This review incorporates latest findings into an extensive description of similarities between insect corpora allata and vertebrate adenohypophysis that should encourage further studies about the onto- and phylogenetic origin of these neuroendocrine glands.

Spatial expression of the mevalonate enzymes involved in juvenile hormone biosynthesis in the corpora allata in Bombyx mori

The developmental expressions of the mRNA of JH synthetic enzymes have been studied using homogenates of the corpora cardiaca-corpora allata (CC-CA) complexes in Bombyx mori [Kinjoh, T., Kaneko, Y., Itoyama, K., Mita, K., Hiruma, K., Shinoda, T., 2007. Control of juvenile hormone biosynthesis in Bombyx mori: cloning of the enzymes in the mevalonate pathway and assessment of their developmental expression in the corpora allata. Insect Biochemistry and Molecular Biology 37, 808-818]. The in situ hybridization analyses in the CC-CA complex showed that the distribution of the mRNAs of all the mevalonate enzymes and juvenile hormone (JH) acid O-methyltransferase occurred only in the CA cells, indicating that the fluctuations of the enzyme mRNA amounts in the CC-CA complexes were derived solely from the CA. In addition, the size of the CA and their nuclei was not associated with the JH synthetic activity by the CA until the pharate adult. Only female adult CA synthesized JH in B. mori, and the CA and the nuclei were significantly larger than those of male CA which do not synthesize JH.

Origin of alimentary tract in embryogenesis of Ixodes ricinus (Acari: Ixodidae)

We studied the embryos of Ixodes ricinus (L.) in the second and third trimester of embryonic development, by using light and transmission electron microscopy. At the beginning of the second trimester, the formation of the foregut and rectal sac, by a process of invagination, was observed. The invagination, which develops into the primordium of the hindgut, forms only in the third trimester. The rectum forms in the last phase of embryogenesis. The development of the midgut is incomplete during embryogenesis. The yolk is surrounded by a wall, formed of an amorphous basal lamina and flattened cells, that gradually accumulate deutoplasmic material. These cells do not acquire the typical features of the gut epithelium until after larval hatching. These features are, however, found in the cells forming the rectal sac.

Specialization of midgut cells for synthesis of male isoprenoid pheromone components in two scolytid beetles, Dendroctonus jeffreyi and Ips pini

Endodermal or midgut cells have only recently been recognized as the site of pheromone synthesis in bark beetles. Midgut cells are not only specialized for digestion, but they have also been recruited to form isoprenoid compounds that function as pheromone components in Ips pini and Dendroctonus jeffreyi. Male bark beetle midgut cells are competent to produce isoprenoid pheromones after feeding or stimulation by juvenile hormone (JH) III. Competent midgut cells share many ultrastructural features with cells that do not secrete isoprenoid pheromone, but they are distinguished from these by abundant and highly ordered arrays of smooth endoplasmic reticula. During secretion, both midgut cells that produce pheromone and cells that do not are characterized by the presence of apical extrusions (apocrine secretion) rather than the presence of vesicles that fuse with the apical membrane and undergo exocytosis (eccrine secretion). Pheromone-producing cells of the midgut do not represent a population of cells that are distinct from cells involved in digestion. All, or most, midgut cells of male I. pini and D. jeffreyi can secrete pheromones as well as digestive enzymes.

Ultrastructural changes in the corpus allatum after azadirachtin and 20-hydroxyecdysone treatment in adult females of Labidura riparia (Dermaptera)

In previous reports, we have shown that the injection of azadirachtin (AZA) as well as 20-hydroxyecdysone (20E) into vitellogenic females of Labidura riparia induces inhibition of vitellogenin synthesis and ovarian development. Juvenile hormone (JH) treatment rescues vitellogenin synthesis and ovarian growth (Sayah et al., 1995, 1996). In this work, we have studied ultrastructural changes of corpus allatum (CA) after injection of 200, 400, and 600 ng of 20E or 1, 3, and 5 microg of AZA. CA cells exhibit signs of inactivity in both AZA and females treated with 20E at doses of 3 microg and 400 ng, respectively. Conspicuous cytological effects consisting of multivesicular bodies with dense contents, abnormally large intercellular spaces comprising myelinic structures, and rare smooth endoplasmic reticula occurred in cytoplasm of CA glandular cells in both experimental females. However, the CA ultrastructure of females injected with 20E differs from CA cells of females injected with AZA in having a cytoplasm containing numerous electron-lucent intracellular areas and marked glycogen zones. They also differ in having abundant microtubules and well-developed junctional membranes. At a dose of 600 ng of 20E or 5 microg of AZA, the intensity of the cytotoxic effects is more apparent. CA cells display pycnotic nuclei, spherical mitochondria, large multivesicular bodies, and vacuolization of the cytoplasm. These results are discussed and compared with observations made on other insect species.

Endocrine and neuroendocrine effects of Azadirachtin in adult females of the earwig Labidura riparia

In previous studies we have shown that injection of the insect growth regulator Azadirachtin (AZA) into young vitellogenic females induces inhibition of vitellogenesis in a dose-dependent manner. Juvenile hormone treatment rescues vitellogenin synthesis and ovarian growth. The cytopathological effects on ovaries and fat body are not linked to an inhibition of feeding. In this work we investigated the effects of AZA on the endocrine and neuroendocrine system. Enzyme immunoassay reveals that ovarian ecdysteroid levels are drastically reduced, in a dose-dependent fashion, by AZA. Ultrastructural study indicates that corpus allatum cells exhibit signs of inactivity and degenerative changes after AZA exposure. Using an antibody against allastostatin-3 of Blatella germanica (BLAST-3), we show the appearance of strong immunoreactivity of numerous cells and axons in the brain of AZA-injected females. We conclude that vitellogenesis inhibition by AZA consists of a direct cytotoxic effect as well as a generalized disruption of endocrine and neuroendocrine functions.

The ultrastructure of the prothoracic gland/corpus allatum/corpus cardiacum ring complex of the Australian sheep blowfly larva Lucilia cuprina (Wied.) (Insecta: Diptera)

The ultrastructure of the constituent endocrine glands within the ring complex of Lucilia cuprina larvae has not been previously described. This study investigated the ring complex of mid-third instar larvae. Three distinct endocrine cell types were identified: (1) prothoracic gland (PTG) cells which constituted the major cell type; (2) corpus allatum (CA) cells, localized in the anterior central region, and (3) corpus cardiacum (CC) cells, located posteriorly, ventral to the aorta. PTG cells were identifiable by their large ovoid nuclei (9-14 microns dia, length 12-18 microns) and numerous cytoplasmic lipid vacuoles. The plasma membrane of peripheral PTG cells were invaginated to form intercellular channels. The CA cells are characterized by ovoid nuclei (6-7.5 microns dia, 6.5-9 microns length) and electron dense staining cytoplasm. Compared to PTG cells the CA cells had smaller nuclei and lower nucleus:cytoplasm cell ratio. Extensive networks of highly irregular, electron-lucent intracellular spaces, dispersed throughout the cytoplasm were also characteristic of CA cells at this developmental stage. These spaces often contained membrane bound lipid vacuoles occurring singly or as aggregates. The CC contained both intrinsic and extrinsic neural components. The intrinsic cells were characterized by circular nuclei (6.5-8.5 microns dia), prominent nucleolus and numerous cytoplasmic electron-dense neurosecretory granules (100-240 nm dia). The extrinsic axons and terminals within the CC contained electron-dense neurosecretory granules (80-150 nm), neurotubules and mitochondria.

Developmental regulation of juvenile hormone synthesis: ovarian synchronization of volumetric changes of corpus allatum cells in cockroaches

The corpus allatum (CA) cells of adult Blattella germanica females undergo cyclic volumetric changes in relation to juvenile hormone (JH) synthesis. In intact females the size of CA cells changes synchronously during the gonotrophic cycle, resulting in cyclic JH synthesis. In ovariectomized females volumetric changes among CA cells become asynchronous, resulting in highly variable but high rates of JH synthesis. Injection of the steroid hormone 20-hydroxyecdysone into ovariectomized females with active CA resulted in a transient decline followed by an increase in both CA volume and JH biosynthesis. This response was due to a change in the size distribution of CA cells and not in the total number of CA cells. In ovariectomized females, CA cells can be re-synchronized into a uniform population of small inactive cells with injection of 20-hydroxyecdysone and implantation of an artificial egg-case, mimicking the successive events of ovulation, oviposition and pregnancy.

Morphometric analysis of corpus allatum cells in adult females of three cockroach species

The number of cells and their sizes in the corpus allatum (CA) of adult female Blattella germanica, Supella longipalpa and Diploptera punctata were determined during oocyte maturation. Cell number and size were directly measured in cell suspensions following enzymatic dissociation of freshly excised CA. Cell numbers were verified by total cell counts in whole-mount CA monolayers and by hemocytometric sampling. In all three species, cell number did not change during the period of CA activation, averaging ca. 2000 cells per gland in B. germanica, 3500 cells per gland in S. longipalpa and 11,000 cells per gland in D. punctata. Cell diameter increased significantly in all three species during this period from a mean value of 8.9 microns to 11.7 microns in B. germanica, from 9.2 microns to 14.6 microns in S. longipalpa and from 10.0 microns to 15.6 microns in D. punctata. During a 4 h incubation period, dissociated CA cells incorporated L-[methyl-3H]-methionine into juvenile hormone-III at rates comparable to intact glands. These data suggest that CA activation in the first ovarian cycle of these species is associated mainly with an increase in cell size with minor changes in cell number.

Axonal projections within the brain-retrocerebral complex of the cricket, Teleogryllus commodus

The cerebral origins and axonal trajectories of neurons projecting to the retrocerebral complex of the cricket, Teleogryllus commodus, were examined in silver-intensified nickel preparations. Spatially separate groups of somata in the pars intercerebralis (PI) and in the pars lateralis (PL), commonly accepted as neurosecretory loci, were found to give rise to axons which terminate in the nervus corporis allati 2, the corpus allatum, or the corpus cardiacum. Additional findings demonstrated a distinct group of somata from the PI whose axons run in the esophageal nerve (stomatogastric nervous system), nine somata in the subesophageal ganglion with axons projecting into the nervus corporis allati 2, and also a small cluster of tritocerebral perikarya with axons terminating in the corpus cardiacum. Somata residing in the PI and PL were found to be compartmentally organized based upon the retrocerebral destinations of their axons. Possible functional consequences of these results with respect to the insect neurosecretory system are discussed.

Ultrastructural identification of neurohaemal sites in a tick: evidence that the dorsal complex may be a true endocrine gland

A fine structural study has been made of the 'paraganglionic plates' associated with the perineurium and of the 'retrocerebral organ' associated with the periganglionic sheath of the tick Boophilus microplus; these structures have been postulated, from descriptions by light microscopy, to be tick neurohaemal organs. Neurosecretory terminals are observed frequently in the neural lamella/perineurial sheath, particularly in a dorso-lateral area which may correspond to the 'paraganglionic plates'. No evidence was found of a discrete peripheral neurohaemal organ such as the corpus cardiacum of insects. The 'retrocerebral organ' is comprised of periganglionic sheath cells, which appear to be glandular rather than neurohaemal, and peripheral ganglionic cells.

The fine structure of Acanthamoeba astronyxis, with special emphasis on encystment

The fine structure of the trophozoite, encysting cells, and the cyst of Acanthamoeba astronyxis has been examined. In the trophic form a microtubule organizing center was associated with a well developed Golgi complex. During encystment the organelles of the amoeba changed considerably. The profiles of rough endoplasmic reticulum elongated and were often arranged in circles of multilayered concentric systems, enclosing mitochondria, the nucleus, or other inclusions. The mitochondria showed a tendency toward elongation and constriction. One or two nucleolus-like bodies appeared in the nucleus. Lipid droplets increased considerably in amount and were distributed individually or as aggregates. The mature cyst was star-shaped and surrounded by an almost circular exocyst and an endocyst that was closely apposed to the cell membrane. Both walls differed in their thickness and granulation. The exocyst was continuous over the entire cyst, while the endocyst was interrupted by gaps, ostioles, in the region of the rays. Within the ostioles was a bell-shaped structure, the operculum. The latter was composed of a granular material comparable in electron density to that of the endocyst.

Enzymic synthesis of juvenile hormone in locust corpora allata: evidence for a microsomal cytochrome P-450 linked methyl farnesoate epoxidase

Homogenates of corpora allata from adult Locusta migratoria in phosphate-buffered EDTA have been analysed by sucrose-density-gradient centrifugation. Succinate-cytochrome c reductase activity (mitochondrial) bands between d20/4 1.13-1.15, whereas NADPH-cytochrome c reductase and NADPH-dependent methyl farnesoate 10.11-epoxidase activities band identically between d20/4 1.06-1.12. We conclude that the methyl farnesoate epoxidase is exclusively microsomal. Farnesoic acid O-methyltransferase is an exclusively soluble enzyme which stoichiometrically transfers the S-methyl group from S-adenosylmethionine to farnesoic acid. No carboxyl esterase activity was found. Isolated microsomes were used to obtain an apparent Km = 7.7 X 10-6 M for the epoxidase, although substrate solubility limits the rate to 0.5 V. As expected, the product (juvenile hormone III) is chiral (10 R). The epoxidase is inhibited by excess NADP+ and oxidised cytochrome c, but neither inhibited nor synergised by NADH. NADH supports less than 10% of the NADPH rate of epoxidation. The epoxidase is inhibited by a carbon monoxide/oxygen atmosphere, half-maximal inhibition occurring at a CO/O2 ratio of 4.0. This inhibition is reversed by white-light irradiation.

Ultrastructural evidence for the endocrine nature of the lateral organs of the cattle tick Boophilus microplus

The lateral organs of the tick Boophilus microplus were previously thought to have a neurohaemal function, but the present study shows that they consist of glandular cells which contain a rich system of smooth endoplasmic reticulum (SER) and Golgi but no indication of neurosecretory production or release. There is acid phosphatase activity throughout the SER as well as in Golgi and a major function of the latter may be the production of lysosomal enzymes. It is suggested that the organs are endocrine glands and that, in engorged females, may secrete a hormone involved in the control of vitellogenesis. The organs are more active in feeding than in unfed males and a related function could be in control of the development of genital organs or spermatogenesis. Also present in the cells are coated vesicles, lipid droplets and microtubules. Coated vesicles close to Golgi are probably primary lysosomes whereas those near the periphery are shown by ferritin tracer to arise from coated pits. Pinocytosis could be involved in membrane retrieval but, in the absence of evidence for exocytosis, this seems unlikely. It is tentatively proposed that, by analogy with vertebrate and insect endocrine glands, the lateral organs may take up hormone precursor via coated vesicles for storage in lipid droplets and conversion to hormone in the SER. As in other SER-rich endocrine glands, the release mechanism for the hormone or other secretory product of the lateral organs is uncertain. Both the steroid, ecdysone and the terpenoid, juvenile hormone, are discussed as possible candidates for the lateral organ hormones.