Maturation and aging of adult fat body and oenocytes in Drosophila as revealed by light microscopic morphometry

A morphological and cytometric analysis of the adult fat body cells and oenocytes was made on sections of abdomens from immature, mature and senescent Drosophila melanogaster of both sexes. There are about 18,000 fat body cells in abdomens of female and mature male flies. Immature and senescent males have about 12,000 and 15,000 cells, respectively. The size of the cells is almost the same for immature flies of both sexes and increases about six-fold to approximately 2600 micron2, so that mature flies of both sexes have equivalent amounts of fat body tissue. The proportions of lipid, glycogen, and background cytoplasm of fat body cells also remain relatively constant throughout adult life, but dense, proteinaceous granules are observed in cells of senescent flies. The amounts of cellular components change dramatically due to change of cell size with age; the amount of lipid shows the greatest sexual difference with about 2x more in the females at all stages studied. The oenocytes number about 6,000 in the abdomens of all but immature male flies, which have approximately 4,000. Although the cells of both sexes triple in size to about 700 micron 2, the oenocytes of males reach maximum size earlier than those of females. The major features of oenocytes appear to be dense background cytoplasm, putative lipid droplets found only in mature flies, and pigmented granules first seen in the cells of mature flies which accumulate with age to 33% of the cytoplasm. The number of cells and their anticipated capacity for protein synthesis is discussed in relation to the production of yolk protein precursors.     

Moulting hormone,juvenile hormone and the ultrastructure of the fat body of adult Sarcophaga bullata (Diptera)

Summary In the ovoviviparous fly, Sarcophaga bullata, vitellogenesis is cyclic; a process reflected in ultrastructural changes in the fat body cells and oenocytes. At eclosion the larval fat body has not yet completely disappeared. During vitellogenesis the fat body cells are specialized for intensive protein synthesis showing a very extensive RER and numerous invaginations of the plasma membrane. These features disappear when the eggs descend into the oviducts to complete embryogenesis. The predominant feature of the oenocytes is their very prominent SER. The fat body cells of the males are never as specialized for protein synthesis as those of the females. Feeding of ecdysterone to males for 3 or more days induces a rather extensive subcellular apparatus for protein synthesis, i.e., invaginations of the plasma membrane and an extensive RER. Juvenile hormone is completely ineffective in this respect. Both ecdysterone and juvenile hormone have pronounced but different effects on the oenocytes of males.     

Development and ultrastructure of the fat body cells and oenocytes of the Queensland fruit fly,Dacus tryoni (Frogg.)

Summary Half-way through the larval period in Dacus tryoni, the fat body cells begin to accumulate protein in the form of granules. Early in the pupal period, both the fat body cells and oenocytes become free in the body cavity. Meanwhile, an imaginal generation of hypodermal cells, while in the process of displacing the larval hypodermis, gives rise to an imaginal generation of oenocytes. Soon after, imaginal fat body cells also appear. A few days after emergence, the larval fat body cells and oenocytes disintegrate and their imaginal equivalents expand to fill the body cavity.This paper also describes the ultrastructure of the larval and imaginal fat body cells and of the imaginal oenocyte. In all three, tubular invaginations of the plasma membrane occupy the peripheral cytoplasm. At most stages, the fat body cells contain a considerable quantity of slightly distended, rough endoplasmic reticulum, which suggests that when these cells are not sequestering protein, they are secreting it into the blood. The imaginal oenocytes are packed with smooth endoplasmic reticulum, which supports other evidence that they participate in the synthesis of cuticular wax.For assistance with the electron microscopy, I thank Mr. Tony Webber and Miss Ann Miller of the Electron Microscopy Unit at Sydney University. For the loan of some sectioned material, I am grateful to Dr. D. T. Anderson.     

Fat body cells of female reproductive castes of Attini ants (Hymenoptera: Formicidae): An ultrastructural and chemical analysis

The ultrastructural study on the fat body of gynes (virgin queens) of the basal ant species Cyphomyrmex rimosus and Mycetarotes parallelus and the derived Acromyrmex disciger and Atta laevigata queens showed vesicular rough endoplasmic reticulum, Golgi complex, and mitochondria in trophocytes, suggesting the involvement of these cells in protein synthesis, in addition to digestive vacuoles associated with the digestion of endocytosed compounds or rejected cell organelles. Oenocytes, another cell type present in the fat body of these species exhibit mitochondria, digestive vacuoles, and vesicles, indicating a mobilization of compounds by these cells. In A. laevigata, oenocytes also exhibited large storage sites of glycogen, in addition to a well-developed vesicular rough endoplasmic reticulum, suggesting an intensive participation of these cells in protein synthesis. The ultrastructural cytochemistry study also revealed electrodense granules of basic proteins present throughout the cytoplasm of trophocytes. The same was observed in oenocytes, although with smaller amounts of proteins. In the cytoplasm of trophocytes and oenocytes were also found droplets or electrodense granules of lipids. In oenocytes of A. disciger and in trophocytes of A. laevigata, lipids were observed in mitochondria, suggesting that this organelle might be a site of synthesis of these compounds. The chemical analysis of lipids revealed that in gynes, the main compounds present in fat body cells were saturated fatty acids, while in queens, saturated as well as unsaturated fatty acids were found. In conclusion, the present study showed that the fat body cells of gynes and queens, in general, maintained the same compounds and original features through the evolution process of the Attini tribe.     

Autoradiographic study of protein synthesis in abdominal tissues of Glossina austeni

Protein synthesis at various times during the pregnancy cycle of G. austeni was determined by autoradiographic measurement of the incorporation of H³-leucine and H³-tyrosine into the cells of the fat body, oenocytes, milk gland and epidermis. The rate of utilization of these molecules is such that the labelled pool in the haemolymph is depleted before 0.5 hr after injection. The incorporation of both amino acids into fat body and oenocytes is high at eclosion and just after larviposition, with the incorporation of tyrosine by the oenocytes being much higher than that in the fat body. The same pattern of incorporation is observed in the epidermal cells. Label also appears in the endocuticle during the first 10 days of adult life. Except during the first 4 days following emergence, the incorporation of the two amino acids into the milk gland is very high, with periods of less intense protein synthesis at about the time of larviposition. The milk gland represents a highly efficient secretory system, with a t₅₀ of less than 30 min.     

Synthesis and release of hydrocarbons by the oenocytes of the desert locust, Schistocerca gregaria

When incubated in vitro, the oenocytes in the peripheral fat body of the desert locust incorporate Na-¹⁴C-acetate into hydrocarbons (paraffins). The presence of haemolymph in the incubation medium greatly stimulates the release of the ¹⁴C-hydrocarbons into the medium. The labelled hydrocarbons appear to be rapidly released by the cells into the incubation medium as a function of time provided that haemolymph is present. The fact that the oenocytes not only synthesize ¹⁴C-hydrocarbons but also release them into the medium supports the hypothesis that the oenocytes of the desert locust synthesize cuticle lipids.     

Drosophila melanogaster Acetyl-CoA-Carboxylase Sustains a Fatty Acid-Dependent Remote Signal to Waterproof the Respiratory System

Fatty acid (FA) metabolism plays a central role in body homeostasis and related diseases. Thus, FA metabolic enzymes are attractive targets for drug therapy. Mouse studies on Acetyl-coenzymeA-carboxylase (ACC), the rate-limiting enzyme for FA synthesis, have highlighted its homeostatic role in liver and adipose tissue. We took advantage of the powerful genetics of Drosophila melanogaster to investigate the role of the unique Drosophila ACC homologue in the fat body and the oenocytes. The fat body accomplishes hepatic and storage functions, whereas the oenocytes are proposed to produce the cuticular lipids and to contribute to the hepatic function. RNA-interfering disruption of ACC in the fat body does not affect viability but does result in a dramatic reduction in triglyceride storage and a concurrent increase in glycogen accumulation. These metabolic perturbations further highlight the role of triglyceride and glycogen storage in controlling circulatory sugar levels, thereby validating Drosophila as a relevant model to explore the tissue-specific function of FA metabolic enzymes. In contrast, ACC disruption in the oenocytes through RNA-interference or tissue-targeted mutation induces lethality, as does oenocyte ablation. Surprisingly, this lethality is associated with a failure in the watertightness of the spiracles-the organs controlling the entry of air into the trachea. At the cellular level, we have observed that, in defective spiracles, lipids fail to transfer from the spiracular gland to the point of air entry. This phenotype is caused by disrupted synthesis of a putative very-long-chain-FA (VLCFA) within the oenocytes, which ultimately results in a lethal anoxic issue. Preventing liquid entry into respiratory systems is a universal issue for air-breathing animals. Here, we have shown that, in Drosophila, this process is controlled by a putative VLCFA produced within the oenocytes.     

Genetic study of the production of sexually dimorphic cuticular hydrocarbons in relation with the sex-determination gene transformer in Drosophila melanogaster

In Drosophila melanogaster, the main cuticular hydrocarbons (HCs) are some of the pheromones involved in mate discrimination. These are sexually dimorphic in both their occurrence and their effects. The production of predominant HCs has been measured in male and female progeny of 220 PGa14 lines mated with the feminising UAS-transformer transgenic strain. In 45 lines, XY flies were substantially or totally feminised for their HCs. Surprisingly, XX flies of 14 strains were partially masculinised. Several of the PGa14 enhancer-trap variants screened here seem to interact with sex determination mechanisms involved in the control of sexually dimorphic characters. We also found a good relationship between the degree of HC transformation and GAL4 expression in oenocytes. The fat body was also involved in the switch of sexually dimorphic cuticular hydrocarbons but its effect was different between the sexes.     

Nutrient transfer during the reproductive cycle in Glossina austeni Newst.: histology and histochemistry of the milk gland, fat body, and oenocytes

Using thick sections cut from Epon-embedded tissues fixed in Karnovsky's fixative, the cytological changes visible in the light microscope were described for milk gland, fat body and oenocytes during the pregnancy cycle of the female of Glossina austeni. Histochemical procedures on paraffin sections were used to explore changes in nucleic acids, proteins, and tyrosine-containing proteins. The milk gland undergoes a cycle of secretion correlated with the pregnancy cycle and the results are consistent with the view that the protein component of the milk is synthesized in the secretioy cells of the gland. Tyrosine is particularly abundant in the oenocytes which appear to play a crucial role in the metabolism of this amino acid.     

Histochemical and biochemical investigations concerning the function of larval oenocytes of Tenebrio molitor L. (Coleoptera,Insecta)

Summary Larval oenocytes of Tenebrio molitor were investigated histochemically. In contrast to the lipid droplets of the fat body, they did not stain with Sudan black. A positive reaction for lipoproteins appeared only after destructive oxidation with sodium hypochlorite. These lipoproteins are the remnants of degenerated membranes, as revealed by ultrastructural analysis. Polyphenols could be identified in the exocuticle of exuvia, and in the newly formed procuticle. Endocuticle, epidermis and oenocytes showed no staining reaction. In oenocytes a great amount of lipase is also present which could be detected with several Tweens as substrates. The significance of these lipases remains unclear, since only few glycerides are synthesized in the cells, as shown below. They may play a role in the extended membrane turnover observed in this cell type. In vitro incubation of oenocytes of the larval generation demonstrated that ¹⁴C-labeled acetate was only incorporated into the paraffin fraction. A negligible amount of the label was found in glycerides; wax esters were free of label. Larval epidermis is also capable of paraffin formation, but only to a small degree. Oenocytes of the imaginal generation located between the sternal epidermis cells of pupae and adults do not synthesize paraffins, but other more polar compounds not yet identified. Labeled waxes in cuticular lipids were detected only when ¹⁴C-acetate was injected into whole larvae, and the lipids extracted some hours later. Autoradiographs demonstrated that ¹⁴C-acetate was intensively incorporated into larval oenocytes, the rate varying in different cells. Incorporation into the epicuticle, probably into the wax layer, was clearly shown. Cuticulin and dense layer do not show an intensive label. The lamellated cuticle also seems to be impregnated with acetate derivatives.Supported by the Deutsche Forschungsgemeinschaft     

Regional and functional differentiation in the fat body of pharaoh's ant queens, Monomorium pharaonis (L.)

The insect fat body is generally described as a uniform tissue with multiple functions, but we have found evidence of cell differentiation in the Monomorium fat body. We show that the fat body of a mature egg-laying pharaoh's ant queen is a result of a preceding remodeling of cell material comprising at least 11 different fat cell types, located at specific positions in the head, alitrunk (thorax) and gaster (abdomen). The cell types are classified based on their position, histochemistry, ultrastructure, and immunoreactivity for vitellogenin/vitellin. Some of these cells are primordial cells present at emergence, others invade the histolysing flight muscle tissue, and still others disappear during the maturation process. Only one type, the subepidermal fat cell of the gaster, is active in vitellogenin synthesis and is the only cell type in close association with oenocytes. Although only this type produces vitellogenin, our material indicates that most fat cell types are essential to support egg production. In some queens vitellogenin was found to form crystals in ventral vitellogenin-producing fat cells. This indicates an imbalance between vitellogenin production in the fat cells and uptake in the oocytes, which is probably related to a cyclic regulation of egg production.     

Juvenile hormone and the ultrastructural properties of the fat body of the adult Colorado beetle,Leptinotarsa decemlineata say

Summary In the fat body of ovipositing female Colorado beetles, two types of lobes occur. The first type, the internal fat body, is highly specialised for protein synthesis. A lobe of the second type, the peripheral fat body, contains two types of cells, oenocytes and glycogen cells. Ovariectomy, performed at adult moult results in hypertrophy of the glycogen cells of the peripheral fat body. The lobes are characterized by the storage of lipid bodies and glycogen and by numerous mitochondria. Short-day conditions ab ovo, which induce diapause in adults, also result in hypertrophy of glycogen cells of the peripheral fat body. Furthermore, only few mitochondria occur but many proteinaceous bodies may be observed, which conditions are in contrast to the observed effects of castration. The fat body of allatectomized long-day females, has the same structure as that of short day beetles. Consequently a lack of juvenile hormone induces the proteinaceous bodies.Dr. A. De Loof gratefully acknowledges a scholarship as Aspirant of the National Foundation of Scientific Research in Belgium. We wish to thank Prof. Dr. h. C. J. de Wilde for his suggestions and helpfull criticism. We also thank Mr. W. Bohijn for his help in operating the EM and Mr. G. Maes for photography.     

Control of pupal fat body disappearance in the female black blow fly,Phormia regina (Meigen) by the brain and the corpus allatum

Neck-ligation, brain implantation, allatectomy, methoprene treatment, and ovariectomy indicated that the disappearance of pupal fat body cells in newly emerged adult female blow fly, Phormia regina, is controlled by the brain and the corpus allatum (CA). Absence of brain or CA greatly lowered the rate of fat body cell disappearance (i.e. death). Dependency on the CA decreased from 0 to 36h post-emregence, indicating that the CA was active during the earlier part of this timespan. Methoprene treatment enhanced pupal fat body cell disappearance in allatectomized females. Brain implantation restored the rate of pupal fat body cell disappearance in neck-ligated flies. Brains from day 1 sugar-fed flies proved to be more effective than those from day 2 sugar-fed flies, indicating that there may be a window after adult emergence that allows the brain to act directly or indirectly on the death of pupal fat body cells. Ovariectomy did not alter the rate of pupal fat body cell death in test animals. Dying pupal fat body cells were smaller in size, less dense (i.e. did not sink in saline like normal pupal fat body cells), and stickier (i.e. attached to other tissues tighter) than the healthy cells. A possible role played by ecdysteroids is also discussed.     

Flexible origin of hydrocarbon/pheromone precursors in Drosophila melanogaster

In terrestrial insects, cuticular hydrocarbons (CHCs) provide protection from desiccation. Specific CHCs can also act as pheromones, which are important for successful mating. Oenocytes are abdominal cells thought to act as specialized units for CHC biogenesis that consists of long-chain fatty acid (LCFA) synthesis, optional desaturation(s), elongation to very long-chain fatty acids (VLCFAs), and removal of the carboxyl group. By investigating CHC biogenesis in Drosophila melanogaster, we showed that VLCFA synthesis takes place only within the oenocytes. Conversely, several pathways, which may compensate for one another, can feed the oenocyte pool of LCFAs, suggesting that this step is a critical node for regulating CHC synthesis. Importantly, flies deficient in LCFA synthesis sacrificed their triacylglycerol stores while maintaining some CHC production. Moreover, pheromone production was lower in adult flies that emerged from larvae that were fed excess dietary lipids, and their mating success was lower. Further, we showed that pheromone production in the oenocytes depends on lipid metabolism in the fat tissue and that fatty acid transport protein, a bipartite acyl-CoA synthase (ACS)/FA transporter, likely acts through its ACS domain in the oenocyte pathway of CHC biogenesis. Our study highlights the importance of environmental and physiological inputs in regulating LCFA synthesis to eventually control sexual communication in a polyphagous animal.     

Tissue-specificity of hemoglobin synthesis: Localization of heme synthesis in the subepidermal fat body of Chironomus thummi (Diptera)

Possible sites of heme synthesis in the fourth instar of Chironomus thummi were investigated by means of autoradiography of specific isotope incorporation. “Body wall” preparations, which include subepidermal and visceral fat body, oenocytes, muscle, epidermis, and cuticle, were cultured for 1 h in a medium containing tritiated-δ-aminolevulinic acid, a specific precursor to heme biosynthesis. Light-microscopic examination of autoradiographs of sections of the body walls indicates that the subepidermal fat body is the major site of incorporation of the precursor into heme. The visceral fat body shows few silver grains. Oenocytes, as well as muscle and epidermis, are characterized by absence of silver deposits. These findings indicate that the subepidermal fat body of Chironomus is the primary site of heme synthesis, and are discussed in relation to specific hemoglobin synthesis.     

Vairimorpha invictae N. Sp. (Microspora: Microsporida), a Parasite of the Red Imported Fire Ant,Solenopsis invicta Buren (Hymenoptera: Formicidae)12

ABSTRACT. Vairimorpha invictae n. sp. infects the red imported fire ant, Solenopsis invicta Buren, in Brazil. The parasite is dimorphic, producing two morphologically distinct types of spores, which develop sequentially in the same fat cells or oenocytes in the fat body. The binucleate free spores develop from disporous sporonts; the uninucleate octospores develop from multinucleate sporonts within a sporophorous vesicle. Infected cells are transformed into large sacs which contain both types of spores in mature adult hosts. Mature free spores are often present by the time the larvae pupate, but mature octospores are found only in adult hosts. Masses of spores may be seen through the intact cuticle by low power phase-contrast microscopy; there are no other physical signs and no behavioral signs of infection. Attempts to transmit the infection in the laboratory failed.     

A comparative study of fat body morphology in five mosquito species

The insect fat body plays major roles in the intermediary metabolism, in the storage and transport of haemolymph compounds and in the innate immunity. Here, the overall structure of the fat body of five species of mosquitoes (Aedes albopictus, Aedes fluviatilis, Culex quinquefasciatus, Anopheles aquasalis and Anopheles darlingi) was compared through light, scanning and transmission electron microscopy. Generally for mosquitoes, the fat body consists of lobes projecting into the haemocoel and is formed by great cell masses consisting of trophocytes and oenocytes. Trophocytes are rich in lipid droplets and protein granules. Interestingly, brown pigment granules, likely ommochromes, were found exclusively in the trophocytes located within the thorax and near the dorsal integument of Anopheles, which is suggestive of the role these cells play in detoxification via ommochrome storage. This study provides a detailed comparative analysis of the fat body in five different mosquito species and represents a significant contribution towards the understanding of the structural-functional relationships associated with this organ.     

Ultrastructural immunocytochemical localization of vitellogenin in the fat body of the blowfly,Calliphora vicina Rob.-Desv. (erythrocephala Meig.) by use of the unlabeled antibody-enzyme method

Summary The unlabeled antibody-enzyme method was used to demonstrate ultrastructurally the specific localization of vitellogenin in the fat body of Calliphora. In control flies the binding sites to vitellogenin were localized in secretory granules situated in the Golgi complex, and in larger bodies named composite secretory granules. These composite granules appear to be formed when a part of a Golgi complex containing secretory granules and a number of small vesicles become surrounded by a common membrane. Ovariectomized flies, which apparently do not produce secretory granules, exhibited no immunocytochemical staining. Ovariectomized flies in which the administration of ecdysterone induced formation of secretory granules, also revealed specific staining on these granules. This is the first ultrastructural evidence of: (a) the specific localization of vitellogenin in secretory granules of the fat body of an insect; (b) the relationship between the presence of the ovary, and of ecdysterone, and the synthesis of vitellogenin by the fat body.