The roles of Centrin 2 and Dynein Light Chain 8a in apical secretory organelles discharge of Toxoplasma gondii

To efficiently enter host cells, apicomplexan parasites such as Toxoplasma gondii rely on an apical complex composed of tubulin‐based structures as well as two sets of secretory organelles named micronemes and rhoptries. The trafficking and docking of these organelles to the apical pole of the parasite is crucial for the discharge of their contents. Here, we describe two proteins typically associated with microtubules, Centrin 2 (CEN2) and Dynein Light Chain 8a (DLC8a), that are required for efficient host cell invasion. CEN2 localizes to four different compartments, and remarkably, conditional depletion of the protein occurs in stepwise manner, sequentially depleting the protein pools from each location. This phenomenon allowed us to discern the essential function of the apical pool of CEN2 for microneme secretion, motility, invasion and egress. DLC8a localizes to the conoid, and its depletion also perturbs microneme exocytosis in addition to the apical docking of the rhoptry organelles, causing a severe defect in host cell invasion. Phenotypic characterization of CEN2 and DLC8a indicates that while both proteins participate in microneme secretion, they likely act at different steps along the cascade of events leading to organelle exocytosis.     

Fine structure of the erythrocytic stages of Plasmodium knowlesi

Summary The fine structure of erythrocytic stages of Plasmodium knowlesi was compared with that of the same parasite isolated from its host cell by a saponin technique. Rhesus monkeys experimentally infected with Plasmodium knowlesi were the source of parasitized red cells. The erythrocytic stages of this Plasmodium showed all the organelles described in other mammalian forms; the nucleus lacked a typical nucleolus but contained a cluster of granules. P. knowlesi did not have protozoan-type mitochondria as do the avian and reptilian forms, but had double-membrane-bounded bodies as observed in other mammalian malarial parasites.The isolation procedure caused a slight swelling of the parasite, but in general, the structure and structural relationships of the parasite were preserved. However, the isolation technique gave a new insight into the connection of the host cell cytoplasm with the large, so-called food vacuoles of the parasite. The parasite freed from its host cell showed clear spaces where the large vacuoles had been. The content of these vacuoles had been removed together with the red cell cytoplasm. As the nature of the isolation procedure precluded any disruption of the parasite itself, these findings support our view that the vacuoles are not true food vacuoles. If these were true food vacuoles, they would be completely enclosed by a parasite membrane within the parasite cytoplasm. However, we have demonstrated that they represent extensions of host cell cytoplasm in direct communication with the rest of the red cell. The outer membrane surrounding the intra-erythrocytic parasites disappeared after isolation of the parasite from the host cell. This strongly suggested that the outer membrane is of host cell origin. The budding process of the merozoites from a schizont was also described and discussed.This paper is contribution No. 558 from the Army Research Program on Malaria and was supported in part by Research Grant AI 08970-01 from the United States Public Health Service.     

THE EVOLUTION OF PARASITISM IN RED ALGAE: CELLULAR INTERACTIONS OF ADELPHOPARASITES AND THEIR HOSTS1

In the initial stages of cell–cell interactions (spore germination and host penetration), the adelphoparasites Gardneriella tuberifera Kyl. and Gracilariophila oryzoides Setch. & Wilson form infection rhizoids that fuse directly with underlying host epidermal or cortical cells. In so doing, parasite nuclei and other organelles enter the cytoplasm of the host. The resulting heterokaryon may fuse with adjacent host cells either directly, via secondary pit connections, or by the dissolution or dislodgment of pit plugs from existing pit connections. The cell fusion events result in a heterokaryotic syncytium in which parasite nuclei replicate. In Gardneriella, formation of the syncytium induces surrounding host tissues to divide to form a photosynthetic callus. The internalized syncytium forms conjunctor and rhizoidal cells that fuse with host callus, eventually transforming the host callus into cells containing parasite nuclei. Gracilariophila does not induce surrounding host tissue to divide. Rather, division of the initial heterokaryotic tissue gives rise to the colorless mantle that protrudes from the host and forms reproductive structures. The heterokaryotic tissue also fuses with underlying host cells, thereby spreading parasite nuclei throughout adjacent host cells. In both these adelphoparasites, transformation of host cells by parasite nuclear invasion results in plastid dedifferentiation, an increase in mitochondria, autolysis of organelles, and accumulation of large amounts of floridean starch. The development and physiology of these parasites is similar to normal post-fertilization processes in the hosts that give rise to carposporophytes and suggests that these adelphoparasites may have originated from perturbations of developmental pathways involved in their host's post-fertilization development.     

Development of the endoplasmic reticulum, mitochondrion and apicoplast during the asexual life cycle of Plasmodium falciparum

Plasmodium parasites are unicellular eukaryotes that undergo a series of remarkable morphological transformations during the course of a multistage life cycle spanning two hosts (mosquito and human). Relatively little is known about the dynamics of cellular organelles throughout the course of these transformations. Here we describe the morphology of three organelles (endoplasmic reticulum, apicoplast and mitochondrion) through the human blood stages of the parasite life cycle using fluorescent reporter proteins fused to organelle targeting sequences. The endoplasmic reticulum begins as a simple crescent-shaped organelle that develops into a perinuclear ring with two small protrusions, followed by transformation into an extensive reticulated network as the parasite enlarges. Similarly, the apicoplast and the mitochondrion grow from single, small, discrete organelles into highly branched structures in later-stage parasites. These branched structures undergo an ordered fission - apicoplast followed by mitochondrion - to create multiple daughter organelles that are apparently linked as pairs for packaging into daughter cells. This is the first in-depth examination of intracellular organelles in live parasites during the asexual life cycle of this important human pathogen.     

Development of the red algal parasite Vertebrata aterrimophila sp. nov. (Rhodomelaceae,Ceramiales) from New Zealand

Parasitic red algae grow only on other red algae and have over 120 described species. Developmental studies in red algal parasites are few, although they have shown that secondary pit connections formed between parasite and host and proposed that this was an important process in successful parasitism. Furthermore, it was recorded that the transfer of parasite nuclei by these secondary pit connections led to different host cell effects. We used developmental studies to reconstruct early stages and any host cell effects of a parasite on Vertebrata aterrima. A mitochondrial marker (cox1) and morphological observations (light and fluorescence microscopy) were used to describe this new red algal parasite as Vertebrata aterrimophila sp. nov. Early developmental stages show that a parasite spore connects via secondary pit connections with a pericentral host cell after cuticle penetration. Developmental observations revealed a unique connection cell that grows into a ‘trunk-like’ structure. Host cell transformation after infection by the parasite included apparent increases in both carbohydrate concentrations and nuclear size, as well as structural changes. Analyses of molecular phylogenies and reproductive structures indicated that the closest relative of V. aterrimophila is its host, V. aterrima. Our study shows a novel developmental parasite stage (‘trunk-like’ cell) and highlights the need for further developmental studies to investigate the range of developmental patterns and host effects in parasitic red algae.     

MAHRP2, an exported protein of Plasmodium falciparum,is an essential component of Maurer's cleft tethers

Upon invasion into erythrocytes, the malaria parasite Plasmodium falciparum must refurbish the host cell. The objective of this study was to elucidate the location and function of MAHRP2 in these processes. Using immunofluorescence and immunoelectron microscopy we showed that the membrane‐associated histidine‐rich protein‐2 (MAHRP2) is exported during this process to novel cylindrical structures in the erythrocyte cytoplasm. We hypothesize that these structures tether organelles known as Maurer's clefts to the erythrocyte skeleton. Live cell imaging of parasite transfectants expressing MAHRP2–GFP revealed both mobile and fixed populations of the tether‐like structures. Differential centrifugation allowed the enrichment of these novel structures. MAHRP2 possesses neither a signal peptide nor a PEXEL motif, and sequences required for export were determined using transfectants expressing truncated MAHRP2 fragments. The first 15 amino acids and the histidine‐rich N‐terminal region are necessary for correct trafficking of MAHRP2 together with a predicted hydrophobic region. Solubilization studies showed that MAHRP2 is membrane associated but not membrane spanning. Several attempts to delete the mahrp2 gene failed, indicating that the protein is essential for parasite survival.     

Polyamine depletion inhibits the autophagic response modulating Trypanosoma cruzi infectivity

Autophagy is a cell process that in normal conditions serves to recycle cytoplasmic components and aged or damaged organelles. The autophagic pathway has been implicated in many physiological and pathological situations, even during the course of infection by intracellular pathogens. Many compounds are currently used to positively or negatively modulate the autophagic response. Recently it was demonstrated that the polyamine spermidine is a physiological inducer of autophagy in eukaryotic cells. We have previously shown that the etiological agent of Chagas disease, the protozoan parasite Trypanosoma cruzi, interacts with autophagic compartments during host cell invasion and that preactivation of autophagy significantly increases host cell colonization by this parasite. In the present report we have analyzed the effect of polyamine depletion on the autophagic response of the host cell and on T. cruzi infectivity. Our data showed that depleting intracellular polyamines by inhibiting the biosynthetic enzyme ornithine decarboxylase with difluoromethylornithine (DFMO) suppressed the induction of autophagy in response to starvation or rapamycin treatment in two cell lines. This effect was associated with a decrease in the levels of LC3 and ATG5, two proteins required for autophagosome formation. As a consequence of inhibiting host cell autophagy, DFMO impaired T. cruzi colonization, indicating that polyamines and autophagy facilitate parasite infection. Thus, our results point to DFMO as a novel autophagy inhibitor. While other autophagy inhibitors such as wortmannin and 3-methyladenine are nonspecific and potentially toxic, DFMO is an FDA-approved drug that may have value in limiting autophagy and the spread of the infection in Chagas disease and possibly other pathological settings.     

A Novel Fluorescent Labeling Method Enables Monitoring of Spatio‐Temporal Dynamics of Developing Microsporidia

The microsporidium, Anncaliia algerae (Brachiola algerae), is a eukaryotic obligate intracellular parasite first isolated from mosquitoes and is an important opportunistic human pathogen that can cause morbidity and mortality among immune‐compromised individuals including patients with AIDS and those undergoing chemotherapy. There is little known about the Microsporidia–host cell interface in living host cells, due to current approaches being limited by the lack of fluorescent reporters for detecting the parasite lifecycle. Here, we have developed and applied novel vital fluorescent parasite labeling methodologies in conjunction with fluorescent protein‐tagged reporters to track simultaneously the dynamics of both parasite and host cell specific components, including the secretory and endocytic trafficking pathways, during the entire infection time period. We have found dramatic changes in the dynamics of host secretory trafficking organelles during the course of infection. The Golgi compartment is gradually disassembled and regenerated into mini‐Golgi structures in parallel with cellular microtubule depolymerization. Importantly, we find that Microsporidia progeny are associated with these de novo formed mini‐Golgi structures. These host structures appear to create a membrane bound niche environment for parasite development. Our studies presented here provide novel imaging tools and methodologies that will facilitate in understanding the biology of microsporidial parasites in the living host.     

THE ROLE OF SECONDARY PIT CONNECTIONS IN RED ALGAL PARASITISM1

Secondary pit connections are common between cells of hosts and parasites in the widespread phenomenon of red algal parasitism. The DNA-specific fluorochrome 4′,-6-diamidino-2-phenylindole (DAPI) reveals that in host-parasite secondary pit connection (SPC) formation between the parasitic red alga Choreocolax polysiphoniae and its host Polysiphonia confusa, a nucleus and other cytoplasmic components of the parasite are delivered into the cytoplasm of a host cell. Host cells receive large numbers of parasite nuclei and these, apparently arrested in G¹, are maintained intact in host cells for periods of several weeks. Within these enlarged, differentiated cells, starch accumulates and cytoplasmic organelles proliferate as the central vacuole decreases in size. Host nuclear DNA synthesis is stimulated in the infected host cell, resulting in an increase in the number of host nuclei, or an increase in DNA in each of the existing host nuclei (i.e. somatic polyploidy). Occasionally, infected host cells will recommence division and engender a new host branch. Microspectrofluorometry of nuclear DNA quantitatively confirms not only the identity and transfer of parasite nuclei to host cells, but also the transfer of parasite nuclei to other parasite cells. Measurements also reveal that the single nucleus of Choreocolax becomes progressively more polyploid as cells become larger and more highly differentiated. Secondary pit connection formation between Choreocolax and Polysiphonia provides the mechanism for the transfer of parasite genetic information (via the parasite nucleus and cytoplasm) into the host. The parasite nuclei may thereby control and redirect the physiology of the host for the benefit of the parasite.     

Positioning of large organelles by a membrane‐ associated cytoskeleton in Plasmodium sporozoites

Cellular organelles are usually linked to the cytoskeleton, which often provides a scaffold for organelle function. In malaria parasites, no link between the cytoskeleton and the major organelles is known. Here we show that during fast, stop‐and‐go motion of Plasmodium sporozoites, all organelles stay largely fixed in respect to the moving parasite. Cryogenic electron tomography reveals that the nucleus, mitochondrion, apicoplast and the microtubules of Plasmodium sporozoites are linked to the parasite pellicle via long tethering proteins. These tethers originate from the inner membrane complex and are arranged in a periodic fashion following a 32 nm repeat. The tethers pass through a subpellicular structure that encompasses the entire parasite, probably as a network of membrane‐associated filaments. While the spatial organization of the large parasite organelles appears dependent on their linkage to the cortex, the specialized secretory vesicles are mostly not linked to microtubules or other cellular structures that could provide support for movement.     

Toxoplasma gondii sequesters lysosomes from mammalian hosts in the vacuolar space

The intracellular compartment harboring Toxoplasma gondii satisfies the parasite's nutritional needs for rapid growth in mammalian cells. We demonstrate that the parasitophorous vacuole (PV) of T. gondii accumulates material coming from the host mammalian cell via the exploitation of the host endo-lysosomal system. The parasite actively recruits host microtubules, resulting in selective attraction of endo-lysosomes to the PV. Microtubule-based invaginations of the PV membrane serve as conduits for the delivery of host endo-lysosomes within the PV. These tubular conduits are decorated by a parasite coat, including the tubulogenic protein GRA7, which acts like a garrote that sequesters host endocytic organelles in the vacuolar space. These data define an unanticipated process allowing the parasite intimate and concentrated access to a diverse range of low molecular weight components produced by the endo-lysosomal system. More generally, they identify a unique mechanism for unidirectional transport and sequestration of host organelles.     

Transfer cells and flange cells in sinkers of the mistletoePhoradendron macrophyllum (Viscaceae), and their novel combination

Summary The unusual thick-walled cells in contact with host and parasite vessels, first noted by Calvin 1967 in sinkers (structures composed of tracheary elements and parenchyma that originate from parasite bark strands that grow centripetally to the host vascular cambium and become embedded by successive development of xylem) of the mistletoePhoradendron macrophyllum (Englem.) Cockerell, have been investigated by modern methods of microscopy. The wall is thickest in cells abutting large-diameter host vessels, less so against smaller host vessels and those abutting sinker vessels. Transmission electron microscopy reveals the wall to be complex, consisting of a basement primary wall, upon which two developments of secondary-wall material occur. These are represented by lignified thickenings, in the form of flanges, and a labyrinth of wall ingrowths characteristic of a transfer cell. The wall ingrowths occur mostly in the primary-wall regions between the flanges, but when in contact with a large host vessel the ingrowths also differentiate on top of the flanges. Cells with such a transfer cell labyrinth have not been previously reported in the endophytic system of a mistletoe. The cells are confined to the xylary portion of the primary haustorium and sinkers. InP. macrophyllum, however, the cells differ from ordinary transfer cells in that they have differentiated as part of a flange parenchyma cell. This arrangement represents a novel anatomical situation. The name flange-walled transfer cell is used for these cells. The xylem of primary haustorium and sinkers also contain numerous ordinary flange cells. In both flange-walled transfer cells and ordinary flange cells the flanges are lignified and form a reticulate pattern of thickenings, separated by rounded areas of primary pit fields. The extent of development of the flange wall can vary in different parts of a sinker. At the host interface, the existence of a flange-walled transfer cell in direct contact with a vessel reflects a site associated with high loading into the parasite. Similarly, a labyrinth against a sinker vessel indicates a site of unloading from surrounding sinker tissue into the vessel for subsequent longdistance transport within the parasite.Dedicated to the memory of Dr. Katherine Esau (1898–1997)     

Ultrastructural enzyme-cytochemical study of the intrinsic glandular cells in the corpus cardiacum of Locusta migratoria: relation to the secretory and endocytotic pathways,and to the lysosomal system

Summary Ultrastructural aspects of the secretory and the endocytotic pathways and the lysosomal system of corpus cardiacum glandular cells (CCG cells) of migratory locusts were studied using morphological, marker enzyme, immunocytochemical and tracer techniques. It is concluded that (1) the distribution of marker enzymes of trans Golgi cisternae and trans Golgi network (TGN) in locust CCG cells corresponds to that in most non-stimulated vertebrate secretory cell types; (2) the acid phosphatase-positive TGN in CCG cells is involved in sorting and packaging of secretory material and lysosomal enzymes; (3) these latter substances are produced continuously; (4) at the same time, superfluous secretory granules and other old cell organelles are degraded; (5) the remarkable endocytotic activity in the cell bodies and the minor endocytotic activity in cell processes are coupled mainly to constitutive uptake of nutritional and/or regulatory (macro)molecules, rather than to exocytosis; (6) plasma membrane recycling occurs mainly by direct fusion of tubular endosomal structures with the plasma membrane and little traffic passes the Golgi/TGN; and (7) so-called cytosomes arise mainly from autophagocytotic vacuoles and represent a special kind of complex secondary lysosomes involved in the final degradation of endogenous (cell organelles) and exogenous material.     

A comparative study of microtubules in some vertebrate and invertebrate cells

Summary An electron microscope study of a variety of invertebrate and vertebrate cell types has supported the postulate that the microtubule is a universal cellular organelle. Microtubules of similar dimensions have been observed in the flagellum and beneath the plasma membrane of Trypanosoma lewisi, in the flagellum, manchette and mitotic spindle of the earthworm (Lumbricus terrestris) spermatid; and in fibroblasts, proximal convoluted and collecting tubule cells of the hypertrophying rat kidney. The specific occurrence and organization of the microtubules in cells undergoing morphological and developmental changes have suggested that these organelles are contractile and that they effectively contribute to the maintenance of cellular form. The possibility that microtubules may function as an intracellular transport system is also suggested.This work was supported by grants CA-04046, GM-08380, and K 3-AM-4932 from the U. S. Public Health Service.     

Cathepsin L occupies a vacuolar compartment and is a protein maturase within the endo/exocytic system of Toxoplasma gondii

Regulated exocytosis allows the timely delivery of proteins and other macromolecules precisely when they are needed to fulfil their functions. The intracellular parasite Toxoplasma gondii has one of the most extensive regulated exocytic systems among all unicellular organisms, yet the basis of protein trafficking and proteolytic modification in this system is poorly understood. We demonstrate that a parasite cathepsin protease, TgCPL, occupies a newly recognized va cuolar c ompartment (VAC) that undergoes dynamic fragmentation during T. gondii replication. We also provide evidence that within the VAC or late endosome this protease mediates the proteolytic maturation of proproteins targeted to micronemes, regulated secretory organelles that deliver adhesive proteins to the parasite surface during cell invasion. Our findings suggest that processing of microneme precursors occurs within intermediate endocytic compartments within the exocytic system, indicating an extensive convergence of the endocytic and exocytic pathways in this human parasite.     

Germinal granules in interstitial cells of the colonial hydroids <Emphasis Type="Italic">Obelia longissima</Emphasis> pallas, 1766 and <Emphasis Type="Italic">Ectopleura crocea</Emphasis> Agassiz, 1862

Electron-dense germinal granules, which are usually regarded as markers and key organelles of germline cells, were revealed in the interstitial (stem) cells of the colonial hydroids Obelia longissima and Ectopleura crocea. The interstitial cells of O. longissima displayed intense alkaline phosphatase activity, a histochemical marker for vertebrate embryonic stem and primary germ cells, as well as positive reaction to proliferating cell nuclear antigen (PCNA), which is an immunochemical marker for cell reproduction. Our findings and the literature data suggest the evolutionary conservation and similarity of the morphological and functional organization of potentially gametogenic stem cells in asexually reproducing invertebrates and germ cells in all studied Metazoa. The self-renewing pool of such stem cells provides the cellular source for blastogenesis and gametogenesis and the cellular basis for life functions, including both asexual and sexual reproduction.     

The LIM protein Ajuba is required for ciliogenesis and left-right axis determination in medaka

Cilia are microtubule-based organelles that are present on the surfaces of almost all vertebrate cells. Most cilia function as sensory or molecular transport structures. Malfunctions of cilia have been implicated in several diseases of human development. The assembly of cilia is initiated by the centriole (or basal body), and several centrosomal proteins are involved in this process. The mammalian LIM protein Ajuba is a well-studied centrosomal protein that regulates cell division but its role in ciliogenesis is unknown. In this study, we isolated the medaka homolog of Ajuba and showed that Ajuba localizes to basal bodies of cilia in growth-arrested cells. Knockdown of Ajuba resulted in randomized left-right organ asymmetries and altered expression of early genes responsible for left-right body axis determination. At the cellular level, we found that Ajuba function was essential for ciliogenesis in the cells lining Kupffer’s vesicle; it is these cells that induce the asymmetric fluid flow required for left-right axis determination. Taken together, our findings identify a novel role for Ajuba in the regulation of vertebrate ciliogenesis and left-right axis determination.     

Immunocytochemical mapping of the hemoglobin biosynthesis site in amphibian erythroid cells.

During the past 25 years, several studies have attempted to determine the site of integration of the heme and the four globin chains in vertebrate erythroid cells that is important in the formation of the hemoglobin molecule. Mitochondrion-like organelles or hemosomes were pointed out as responsible for this task. We performed several experiments to investigate this hypothesis. The intracellular distribution of hemoglobin in amphibian erythroid cells was detected by post-embedding immuno-electron microscopy, using a polyclonal anti-human hemoglobin-proteinA-gold complex. Hemoglobin mapping showed an intense labeling in the cell cytoplasm, but none in cytoplasmic structures such as endoplasmic reticulum, mitochondria, mitochondrion-like organelles, Golgi complex, ribosomes or ferruginous inclusions. The mitochondrial fraction obtained according to the protocol described for some authors, showed by ultrastructural examination that this fraction has a heterogeneous content, also composed by microvesicles rich in cytoplasmic hemoglobin, an artifact generated by mechanical action during cell fractionation. Thus, when this fraction is lysed and its content submitted to electrophoresis, hemoglobin bands would be found inevitably, causing false-positive results, erroneously attributed to hemoglobin content of mitochondrion-like organelles. Our data do not confirm the hypothesis that the final hemoglobin biosynthesis occurs inside mitochondrion-like organelles. They suggest that the hemoglobin molecule be assembled in the erythrocyte cytoplasm outside of mitochondria or hemosomes.     

Cell death in trichomonads: new insights

Tritrichomonas foetus is an amitochondriate parasite that possesses hydrogenosomes, unusual anerobic energy-producing organelles. In these organisms the “mitochondrial cell death machinery” is supposed to be absent, and the mechanisms that lead to cell demise remain to be elucidated. The presence of a cell death program in trichomonads has already been reported, suggesting the existence of a caspase-like execution pathway in such organisms. Here we demonstrate the alterations provoked by the fungicide griseofulvin and raise the possibility that other cell death pathways may exist in T. foetus. Dramatic changes in trichomonads morphology are presented after griseofulvin treatment, such as intense plasma membrane and nuclear envelope blebbing, nucleus fragmentation, and an abnormal number of oversized vacuoles. One important finding was the exposition of phosphatidylserine (PS) in the outer leaflet of the plasma membrane in cells after drug treatment, and also the presence of a high amount of misshapen flagella and tubulin precipitates as vacuolar contents, suggesting an autophagic process of abnormal cellular elements. Interestingly, immunoreactivity for activated caspase-3 was not detected during griseofulvin treatment, a finding distinct from the observed when this cell was treated with H₂O₂. The possibility of the existence of different pathways to cell death in trichomonads is discussed.     

Ommochromes in invertebrates: biochemistry and cell biology

Ommochromes are widely occurring coloured molecules of invertebrates, arising from tryptophan catabolism through the so‐called Tryptophan → Ommochrome pathway. They are mainly known to mediate compound eye vision, as well as reversible and irreversible colour patterning. Ommochromes might also be involved in cell homeostasis by detoxifying free tryptophan and buffering oxidative stress. These biological functions are directly linked to their unique chromophore, the phenoxazine/phenothiazine system. The most recent reviews on ommochrome biochemistry were published more than 30 years ago, since when new results on the enzymes of the ommochrome pathway, on ommochrome photochemistry as well as on their antiradical capacities have been obtained. Ommochromasomes are the organelles where ommochromes are synthesised and stored. Hence, they play an important role in mediating ommochrome functions. Ommochromasomes are part of the lysosome‐related organelles (LROs) family, which includes other pigmented organelles such as vertebrate melanosomes. Ommochromasomes are unique because they are the only LRO for which a recycling process during reversible colour change has been described. Herein, we provide an update on ommochrome biochemistry, photoreactivity and antiradical capacities to explain their diversity and behaviour both in vivo and in vitro. We also highlight new biochemical techniques, such as quantum chemistry, metabolomics and crystallography, which could lead to major advances in their chemical and functional characterisation. We then focus on ommochromasome structure and formation by drawing parallels with the well‐characterised melanosomes of vertebrates. The biochemical, genetic, cellular and microscopic tools that have been applied to melanosomes should provide important information on the ommochromasome life cycle. We propose LRO‐based models for ommochromasome biogenesis and recycling that could be tested in the future. Using the context of insect compound eyes, we finally emphasise the importance of an integrated approach in understanding the biological functions of ommochromes.