Andrew B. Shatrov

Stylostome formation in trombiculid mites (Acariformes: Trombiculidae)

Stylostomes of the trombiculid mite larvae Neotrombicula pomeranzevi (Schluger), Hirsutiella zachvatkini (Schluger), Miyatrombicula esoensis (Sasa and Ogata) and Euschoengastia rotundata (Schluger) (Acariformes: Trombiculidae), formed in the host skin during feeding of the parasites on their natural hosts (voles) were studied histologically and histochemically. A stylostome is a variously shaped tube formed of solidified mite saliva that extends from the mouthparts of the parasite through the epidermis into the dermis of the host, and allows the mite to obtain its liquid food. The first step of stylostome formation is deposition of an eosinophilic cone, to which the larva’s chelicerae are glued. Organization of the stylostome depends on the mite species, and its walls may show weakly expressed longitudinal or transverse stratification. Histochemically, the stylostome is composed of complex glycoprotein with varying tinctorial properties through the width or the length of the stylostome’s walls. Beneath the distal end of the stylostome, irrespectively of its localization either in the epidermis or in the dermis of the host, a feeding cavity is formed as a result of the action of the hydrolytic components of the mite’s saliva forced through the stylostome into the wound. An inflammatory dermal reaction of moderate intensity is evolved during larval feeding and stylostome formation. It is manifested by the infiltration of the foci with neutrophiles, lymphocytes and macrophages and by dilation of capillaries of the terminal vessel bed and filling them by erythrocytes and other blood elements. Around the stylostome, necrosis of the epidermal cells occurs, leucocytes come to the damaged area and fuse with the necrotic epidermal cells, leading to the formation of the large scabs on the surface of the host’s skin. In the case of E. rotundata, single capsules having a terminal opening and containing feeding larva are formed on the abdomen of the hosts. The walls of the capsules are composed of the mite’s saliva flowing upon the surface of the host’s skin. At the bottom of the capsule, a stylostome perforating the epidermis is also present.

Stylostome organization in feeding Leptotrombidium larvae (Acariformes: Trombiculidae)

The stylostome of larvae of the trombiculids Leptotrombidium scutellare (Nagayo et al.), Leptotrombidium fletcheri (Womersley et Heaslip) and Leptotrombidium deliense (Walch) was studied experimentally at different time intervals after larval attachment using the histological method. The stylostome of these species has the same organization and belongs to the epidermal combined with the mixed type, developing more in width than in length. Neither transverse nor conspicuous longitudinal layers are present within the stylostome walls, which stain predominantly in red with Azan, also showing longitudinal portions with blue staining. Larvae tend to attach closely to each other and scabs, consisting of the hyperkeratotic epidermal layers fusing with migrating inflammatory cells, develop around the attachment sites. The dermis shows inflammatory foci with dilated capillaries and inflammatory cells inserting in the connective tissue layer underneath the stylostome. The feeding cavity, which is moderately expressed, may be found either in the epidermis or in the dermis. It contains inflammatory cells and their debris in the liquefied host tissues. The stylostome length depends on the character of the attachment site (the thicker epidermis or scab the longer the stylostome), and does not directly correspond to the stages of larval feeding. Nevertheless, at the 48-h time interval, nearly all attached larvae are found to be fully fed and their midgut cells are filled with nutritional globules.

Anatomy and ultrastructure of dermal glands in an adult water mite, Teutonia cometes (Koch, 1837) (Acariformes: Hydrachnidia: Teutoniidae).

Organization of dermal glands in adult water mites Teutonia cometes (Koch, 1837) was studied using light-optical, SEM and TEM methods for the first time. These glands are large and occur in a total number of ten pairs at the dorsal, ventral and lateral sides of the body. The slit-like external openings of the glands (glandularia) are provided with a cone-shaped sclerite, and are combined with a single small trichoid seta (hair sensillum), which is always situated slightly apart from the anterior aspect of the gland opening. Each gland is formed by an epithelium encompassing a very large lumen (central cavity) normally filled with secretion that stains in varying intensity on toluidine blue stained sections. The epithelium is composed of irregularly shaped secretory cells with an electron-dense cytoplasm and infolded basal portions. The cells possess a large irregularly shaped nucleus and are filled with tightly packed slightly dilated cisterns and vesicles of rough endoplasmic reticulum (RER) with electron lucent contents. Dense vesicles are also present in the apical cell zone. Some cells undergo dissolution, occupy an upper position within the epithelium and have a lighter cytoplasm with disorganized RER. Muscle fibers are regularly present in the deep folds of the basal cell portions and may serve to squeeze the gland and eject the secretion into the external milieu. The structure of these dermal glands is compared with the previously described idiosomal glands of the same species and a tentative correlation with the glandularia system of water mites is given. Possible functions of the dermal glands of T. cometes are discussed.

Redescription of a human-infesting European trombiculid mite Kepkatrombicula desaleri (Acari: Trombiculidae) with data on its mouthparts and stylostome

ABSTRACT External morphology of trombiculid larvae Kepkatrombicula desaleri (Methlagl, 1928) collected on chamois in Alps with special consideration of the mouth apparatus was studied by scanning electron microscopy (SEM). Standard morphological description of this species using optical microscopy is also given, with the discussion on its taxonomy. Neotype of K. desaleri is designated. Feeding tube or stylostome formed by larvae in the host skin was examined using histological methods. Larvae of this species possess an unusual organization of the apical portions of the hypostome forming a soft sucker disk. The latter being applied to the stratum corneum of the host epidermis provides an additional pumping effect for engorgement of food through the long stylostome that may extend deep into the dermis at different angles to the surface of the epidermis. Stylostome corresponds to the mesenchymal type of stylostome. Skin reaction is strong and is expressed in the intensive infiltration of the inflammatory foci with leukocytes and erythrocytes. Besides fluid components of the inflammatory focus and cellular debris, larvae also may uptake the whole erythrocytes. Edema, scab formation, as well as the epidermal hyperplasia and hyperkeratosis are not characterized for feeding of larvae of this species. Stylostome organization and the character of the skin inflammatory reaction suggest that big animals like chamois and probably humans are not natural hosts for larvae of this species that may serve as vector of Rickettsiae and causes trombidiosis in man.

Comparative midgut ultrastructure of unfed larvae and adult mites of Platytrombidium fasciatum (C.L. Koch, 1836) and Camerotrombidium pexatum (C.L. Koch, 1837) (Acariformes: Microtrombidiidae).

The midgut of unfed larvae and adult mites of Platytrombidium fasciatum (C.L. Koch, 1836) and Camerotrombidium pexatum (C.L. Koch, 1937) (Acariformes: Microtrombidiidae) was investigated by electron microscopy. The sac-like midgut occupies the entire body volume, ends blindly and is not divided into functionally differentiated diverticula or caeca. The midgut walls are composed of one type of digestive cell that greatly varies in shape and size. In larvae, the lumen of the midgut is poorly recognizable and its epithelium is loosely organized, although yolk granules are already utilized. In adults, the midgut forms compartments as a result of deep folds of the midgut walls, and the lumen is well distinguished. The epithelium is composed of flat, prismatic or club-like cells, which may contain nutritional vacuoles and residual bodies in various proportions that depend on digestive stages. In both larvae and adult mites, parts of cells may detach from the epithelium and float within the lumen. The cells contain a system of tubules and vesicles of a trans-Golgi network, whereas the apical surface forms microvilli as well as pinocytotic pits and vesicles. Lysosome-like bodies, lipid inclusions and some amount of glycogen particles are also present in the digestive cells. Spherites (concretions) are not found to be a constant component of the digestive cells and in adult mites occur for the most parts in the midgut lumen.

Comparative morphology and ultrastructure of the mouthparts in unfed larvae of Platytrombidium fasciatum and Camerotrombidium pexatum (Acariformes: Microtrombidiidae)

The mouthparts of unfed larvae of Platytrombidium fasciatum (C. L. Koch, 1836) and Camerotrombidium pexatum (C. L. Koch, 1837) (Acariformes: Microtrombidiidae) were studied using both light optical (whole-mounted specimens, toluidine blue stained semi-thin sections) and electron microscope (TEM, SEM) methods. The mouth apparatus incorporated within the gnathosoma occupies an axial position and is covered from above by the arched dorsal shield, or scutum. The chelicerae are comparatively long and separated, and the lateral lips form a permanent apomorphic sucker provided with an internal cuticular sclerite. The pharynx is extremely wide and totally fused with the bottom of the infracapitulum. The pharyngeal dilators originate on the posterior portions of the cervix (epistome) and on the capitular apodemes and run nearly parallel to the cervix to the dorsal pharyngeal wall. Comparatively short sigmoid pieces serve as origin of the muscles—cheliceral levators inserting on the posterior wall of the basal cheliceral segments. There are two sets of the extrinsic gnathosomal muscles originating on the posterior portion of the scutum: retractors of chelicerae inserting on the posterior portions of the basal cheliceral segments, and retractors of the gnathosoma inserting on the very posterior parts of the capitular apodemes. The labrum and the cervix delimit the pharynx and the subcheliceral space. The labrum and the cervix for the most part are weakly sclerotized cuticular plates and do not have own muscles. The larval mouth apparatus, in comparison with that of adult microtrombidiid mites, is simply organized and more specialized for ingestion of large masses of liquid food.

Organization of unusual idiosomal glands in a water mite, Teutonia cometes (Teutoniidae).

The unusual idiosomal glands of a water mite Teutonia cometes (Koch 1837) were examined by means of transmission and scanning electron microscopy as well as on semi-thin sections. One pair of these glands is situated ventrally in the body cavity of the idiosoma. They run posteriorly from the terminal opening (distal end) on epimeres IV and gradually dilate to their proximal blind end. The terminal opening of each gland is armed with the two fine hair-like mechanoreceptive sensilla (‘pre-anal external’ setae). The proximal part of the glands is formed of columnar secretory epithelium with a voluminous central lumen containing a large single ‘globule’ of electron-dense secretory material. The secretory gland cells contain large nuclei and intensively developed rough endoplasmic reticulum. Secretory granules of Golgi origin are scattered throughout the cell volume in small groups and are discharged from the cells into the lumen between the scarce apical microvilli. The distal part of the glands is formed of another cell type that is not secretory. These cells are composed of narrow strips of the cytoplasm leaving the large intracellular vacuoles. A short excretory cuticular duct formed by special excretory duct cells connects the glands with the external medium. At the base of the terminal opening a cuticular funnel strengthens the gland termination. At the apex of this funnel a valve prevents back-flow of the extruded secretion. These glands, as other dermal glands of water mites, are thought to play a protective role and react to external stimuli with the help of the hair-like sensilla.

The ultrastructure and possible functions of nephrocytes in the trombiculid mite Hirsutiella zachvatkini (Acariformes: Trombiculidae)

The ultrastructure of nephrocytes during the post-larval development of the trombiculid mite Hirsutiella zachvatkini (Schluger) was investigated by means of transmission electron microscopy. Nephrocytes are situated either individually or in groups inside the haemocoelic space and are always ensheathed by a basal membrane. They contain numerous tubular elements, electron-dense inclusions and sometimes electron-lucent vacuoles. Invaginations of the plasma membrane, often in the form of labyrinthine channels, with coated pits of plasma membrane and characteristic slit diaphragms linking adjacent pedicels, typically occupy the peripheral parts of the cells. However, no distinct zonation of organelles in the nephrocytes of trombiculids was observed. The number of nephrocytes in the haemocoel and the intensity of development of their tubular elements and in particular of labyrinthine channels vary significantly during the ontogenesis of mites. The possible functions of nephrocytes and their structure and differentiation in comparison with other arachnids are discussed.

Oogenesis in ovipositing females of the microtrombidiid mite Platytrombidium fasciatum (C.L. Koch) (Acariformes: Microtrombidiidae)

Summary The process of oogenesis in ovipositing females of the microtrombidiid mite Platytrombidium fasciatum (C.L. Koch, 1836) was investigated by means of transmission electron microscopy. Two ovaries do not fuse at their posterior ends and may be conventionally divided into centrally located compact germarium where oocytes undergo previtellogenesis and prominent peripheral vitellarium consisting of growing oocytes protruding on either sides of the ovaries into surrounding tissues are encompassed only by a basal lamina (tunica propria) (solitary type of oogenesis). Previtellogenic oocytes subsequently grow and contain multiplying mitochondria and prominent Golgi complexes and are characterized by a high rate of synthetic activity. To the end of previtellogenesis, large oval oocytes, tightly packed with elements of rough endoplasmic reticulum and free ribosomes and containing large nucleus, gradually migrate to the ovarian boundaries, coming into contact with a basal lamina. At the onset of vitellogenesis, oocytes form short microvilli on their surface. The processes of yolk accumulation and vitelline envelope formation begin simultaneously and carry out due to endogenous sources. Yolk granules are deposited within the membrane profiles of endoplasmic reticulum and originally occupy a zone just beneath a rim of the cortical ooplasm. In addition to yolk bodies, lipid inclusions are accumulated in the cytoplasm. In the late vitellogenesis, the rarely observed process of extraoocytic obtaining of yolk precursors via pinocytotic activity adds to the main mode of yolk accumulation. The vitelline envelope formation proceeds also intraoocytically due to activity of Golgi complexes, which produce electron-dense globules during early vitellogenesis and small vesicles in the late period. They fuse with plasmolemma to form a vitelline envelope that possesses two distinct layers and in mature egg is devoid of pore canals.

Comparative morphology and ultrastructure of the prosomal salivary glands in the unfed larvae Leptotrombidium orientale (Acariformes, Trombiculidae), a possible vector of tsutsugamushi disease agent

The prosomal salivary glands of the unfed larvae Leptotrombidium orientale (Schluger) were investigated using transmission electron microscopy. In total, four pairs of the prosomal glands were identified—three pairs, the lateral, the medial and the anterior, belong to the podocephalic system, and one pair, the posterior, is separate having an own excretory duct. All glands are simple alveolar/acinous with prismatic cells arranged around a relatively small intra-alveolar lumen with the duct base. The cells of all glands besides the lateral ones contain practically mature electron-dense secretory granules ready to be discharged from the cells. The secretory granules in the lateral glands undergo formation and maturation due to the Golgi body activity. The cells of all gland types contain a large basally located nucleus and variously expressed rough endoplasmic reticulum. Specialized duct-forming cells filled with numerous freely scattered microtubules are situated in the middle zone of each gland’s acinus and form the intra-alveolar lumen and the duct base. Both the acinar (secretory) and the duct-forming cells contact each other via gap junctions and septate desmosomes. Axons of nerve cells come close to the basal extensions of the duct-forming cells where they form the bulb-shaped synaptic terminations. The process of secretion is under the control of the nerve system that provides contraction of the duct-forming cells and discharge of secretion from the secretory cells into the intra-alveolar lumen and further to the exterior. Unfed larvae of L. orientale, the potential vector of tsutsugamushi disease agents, contain the most simply organized salivary secretory granules among known trombiculid larvae, and this secretion, besides the lateral glands, does not undergo significant additional maturation. Thus, the larvae are apparently ready to feed on the appropriate host just nearly after hatching.