A function for plasma membrane reticular systems

Segmental pairs of dermal secretory cells release proteins into the hemolymph at the larval-pupal molt

At each molt of Manduca, the large dermal secretory cells expel the protein contents of their vacuoles into the hemocoel. The constellation of proteins expelled at the last larval-pupal molt, however, differs qualitatively from those proteins released at earlier larval-larval molts. Secretory cells at the two stages not only have different lectin staining properties but also have different proteins that separate on two-dimensional gels. Numerous physiological changes accompany the termination of the last larval instar, including increased chitin synthesis, diminished oxygen delivery, and reduced humoral immunity. Secretion of trehalase that is essential for chitin synthesis and the release of hypoxia up-regulated protein to ameliorate oxygen deprivation help ensure normal transition from larva to pupa. Proteins released by dermal secretory cells at this last molt could supplement the diminished immune defenses mediated by fat body and hemocytes at the end of larval life. Additional immune defenses provided by dermal secretory cells could help ensure a safe transition during a period of increased vulnerability for the newly molted pupa with its soft, thin cuticle and reduced mobility.

Ultrastructure of the larval midgut of Bittacus planus (Mecoptera: Bittacidae) and Neopanorpa longiprocessa (Mecoptera: Panorpidae)

Bittacidae and Panorpidae are the two largest families in Mecoptera. The larvae of Bittacidae are different from those of Panorpidae in external morphology and habits, and have an interesting habit of spraying the body surface with soil through the anus. However, it remains unknown to date whether the larval midguts are different in structure between the two families. Here the ultrastructure of the larval midguts of the hangingfly Bittacus planus Cheng and the scorpionfly Neopanorpa longiprocessa Hua & Chou were compared using light, scanning, and transmission electron microscopy. The midguts of both species are simple tubes of single layered epithelia with digestive and regenerative cells but without diverticula. The basal plasma membrane of epithelial cells exhibits infolding in B. planus, but is closely apposed to its basal lamina in N. longiprocessa. Lymph spaces are present between adjacent epithelial cells in B. planus, but are absent in N. longiprocessa. The regenerative cells are scattered among the digestive cells in B. planus, but are aggregated in N. longiprocessa. The longitudinal muscle bands are compact in B. planus, but are sparse in N. longiprocessa. The compact longitudinal muscle bands are likely associated with their soil-spraying habit in Bittacidae.

The development and functions of oenocytes

Oenocytes have intrigued insect physiologists since the nineteenth century. Many years of careful but mostly descriptive research on these cells highlights their diverse sizes, numbers, and anatomical distributions across Insecta. Contemporary molecular genetic studies in Drosophila melanogaster and Tribolium castaneum support the hypothesis that oenocytes are of ectodermal origin. They also suggest that, in both short and long germ-band species, oenocytes are induced from a Spalt major/Engrailed ectodermal zone by MAPK signaling. Recent glimpses into some of the physiological functions of oenocytes indicate that they involve fatty acid and hydrocarbon metabolism. Genetic studies in D. melanogaster have shown that larval oenocytes synthesize very-long-chain fatty acids required for tracheal waterproofing and that adult oenocytes produce cuticular hydrocarbons required for desiccation resistance and pheromonal communication. Exciting areas of future research include the evolution of oenocytes and their cross talk with other tissues involved in lipid metabolism such as the fat body.

The formation of plasma membrane reticular systems in the oenocytes of an insect

Plasma membrane reticular systems (RSs) are infolds of the plasma membrane found in cells of several insect tissues that are not transporting epithelia. They form a subsurface reticular lymph space that may be involved in the loading and unloading of hemolymph carrier molecules. The development of a new RS during the fifth larval stadium has been studied in the oenocytes of Calpodes ethlius by scanning electron microscopy. The RS forms by the extension and progressive apical fusion of cell processes leaving a reticular lymph space below. Reticular system formation occurs in a front moving over the cell surface. The RS made in the 4th stadium persists through the moult to the 5th stage but diminishes for the next 3 days. A new intermoult RS then forms very quickly. Its time of formation follows the commitment ecdysteroid peak rather than the beginning of secretion by the wax glands. This new 5th stage RS is maintained during the period of intermoult synthesis, after which it declines and is nearly absent by the time of pupation.

Communities of microbes that inhabit the changing hindgut landscape of a subsocial beetle

Microbes that have adopted endosymbiotic life styles not only have evolved to live in specialized habitats within living organisms, but the living habitats also have evolved to accommodate them. The hindgut of the passalid beetle (Odontotaenius disjunctus) is lined with a cuticle that undergoes dramatic topographic changes during the life cycle of the beetle. This manuscript addresses the changes that have been observed in time and space for the cuticular landscape of the hindgut as well as for the microbial communities within the hindgut. Microbial identity is based on morphology, culture, and extrapolation from previously reported passalid gut inhabitants.

Surface membranes, Golgi complexes, and vacuolar systems

In the absence of fossils, the cells of vertebrates are often described in lieu of a general animal eukaryote model, neglecting work on insects. However, a common ancestor is nearly a billion years in the past, making some vertebrate generalizations inappropriate for insects. For example, insect cells are adept at the cell remodeling needed for molting and metamorphosis, they have plasma membrane reticular systems and vacuolar ferritin, and their Golgi complexes continue to work during mitosis. This review stresses the ways that insect cells differ from those of vertebrates, summarizing the structure of surface membranes and vacuolar systems, especially of the epidermis and fat body, as a prerequisite for the molecular studies needed to understand cell function. The objective is to provide a structural base from which molecular biology can emerge from biochemical description into a useful analysis of function.

Concomitant primary infection of the midgut epithelial cells and the hemocytes of Trichoplusia ni by Autographa californica nucleopolyhedrovirus

We have constructed a modified Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) to express the green fluorescent protein (GFP) under the polyhedrin promoter and used it to study the infection process of AcMNPV in Trichoplusia ni larvae. T. ni larvae that ingested the virus showed localized expression of GFP in the midgut epithelial cells and the hemocytes at 12 h post infection (hpi). The presence of GFP-related fluorescence in the midgut columnar cells indicated that the virus was not only replicating, but also synthesizing the late viral proteins. Studies using the transmission electron microscope showed that the virus infected the midgut columnar cells. At the same time a proportion of the parental virus travelled through the midgut epithelial layer, possibly utilizing the plasma membrane reticular system, entered the hemocoel and infected the hemocytes. This resulted in the simultaneous infection of the midgut epithelial cells and the hemocytes. Subsequently, the budded virus (BV) released from the infected hemocytes into the hemolymph caused secondary infection within the tracheal epithelial cells. The virus then rapidly spread through the tracheal system allowing the infection of a variety of other tissues such as the epidermis and the fat body.

Studies of the nucleopolyhedrovirus infection process in insects by using the green fluorescence protein as a reporter

A recombinant Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) expressing the green fluorescence protein (GFP) under the control of the AcMNPV polyhedrin promoter was constructed to study the spatial and temporal regulation of baculovirus infection in a permissive host. Larvae that ingested AcMNPV-GFP showed localized expression of GFP in the midgut epithelial cells, as well as hemocytes, at 24 h postinfection. The presence of fluorescence in these tissues indicated not only that the virus was replicating but also that the very late viral proteins were being synthesized. Secondary infection occurred within the tracheal cells throughout the body cavity, confirming earlier reports, and these foci of infection allowed entry of the virus into other tissues, such as the epidermis and the fat body.

Evidence for a transcellular cisternal route across the caecal epithelium of an insect

The cells of the mesenteric caeca in the midgut of certain insects possess a labyrinth of transepithelial cisternae. Their existence can be seen in thin sections of lanthanum-incubated tissue, where the tracer enters not only the intercellular clefts but also membranous cisternae which are inpocketings from, and, in continuity with, both the lateral clefts and basal membrane. These infoldings, which are numerous, run from the basal or lateral surfaces into the perinuclear region of the cells, where they are found, laden with lanthanum, as smooth cisternae or vesicles in the peripheral cytoplasm near the plasma membrane. These can be followed in serial sections and are quite distinct from other sub-surface cisternae of the lateral borders which are studded with ribosomes on the cytoplasmic surface. Near the luminal surface, tracer-laden structures in the form of vesicles and granules become increasingly predominant over those in the form of cisternae. Freeze-fracture replicas confirm the above observations, in that the plasma membrane of the intercellular cleft can be characterized as such unequivocally, since it exhibits smooth septate junctional E face grooves and P face ridges. Lateral infoldings, cisternae and vesicles can be seen arising directly from these junction-bearing membranes. The transepithelial cisternae and vesicles may be the morphological basis of an insect transcellular transport system, comparable to the tubulo-cisternal endoplasmic reticulum present in the transporting secretory and absorptive epithelia of vertebrate tissues. However, in insect midgut caecal epithelia, the cisternae appear to be, albeit presumably transiently, in direct continuity with the extracellular space, forming a plasma membrane reticular system which seems not to be the case with the tubulo-cisternal endoplasmic reticulum which terminates in subsurface cisternae.

Ultrastructure of the midgut and blood meal digestion in the adult tick Dermacentor variabilis

Digestive cells in the midgut of male and female Dermacentor variabilis (Say) took up the blood meal in coated vesicles and smooth flask-shaped vesicles, and deposited it in endosomes which were digested via heterophagy. Iron was concentrated in residual bodies. Digestion occurred in three distinct phases in mated females: (1) continuous digestion (initiated by feeding) occurred during slow engorgement; (2) reduced digestion (initiated by mating) occurred in mated females during the period of rapid engorgement; (3) a second continuous digestion phase (initiated by detachment from the host) occurred throughout the post-feeding periods of preoviposition and oviposition. It proposed that the stem cells in the midguts of unfed females were progenitor of digestive, replacement, and presumed vitellogenic cells in midguts of mated feeding females. Digestive cells were present in all three digestion phases. Only during the first continuous digestion phase did digestive cells fill up with residual bodies, rupture and slough into the lumen, or did whole cells slough into the lumen. During the other two digestion phases no sloughing of digestive cells was observed. At the end of oviposition the digestive cells were filled with residual bodies. Replacement cells were present only during the first continuous-digestion phase. Presumed vitellogenic cells were present only during the reduced-digestion phase and during the second continuous-digestion phase. Stem cells in unfed males developed only into digestive cells in feeding males. Fed males and fed unmated females had only the first continuous-digestion phase. After being hand-detached from the host, unmated 13-day-fed females went through cellular changes associated with the reduced-digestion phase and second continuous-digestion phase of fed mated females, then began ovipositing. Maximum development of the basal labyrinth system and lateral spaces matched the known time of maximum water and ion movement across the midgut epithelia. Spectrophotometric analyses of lumen contents and midgut cells, sampled after detachment from the host, showed that concentrations of protein and hemoglobin at day 1 post-detachment decreased by one-half at the beginning of oviposition, while hematin increased about twofold by the end of oviposition. This supported the idea of the presence of a second continuous-digestion phase.

Sequential structural changes in the fat body of the tobacco hornworm, Manduca sexta, during the fifth larval stadium

Light and electron microscopy revealed a series of structural changes that occur in the fat body of the tobacco hornworm, Manduca sexta, during the fifth, i.e. the final, larval stadium. At each developmental stage studied, the cells of the fat body were homogeneous in structure. We found no evidence suggesting the presence of more than one type of fat body cell. Our structural data are consistent with published observations on biochemical activities of M. sexta fat body at particular developmental stages. Specific points of agreement include: (a) acquisition of Golgi complex (GC) and rough endoplasmic reticulum (RER) concomitant with the time of major protein production; (b) loss of many cellular organelles (such as GC and RER) as protein production drastically decreases; (c) accumulation of protein granules and urate granules after the onset of wandering (i.e. during the pre-pupal period); (d) accumulation of lipid and glycogen throughout the feeding period. In addition we found that (a) the plasma membrane reticular system (PMRS) developed during the period when protein secretion was great; (b) the PMRS was lost abruptly at the onset of wandering; and (c) the nucleus changed in shape from being roughly spherical to elliptoid in the pre-pupal stage. We found that the structure of M. sexta fat body is similar to that published for other Lepidoptera. However, it differs from that of Heliothis zea in that regional differences are not obviously apparent.

Lenticles: innervated secretory structures that are expressed at every other larval moult

Lenticles are dome-shaped circles or ovals of cuticle with a dark rim. They occur with a precise segmental arrangement in the larvae and pupae of lycaenid and hesperiid butterflies. In Calpodes ethlius (Lepidoptera, Hesperiidae) each lenticle is secreted by a pair of large polyploid epidermal cells. The dark rim or annulus is formed from a ring-shaped cell. The dome, which consists of an epicuticle with a perforate intermediate layer like a pepper-pot, is formed by a central goblet cell. Between the perforate intermediate layer and the cell surfaces there is a cavity that contains material presumed to be secretion. Both cells have elaborate basal plasma membrane reticular systems and the apical microvilli associated with an extensive smooth endoplasmic reticulum that is typical of lipid secreting cells. In addition, there is a plasma membrane reticular system in the ring cell and between it and the goblet cell that contains the endings of nerves having neurosecretory vesicles. Lenticles thus have a structure appropriate for an innervated organ of lipid secretion. However, in their development, lenticles arise from bristles that are presumed to be sensory. Lenticles or their precursors are segmentally arranged in the five larval instars and the pupa, but the pattern changes at each moult. The cells that form a lenticle at one moult have a rest period at the next one when they only secrete surface cuticle. Many lenticles are paired in their cycle of development, with only one of the pair making a lenticle at a particular moult. For example, the dorsal and lateral lenticles alternate in position between anterior and posterior. The second and fourth instar segments have anterior and the third and fifth instars have posterior lenticles. In the first instar the cells that will make lenticles for the second and third instars both make bristles. Lenticles are thus formed by cells that not only change their response to ecdysone qualitatively by switching from bristle to lenticle but also alternate in their later responses, switching back and forth at alternate moults between the formation of a lenticle and the secretion of surface cuticle.

The induction and distribution of an insect ferritin--a new function for the endoplasmic reticulum

Three insect tissues have particular roles as filters to maintain the fluid composition of the hemolymph. Water and ions enter and leave through the midgut. The pericardial cells filter circulating hemolymph. Malpighian tubules, often with the rectum, allow resorption from a hemolymph filtrate that passes to the hindgut. All three tissues have plasma membrane infolds making a reticulum on their hemolymph surfaces, and all three have RER leading to SER extensions into their reticula. SER is a catch-all description for membranes lacking ribosomes in the pre-Golgi complex set of compartments of the vacuolar system. Some kinds of SER are well known for their role in housing enzymes for steroid metabolism and for detoxification. The SER ramifying within the plasma membrane reticular systems of tissues concerned with hemolymph filtration contains ferritin, suggesting that this SER has another, different function. In contrast to vertebrate cells, where ferritin is confined to the cytosol and lysosomes, we have found that in Calpodes and perhaps in most insects, ferritin occurs in the vacuolar system and not in the cytosol. Ferritin occurs naturally in the RER and SER of cells at the hind end of the midgut, in pericardial cells and in the yellow region of the Malpighian tubules. Additional ferritin is induced by loading the gut or hemolymph with iron. Overloading with iron causes ferritin secretion to the gut lumen. We propose that the SER in these cells functions in iron homeostasis by holding ferritin for loading and unloading as it moves to and from the reticulum at the cell surface where it can be maximally exposed to extracellular fluid flow.