Electron microscope studies of Fasciola hepatica. X. Egg formation

Evidence obtained would suggest that the processes involved in egg formation in Fasciola hepatica occur in the following sequence. The egg constituents, namely an ovum and some vitelline cells, pass through the proximal ootype and as they do so they are smeared by the secretions of Mehlis' gland which have accumulated there. A temporary interface is set up between the Mehlis' gland secretion and the fluid which surrounds the egg constituents. Shell globules are released by the vitelline cells and coalesce on the inner aspect of the interface. At the same time some of the Mehlis' secretion diffuses across the interface, thereby bringing about the dissociation of the interface. It is suggested that Mehlis' gland secretion brings about the fusion of the shell layer. During the initial stages of the process the developing egg is supported by the cells of the ootype epithelium. Later the egg passes into a wider, more distal part of the ootype where the process of shell deposition is continued. When complete, or almost complete, the egg passes into the uterus where changes indiciating the initial process of tanning are seen to take place. A thin, very uniform layer of Mehlis' gland secretion can still be identified on the surface of mature eggs.     

ULTRASTRUCTURAL DEVELOPMENT OF CAPITATE GLANDULAR HAIRS OF CANNABIS SATIVA L. (CANNABACEAE)

The capitate-sessile and capitate-stalked glands of the glandular secretory system in Cannabis, which are interpreted as lipophilic type glandular hairs, were studied from floral bracts of pistillate plants. These glands develop a flattened multicellular disc of secretory cells, which with the extruded secretory product forms the gland head and the auxiliary cells which support the gland head. The secretory product accumulates beneath a sheath derived from separation of the outer wall surface of the cellular disc. The ultrastructure of secretory cells in pre-secretory stages is characterized by a dense ground plasm, transitory lipid bodies and fibrillar material, and well developed endoplasmic reticulum. Dictyosomes and dictyosome-derived secretory vesicles are present, but never abundant. Secretory stages of gland development are characterized by abundant mitochondria and leucoplasts and by a large vacuolar system. Production of the secretory product is associated with plastids which increase in number and structural complexity. The plastids develop a paracrystalline body which nearly fills the mature plastid. Material interpreted as a secretion appears at the surface of plastids, migrates, and accumulates along the cell surface adjoining the secretory cavity. Extrusion of the material into the secretory cavity occurs directly through the plasma membrane-cell wall barrier.     

Structural and functional changes of the Dufour gland in gynes of the amazon ant Polyergus rufescens (Hymenoptera, Formicidae)

Colony foundation by the slave-making amazon ant Polyergus rufescens requires penetration of the young gyne soon after mating into a colony of the slave species. During this process, she uses decyl butyrate from her large Dufour gland as an effective appeasement allomone. The structural appearance and development of this gland shows a clear age-dependent evolution that is in line with its behavioural function. At the moment of eclosion, young females have a gland with an empty lumen but thick lining epithelium, of which the active secretory cells are characterised by a well-developed Golgi apparatus, abundant mitochondria and smooth endoplasmic reticulum. The basal plasmalemma shows deep invaginations that facilitate the uptake of precursors from the haemolymph, while intercellular contacts display conspicuous interdigitations in the apical cell part. During the first days of adult life, secretion starts to accumulate in the lumen. At the age of 3 weeks, the gland displays a large lumen filled with secretion and a thin epithelium that no longer displays secretory activity. By this time, the gynes are ready for penetration into a slave colony, being loaded with large quantities of the appeasement allomone that are necessary at this initial stage of the usurpation process. Accepted: 13 March 2001     

Mitochondrial deformation and apocrine secretory mechanism in the rabbit submandibular organ as revealed by electron microscopy

Summary The submandibular organ (a sort of apocrine sweat glands) of the rabbit was observed with the electron microscope. The cell structure of glandular tubules varies depending upon the secretory activity; there are three functional stages. The secretory cells at the resting stage are characterized by low height, absence of secretory substance, and presence of small and slender mitochondria.In the synthesizing stage, enlargement and peculiar deformation of mitochondria are observed. Secretory substance always occurs near the deformed mitochondria. The part of a mitochondrion closely abutting on the secretion mass is extremely thin, and contains longitudinally oriented cristae. Sometimes a direct continuity is observed between the thinned portion of the deformed mitochondria and the mass of secretory substance. It is presumed that the secretion is initially produced in the mitochondria and then discharged from them. The Golgi apparatus and the rough surfaced endoplasmic reticulum may be involved indirectly. Smooth surfaced vesicles, probably related to the transport of raw material, are extremely abundant in the cells of this stage.The development of a generally homogeneous projection into the gland lumen is characteristic of the stage of secretion discharge. The mitochondria are again small and slender, and the secretion is liquefied. At the base of the full-grown projection, cytoplasm is condensed to form a demarcation zone from which the projection may become detached. This mechanism of release of secretory product is quite the same as the so-called apocrine secretory process long postulated by light microscopists.     

Ultrastructural changes of the teleostean hatching gland cell during natural and electrically induced precocious secretion.

Ultrastructural changes of the hatching gland during electrically induced precocious secretion were compared with those during natural secretion in the medaka, Oryzias latipes. The gland cells are covered by a layer of epithelial cells, which adjoin one another just on the apical center of each gland cell. When the natural as well as the precocious secretion occurred, each gland cell was swollen upward and rounded, and separation of the epithelial joints occurred, giving rise to an exposure of the apical portion of the gland cells. There were marked differences between these two kinds of secretion process in the behavior of the secretory granules prior to secretion and in the mode of discharge of the secretory substances. The changes which occurred during both types of secretion and which, therefore, seemed to be essential to the secretory processes of this gland cell were the swelling up of the gland cells in the initiation of secretion and the reduction of the electron density of the zymogen granules. These secretion-associated ultrastructural changes are discussed in view of the difference in the maturation of the gland cells.     

Fine structure of the silk gland in the mite Ornithocheyletia sp. (Prostigmata,Cheyletidae)

Silk spinning is widely-spread in trombidiform mites, yet scarse information is available on the morphology of their silk glands. Thus this study describes the fine structure of the prosomal silk glands in a small parasitic mite, Ornithocheyletia sp. (Cheyletidae). These are paired acinous glands incorporated into the podocephalic system, as typical of the order. Combined secretion of the coxal and silk glands is released at the tip of the gnathosoma. Data obtained show Ornithocheyletia silk gland belonging to the class 3 arthropod exocrine gland. Each gland is composed of seven pyramidal secretory cells and one ring-folded intercalary cell, rich in microtubules. The fine structure of the secretory cells points to intensive protein synthesis resulted in the presence of abundant uniform secretory granules. Fibrous content of the granules is always subdivided into several zones of two electron densities. The granules periodically discharge into the acinar cavity by means of exocytosis. The intercalary cell extends from the base of the excretory duct and contributes the wall of the acinar cavity encircling the apical margins of the secretory cells. The distal apical surface of the intercalary cell is covered with a thin cuticle resembling that of the corresponding cells in some acarine and myriapod glands. Axon endings form regular synaptic structures on the body of the intercalary cell implying nerve regulation of the gland activity.     

Transformation of the endostyle of the anadromous sea lamprey,Petromyzon marinus L., during metamorphosis. II. Electron microscopy

Electron microscopy was used to follow the transformation of the endostyle to a thyroid gland in the anadromous sea lamprey, Petromyzon marinus L., throughout metamorphosis (stages 1–7). Transformation of the larval (ammocoete) endostyle begins at the first signs of external change (stages 1–2), and the adult form of the gland is reached by stage 5. Only slight modifications of the gland accompany further development to the end of metamorphosis. Development of the thyroid gland involves degeneration, proliferation, and reorganization of the cells in the endostyle, and changes in their fine structure. Ultrastructural changes during early stages are most obvious in the type 1 cells that make up the shrinking glandular tracts, and involves the accumulation of cytoplasmic microfilaments and a variety of cytoplasmic inclusions. The glandular tracts and their cells gradually disappear through autolysis and, apparently, through phagocytosis by neighboring epithelial cells and macrophages. Although the fine structure of the type 2, 3, 4, and 5 cells is not altered in the early stages, by stage 3, many of these cells become either vacuolated, undergo autolysis, or are extruded. Phagocytosis of some of each of these cell types likely occurs. Thyroid follicles are first observed during stage 4. Some of their lumina seem to arise from the accumulation of material in intercellular spaces and from vacuoles among cell clusters. Other lumina may represent a portion of the original lumen of the endostyle. Many follicles appear to be comprised of cells with cytological characteristics similar to those of larval cell types 3 and 2c. Some of the other larval cell types, such as type 5, may also be involved. In young adult lampreys follicles are composed of cuboidal to columnar cells that lack the dilated cisternae of rough endoplasmic reticulum seen in follicular cells of higher vertebrates. Dense collagenous connective tissue surrounding the follicles contains relatively few blood vessels. The transformation process described may have some relevance to our understanding of the development and evolution of the vertebrate thyroid gland.     

The embryonic development of the triclad <Emphasis Type="Italic">Schmidtea polychroa</Emphasis>

Triclad flatworms are well studied for their regenerative properties, yet little is known about their embryonic development. We here describe the embryonic development of the triclad Schmidtea polychroa, using histological and immunocytochemical analysis of whole-mount preparations and sections. During early cleavage (stage 1), yolk cells fuse and enclose the zygote into a syncytium. The zygote divides into blastomeres that dissociate and migrate into the syncytium. During stage 2, a subset of blastomeres differentiate into a transient embryonic epidermis that surrounds the yolk syncytium, and an embryonic pharynx. Other blastomeres divide as a scattered population of cells in the syncytium. During stage 3, the embryonic pharynx imbibes external yolk cells and a gastric cavity is formed in the center of the syncytium. The syncytial yolk and the blastomeres contained within it are compressed into a thin peripheral rind. From a location close to the embryonic pharynx, which defines the posterior pole, bilaterally symmetric ventral nerve cord pioneers extend forward. Stage 4 is characterized by massive proliferation of embryonic cells. Large yolk-filled cells lining the syncytium form the gastrodermis. During stage 5 the external syncytial yolk mantle is resorbed and the embryonic cells contained within differentiate into an irregular scaffold of muscle and nerve cells. Epidermal cells differentiate and replace the transient embryonic epidermis. Through stages 6–8, the embryo adopts its worm-like shape, and loosely scattered populations of differentiating cells consolidate into structurally defined organs. Our analysis reveals a picture of S. polychroa embryogenesis that resembles the morphogenetic events underlying regeneration.Edited by D. Tautz     

The terpene-producing glands of a Phasmid insect. Cell morphology and histochemistry

The defensive glands of Anisomorpha buprestoides produce the terpene toxicant anisomorphal. Each gland consists of a cuticular secretion reservoir surrounded by the secretory epithelium and the musculature which serves to compress the gland and expel the secretion. Two types of cells make up the secretory epithelium: a squamous layer next to the cuticular reservoir and a layer of larger secretory cells responsible for production of the toxicant. The microvilli-laden plasma membrane of each secretory cell is invaginated to form a central cavity. It appears that secretory products pass into the central cavity and then flow out to the gland reservoir via an efferent cuticular ductule contained within the squamous epithelial cell. Histochemical techniques demonstrate lipid reserves, carboxylic esterases, a variety of phosphatases, and an alcohol dehydrogenase, within the secretory cells. It is suggested that the lipid reserves are precursors of the terpenoid toxicant, that a mevalonic kinase has been histochemically demonstrated by the phosphatase test, and that an unusual alcohol dehydrogenase is active in the final steps of toxicant synthesis. The histochemical evidence is consistent with the hypothesis that anisomorphal is produced via the mevalonic acid pathway.     

Dufour gland of the digger wasp<Emphasis Type="Italic"> Liris niger:</Emphasis> structure and developmental and biochemical aspects

The tubiform Dufour gland in the digger wasp species Liris niger is about 1.0 mm long ( 0.15 mm). An alternating arrangement of longitudinal and circumferential bundles of striated muscle fibers surrounds the gland. The Dufour gland, together with the venom gland, enters the sting base and terminates in the sting. The glandular epithelium is monolayered. Glands about 3 day after imaginal ecdysis have an empty lumen but a thick lining epithelium. The gland cells are characterized by a well-developed vesicular smooth endoplasmic reticulum, sparse rough ER and numerous free ribosomes. They also exhibit several electron-lucent vesicles and autophagic vacuoles. Secretion of electron-dense material via the gland cuticle into the gland lumen is apparent. Glands more than 20 days after imaginal ecdysis display a large lumen and a thin epithelium. The cells show signs of degeneration with numerous cytolytic inclusions. Dufour gland liquid contains numerous polypeptides of molecular weights ranging from 14 to about 200 kDa. In addition the secretion consists predominantly of straight-chain hydrocarbons, accompanied by small amounts of esters. The major hydrocarbons are pentadecane and (Z)-8-heptadecene. Dufour gland secretion may have several functions: (1) the polypeptides might be involved in the gluing process of the eggs, while (2) the hydrocarbon oils may function as lubricants for the lancets and (3) might soften the secretion, thus allowing easier application of the glue. The lipophilic volatile material (4) might also be involved in pheromonal signaling.     

Gonadotropin-releasing hormone-like immunoreactivity in the planarian Bdelloura candida (Platyhelminthes, Tricladida)

Abstract. Although peptides similar to gonadotropin‐releasing hormone (GnRH) peptides have been reported in various taxa of invertebrates, no such evidence has yet been identified for platyhelminths. Antibodies raised against mammalian GnRH were used to investigate the distribution of GnRH immunoreactivity in specimens of the triclad turbellarian Bdelloura candida (Maricola, Bdellouridae). While no GnRH immunoreactivity was detected in the brain and nerve cords, both the putative subepidermal and submuscular nerve plexuses appeared to be immunoreactive. GnRH immunoreactivity was also present in epidermal mucous cells, in parenchymal cells, including cells surrounding the branches of the intestine, in putative neuroendocrine cells associated with the testes, and in the vitellogenic gland cells associated with the oviduct. These observations suggest that the GnRH‐like material found in specimens of B. candida may be involved in a variety of functions, including a possible pheromone‐like role for mucous cells and the control of specific reproductive activities.     

Histochemistry of the glands associated with the reproductive tract ofLymnaea stagnalis

Summary The albumen gland, the muciparous gland and the oöthecal gland of female genital tract of Lymnaea stagnalis, collected in spring, autumn and winter have been studied.The reactions for polysaccharides, proteins and RNA have been performed in order to characterise the secretion of the glands.The albumen gland secretion consists almost exclusively of slightly acid polysaccharides whose histochemical reactions, according to Lison and Grainger and Shillitoe confirm the presence of galactogen. Proteins are also present in the secretion. The muciparous gland secretion consists of strongly acid mucopolysaccharides (non sulphated) produced by large cells among which small cells containing sulphated mucopolysaccharides are present.In the oöthecal gland two zones are present, one with a single type of cells containing acid mucopolysaccharides, and the other with two different types of cells: the first with mucopolysaccharides and the second with sulphated mucopolysaccharides, proteins and glycogen at the basis of the cell. Sialic acids are not present in the secretion of the glands studied.The polysaccharidic composition of the secretion of the glands is different from gland to gland. The secretion of the glands gradually changes and gets acid according to the composition of the various membranes and envelopes wrapping up the eggs.     

Comparison of the secretory processes in the parotid and sublingual glands of the mouse. 1. Regulation of the secretory processes.

1. Secretion from the mucous sublingual gland of the mouse has been investigated and compared with the serous parotid gland. The influence of acetylcholine, noradrenalin and adrenalin on the secretion of glycoproteins (e.g. mucins) and proteins (e.g. amylase) from these glands in vitro, and the involvement of cyclic AMP and Ca2+ has been studied. 2. Secretion from the parotid gland could be stimulated by both acetylcholine and the catecholamines. It appears that cyclic AMP plays an important role in the adrenergic secretory process, but not in the cholinergic-induced secretion. In the latter case, exogenous Ca2+ strongly increased the secretion. 3. Mucin secretion from the sublingual gland could be affected by acetylcholine in the presence of exogenous Ca2+. Noradrenalin and adrenalin induced only a slow mucin secretion and, for this secretory process, exogenous Ca2+ is also required. Though cyclic AMP is present in the sublingual gland, no influence on its level could be detected in this gland after stimulation of the adrenergic beta-receptor, whereas, in contrast to the parotid gland, dibutyryl cyclic AMP induced only a slow secretion. Because it was observed that the sublingual gland of the mouse is not innervated sympathetically, it seems reasonable to suppose that the catecholamines stimulate the mucin secretion from this gland via hormonal receptors and not via the adrenergic beta-receptor. 4. The protein secretion from the sublingual gland could be stimulated by both acetylcholine and the catecholamines. An involvement of cyclic AMP in this process was not observed. Addition of exogenous Ca2+ is less important, as was found for the mucin secretion. So it has been concluded that protein and mucin secretion from the sublingual gland are regulated via different pathways.     

Secretory Mechanism of Fibroin,a Silk Protein,in the Posterior Silk Gland Cells of Bombyx mori

There are two microtubule-microfilament systems in the posterior silk gland cells of Bombyx mori. One is a radial microtubule system; the other is a circular microtubule-microfilament system. These two systems are presumably concerned with the intra-cellular transport of secretory granules of fibroin and the secretion of fibroin into the lumen, respectively. Conventional and scanning electron microscopic observations of the two microtubule-microfilament systems in the posterior silk gland cells are reported. Scanning electron micrographs showed that a number of parallel linear cytoplasmic processes ran circularly on the luminal surface of the posterior silk gland cells. These processes were assumed to correspond to the circular microtubule-microfilament systems. The effects of cytochalasin (B or D), a secretion stimulating agent of fibroin, on the intracellular recording of membrane potential from the posterior silk gland cells are also reported. Exposure to cytochalasin resulted in depolarization of the membrane potential of the gland cells. Possible functional roles of the two microtubule-microfilament systems in the secretory mechanism of fibroin are discussed with reference to the effects of antimitotic reagents and cytochalasin on these two systems.     

Glandular scale development and essential oil secretion in Origanum dictamnus L.

Glandular scales of Origanum dictamnus L. originate from a single protodermal cell. They are composed of a 12-celled head and an unicellular stalk and foot. During the early stages of gland differentiation, the head cells possess a small number of plastids which contain globular inclusions. Similar inclusions are also observed in the plastids of the stalk and the foot cell. The lateral walls of the stalk cell progressively undergo cutinization which does not extend to the upper and lower periclinal walls. At the onset of secretion the electron density of the plasmalemma region lining the apical walls of the head cells remarkably increases. These walls are impregnated with an osmiophilic substance identical in appearance to the content of the subcuticular space. In a following stage of the secretory process osmiophilic droplets of various size arise in the cytoplasm of the secretory cells which undergoes simultaneously a reduction of its initial density. After secretion has been concluded the protoplast of the head cells becomes gradually degenerated. The chlorenchyma cells of the mesophyll possess numerous microbodies closely associated with various organelles. In the cytoplasm of these cells crystalloids occasionally occur.     

Anatomical and ultrastructural changes of the floral nectary ofPisum sativum L. during flower development

Summary The floral nectary ofPisum sativum L. is situated on the receptacle at the base of the gynoecium. The gland receives phloem alone which departed the vascular bundles supplying the staminal column. Throughout the nectary, only the companion cells of the phloem exhibited wall ingrowths typical of transfer cells. Modified stomata on the nectary surface served as exits for nectar, but stomatal pores developed well before the commencement of secretion. Furthermore, stomatal pores on the nectary usually closed by occlusion, not by guard-cell movements. Pore occlusion was detected most frequently in post-secretory and secretory glands, and less commonly in pre-secretory nectaries. A quantitative stereological study revealed few changes in nectary fine structure between buds, flowers secreting nectar, and post-secretory flowers. Dissolution of abundant starch grains in plastids of subepidermal secretory cells when secretion commenced suggests that starch is a precursor of nectar carbohydrate production. Throughout nectary development, mitochondria were consistently the most plentiful organelle in both epidermal and subepidermal cells, and in addition to the relative paucity of dictyosomes, endoplasmic reticulum, and their associated vesicles, the evidence suggests that floral nectar secretion inP. sativum is an energy-requiring (eccrine) process, rather that granulocrine.Abbreviations ER endoplasmic reticulum - GA glutaraldehyde - SEM scanning electron microscopy     

Ultrastructural studies on the secretory cavities ofCitrus deliciosa ten. I. Early stages of the gland cells differentiation

Summary Secretory cavities ofCitrus deliciosa seem to originate from a pair of meristematic cells (an epidermal cell and a second one placed under it). These cells undergo successive divisions resulting in the formation of a globular/oval gland situated in the parenchyma, and a conical stalk, which joins the gland with the epidermis. The young gland consists of a central group of polyhedral cells ensheathed by layers of radially flattened cells.During the early differentiation stages of the gland cells a close association of cytoplasmic microtubules with various organelles is observed. Plastids increase progressively in number and size and their matrix locally contains tubular networks accompanied by small oil droplets. In peripheral cytoplasm numerous myelin-like lomasomes have been observed. Peripheral cells of the gland are gradually modified from the inner cells following a different developmental pattern. Their walls become thicker and plastids do not contain tubular complexes, but only a few thylakoids partly surrounding the newly formed starch grains.     

An electron microscopic study of the salivary gland of Rhynchosciara angelae

Summary The structure of the salivary gland of the dipteran insect Rhynchosciara angelae in a defined stage of the larval development, characterized by the synthesis and storage of secretion product, is described. Observations were made with both Nomarski optics and electron microscopy. Filiform projections extending into the lumen of the gland were observed in the apical portion of the cells. At the basal region junctions, characterized as hemidesmosomes, were observed between the membrane of the cell and the basal lamina. The plasma membrane presents numerous infoldings into the cell increasing considerably the surface area at this region. Throughout the cytoplasm of the gland cells numerous mitochondria, Golgi complexes, microtubules, profiles of endoplasmic reticulum, secretion granules and glycogen granules were observed. Carbohydrates were detected on ultrathin sections by using the periodic acid-silver methenamine and the periodic acid-thiosemicarbazide-silver proteinate techniques.     

Mode of Action of some Stimulants of the Hatching Enzyme Secretion in Fish Embryos*

Mode of action of two stimulants of the hatching enzyme secretion, electric current (AC) and potassium cyanide, was analyzed by applying them to Medaka embryos in the presence or absence of suppressants of nervous system-mediated secretion, tetrodotoxin or MS–222. Electric current (AC) stimulated the secretion of the hatching gland of the embryos that had been treated with these suppressants, while potassium cyanide did not. These results strongly suggest that electric current acts as a stimulant of hatching enzyme secretion directly on the gland cell itself, while potassium cyanide stimulates the secretion indirectly, probably through nervous system of the embryo. In the present experiments, it was also shown that Ca²⁺ and ionophore, X-537A, when applied directly to the hatching gland extracellularly, induced a marked secretion-associated morphological change of the gland cells instantaneously. However, it was found that chum salmon prolactin did not induce the secretion-associated morphological changes in the hatching gland cells when it was applied directly to the gland cells in situ or indirectly through embryonic circulation.     

Post-mating effects in the grasshopper, Gomphocerus rufus L. mediated by the spermatheca

In the female grasshopper Gomphocerus rufus mating replaces copulatory readiness with immediate and long-lasting `secondary defense', during which further mating attempts are efficiently repelled. The behavioral change is caused by secretions from the male accessory glands' white secretory tubule 1 which is injected with the spermatophore material into the female's spermathecal duct. A bristle field of contact chemoreceptors at the entry of the spermathecal duct into the endbulb is assumed to be stimulated by the secretion. Ablation of the bristle field, interruption of the nervous pathway between the spermatheca and the ventral nervecord, or severance of the latter sustains sexual receptivity after mating. Both the secretion from white secretary tubule 1 and the spermatophore contained in the spermatheca of a mated female are digested by proteolytic enzymes from spermathecal gland cells. Dissolved material is resorbed by similar glandular-like cells. The intersexual conflicts of interest and their evolutionary consequences are discussed. Accepted: 4 December 1998