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.     

Schistosoma mansoni: penetration apparatus and epidermis of the miracidium

Free swimming miracidia of Schistosoma mansoni were studied with the electron microscope for the purpose of describing the penetration apparatus and the epidermis. The penetration apparatus was composed of 3 unicellular glands which contain membrane-bound vesicles containing macromolecular diglycols. Each gland contained a nucleus, rough endoplasmic reticulum, Golgi complexes, numerous mitochondria, and glycogen stores. Each gland cell opened to the exterior through the apical papilla.The surface of the miracidium, with the exception of the apical papilla, was covered with ciliated epidermal cells containing numerous mitochondria, membranous bodies, and glycogen. No nuclei were detected within these epidermal cells. Intercellular ridges connecting with subepidermal cytons interrupted the epidermal cells at numerous points. The ridges were joined to the epidermal cells by septate desmosomes. Beneath the epidermal cells were found circular and longitudinal muscle bundles.Sensory structures of various types were associated with the outer covering. These consisted of (1) numerous “knob-like” cytoplasmic projections associated with epidermal cells, (2) bulbous, lamelloid structures with external cytoplasmic projections, and (3) ciliated nerve endings with posterior epidermal tiers and ciliated nerve pits associated with apical papilla.     

Electron microscopic studies of coated membranes in two types of gill epithelial cells of lamprey

Summary Coated membranes in two types of gill epithelial cell of adult lamprey, Lampetra japonica, were studied by electron microscopy. The type 3 gill epithelial cells possess well-developed microvilli or microfolds, apical vesicles and abundant mitochondria. The cytoplasmic surface of the microvillous plasma membrane is covered by a coat of regularly spaced particles with a center-to-center distance of about 15 nm. Each particle consists of a bulbous free end, about 10 nm in diameter, and a connecting piece, about 5 nm long. Apical vesicles are covered by a surface coat which consists of fine filamentous material but lack any special coating on their cytoplasmic surface.The type 4 cells (chloride cells) are characterized by apical vesicles, abundant mitochondria and cytoplasmic tubules. These tubules possess a coat on their luminal surface which consists of spirally wound parallel rows of electron-dense materials. The rows are about 16 nm apart and wound at a pitch of about 45°. The cytoplasmic surface of these tubules does not display a special coat. These coated membranes are assumed to be the sites of active ion transport across the plasma membrane. In particular, particles in type 3 cells and linear coat materials in chloride cells may be either loci of transport enzymes or energy generating systems. Apical vesicles lack any coating on their cytoplasmic surface but a fine filamentous coat is present on their luminal surface. They contain intraluminal vesicles and are continuous with apical ends of cytoplasmic tubules.     

Correlative light and scanning electron-microscopic study of feline gastric mucosa: the cardiac region (pars cardiaca)

The cardiac region (pars cardiaca) of the cat's stomach was examined with light and scanning electron microscopy. The glands are simple, coiled tubular, and contain mucus-secreting cells. Their surfaces are covered with microvilli which are concentrated on the boundaries of the mucus-secreting cells. A few cells interposed between the glandular cells are probably G cells. They are identified by apical projections of long microvilli into the lumen of the gland. The surface epithelial cells lining the cardiac region are covered by minute microvilli. The muscularis mucosae is not distinctly divided into two layers. However, a group of smooth muscle cells which are directed in a circular manner around the gastroesophageal junction is considered to be the distal esophageal sphincter.     

Comparative ultrastructural investigations of the uterine epithelium in the viviparous Salamandra atra Laur. and the ovoviviparous Salamandra salamandra (L.) (Amphibia,Urodela)

The uterine epithelium of the viviparous Salamandra atra and the ovoviviparous Salamandra salamandra was studied in non pregnant and ovulating females and in females during different stages of pregnancy. The epithelium of both species is organized in a monolayer. The epithelial cells are characterized by a moderate secretory activity, a variable amount of apical granules which include PAS-positive material and by some apical and basal exo- or endocytotic vesicles. Adjacent cells are joined by junctional complexes. The lateral surfaces form a tortuous boundary with adjoining cells which suggest that the epithelium is involved in transport. Sporadic light cells possess highly variable cytoplasmic inclusions and are not joined with neighbouring cells. Possibly they represent migratory cells. The entire epithelium, except for a small cranial portion of the uterus in S. atra, undergoes no remarkable morphological changes during the different physiological stages examined except that flattened cells seem to be more numerous in pregnant females. The results are discussed with regard to the possible supply of the developing young by the mother.     

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.     

The role of exocytosis in the apocrine secretion of milk lipid globules in mouse mammary gland during lactogenesis.

Functional relations between exocytotic vesicle membranes, plasmalemma and milk fat globule membranes (MFGM) were studied during the final stages of mouse mammary gland differentiation, in the gland during full lactation and in the postpartum gland in which the synthesis of secretory products was partly inhibited by application of 2-Br-alpha-ergocryptine. Analysis of ultrathin sections, freeze-fracture replicas, scanning electron microscopy and application of a cytochemical marker filipin showed that the apocrine secretion of lipid globules was closely related to the exocytosis of milk proteins. During the last days of gestation the secretion of lipid globules resulted from many exocytotic events of the secretory vesicles that accumulated and fused around the cytoplasmic lipid droplets. Seldom the lipid droplet protruded partly into the gland lumen and a part of its surface became covered with the apical plasmalemma. Although apical plasmalemma became more important in the formation of MFGM in the postpartum period, we could still confirm a direct contribution of secretory vesicle membranes to the final detachment of the lipid globule. The application of 2-Br-alpha-ergocryptine hindered the apocrine secretion of the lipid globules and a situation similar to the situation in the prepartum gland was observed.     

Apical surface topography of invaginating and noninvaginating cells. A scanning-transmission study of amphibian neurulae

A comparative study on scanning and transmission electron microscopy of apical ultrastructure in epithelia of the axolotl neurula (Ambystoma mexicanum, Cope) is presented. The aim of the work is to determine whether apical surface topography is correlated to other morphological features of the cells, and whether there are any ultrastructural differences between surfaces of invaginating and noninvaginating regions. Scanning specimens are prepared by critical point drying and benzene freeze-drying. Comparisons show that the scanning specimens are comparable to the standard transmission specimens with regard to surface topography.Apical surfaces are sculptured by folds and microvilli-like processes. Assessment of the relative abundance of surface projections shows that these occur in largest numbers on invaginating, bottle-shaped cells in the neural plate and in the notochord rudiment of stage-16 larvae. It is proposed that the surface projections may support apical narrowing in these cells, by facilitating endocytosis, and that they may be lateral attachment organelles. It is suggested that the morphogenesis of the invaginating cells may be the result of coordination between microfilaments, apical endocytosis, microtubules, and “adhesive peripheral surface projections.”     

Scanning Electron microscopic Study on the Early Development of Silkworm Eggs (Bombyx mori L.)

Morphological changes of the surface of eggs of the silkworm Bombyx mori L. were studied during early developmental stages by scanning electron microscopy. The egg surface was covered with numerous microvilli at least until 4 h after oviposition. At 6 h the microvilli were replaced by ruffle-like microprojections. This suggests that developmental changes of the surface structure may occur without direct influence of cleavage nuclei. Immediately before blastoderm cell formation, microvilli reappeared in the presumptive groove area. The ruffles seen on the apical portion of newly-formed blastoderm cells gradually became flattened, while microvilli developed on the lateral side of the cells. The mode of blastoderm cell formation is different from the typical one seen in most species of insects.     

The tarsal gland of ixodid ticks <Emphasis Type="Italic">Ixodes persulcatus</Emphasis> and <Emphasis Type="Italic">Ixodes ricinus</Emphasis> (Mesostigmata,Ixodidae)

A complicated multicellular gland is situated in all the leg tarsi, occupying from one third to half the segment. The glandular cells form a single-layer sack; the inner surface of the gland cavity is covered with the multi-layer membrane. Cuticular rods (“sinews”) of muscles moving the claw pass inside the gland cavity. The glandular cells are characterized by the presence of numerous microvilli on their apical surfaces and by the presence of secretory vacuoles. The basal part of each secretory cell is characterized by accumulations of lipid vacuoles and glycogen granules. Problems concerning the possible role of tarsal gland in the production of the trace pheromone are discussed.     

Morphology and ultrastructure of a bacteria cultivation organ: the antennal glands of female European beewolves, Philanthus triangulum (Hymenoptera, Crabronidae)

Females of a solitary digger wasp, the European beewolf (Philanthus triangulum F.), cultivate symbiotic bacteria of the genus Streptomyces in specialized antennal glands. The streptomycetes are secreted in the subterranean brood cells and protect the offspring against mould fungi. We reconstructed the complex morphology of the antennal glands using 3D-visualization software, investigated the ultrastructure of the glands, and examine the role of the antennal glands as organs for the cultivation of the symbiotic bacteria. The bacteria are cultivated in five antennomeres within large reservoirs that consist of two slightly bent lobes. Each gland reservoir is bordered by a monolayered epithelium lined with a partially reinforced cuticle and when completely filled with bacteria it comprises about half of the antennomere's volume. The opening of the reservoir is covered by gelatinous appendage of the cuticle. The cells of the monolayered epithelium bordering each reservoir show basal invaginations, apical microvilli and numerous vesicles. Each reservoir is surrounded by approximately 400 class 3 gland units that are connected to the reservoir lumen through conducting canals. The class 3 gland cells contain numerous vesicles and a high density of rough endoplasmatic reticulum. In the reservoir lumen, large numbers of symbiotic Streptomyces bacteria are embedded in secretion droplets. Thus, the bacteria are apparently provided with large amounts of nutrients via the gland epithelium and the class 3 gland cell units.     

Glomeruli and renal tubules are restricted to the cranial kidney of the adult estuarine Sphoeroides testudineus

The kidney of the pufferfish Sphoeroides testudineus , an abundant tropical euryhaline estuarine species of the western Atlantic Ocean found in southern Brazil (in salinities ranging from 0 to 34) has a large and laterally spread cranial red portion, and a very thin and pale caudal portion. When studied under light and transmission electron microscopy, the cranial kidney displayed glomeruli and renal tubules surrounded by haematopoietic tissue. These tubules appeared to drain into a single large convoluted collecting duct with a wide lumen and thick pseudostratified epithelium, the mesonephric duct, which constituted the sole structure of the caudal kidney. Apical microvillae were viewed in the renal tubules, as well as in the mesonephric duct. Basal mitochondria and membrane infoldings were observed in the renal tubules. Abundant more basally‐located mitochondria and electron‐dense vesicles, mainly in the apical cytoplasm, were observed along the entire length of the mesonephric duct. Aposomes (blebs) were frequently observed in the mesonephric duct, both by light‐ and electron‐microscopy. This euryhaline estuarine pufferfish has thus been revealed to possess a rare type of kidney.     

Function of the cytoskeleton in cells with microridges from the oral epithelium of the carp Cyprinus carpio

Superficial cells of the oral mucosal epithelium in the carp and the cytoskeleton of the epithelial cells are examined by scanning and transmission electron microscopy. Microridges are formed on the surface of the epithelium. Epithelial cells contain two types of vesicles: mucous secretory vesicles and coated vesicles. Most of the mucous vesicles are situated in the center of the cell near the Golgi apparatus. In freeze-fracture replicas, intramembranous particles are abundant in the membranes of the secretory vesicles but rare in the apical plasma membrane. Coated vesicles are situated in the apical and subapical cytoplasm. A great number of thick filaments, considered to be keratin filaments, run randomly throughout the cell to form a meshwork. Thick filaments, which are sparse in the central cytoplasm, are connected to the membranes of the secretory vesicles and other membranous organelles. A layer of closely packed thin filaments, considered to be actin filaments, is found just beneath the apical plasma membrane. Microtubules also occur in the apical cytoplasm and run almost parallel to the cell surface. Both kinds of vesicles are connected to the thin and thick filaments. Their functional significance in the regulation of membrane at the free surface is discussed.     

Ultrastructure of lineus ruber (rhyncocoela) epidermis

Recent evidence indicates that nemertean epidermis is capable of absorbing certain organic solutes from sea water via mediated transport mechanisms, as well as secreting mucoid substances. Morphological studies suggest that these functions may be restricted to distinct epidermal cell populations.Mucous secretion at the free surface of the epidermis is the result of synthesis and release activities of cells in both the epidermis and dermis (cutis). Secretion of dermal origin passes through the epidermis to the worm's exterior in slender cytoplasmic processes (canaux d'evacuation) in the form of membrane bound vesicles. A single gland cell type, located entirely within the epidermis, releases externally a granular product histochemically identified as largely protein plus some amount of carbohydrate with low periodic acid-Schiff's reactivity. The close juxtaposition of granular endoplasmic reticulum and Golgi apparati to the secretory material is consistent with the composition of this secretory product.Interstitial cells possess microvilli projecting from their apical surface, in addition to cilia. The outer surface of the plasmalemma covering these ciliary projections is unadorned, but microvilli possess a fuzzy coat. At the peripheral ends of the microvilli, the coat is filamentous, while at their base the coat consists of foliate structures. Cationic colloidal iron binding suggest that the filamentous portion of the fuzzy coat contains the greatest proportion of the acidic surface charge. The presence of periodic acid-Schiff's positive material in this region suggests that the fuzzy coat also contains carbohydrate. Lateral boundaries of the interstitial cell lacks obvious junctional specializations; however, the apical 150 nm intracellular space narrows to 40 nm and continues in a tortuous interdigitating path to the base of the adjacent interstitial cells. Where the apex of these cells is broad, the interdigitations are shallow, but the basal half of the interstitial cells have deep complex infoldings. Cytoplasmic organelles other than the nucleus, mitochondria and some granular endoplasmic reticulum, are restricted to the apical half of the cytoplasm. The presence of closely apposed Golgi complexes and smooth endoplasmic reticulum, multivesicular bodies, lysosome-like dense vesicles and coated vesicles suggests that these cells may play a role in intracellular digestion of phagocytized paniculate matter from the external environment. The amplification of the interstitial cell's free surface suggests that these cells are primarily responsible for mediated solute transport across the epidermis.     

The distribution of MUC1, an apical membrane glycoprotein, in mammary epithelial cells at the resolution of the electron microscope: implications for the mechanism of milk secretion

The distribution of the glycoprotein, mucin 1 (MUC1), was determined in lactating guinea-pig mammary tissue at the resolution of the electron microscope. MUC1 was detected on the apical plasma membrane of secretory epithelial cells, the surface of secreted milk-fat globules, the limiting membranes of secretory vesicles containing casein micelles and in small vesicles and tubules in the apical cytoplasm. Some of the small MUC1-containing vesicles were associated with the surfaces of secretory vesicles and fat droplets in the cytoplasm. MUC1 was detected in much lower amounts on basal and lateral plasma membranes. By quantitative immunocytochemistry, the ratio of MUC1 on apical membranes and milk-fat globules to that on secretory vesicle membranes was estimated to be 9.2:1 (density of colloidal gold particles/microm membrane length). The ratio of MUC1 on apical membranes compared with basal/lateral membranes was approximately 99:1. The data are consistent with a mechanism for milk-fat secretion in which lipid globules acquire an envelope of membrane from the apical surface and possibly from small vesicles containing MUC1 in the cytoplasm. During established lactation, secretory vesicle membrane does not appear to contribute substantially to the milk-fat globule membrane, or to give rise in toto to the apical plasma membrane.     

Fine Structure of the ‘Slit Organs’ of the Pycnogonid Anoplodactylus petiolatus (Anoplodactylidae)

Abstract The ‘slit organs’ of Anoplodactylus petiolatus are found all over the body cuticle. They are composed of a cuticular pore apparatus, an inner and an outer canal cell, and of four large and one to three small compartment cells. Plasma of the latter seven cells is almost completely filled with large membrane-enclosed compartments that contain either numerous small vesicles (one of the large cells) or homogeneous material of varying electron density (three large and all the small cells). Microvilli are found in the apical region of the compartment cells. The nucleus is situated basally where Golgi-cisternae, coated vesicles and free ribosomes are frequently found. Apical microvilli and vesicles are also formed by the inner canal cell indicating that it might directly be involved in transport. Anatomically the ‘slit organs’ are similar to class III glands described for many arthropods. In addition, discharge of secretion via large intracellular compartments is also a feature found in arthropod glands. Although pycnogonids appear to take up substances across the cuticle, a genuine secretion rather than a more generalized transport function is suggested for the ‘slit organs’.     

Ultrastructure of the full-term shark yolk sac placenta. II. The smooth, proximal segment

The smooth, proximal portion of the yolk sac placenta of the sandbar shark, Carcharhinus plumbeus is comprised of: (1) An outermost epithelial ectoderm; (2) an intervening collagenous stroma; and (3) an inner mesothelium. The surface epithelium may be one to three cell layers thick. The surface epithelium comprises two cell types. A cuboidal cell that has a dome-like apical surface covered with microvilli and an ovoid nucleus predominate. These cells contain lipid inclusions, many cytoplasmic filaments, and are joined by desmosomes. The second cell type has a convoluted nucleus and a flattened cell apex with microvilli, cilia, and paddle cilia. Golgi complexes and elements of the endoplasmic reticulum are relatively uncommon in the cytoplasm of both cell types. Microplicae also occur on the surface of some cells. The smooth, proximal portion of the placenta is sparsely vascularized. The innermost cellular elements of the surface epithelium rest on a prominent basal lamina. A collagenous zone separates the epithelial basal lamina from the basal lamina of the mesothelium. The mesothelial cells are squamous with a fusiform nucleus, many pinocytotic pits and vesicles, and a large number of cytoplasmic filaments. The endoplasmic reticulum, except for occasional patches of the rough type, and the Golgi complex are poorly developed. Ultrastructural tracer studies show that this portion of the placenta does not absorb horseradish peroxidase (HRP) and trypan blue.     

Ultrastructure of human labial salivary glands. I. Acinar secretory cells

The structure of human labial salivary gland acini was studied by light and electron microscopy. Contrary to previous reports, these glands were pure mucous in nature; no serous elements were present. The acinar cells were found in all stages of maturation. Immature cells were characterized by an extensive and highly organized rough-surfaced endoplasmic reticulum. The Golgi complex was extremely prominent, consisting of stacks of flattened cisternae and swarms of small vesicles. Mucous droplets were almost completely absent. As secretory activity progressed, the endoplasmic reticulum involuted, while the Golgi cisternae became distended and formed many vacuoles. In mature mucous cells, the apical cytoplasm was filled with membrane-bounded mucous droplets, and the nucleus was displaced basally. The droplets frequently showed great variation in density from cell to cell, and even within the same cell they sometimes were quite heterogeneous. They were liberated from the acinar cells by an apocrine process, so that droplets with intact limiting membranes were often observed in the acinar lumen. These droplets soon lysed, their contents fusing into streams of mucus. Occasionally during apocrine secretion a mucous cell failed to reconstitute its apical surface, and its entire contents spilled into the acinar lumen. Unusual cytoplasmic inclusions were present in many of the acinar cells. These inclusions, which were surrounded by a single membrane, consisted of lipid droplets closely associated with bundles of fine filaments.     

An ultrastructural study of the cysts in chicken ultimobranchial glands,with special reference to C-cells

Summary The ultimobranchial glands of the chicken were examined by electron microscopy and immunocytochemistry using a calcitonin antiserum. Electron microscopy confirmed the presence of C-cells, containing numerous secretory granules storing calcitonin, in the luminal lining of cyst-like structures found in these glands. These cells were furnished with prominent microvillar projections at their luminal surface, and the cytoplasm of the apical region was filled with fibril material. Furthermore, the cells contained prominent junctional complexes and desmosomes at their apico-lateral surfaces. In these C-cells, secretory granules were concentrated near the lumen and some were attached to the apical cell membrane. The luminal content of the cysts had a colloid-like and flocculent appearance, and was frequently seen attached to the cytoplasmic projections or apical cell membrane of the C-cells. Since the cysts progressively increase in volume and number with age, it is suggested that they may partly play a role in the storage of excess or unneeded hormonal products.     

Fine structure of the accessory glands of the female genital tract of the ichneumonid Pimpla turionellae (Insecta,Hymenoptera)

Summary The female accessory glands include the tubular poison gland, the paired, lemon-shaped uterus glands, and Dufour's gland, an unbranched tubular organ. They consist essentially of a single layer of epithelium cells surrounded by a basement membrane. The lumen is lined by cuticle. The proteinaceous secretion of the poison gland is released into intracellular ducts provided with microvilli, each connected to a channel lined with cuticle which leads to the central lumen of the gland. The channel is formed by special canal cells. Nerve endings are interspersed among the gland cells. The uterus gland consists of four cell types derived from a single type of precursor cell found in newly hatched wasps. Type I cells are covered by type II cells and are thus without contact to the luminal surface of the gland. They contain stacks or whorls of mitochondria and smooth cisternae in an alternating arrangement. Vesicles with a secretory product are found in cells of types II and III. Deep anastomosing infoldings of the plasmalemma, stabilized by microtubules and dense material at the branchings, are characteristic for type II cells. Most secretory vesicles are found in type III cells, the prevalent cell type which is thought to be the source of the lipoprotein secretion. Coated vesicles are present at deep infoldings of the plasmalemma. The greatly enlarged apical surface area of type IV cells and the presence of mitochondria in slender outgrowths is suggestive of an osmoregulatory function. In Dufour's gland, two cell types appear in succession, the first with a very dense cytoplasm, the second with dense inclusions and many seemingly empty vesicles of smooth endoplasmic reticulum. The secretion products, lecithin and a cholesterol ester, are thought to be formed by the second cell type. The dense inclusion might be lecithin, which reacts with osmium tetroxide. The cholesterol ester could have been washed out of the empty vesicles by the embedding procedure.