L.T. Threadgold

Electron-microscope studies on Fasciola hepatica: VIII. The development of the vitelline cells

Abstract An electron microscope study of the vitelline follicles of Fasciola hepatica indicates that they are composed of cells in various stages of cytomorphosis. The immature cells are found at the periphery and the most mature are found at the center of the follicle, from where they are released into the vitelline duct. Immature cells are undifferentiated and have a morphology characteristic of embryonic cells. Differentiation involves the formation of large amounts of granular endoplasmic reticulum. Golgi complexes give rise to globules of shell protein, which migrate to form clusters at the periphery of the cell. Further maturation results in the disintegration of the granular reticulum and the cell becomes filled with glycogen. The vitelline cell, therefore, develops and functions first as a protein secretory cell and subsequently as a food store for the developing embryo; it clearly fulfills a dual function. Nurse cells are present and have cytoplasmic processes which envelop the developing vitelline cells. It is suggested that the nurse cells may have a role in the selection and transport of materials necessary for the protein secretory and glycogen storage functions of the vitelline cells.

Fasciola hepatica: Development of tegument during migration in mouse

Abstract T-0 granules of the type 0 tegumental cells of newly excysted juveniles appear in the syncytium of juveniles recovered from the abdominal cavity of mice 12 hr postinfection (p.i.). They undergo exocytosis and/or add their contents to the glycocalyx of the syncytial apical plasma membrane. While in the abdominal cavity the syncytial ground cytoplasm has an increased electron density. After arrival in the liver the type 0 cells metamorphose into type 1 cells of the adult and begin to synthesize T-1 granules. The type 2 cells of the adult arise by differentiation of embryonic cells in the parenchyma, 2–3 days p.i., and subsequently form protoplasmic tubular connections with the syncytium. On arrival in the bile ducts, 4 wk p.i., T-2 granules, formed in the type 2 cells, congregate in the apical cytoplasm of the thickened syncytium and the apical plasma membrane becomes much invaginated. The discussion correlates the development of the tegument with the changes in environment and a mechanism of spine growth is proposed.

Electron microscope studies of Fasciola hepatica. XIII. Fine structure of newly excysted juvenile

Abstract The gross morphology and fine structure of the newly excysted juvenile fluke are described. The tegument is organized as in the adult, with a spine-containing surface syncytium connected to a perinuclear region lying below the muscle layers. It differs from the adult, however, in having only one type of perinuclear region and one type of secretory body. The digestive system has all the morphological characteristics of a secretory epithelium, rather than an absorptive one. It has numerous dense secretory bodies, ribosomes, and GER, an irregular apical surface due to eccrine secretion, and a lumen filled with a moderately dense material derived from the dispersed secretory granules. The excretory system closely resembles that of the adult, but has, in addition, part of its ascending ducts ciliated and concretions as well as lipid droplets as visible excretory products. The muscle is identical with that of the adult and the parenchyma is also very similar to the adult, although it has more lipid droplets and is associated in a less complex way with cells of other organ systems. Groups of embryonic cells are present and are characterized by a dense, invaginated nucleus surrounded by a very thin layer of cytoplasm.

Fasciola hepatica: basal infolds and associated vacuoles of the tegument.

Abstract The tegument of Fasciola hepatica has been shown to contain long invaginations of its basal plasma membrane, hereafter called basal infolds. Associated with the membranes of the infolds, and with the apical and basal plasma membranes, is a Na + K + ATPase (EC 3.6.1.3). Furthermore, polymorphic masses of acid mucopolysaccharide lie close to or against the sides of the infolds and the basal plasma membrane and also fill cytoplasmic tubules which connect the tegument with the tegumental cells. Fixation and incubation of flukes in hypertonic and hypotonic media have shown that the infolds respond to changes in external osmolarity by collapsing in the former and swelling in the latter. This is not simply a passive response, however, since the infolds return to near normal configuration and morphology within 1 hr in either hypertonic or hypotonic media, even though the whole fluke may be shrunken or turgid depending on medium osmolarity. The tegument, therefore, has many of the characteristics of a transporting epithelium. A theory outlining the possible mode of operation of the tegument as a transporting epithelium is proposed on the basis of the present structural, chemical, and physiological findings combined with current ideas of the role and functioning of standing gradients, and forward and backward channel systems in other transporting epithelia.

Electron microscope studies of Fasciola hepatica. XII. The fine structure of the gastrodermis.

Abstract The gastrodermis of Fasciola hepatica comprises a single continuous layer of epithelial cells which show considerable variation in fine structure. These differences in structure, however, reflect only the different functional states of a single cell type. Using ultrastructural morphology as a basic criterion, the cells can be classified into one of three groups, arbitrarily termed Groups I, II, and III. Group I cells are considered to be predominantly secretory in function and Group II cells predominantly absorptive. Each cell shows a cyclical transformation between the secretory and absorptive forms, but since all the cells in any particular area of the diverticula are not all in the same state, secretion, absorption and digestion are occurring simultaneously and continuously. This situation has been observed only in the cells of the lateral diverticula. The main caeca, lined by Group III cells, appear to be more concerned with the movement of material back and forth within the lumen of the gut system, although they must also both secrete and absorb.

Fasciola hepatica: Ultrastructure and histochemistry of the glycocalyx of the tegument

Abstract The morphology and histochemistry of the glycocalyx of the tegument was investigated by electron microscopy. The results showed that both the morphology and histochemistry of the glycocalyx varied depending on the environment immediately prior to fixation and also on postfixation treatments. Conventional electron microscope fixation appeared to preserve only about half the total thickness of the glycocalyx, and only histochemical tests applied en bloc gave a true morphological and histochemical picture. The glycocalyx, therefore, consists of two parts, an inner continuous layer which is tightly bound to the apical plasma membrane and is always preserved, and an outer fibrillar layer, which is not always preserved by conventional fixation. Both layers are anionic and carbohydrate-rich and therefore contain glycoproteins and sialic acids. The surface of Fasciola hepatica , therefore, has morphological and chemical features very like those proposed for the “greater membrane” by Lehninger.

Fasciola hepatica: stereological analysis of vitelline cell development.

Abstract Cytomorphosis of vitelline cells in Fasciola hepatica has been studied quantitatively by means of a Kontron Videoplan computerized image analyser. The process of vitelline cell development was subdivided into four characteristic phases: the stem cell, the intermediate type 1 cell, the intermediate type 2 cell, and the mature cell. The whole cell and the following constituent organelles, the nucleus, nucleolus, mitochondria, granular endoplasmic reticulum (GER), secretory granules, glycogen, heterophagosomes, and lipid, were analysed at each phase. Results indicated that there was a significant increase in nuclear size between the two intermediate cell stages, and a significant increase in nucleolar size between the stem cell and intermediate type 1 cell; the changes were related to possible gene activation, ribosome production, and cell synthesis required for both cell growth and secretory production. Mitochondria increased in number and showed changes in volume and surface area of whole organelles and their cristae such that these correlated with the energy demands of growth and synthesis. The GER increased its average total volume some 16 times and its average total surface area some 25 times between the stem cell and mature cell stages. Even this increase masked the true extent of membrane production by this system, since membrane was transferred to secretory granules; in reality the GER or GER-derived membranes increased more than 42 times between the stem cell and mature cell stages. Shell globules and glycogen eventually contributed 21 and 20%, respectively, to cytoplasmic volume, while heterophagosomes contributed 14% and lipid only 3%. Residual cytoplasm was shown to be an important component in cytomorphosis, comprising never less than 40% of the total cytoplasmic volume and it was shown to increase prior to or parallel with the development and expansion of other systems.

Electron microscope studies of Fasciola hepatica: XI. Autophagy and parenchymal cell function

Abstract An electron microscope study of the function of parenchymal cells in Fasciola hepatica in relation to glycogen storage and metabolism was undertaken. Although no evidence of a relationship between morphological changes in parenchymal cells and glycogen was apparent, a correlation between glycogen depletion and autophagy was observed. The autophagic process started with the synthesis and “budding off” of membranes by mitochondria to form small vesicles (M bodies), either of a simple type (limited by a single membrane) or a complex type (limited by two or more membranes). These M bodies fused to form narrow, smooth cisternae (SCM), which wrapped around areas of cytoplasm containing glycogen; this process gave rise to β bodies. The β bodies (autophagosomes) were at first irregular in shape and limited by two or more membranes separated by a space of varied width. Older β bodies became more smooth and oval in outline and were limited by a single membrane due to membrane fusion. Primary lysosomes synthesized by the GER-GA system united with the late β bodies and formed secondary lysosomes (autolysosomes). Following the addition of these lysosomal hydrolases, the glycogen content of the autolysosomes was reduced and eventually disappeared. This resulted in the develpment of residual bodies containing unhydrolysed material in the form of dense granules, tubules, and myelin figures in a matrix of varied density. It was concluded that at least some glycogen in the parenchymal cells of F. hepatica is mobilized by autophagy. All the morphological structures observed in the experimental material were also present in controls, although in fewer numbers, and it is believed that autophagy is a normal process in this fluke.

Schistocephalus solidus and Ligula intestinalis: pinocytosis by the tegument.

Abstract Using ruthenium red as a macromolecule, endocytosis was demonstrated in the plerocercoid of Ligula intestinalis and adult Schistocephalus solidus . Uptake, transport across the tegument, and exocytosis across the basal plasma membrane occurred within 6 min. The types of vesicles in the tegument of L. intestinalis are redescribed and their former classification is modified. The vertical and longitudinal distribution of pinosomes in the tegument of adult S. solidus and L. intestinalis plerocercoids were determined. The possible role of macromolecular uptake in the economy of pseudophyllidean tapeworms is discussed with particular reference to growth and defence of an unencysted larval stage in the tissues of the intermediate host.

Electron microscope studies of Fasciola hepatica: III. Fine structure of the prostate gland

Abstract An ultrastructural study of the prostate gland of Fasciola hepatica shows it to be composed of numerous unicellular glands. These gland cells contain an extensive granular endoplasmic reticulum (GER) system parts of which are intimately associated with septum-like invaginations of the plasma membrane extending almost to the nucleus. Also associated with the GER are many Golgi complexes which secrete large electron-lucid carbohydrate-rich secretory vesicles. The secretion passes up the gland ducts along with a very dense granular and fibrillar material. The ducts have a peripheral microtubular skeleton and are tightly bound to the epithelium of the ejaculatory duct by septate desmosomes. Secretory vesicles are stored in the expanded ends of the ducts where they pass through the ejaculatory epithelium and their content is discharged by the bursting of their limiting membrane.