Richard L. Wood

The cell junctions of hydra as viewed by freeze-fracture replication.

The cell junctions of hydra have been examined in freeze-fracture replication. Septate junctions appear as linear arrays of particles and complementary furrows containing fewer particles. In the epidermis, the particles are most numerous on the cytoplasmic fracture face (A-face), and the furrows are most obvious on the external fracture face (B-face). The polarity of particle distribution is reversed in the gastrodermis. Gap junctions are of the “B” type, having an aggregation of particles arranged in a plaque on the B fracture face. The particles are embedded in a matrix material. A complementary image appears on the A fracture face. Focal aggregates of particles appear on the A fracture face of epidermal myoid processes. No complementary B-face profile of pits has been identified. These observations support the view that the septate junction of hydra corresponds to a tight and intermediate junction of higher organisms and that the B-type gap junction is strongly adhesive. The significance of the focal particle aggregates is not known, but they may represent a punctate gap junction, a special receptor site, or an anchorage site for intracellular components.

Hormonal Support of Lacrimal Function, Primary Lacrimal Deficiency, Autoimmunity, and Peripheral Tolerance in the Lacrimal Gland

Several causes of lacrimal insufficiency have been recognized, including Sjögrens syndrome and other immune-related processes as well as a disparate group of non-immune related disorders. However, the mechanisms underlying primary lacrimal deficiency (PLD), the most common cause of dry eye, have remained obscure. After summarizing mechanisms of lacrimal secretion and stimulus-secretion coupling, the authors review the thesis that optimal lacrimal gland function depends on a hormonal milieu in which androgens play a crucial role. According to this thesis, simple acquired PLD results when bioavailable androgen levels decrease below critical values. However, it is noted that PLD also may be complicated by local autoimmune processes, and hypothetical pathways leading to such processes are discussed. Cell death following withdrawal of hormonal support may lead to processing and presentation of parenchymal cell antigens. Normal intracellular membrane traffic patterns may cause acinar cells to secrete autoantigens into the interstitium. When acinar cells have been induced to express major histocompatibility complex Class II molecules, their intracellular membrane traffic may allow them to process and present autoantigens, essentially mimicking the functions of professional antigen presenting cells. The possibility is discussed that perturbations of the spectra of released and presented autoantigens upset the equilibria of idiotypic networks arising to establish peripheral tolerance. The resulting incremental increases in lymphocytic infiltration are suggested to represent essentially cryptic autoimmune processes which may impair lacrimal secretory function and regeneration. Failure to establish peripheral tolerance is predicted to permit unrestrained CD(4) cell proliferation and an environment favoring B cell activation. Recruitment of B lymphocytes, perhaps in events influenced by re-activated viruses, is predicted to lead to Sjögrens autoimmunity as recognized by stringent diagnostic criteria. Finally, the possibility is discussed that androgen supplementation or hormone replacement therapy might prevent simple PLD and avoid the initiation of autoimmune processes.

MHC class II molecules, cathepsins, and La/SSB proteins in lacrimal acinar cell endomembranes

Sjögrens syndrome is a chronic autoimmune disease affecting the lacrimal glands and other epithelia. It has been suggested that acinar cells of the lacrimal glands provoke local autoimmune responses, leading to Sjögrens syndrome when they begin expressing major histocompatibility complex (MHC) class II molecules. We used isopycnic centrifugation and phase partitioning to resolve compartments that participate in traffic between the basolateral membranes and the endomembrane system to test the hypothesis that MHC class II molecules enter compartments that contain potential autoantigens, i.e., La/SSB, and enzymes capable of proteolytically processing autoantigen, i.e., cathepsins B and D. A series of compartments identified as secretory vesicle membranes, prelysosomes, and microdomains of the trans-Golgi network involved in traffic to the basolateral membrane, to the secretory vesicles, and to the prelysosomes were all prominent loci of MHC class II molecules, La/SSB, and cathepsins B and D. These observations support the thesis that lacrimal gland acinar cells that have been induced to express MHC class II molecules function as autoantigen processing and presenting cells.Sjögrens syndrome is a chronic autoimmune disease affecting the lacrimal glands and other epithelia. It has been suggested that acinar cells of the lacrimal glands provoke local autoimmune responses, leading to Sjögrens syndrome when they begin expressing major histocompatibility complex (MHC) class II molecules. We used isopycnic centrifugation and phase partitioning to resolve compartments that participate in traffic between the basolateral membranes and the endomembrane system to test the hypothesis that MHC class II molecules enter compartments that contain potential autoantigens, i.e., La/SSB, and enzymes capable of proteolytically processing autoantigen, i.e., cathepsins B and D. A series of compartments identified as secretory vesicle membranes, prelysosomes, and microdomains of the trans-Golgi network involved in traffic to the basolateral membrane, to the secretory vesicles, and to the prelysosomes were all prominent loci of MHC class II molecules, La/SSB, and cathepsins B and D. These observations support the thesis that lacrimal gland acinar cells that have been induced to express MHC class II molecules function as autoantigen processing and presenting cells.

The anchoring of nematocysts and nematocytes in the tentacles of hydra

The ultrastructure of nematocyte junctions in hydra tentacles was examined by electron microscopy using thin sections and freeze-fracture replicas. The apical region of a nematocyte is joined to the battery cell by septate junctions. A second basal junctional relationship simultaneously anchors a nematocyte to a surrounding battery cell and the underlying mesoglea. We term this basal junction the NBM complex. Each nematocyte contains 10- to 14-nm cytoplasmic filaments basally; these do not bind NBD-phallacidin, an actin-specific probe. A tongue of battery cell cytoplasm containing microtubules intervenes between a nematocyte and the mesoglea. A condensation of extracellular filaments extends from the battery cell into the underlying mesoglea. We interpret the NBM complex as a tandem arrangement of a fascial desmosome and a fascial hemidesmosome. Nematocyte-free hydra possess epidermal cell-mesogleal junctions resembling the NBM complex, suggesting that sites for nematocyte positioning may be predetermined.

Ultrastructure of the coelomic lining in the podium of the starfish Stylasterias forreri

SummaryUltrastructural examination of the podium of the asteroid echinoderm Stylasterias forreri reveals that cells of the coelomic epithelium and cells of the retractor muscle are, in fact, components of a single epithelium. The basal lamina of this unified epithelium adjoins the connective tissue layer of the podium.The principal epithelial cells in the coelomic lining are the flagellated adluminal cells and the myofilament-bearing retractor cells. Adluminal cells interdigitate extensively with each other and form zonular intermediate and septate junctions at their apicolateral surfaces. The adluminal cells emit processes which extend between the underlying retractor cells and terminate on the basal lamina of the epithelium. Retractor cells exhibit unregistered arrays of thick and thin myofilaments. The periphery of the retractor cell is characteristically thrown into keel-like folds which interdigitate with the processes of neighboring cells. Specialized intermediate junctions bind the retractor cells to each other and anchor the retractor cells to the basal lamina of the epithelium. The retractor cells are not surrounded by external laminae or connective tissue envelopes.It is concluded that the coelomic lining in the podium of S. forreri is a bipartite epithelium and that the retractor cells of the podium are myoepithelial in nature. There are no detectable communicating (gap) junctions between the epithelial cells of the coelomic lining.

Subsurface cisterns in the Purkinje cells of cerebellum of Syrian hamster.

SummaryThree types of subsurface cisterns were observed in Purkinje cells of the cerebellum of the Syrian hamster. The type-1 cisterns are subsynaptic, related to axosomatic synapses, and are separated from the postsynaptic cell membranes with distances of 400–800 Å. These are probably modified rough surfaced endoplasmic reticulum. The type-2 cisterns are closely apposed to the surface membranes of Purkinje cells, and have very little intracisternal space except at the dilated lateral edges. The type-3 cisterns are similar in structure to the type-2 cisterns but in addition are closely associated with mitochondria. The type-2 and type-3 cisterns appear between one and two weeks after birth and are still present in adults, having almost the same frequency of occurrence. Thin cell processes opposite the type-2 and type-3 cisterns are considered to be glial cell processes. The morphological details of these types of subsurface cisterns are described here, and their possible functional significance is briefly discussed.

The fine structure of the hypostome and mouth of hydra

SummaryThe hypostome and mouth of fresh-water Hydra were examined by scanning electron microscopy. The external surface of the hypostome possesses cnidocils, possibly sensory hairs, and small spiny protrusions surrounding the mouth; the internal surface has cylindrical microvilli, free flagella and adherent flagella. The adherent flagella are most numerous close to the mouth where they cause the cell surface to appear smooth when viewed at low magnifications. Free flagella and leaf-like microvilli increase in prominence towards the tentacles and enter on proper. The edge of the mouth has an abrupt boundary marking the apposition of epidermal and gastrodermal cells. A transitional groove occurs at the boundary and the cells underlying the groove are smaller than those on other regions of the hypostome. The transition groove may represent a site of cell loss in normal cell turnover. Some of the small underlying cells may represent nervous elements involved in regulating hypostome activity during the feeding reation.

Stimulation-Associated Redistribution of Na,K-ATPase in Rat Lacrimal Gland

SummaryTo test the possibility that stimulation of secretion leads Na,K-ATPase to be recruited from cytoplasmic pools and inserted into basal-lateral plasma membranes, we surveyed the subcellular distributions of Na, K-ATPase in resting and stimulated fragments of rat exorbital lacrimal gland. After a two-dimensional separation procedure based on differential sedimentation and density gradient centrifugation, we defined sixdensity windows, which differ from one another in their contents of biochemical markers. The membranes equilibrating inwindow I could be identified as a sample of basal-lateral membranes; in resting preparations these membranes contained Na,K-ATPase enriched 16.6-fold with respect to the initial homogenates.Windows II throughVI contained various cytoplasmic membrane populations; these accounted for roughly 80% of the total recovered Na,K-ATPase activity. Thirty-minute stimulation with 10 μm carbachol caused a 1.4-fold increase (P<0.05) in the total Na,K-ATPase content ofwindow I; this increase could be largely accounted for by a 1.7-fold decrease in the total Na,K-ATPase content ofdensity window V. Acid phosphatase activity also redistributed following stimulation, but it was recruited from a different source, and it was inserted into membranes equilibrating inwindows II andIII as well as into the membranes ofwindow I.

Uveoscleral permeability to intracamerally infused ferritin in eyes of rabbits and monkeys

SummaryThe permeability of the uveoscleral outflow pathway from the anterior ocular chamber was examined in rabbit and monkey eyes using anionic ferritin as a tracer. Ferritin, infused intracamerally, had ready access to the choroidal interstitium, and the degree of penetration was generally correlated with the time and pressure relationships during infusion. In both species, there were accumulations of tracer in intercellular spaces at the lamina fusca, but tracer was also present in the sclera. Thus, in contrast to the situation in the eyes of hamsters, the uveoscleral outflow pathway in the eyes of rabbits and monkeys includes the choroidal connective tissue and allows passage of relatively large molecular weight substances.

The septate junction limits mobility of lipophilic markers in plasma membranes ofHydra vulgaris (attenuata)

SummaryFluorescent lipophilic probes were used to study the role of septate junctions in maintaining distinct apical and basolateral domains of plasma membranes in epithelial cells of hydra. In short-term experiments, a 16-carbon chain aminofluorescein probe (AFC16) was localized to the apical plasma membranes of ectodermal and endodermal epithelial cells when presented in the culture medium or injected into the gastric lumen, but did not demarcate basolateral membranes. In longer term experiments, basolateral membranes were stained and the staining was independent of temperature conditions. A dual 18-carbon chain indocarbocyanine probe (DiIC18) gradually diffused across the septate junction to label basolateral membranes at room temperature, but not at 4°C. DiIC18 also filled and stained certain mounted nematocytes. The results indicate that in hydra, lipophilic probes may be limited in mobility within the membrane plane by the septate junctions in a manner similar to vertebrate tight junctions, and that apical membranes of mature nematocytes are differentially permeable.