Iron-containing cells in the honey-bee (Apis mellifera). I. Adult morphology and physiology

Particulate iron was found within the trophocytes of the fat body of the adult honey-bee. These iron granules differed in their structure and composition from iron granules found in other biological systems. The granules had an average diameter of 0.32 +/− 0.07 micron and were composed of iron, calcium and phosphorus in a non-crystalline arrangement. The granules were apparently randomly distributed within the cytoplasm of the cells, and were not associated with any particular cellular organelle. Electron microscopy revealed the presence of cell junctions between the trophocytes. In tissues treated with colloidal lanthanum, 20-nm gaps were seen between the outer leaflets of the cells forming the cell junction. Physiological studies showed that these cells are electrically coupled, but the coupling ratio is low, as a result of extensive coupling to many cells.

Definitive Evidence for the existence of tight junctions in invertebrates

Extensive and unequivocal tight junctions are here reported between the lateral borders of the cellular layer that circumscribes the arachnid (spider) central nervous system. This account details the features of these structures, which form a beltlike reticulum that is more complex than the simple linear tight junctions hitherto found in invertebrate tissues and which bear many of the characteristics of vertebrate zonulae occludentes. We also provide evidence that these junctions form the basis of a permeability barrier to exogenous compounds. In thin sections, the tight junctions are identifiable as punctate points of membrane apposition; they are seen to exclude the stain and appear as election- lucent moniliform strands along the lines of membrane fusion in en face views of uranyl-calcium-treated tissues. In freeze-fracture replicas, the regions of close membrane apposition exhibit P-face (PF) ridges and complementary E-face (EF) furrows that are coincident across face transitions, although slightly offset with respect to one another. The free inward diffusion of both ionic and colloidal lanthanum is inhibited by these punctate tight junctions so that they appear to form the basis of a circumferential blood-brain barrier. These results support the contention that tight junctions exist in the tissues of the invertebrata in spite of earlier suggestions that (a) they are unique to vertebrates and (b) septate junctions are the equivalent invertebrate occluding structure. The component tight junctional 8- to 10-nm-particulate PF ridges are intimately intercalated with, but clearly distinct from, inverted gap junctions possessing the 13-nm EF particles typical of arthropods. Hence, no confusion can occur as to which particles belong to each of the two junctional types, as commonly happens with vertebrate tissues, especially in the analysis of developing junctions. Indeed, their coexistance in this way supports the idea, over which there has been some controversy, that the intramembrane particles making up these two junctional types must be quite distinct entities rather than products of a common precursor.

Cell membrane permeability defects demonstrated with colloidal lanthanum early in ischemic cell injury in dog heart

Lanthanum hydoxide, an electron-dense colloid with a nominal average particle size of 20 A, has been used for many years to label the extracellular space as a tracer applied during aldehyde fixation. If ischemic cell injury were to produce large defects in the sarcolemma, lanthanum would be expected to enter the cell. We studied the distribution of various preparations of lanthanum in thin free-hand slices of dog posterior papillary muscle after 0, 15, 40 or 240 minutes of severe ischemia caused by surgical occlusion of the circumflex coronary artery. Cell death regularly develops after 40 or more minutes of severe ischemia. Ionic lanthanum, lanthanum hydroxide according to Revel and Karnovsky and lanthanum silicate were used with and without pretreatment with alcian blue. Best results were obtained with freshly made solutions of La (NO)3 titrated carefully to pH 7.8, to prepare a clear fixative approximately 1% in lanthanum and 4%, in buffered glutaraldehyde.

EVIDENCE FOR A BLOOD-THYMUS BARRIER USING ELECTRON-OPAQUE TRACERS

In order to verify the existence of a blood-thymus barrier to circulating macromolecules, the permeability of the vessels of the thymus was analyzed in young adult mice using electron opaque tracers of different molecular dimensions (horseradish peroxidase, cytochrome c, catalase, ferritin, colloidal lanthanum). Results show that although blood-borne macromolecules do penetrate the thymus, their parenchyma] distribution is limited to the medulla of the lobe by several factors: (a) the differential permeability of the various segments of the vascular tree; (b) the spatial segregation of these segments within the lobe; (c) the strategic location of parenchymal macrophages along the vessels. The cortex is exclusively supplied by capillaries, which have impermeable endothelial junctions. Although a small amount of tracer is transported by plasmalemmal vesicles through the capillary endothelium, this tracer is promptly sequestrated by macrophages stretched out in a continuous row along the cortical capillaries and it does not reach the intercellular clefts between cortical lymphocytes and reticular cells. The medulla contains all the leaky vessels, namely postcapillary venules and arterioles. Across the walls of the venules, large quantities of all injected tracers escape through the clefts between migrating lymphocytes and endothelial cells; also the arterioles have a small number of endothelial junctions which are permeable to peroxidase, but do not allow passage of tracers of higher molecular weight. The tracers released by the leaky vessels penetrate the intercellular clefts of the medulla, but they never reach the cortical parenchyma, even at long time intervals after the injection. Therefore, a blood-thymus barrier to circulating macromolecules does exist, but is limited to the cortex. Medullary lymphocytes are freely exposed to blood-borne substances.

LOCALIZATION OF PERMEABILITY BARRIERS IN THE FROG SKIN EPITHELIUM

Ruthenium red and colloidal lanthanum were used to determine the site of the structural barriers to diffusion within the intercellular spaces of frog skin epithelium. Electron micrographs show that occluding zonules located at the outer border of the stratum corneum and at the outer layer of the stratum granulosum are true tight junctions since they are impermeable to these tracers. Measurement of 140La uptake by the living skin shows that lanthanum moves across the external surface of the skin readily, into and out of a compartment that has a limited capacity and is bounded on its internal side by a barrier impermeable to lanthanum. Examination of these skins with the electron microscope suggests that the compartment is localized between the external membrane of the cells at the outer layer of the s. granulosum and at the outermost surface of the skin. These observations and other findings described in the literature indicate that the site of the external high resistance barrier of the frog skin is localized at the outer border of the s. granulosum.

Vascular Transfer Cells in the Wheat Spikelet

Xylem and phloem transfer cells are present at the nodal regions where the sterile glumes, lemma, palea, and caryopsis are attached to the rachis and rachilla. The course and distribution of the vascular tissues and the structure of the transfer cells are described. Experiments with colloidal lanthanum tracer fed via the trans-piration stream have indicated that the walls of the transfer cells are relatively porous. The possibility that the transfer cells play an important role in the regula-tion of solute transfer to the developing grain is discussed.