Close association of centroacinar/ductular and insular cells in the rat pancreas.

Close contacts between endocrine insular cells and exocrine acinar, centroacinar and ductular cells occur frequently in the rat pancreas as seen by both light and electron microscopy. Islets of Langerhans are surrounded incompletely by a thin connective tissue capsule or mantle but numerous exocrine-endocrine cell contacts occur at the periphery, which is irregular with considerable "intermingling" of the two cell types. Centroacinar and ductular cells are seen to be in contact with all endocrine cell types but most commonly insulin-secreting B-cells. The basal surface of centroacinar cells in the region of contact may be extensive, sometimes with overlap of basal processes of these cells and their lateral extension between acinar and insular cells. The areas of contact contain no connective tissue or basal lamina and show no surface specializations. The presence of both the "open" and "closed" type of enteroendocrine cells within acini is confirmed, some also being in contact with centroacinar cells. The functional significance of these exo-endocrine cell contacts is discussed in terms of the endocrine-acinar portal system, possible direct paracrine secretion, compartmentalization within the islet, and the known effects of islet hormones on exocrine secretion. Also relevant is the developmental origin of islets from ductal tissue and the cellular origin of some tumours, e.g., insulinomas, from duct cells.

An unusual cylindrical body in interstitial cells of rat testis.

Testes of 6 rats were processed routinely for electron microscopy and, as an incidental finding, interstitial (Leydig) cells were found to contain bundles of unusual cylindrical bodies or macrotubules. These cytoplasmic structures were found in testes of only 2 of the 6 rats and varied in number from 2 to 346 per cell profile, usually in parallel array but with an irregular orientation in approximately 5% of the cells. The macrotubules were up to 11 microns long, showed a diameter of 130 nm with a wall of 18-20 nm and an inside diameter of about 94 nm. The wall appeared to be formed by two membranes 6-7 nm thick showing a unit membrane structure with a central electron-lucent space of 6 nm. In several instances, the wall membranes appeared corrugated, which may account for the apparent formation of the wall by a spiralling small tubule or tubules of 18 nm diameter. Several macrotubules showed continuity between their wall membranes and elements of endoplasmic reticulum and occasionally two adjacent macrotubules showed continuity at their ends in a U-form. Similar if not identical structures have been described previously in four studies of interstitial cells of the rat renal medulla, in one instance correlated with water-deprivation. They have been considered an alteration in the endoplasmic reticulum. Also, they have been reported in pig uterine glands in pregnancy. They may represent a cellular response to an unrecognized physiopathological state.

Postnatal development of sarcolemmal invaginations in right atrial myocardium of the rat.

Postnatal development of the sarcolemmal invaginations of right atrial cells of the rat has been studied using standard fixation combined with tannic acid mordanting. T tubules were seen to form at Z lines as simple tubular invaginations starting at the 14th postnatal day. T tubules were present in most cells by the 18th postnatal day but, as in the adult, were restricted to peripheral regions. Also, between the 16th and 18th postnatal day a proliferation of caveolae was seen, both as single vesicles and as complexes with up to a dozen caveolae sharing the same neck. The caveolar complexes persisted in the adult and did not seem to contribute significantly to the formation of the T tubules. Dyadic couplings were seen to become more abundant as T tubules and caveolae proliferated. These findings are discussed in relation to transsarcolemmal Ca2+ movements and excitation-contraction coupling during postnatal development.

Lanthanum as an intracellular stain for electron microscopy.

Results obtained after the normal aldehyde fixation of duodenal enterocytes for electron microscopy have been compared with results obtained when 0.1% Malachite Green or 10 mM lanthanum chloride had been added during aldehyde fixation. Sections were examined without further staining, and after counterstaining with lead citrate and uranyl acetate. In unstained sections, lanthanum-treated material showed improved contrast when compared to results from the other two methods. Also, after counterstaining, areas showing excellent contrast were much more frequent and more readily detected in the lanthanum-treated material. In the microvilli of enterocytes fixed in the presence of lanthanum, the plasmalemma-glycocalyx was defined more clearly and the results were more pleasing subjectively. When Malachite Green was present in the fixative, good contrast was observed more frequently than in routinely fixed tissues, but less often than in those treated with lanthanum. It is suggested that the addition of lanthanum chloride or Malachite Green to the fixative may prove useful in many ultrastructural studies.

The use of lanthanum chloride as a marker for intercellular junctions in rat exocrine pancreas.

A simple technique of perfusion and immersion of tissue in fixative containing lanthanum chloride as an extracellular tracer is described. In addition to functioning as a tracer, the lanthanum chloride appears to enhance electron staining. In rat exocrine pancreas, intercellular spaces between exocrine and centroductular cells were outlined clearly be electron dense material and, at cellular interfaces, spot desmosomes, gap junctions, and tight junctions were demonstrated. The technique proved simple and effective and should prove useful in studies of epithelial and other tissues.

The terminal web of the duodenal enterocyte.

The terminal web-brush border complex of rodent duodenal enterocytes has been studied by electron microscopy to investigate its structure in relation to currently accepted models of motility in this region. The main adherens zone is composed chiefly of a fine feltwork of 5 to 7 nm filaments, some of which originate in zonulae adherentes. In some cells, this is not a complete layer or sheet. Passing into it from its deep aspect are 10 nm tonofilaments, which also form the basal zone. The filament density in the basal zone is less than that of the adherens zone, and many of the tonofilaments are associated with spot desmosomes. The apical zone contains a loose meshwork of 5 to 7 nm filaments with more filaments lying adjacent to plasmalemmae of the zonula occludens. The core of each microvillus contains a bundle of 17 to 48 microfilaments, 5-7 nm in diameter, apparently attached to the apical plasmalemma and with some slender cross filaments between core filaments and the plasmalemma. In the main, these core bundles of microfilaments pass deeply into and often through the adherens zone of the terminal web where they terminate abruptly. Filaments of the terminal web appear to interconnect microfilaments of adjacent core bundles but without positive evidence of 'splaying' of microfilaments of a core bundle within the adherens zone. These findings are discussed in relation to movement of microvilli.

The fine structure of snake myocardium.

Snake myocardium has been studied using standard methods and tannic acid mordanting. In both atria and ventricle, cells were arranged in fascicles with little connective tissue, up to 20 cells per fascicle, and few differences between cells of atria and ventricle. Cell (fiber) size varied from 8 to 12 micrometers in the nuclear area with a few cells up to 14 micrometers in ventricle, and cells generally were spindle-shaped, tapering toward the extremities where relatively simple intercalated discs were seen. Few myofibrils per cell were present and poorly delineated. Transverse tubules were absent and sarcoplasmic reticulum was poorly developed and, apparently, even absent from some cells. All cells contained micropinocytotic vesicles associated with the surface plasmalemma and subsarcolemmal caveolae. In addition to intercalated discs, intercellular junctions showed spot desmosomes, small and sparse gap junctions and fasciae adherentes of two types, one associated with Z-bands, one independent of Z-bands. Nerve fibers and nerve endings were numerous, particularly in atrium. These findings are discussed in relation to the structure of mammalian myocardium.

T-tubules, couplings and myofibrillar arrangements in rat atrial myocardium.

Myocardial cells of rat atria have been studied using standard fixation and tannic acid mordanting. Cells were arranged in bundles or fascicles with little connective tissue. Fiber size varied, with complex cell junctions and in many fibers, particularly those of small diameter, no T-tubules or internal couplings were seen. Most cells, however, contained T-tubules, mainly coupled as triads at Z-lines but showing considerable variation in morphology. Direct continuity of T-tubules with the sarcolemma was demonstrated. Apparently unrelated to T-tubules and internal couplings, all cells showed the presence of peripheral couplings between sarcoplasmic reticulum and sarcolemma and, often, the subsarcolemmal element of reticulum was observed to pass internally in the sarcoplasm from a peripheral coupling. Also present were subsarcolemmal caveolae. The occasional presence within a single cell of myofibrils orientated in different planes was noted also. The findings are discussed in relation to the role of T-tubules and couplings in excitation-contraction.

Rat retinal blood vessels.

The largest arteries in the rat retina are the arterioles in the nerve fiber layer adjacent to the optic papilla. They are 50 to 100 micrometer in diameter, have an incomplete internal elastic lamina and usually a single layer of smooth muscle. Smaller arterioles of 10 to 50 micrometer have no internal elastic lamina and the media is formed by one or two layers of slender smooth muscle cells. In these vessels, myoendothelial junctions and close contact areas between smooth muscle cells are numerous. Capillaries are present in all layers of the rat retina and from plexuses in the nerve fiber, outer plexiform and exterior part of the inner plexiform layers. In nearly all capillaries, pericytes and their processes from a single layer external to the endothelium with numerous contact points or zones between endothelium and pericytes without any intervening basal laminar material. Areas of close contact between adjacent pericyte processes are frequent. The possible functions of myoendothelial junctions and pericyte-endothelial contacts in relation to vessel tone, mechanical stabilization and metabolic exchange are discussed.

Sarcotubules and subsarcolemmal caveolae and their continuity with the sarcolemma in frog striated muscle.

After using tannic acid mordanting, T-tubules and subsarcolemmal caveolae show a dense staining with lead citrate, with some increase in contrast also of the sarcolemma, in frog muscle. While a few T-tubules show direct continuity with the extracellular space, the majority open indirectly via caveolae. Caveolae lie immediately beneath the sarcolemma, mainly in a single row, and are more numerous in relation to I-bands.

Smooth muscle cells in the rat testicular capsule: a developmental study.

This study of the postnatal development (from 1 to 60 days) of smooth muscle elements in the rat testicular capsule has demonstrated that while such elements are identifiable by light microscopy at 30 days, myocytes are present at birth as seen by electron microscopy. The differentiation of smooth muscle from birth to 30 days has been described, by which time it is of adult morphology and content. Perhaps significantly, it is at 30 days that the testis achieves a scrotal position, although sexual maturity does not occur until about 60 days. Presumably, at 30 days the testicular capsule of the rat is capable of the spontaneous contractions which are known to occur in the adult and which are assumed to aid the transport of non-motile spermatozoa from the testis to the spididymis. The presence of occasional striated muscle fibers in the rat testicular capsule as reported previously has not been confirmed by this investigation, although their possible origin is discussed.

Masking of pleomorphic glycogen sites by methanolic uranyl acetate.

Rat liver tissue was fixed in 2.5% glutaraldehyde buffered with cacodylic acid (pH 7.3) for 2 hr, washed twice in buffer, and postfixed in 2% osmium tetroxide at 4 C for 1 hr. The tissue then was dehydrated, infiltrated with and embedded in Epon by routine procedures. The ultrathin sections from this tissue, when stained with spectroscopic grade methanol saturated with uranyl acetate (SMUA) for 1 min followed by aqueous lead citrate (PbCi) (Reynolds 1963) for 5 min at room temperature, showed a uniform staining of all major cellular components except glycogen. The SMUA appeared to be specific for ribonuceloprotein granules, rendering them more prominent in the cytoplasm due to the lack of glycogen staining. The question of glycogen removal from the sections due to SMUA treatment was evulated using various extractions and staining methods. It appeared that SMUA pretreatment alters the subsequent binding ability of lead salts, resulting in lack of glycogen staining, although it does not remove the glycogen from the sections.

Microfibrillar structures in the nucleus and cytoplasm of amoeba proteus.

The presence of microfibrillar structures in the nucleus and cytoplasm of Amoeba proteus has been described after glutaraldehyde and osmium fixation. The possible roles of cytoplasmic microfibrils in the contraction process of amoeba and nuclear microfibrils in the formation of the honeycomb nuclear lamina are discussed.

Amoeba proteus: the nuclear periphery.

This study extends previous work on the nuclear envelope and associated structures. It illustrates that the cylindrical structures of the honeycomb lattice are not attached to the nuclear envelope, although generally perpendicular and closely apposed to it, and that there is a complex arrangement of fibrillar material between the cylinders of the lattice. The relationship of nuclear helices to these structures is described and the possible mode of their transfer from nucleus to cytoplasm is discussed.