Development of P-protein in sieve elements ofMimosa pudica

Summary The P-protein in sieve elements of leaves ofMimosa pudica L. is first discernible as fine fibrous material which forms homogeneous aggregates. Ribosomes, rough endoplasmic reticulum, and dictyosomes with associated vesicles occur in the cytoplasm surrounding the aggregates. The plastids and mitochondria are in a parietal position in the parts of the cell where the nascent P-protein accumulates. In a later stage, the fibrillar material is organized into a three-dimensional system of five- and six-sided elongated compartments. The corners of the compartments appear solid at first, then they become electron lucent in the center and assume tubular form. Aggregates of mature P-protein tubules usually occur near the compartmentalized system. Tubules in pentagonal or hexagonal arrangements may be present in the aggregates and may be partly interconnected. The conclusion was drawn that the P-protein tubules are assembled at the corners of compartments within a continuous orderly system. The fully formed tubules occur first in aggregates, the P-protein bodies. Later the aggregates become loose and partly dispersed. Many of the dispersed tubules assume a loose, extended, helical form characteristic of P-protein in older sieve elements.This work was supported in part by National Science Foundation grant GB-5506. I am also grateful to MissHatsume Kosakai and Mr.Robert H.Gill for technical assistance.     

Starvation effects on the ultrastructure of amoeba mitochondria

Summary Mitochondria in non-starved giant amoebae, Pelomyxa carolinensis, contain tubules lying at random in the matrix. Many mitochondria in starved amoebae have enlarged tubules aligned in a zigzag pattern. Tubules within the zigzag region are separated by very little matrix material. Some of these altered mitochondria are found in 70% of amoebae starved for only 24 hours, and in nearly all P. carolinensis starved for 8 days or longer. The percentage of such altered mitochondria increases from zero in most well-fed amoebae, to about 60% after two weeks of continuous starvation. Most P. carolinensis starved at 25° C survive less than three weeks. Microfilament bundles are observed in the matrix of some mitochondria in amoebae starved for more than two days.Work supported by the U. S. Atomic Energy Commission.The authors acknowledge the assistance of Miss Doris Jean Buer and Miss Patricia Ann Sustarsic.     

Studies on the receptors in Ciona intestinalis

Summary A photoreceptor type structure not previously described has been found in the dorsal wall of the cerebral vesicle of the tadpole larva of Ciona intestinalis. The membranes of this receptor are organised as tubules some 60–100 nm in diameter and up to 1.5 m long. The tubules are confined in bundles about 1.5 m in diameter, which extend from the cell surface into the cavity of the cerebral vesicle. These tubules are similar to those in the rhabdomeric type of photoreceptor. However, in the cells from which the tubule processes arise are structures typical of the bases of cilia, and found in ciliary type photoreceptors.I should like to thank Professor J. Z. Young, F. R. S. for his continuing encouragement and help, and Dr. R. Bellairs for the use of electron microscope facilities. Mr. R. Moss and Mrs. J. Hamilton gave excellent technical assistance.     

Synapses in the cerebral ganglion of adult Ciona intestinalis

Summary Electron microscopy of the synaptic morphology of synapses in the cerebral ganglion of the adult ascidian (sea squirt) Ciona intestinalis reveals that the synapses are restricted to the central neuropil of the ganglion. Many of the synapses show a polarity of structure such that pre and post synaptic parts can be identified. The vesicles in the presynaptic bag are of two main diameters 80 and 30 nm respectively. The large vesicles have electron dense contents that vary both in their capacity and dimensions.The pre and postsynaptic membranes are more electron dense than the surrounding membranes, but they are only slightly thicker. Both the pre and post synaptic membranes have electron dense dots some 10 nm in diameter associated with their cytoplasmic surfaces. Sometimes the presynaptic membrane has larger peg-like projections between the vesicles. Associated with the post synaptic membrane are tubules some 10 nm in diameter. These tubules may be the dots cut obliquely.The synaptic cleft material is more electron dense than the surrounding intercellular material, and in it there is a dense line made up of granules about 3–5 nm in diameter. This dense line is usually mid way between the pre and post synaptic membranes, but may be nearer the postsynaptic membrane.No tight junctions between adjacent nerve process profiles have been observed.I wish to thank Professors J. Z. Young, F. R. S. and E. G. Gray for much advice and encouragement, also Dr. R. Bellairs for the use of electron microscope facilities and Mr. R. Moss and Mrs. J. Hamilton for skillful technical assistance.     

A ribonucleoprotein inclusion body in Entamoeba invadens

Summary Cytochemical tests have shown that the chromatoid bodies contain mainly ribonucleic acid and protein. There is no evidence to suggest the presence of any desoxyribonucleic acid, lipid, glycogen, neutral fat, or metaphosphate. The cyclic changes in the chromatoid body have been studied by means of light and electron microscopy and estimations of the total RNA present. The results indicate that the chromatoid bodies arise by a process of aggregation of small groups of 250–300 A units form polycrystalline masses in precystic and early cysts. At the same time there is a steady rise in the amount of RNA present. In the maturing cysts, the crystalline masses fragment into separate particles, but there is no corresponding fall in the amount of RNA. The significance of these results in relation to the previous literature on chromatoid bodies and similarly named cellular inclusions in spermatogenic and plant cells, as well as in the protozoa, is discussed. Attention is drawn to the striking similarity of composition, formation, development and ultimate fate of these various inclusion bodies but as the terminology is still confused it is proposed that the term chromatoid body be retained solely for particulate or lamellar ribonucleo-protein accumulations. Acknowledgements. Most of this study was presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Edinburgh. I should like to express my deep appreciation to Prof. M. M. Swann F. R. S. for his interest and helpful guidance in this work. I am grateful to the Melville Trust for Cancer Research who provided the electron microscope.     

Studies on the fine structure of teleost blood cells

Summary Electron microscopic study of nucleated erythrocytes of the goldfish, Carassius auratus, reveals the microtubular elements comprising the marginal band which encircles the cell. Six to ten units are visible at each pole of the cell, immediately within the plasmalemma. Each tubular unit is composed of an electron dense membrane enclosing a less dense core. Cross-sectional units average 264 Å outer diameter, whereas tubules measured in longitudinal sections average 237 Å.The functions of the microtubules of the marginal bands are analyzed in view of Meves' original interpretation of maintenance of the discoidal form of the nucleated erythrocyte, and the more recent investigations in cell physiology of Trotter and Tilney. It is proposed that the microtubules possess a dual function: the support of the cell which is attributed to the hydroelastic properties of the turgid microtubules resulting from intratubular hydrostatic pressures; and the intracellular transport of materials via the intratubular fluid. The microtubules may, therefore, be considered as a skeletal system and part of an intracellular circulatory system.This project was supported by grants 2 G-895 and 2 G-505 from the United States Public Health Service.     

An electron microscopical study of the ventral nerve cord of the leech

Summary Three types of fibrillar structure can be seen with the electron microscope in nerve cells of the vental nerve cord of the leech: the neurofibrillar bundles, the tubules and the tonofibrils. In neuroglial cells only the tonofibrils are present. The three types are structurally distinct, and, contrary to past suggestions, there is no evidence that neurofibrillar bundles may consist of tightly packed or badly fixed tubules.In vertebrates the electron microscope reveals bundles of discrete neurofilaments that form the basis for the argyrophilic neurofibrillae seen by light microscopy. Each neurofilamentous unit appears as a dot in cross section. In contrast, in the leech, the electron microscope shows compact fibrillar bundles that clearly correspond to the neurofibrils described by light microscopists. These bundles are made up of closely packed units rather than discrete filaments and where the units occur singly they are seen to have an angular or stellate outline in cross section. To make this distinction clear these have been termed neurofibrillar bundles rather than neurofilaments.Attachment plaques occur in both neurons and neuroglia. These plaques have tonofibrils attached, and the glial tonofibrils are far more numerous than the neuronal tonofibrils. The glial fibrils are identical with the tonofibrils in the glial cells.The attachment plaques are invariably related to an extracellular space that contains material identical with the basement membrane. This material is continuous, by a complex system of channels and diverticulae, with the outer basement membrane in the neuron packets, but forms isolated patches in the other parts of the nervous system.We are grateful to Prof. J. Z. Young, F. R. S., for his encouragement to Mrs. Astafiev for the drawings, to Miss B. Shirra and Mr. K. Watkins for technical assistance and to Mr. S. Waterman for photography.     

Morphological pattern of response after administration of procarbazine: Alteration of specific cell associations during the cycle of the seminiferous epithelium of the rat

Procarbazine, an anti-cancer agent, administered intraperitoneally to adult, male rats induced a characteristic morphological pattern of response in the seminiferous epithelium. Seminiferous tubules of rats receiving 100 mg/kg procarbazine and higher dosages displayed spermatids which were less mature than those normally found within seminiferous tubules which show a particular cell association. Early spermatids in steps 1–7 of spermiogenesis appeared arrested in their development and were present in cell associations which had advanced normally. The most probable cause of this apparent germ cell arrest was a retardation of acrosomal development since procarbazine is known to affect RNA and consequently protein synthesis. Other features which indicated defective RNA synthesis were the presence of abnormal spermatid nucleoli and abnormally configured chromatoid bodies. This study demonstrates, in contrast to what is indicated by present dogma, that apparent and temporary germ cell arrest may occur under certain deleterious conditions. It also illustrates that particular cell types within a cell association may be out of synchrony with the remainder of the cells in a cell association.     

The integument of the Queensland fruit fly,Dacus tryoni (Frogg.)

Summary Testes of the Japanese freshwater turtle Clemmys japonica Temmnick et Schlegel were fixed in 3% potassium permanganate buffered to pH 7.2 with Veronal-acetate buffer, and thin sections of the tissue, embedded in epoxy Epon resin, were studied under the electron or light microscope.At the early stage of differentiation of the spermatid, the cytoplasm contains a few mitochondria provided with cristae which are oriented transversely or longitudinally. As the differentiation of spermatids proceeds, the mitochondrion has been modified into a cupshaped body with a wall consisting of several concentric layers. Such body has been referred to the mitochondrial lamellar body. The formation of such a body is mainly attributed to the mitochondrial cristae, and subsequently to the membrane system of the endoplasmic reticulum. In a more advanced stage of differentiation, the mitochondrial lamellar bodies appear wrapped around a bundle of tail filaments, and seem to present a very wide surface available for the localization of organized enzyme systems to facilitate the motion of spermatozoa.Prior to the formation of the mitochondrial lamellar bodies, the Golgi apparatus has been reorganized into a peculiar body with a floral appearance, consisting of numerous tubular elements, and revealing to be positive in PAS-reaction. The body has been designated as the tubular body which has never been demonstrated in any spermatogenic cells through animal kingdom.One to three tubules oval in cross section, approximately 430 × 700 Å in diameter, have been found in the nucleoplasm along the longitudinal axis of a greatly elongated, cone-shaped nucleus of the spermatid. The tubules open on the apex surface of the nucleus, but they are not encountered in the acrosome. A possible physiological significance of the tubules has been discussed in view of the function of the acrosomal tubules in the decapod and other species spermatozoa as well as on the basis of the metabolism of nucleus.This study was supported by Grant GM-8327 from the United States Public Health Service.We wish to express our gratitude to Dr. B. A. Afzelius, Wenner-Gren Institute, University of Stockholm, for his valuable suggestion to the present work.     

Silver ion (Ag+)-Induced increases in cell membrane K+ and Na+ permeability in the renal proximal tubule: Reversal by thiol reagents

Summary The initial mechanisms of injury to the proximal tubule following exposure to nephrotoxic heavy metals are not well established. We studied the immediate effects of silver (Ag⁺) on K⁺ transport and respiration with extracellular K⁺ and O₂ electrodes in suspensions of renal cortical tubules. Addition of silver nitrate (AgNO₃) to tubules suspended in bicarbonate Ringer's solution caused a rapid, dose-dependent net K⁺ efflux (K _m =10^–4 m,V _max=379 nmol K⁺/min/mg protein) which was not inhibited by furosemide, barium chloride, quinine, tetraethylammonium, or tolbutamide. An increase in the ouabain-sensitive oxygen consumption rate (QO₂) (13.9±1.1 to 25.7±4.4 nmol O₂/min/mg,P<0.001), was observed 19 sec after the K⁺ efflux induced by AgNO₃ (10^–4 m), suggesting a delayed increase in Na⁺ entry into the cell. Ouabain-insensitive QO₂, nystatin-stimulated QO₂, and CCCP-uncoupled QO₂ were not significantly affected, indicating preserved function of the Na⁺, K⁺-ATPase and mitochondria. External addition of the thiol reagents dithiothreitol (1mm) and reduced glutathione (1mm) prevented and/or immediately reversed the effects on K⁺ transport and QO₂. We conclude that Ag⁺ causes early changes in the permeability of the cell membrane to K⁺ and then to Na⁺ at concentrations that do not limit Na⁺, K⁺-ATPase activity or mitochondrial function. These alterations are likely the result of a reversible interaction of Ag⁺ with sulfhydryl groups of cell membrane proteins and may represent initial cytotoxic effects common to other sulfhydryl-reactive heavy metals on the proximal tubule.     

Apical Nonspecific Cation Channels in Everted Collecting Tubules of Potassium-Adapted Ambystoma

We observed intermediate conductance channels in approximately 20% of successful patch-clamp seals made on collecting tubules dissected from Ambystoma adapted to 50 mm potassium. These channels were rarely observed in collecting tubules taken from animals which were maintained in tap water. Potassium-adaptation either leads to an increase in the number of channels present or activates quiescent channels. In cell-attached patches the conductance averaged 30.3 ± 2.4 (9) pS. Since replacement of the chloride in the patch pipette with gluconate did not change the conductance, the channel carries cations, not anions. Notably, channel activity was observed at both positive and negative pipette voltages. When the pipette was voltage clamped at 0 mV or positive voltages, the current was directed inward, consistent with the movement of sodium into the cell. The pipette voltage at which the polarity of the current reversed (movement of potassium into the pipette) was −29.6 ± 6.5(9) mV. Open probability at 0 mV pipette voltage was 0.08 ± 0.03 and was unaffected when the apical membrane was exposed to either 2 × 10⁻⁶ or 2 × 10⁻⁵ m of amiloride. Exposure of the basolateral surface of the tubule to a saline containing 15 mm potassium caused a significant increase (P less than 0.001) in the open probability of these channels to 0.139 ± 0.002 without affecting the conductance of the apical channel. These data illustrate the presence of an intermediate conductance, poorly selective, amiloride-insensitive cation channel in native vertebrate collecting tubule. We postulate that, at least in amphibia, this channel may be used to secrete potassium. Received: 14 January 2000/Revised: 16 June 2000     

Characterization of the Na+/H+ Exchanger in the Luminal Membrane of the Distal Nephron

In the rabbit as well as the rat, a Na⁺/H⁺ exchanger is expressed in the apical membrane of both the proximal and distal tubules of the renal cortex. Whereas the isoform derived from the proximal tubule has been extensively studied, little information is available concerning the distal luminal membrane isoform. To better characterize the latter isoform, we purified rabbit proximal and distal tubules, and examined the ethylpropylamiloride (EIPA)-sensitive ²²Na uptake by the luminal membrane vesicles from the two segments. The presence of 100 μm EIPA in the membrane suspension decreased the 15 sec Na⁺ uptake to 75.70 ± 4.70% and 50.30 ± 2.23% of the control values in vesicles from proximal and distal tubules, respectively. The effect of EIPA on 35 mm Na⁺ uptake was concentration dependent, with a IC₅₀ of 700 μm and 75 μm for the proximal and distal luminal membranes. Whereas the proximal tubule membrane isoform was insensitive to cimetidine and clonidine up to a concentration of 2 mm, the 35 mm Na⁺ uptake by the distal membrane was strongly inhibited by cimetidine (IC₅₀ 700 μm) and modestly inhibited by clonidine (IC₅₀ 1.6 mm). The incubation of proximal tubule suspensions with 1 mm (Bu₂) cAMP decreased the 15-sec EIPA-sensitive Na⁺ uptake by the brush border membranes to 24.1 ± 2.38% of the control values. Unexpectedly, the same treatment of distal tubules enhanced this uptake by 46.5 ± 10.3%. Finally, incubation of tubule suspensions with 100 nm phorbol 12-myristate 13-acetate (PMA) decreased the exchanger activity to 58.6 ± 3.04% and 79.7 ± 3.21% of the control values in the proximal and distal luminal membranes, respectively. In conclusion, the high sensitivity of the distal luminal membrane exchanger to various inhibitors, and its stimulation by cAMP-dependent protein kinase A, indicate that this isoform differs from that of the proximal tubule and probably corresponds to isoform 1. Received: 6 March 1998/Revised: 6 July 1998     

Effect of amiloride on the apical cell membrane cation channels of a sodium-absorbing,potassium-secreting renal epithelium

Summary The effect of the K-sparing diuretic amiloride was assessed electrophysiologically in the isolated cortical collecting tubule of the rabbit, a segment which absorbs Na and secretes K. Low concentrations of amiloride in the perfusate caused a rapid, reversible, decrease in the magnitude of the lumen negative transepithelial potential difference,V _te, transepithelial conductanceG _te, and equivalent short-circuit current,I _sc, with an apparentK _1/2 of approximately 7×10^–8 m. The effects of a maximum inhibitory concentration of amiloride (10^–5 m) were identical to those observed upon Na removal from lumen and bath (Na removal from the bath alone has no effect). Removal of Na in the presence of 10^–5 m amiloride had no affect onV _te,G _te, orI _sc, and is consistent with the view that amiloride blocks the Na conductive pathways of the apical cell membrane. Further, in the absence of Na, the subsequent addition of amiloride had no influence. In tubules where active Na absorption was either spontaneously low, or abolished by removal of Na from lumen and bath, the elevation of K from 5 to 155 meq/liter in the perfusate caused a marked change of theV _te in the negative direction and an increase in theG _te. These effects could be attributed to a high K permeability of the apical cell membrane and not of the tight junctions. Amiloride (10^–5 m) had no effect on these responses to K. It is concluded that amiloride selectively blocks the apical cell membrane Na channels but has no effect on the K conductive pathways(s). This selective nature of amiloride may indicate that Na and K are transported across the apical cell membrane via separate conductive pathways.     

Marine Gammaridea (Crustacea: Amphipoda) from the Indian region; family — Ampeliscidae

The paper deals with 4 species of ampeliscid amphipods. For the first time among ampeliscids, incomplete coalescence of urosomites 2 and 3 is noticed in two of these viz. Ampelisca cyclops Walker and Amplisca scabripes Walker. It is suggested that this unique feature may be taken note of by those coming across this genus so that eventually we may be able to bring about a division of the genus Ampelisca. The study supports Pirlot's (1936) observation on the validity of A. chevreuxi Walker and points out that the distinction made by J. L. Barnard (1960) for separating this species from A. zamboangae Stebbing is only a case of sexual dimorphism.     

On orderly domains of particles associated with cytomembranes during spermatogenesis in Helix aspersa

Summary In an electron microscopic study of the maturation of the spermatid of the snail Helix aspersa, it was found that: 1. there is persistence and utilization of the mitochondrial architecture in the formation of the middle piece, and 2. particles originate in orderly domains within the pseudomatrix of the mitochondria. Other orderly domains of cytomembranes (Golgi system and peripheral tubules of Grassé) are discussed.Dedicated to Professor Friedrich Wassermann with admiration and affection on the occasion of his eightieth birthday.Work supported by U.S. Atomic Energy Commission.     

Der O2-Austausch zwischen Capillaren und Gewebe I. O2-Konzentrationsprofile in Blutcapillaren und deren Umgebung

Summary The differential equations valid for technical heat exchangers can also describe the O₂ exchange in the blood capillaries and the exchange of molecules like THO and acetamid in the renal tubules. Differences in the boundary conditions occur, however. Hence, these differential equations were resolved for the corresponding boundary conditions. The results permit us to conclude that the concentration profiles occurring in the capillaries and renal tubules, as a result of diffusion in the capillary cross-section, can, generally speaking, be disregarded for the following reason: Although the differences in partial pressure between the capillary wall and capillary centre, at the beginning of the capillaries come to 40–60 mm Hg, they descrease rapidly. The calculations have shown, that the time constant for the saturation process of the plasma (10 msec), is small in comparison with the contact time of the blood (100 msec). In the tissue capillaries, the differences in partial pressure between the capillary wall and the centre come to about 4–6 mmHg. This difference remains constant over the total capillary length.

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Bedeutung der Symbole A Atmungsintensität des Gewebes - R _a Radius des Standardzylinders - r _i Capillarradius - K ₁ Kroghscher Diffusionskoeffizient im Capillarinnern Mit Unterstützung durch die Deutsche Forschungsgemeinschaft.Herrn Prof. Dr. Dörr, Herrn Prof. Dr. Passow und Herrn Prof. Dr. Dr. Thews danke ich für wertvolle Anregungen.     

Histochemical demonstration of l-amino acid-tetrazolium reductase

Summary A method for the histochemical demonstration of l-amino acid-tetrazolium reductase is described. The diformazan deposition was obtained with l-leucine as substrate and no precipitate was present when l-serine and l-lisine were used. In the kidney the reaction was positive in the podocytes, the cells of proximal and distal convoluted tubules, in the ascending limbs of Henle and in the cells of the collecting tubules. In the liver the reaction was positive in the cytoplasm of the hepatocytes. The reaction pattern suggests that it is predominantly extramitochondrial. The specificity of this method is discussed.     

Some new or rare Chrysophyceae from the English Lake District

Summary Tetrasporopsis pseudofenestrata and Chrysolykos gracilis are new species. Chrysococcus cystophorus f. astigmata Skuja and Cyclonexis uraliensis Pochmann are new British records. Chrysolykos planctonicus Mack was recorded but not named by Scourfield (1930).The taxonomy of Cyclonexis Stokes is confused. It is uncertain whether C. annularis Stokes and C. erinus Jane are separate species but, if they are, the only definite records of the former are from the U.S.A. The known British populations of C. erinus seem to lack discobolocystes though Hovasse (1949) doubts this. The other European populations may or may not possess these organs.     

Flow-Dependent Activation of Maxi K+ Channels in Apical Membrane of Rabbit Connecting Tubule

The Ca²⁺-activated maxi K⁺ channel was found in the apical membrane of everted rabbit connecting tubule (CNT) with a patch-clamp technique. The mean number of open channels (NP _o ) was markedly increased from 0.007 ± 0.004 to 0.189 ± 0.039 (n= 7) by stretching the patch membrane in a cell-attached configuration. This activation was suggested to be coupled with the stretch-activation of Ca²⁺-permeable cation channels, because the maxi K⁺ channel was not stretch-activated in both the cell-attached configuration using Ca²⁺-free pipette and in the inside-out one in the presence of 10 mm EGTA in the cytoplasmic side. The maxi K⁺ channel was completely blocked by extracellular 1 μm charybdotoxin (CTX), but was not by cytoplasmic 33 μm arachidonic acid (AA). On the other hand, the low-conductance K⁺ channel, which was also found in the same membrane, was completely inhibited by 11 μm AA, but not by 1 μm CTX. The apical K⁺ conductance in the CNT was estimated by the deflection of transepithelial voltage (ΔV _t ) when luminal K⁺ concentration was increased from 5 to 15 mEq. When the tubule was perfused with hydraulic pressure of 0.5 KPa, the ΔV _t was only −0.7 ± 0.4 mV. However, an increase in luminal fluid flow by increasing perfusion pressure to 1.5 KPa markedly enhanced ΔV _t to −9.4 ± 0.9 mV. Luminal application of 1 μm CTX reduced the ΔV _t to −1.3 ± 0.6 mV significantly in 6 tubules, whereas no significant change of ΔV _t was recorded by applying 33 μm AA into the lumen of 5 tubules (ΔV _t =−7.2 ± 0.5 mV in control vs.ΔV _t =−6.7 ± 0.6 mV in AA). These results suggest that the Ca²⁺-activated maxi K⁺ channel is responsible for flow-dependent K⁺ secretion by coupling with the stretch-activated Ca²⁺-permeable cation channel in the rabbit CNT. Received: 21 August 1997/Revised: 20 March 1998     

The tubular network of the Golgi apparatus

 Golgi apparatus of both plant and animal cells are characterized by an extensive system of approximately 30 nm diameter peripheral tubules. The total surface area of the tubules and associated fenestrae is thought to be approximately equivalent to that of the flattened portions of cisternae. The tubules may extend for considerable distances from the stacks. The tubules are continuous with the peripheral edges of the stacked cisternae, but the way they interconnect differs across the stack. In plant cells, for example, tubules associated with the near-cis and mid cisternae often begin to anastomose close to the peripheral edges of the stacked cisternae, whereas the tubules of the trans cisternae are less likely to anastomose and are more likely to be directly continuous with the peripheral edges of the stacked cisternae. Additionally, the tubules may blend gradually into fenestrae that surround some of the stack cisternae. Because of the large surface area occupied by tubules and fenestrae, it is reasonable to suppose that these components of the Golgi apparatus play a significant role in Golgi apparatus function. Tubules clearly interconnect closely adjacent stacks of the Golgi apparatus and may represent a communication channel to synchronize stack function within the cell. A feasible hypothesis is that tubules may be a potentially static component of the Golgi apparatus in contrast to the stacked cisternal plates which may turn over continuously. The coated buds associated with tubules may represent the means whereby adjacent Golgi apparatus stacks exchange carbohydrate-processing enzymes or where resident Golgi apparatus proteins are introduced into and out of the stack during membrane flow differentiation. The limited gradation of tubules from cis to medial to trans offers additional possibilities for functional specialization of Golgi apparatus in keeping with the hypothesis that tubules are repositories of resident Golgi apparatus proteins protected from turnover during the flow differentiation of the flattened saccules of the Golgi apparatus stack. Accepted: 3 November 1997