Ultrastructural features of cardiac muscle cells in a tarantula spider

The ultrastructural features of cardiac muscle cells and their innervation were examined in the tarantula spider Eurypelma marxi Simon. The cells are transversely striated and have an A band length of about three microns. H zones are indistinct and M lines are absent. Thick and thin myofilament diameters are approximately 200 and 70 Å respectively. Eight to 12 thin filaments usually surround each thick one. Accumulations of thick and thin myofilaments occur perpendicular to the bulk of the myofilaments in some cells. The Z line is discontinuous and thick filaments may pass through the spaces in the Z line. Extensive systems of sarcoplasmic reticulum and transverse tubules are present; these form numerous dyadic junctions in both A and I band regions. Sarcolemmal invaginations form Z line tubules; lateral extensions of the plasma membrane portion of these invaginations form dyads. Nerve branches of the cardiac ganglion make multiple neuromuscular synapses with at least some of the cardiac muscle cells. Both large granular and small agranular vesicles are present in the presynaptic terminals. Intercalated discs similar to those present in other arthropod hearts occur between the ends of adjacent cardiac muscle cells.     

MORPHOLOGY OF THE OMMATIDIA OF THE COMPOUND EYE OF LIMULUS

The sensory portion of the ommatidium of the compound eye of Limulus has been studied with the electron microscope. In axial longitudinal section the rhabdom appears to be made up of small polygons, and in transverse section the rhabdom appears as a banded structure of dark lines. Thus in three dimensions the rhabdom resembles a honeycomb composed of tubular units, the long axes of which lie in transverse planes and are oriented perpendicular to the retinula cell's contours. The tubular units, which are about 140 mµ in diameter in Limulus (70 mµ in diameter in the spider and Scutigera), are microvilli of the borders of the retinula cells. The walls of these microvilli are continuous with fine linear structures (membranes) in the cytoplasm of the retinula cells. In transverse sections of the ommatidium oval bodies interpreted as mitochondria are observed in an annular zone at the tips of the rhabdom's rays. These mitochondria, which are 2 to 10 µ in diameter, are crowded with irregular closed outlines about 100 mµ in diameter. Possible functions of components of the ommatidium are discussed.     

A study of the fine structure of the accessory muscle of the horseshoe crab,Tachypleus gigas

The accessory muscle of the walking leg of the horseshoe crab, Tachypleus gigas, was examined electron microscopically. The muscle fibers vary in size but are small in diameter, when compared with other arthropod skeletal muscles. They are striated with A, I, Z and poorly defined H bands. The sarcomere length ranges from 3-10 μm with most sarcomeres in the range of about 6 μm. The myofilaments are arranged in lamellae in larger fibers and less well organized in the smaller ones. Each thick filament is surrounded by 9-12 thin filaments which overlap. The SR is sparse but well organized to form a fenestrated collar around the fibrils. Individual SR tubules are also seen among the myofibrils. Long transverse tubules extend inward from the sarcolemma to form dyads or triads with the SR at the A-I junction. Both dyads and triads coexist in a single muscle fiber, a feature believed to have evolutionary significance. The neuromuscular relationship is unique. In the region of synaptic contact, the sarcolemma is usually elevated to form a large club-shaped structure containing no myofilaments and few other organelles. The axons or axon terminals and glial elements penetrate deep into the club-shaped sarcoplasm and form synapses with the fiber. As many as 13 terminals have been observed within a single section. Synaptic vesicles of two types are found in the axon terminals.     

The muscle of a monogonont rotifer, Trichocerca rattus. II. The central retractor muscles

This work concerns the ultrastructural characteristics of the two cells of the paired, bicellular, central retractor muscles (CRM) of the rotifer Trichocerca rattus. By transverse ultrathin serial sections, precise measurements where made on the following cytological characteristics: the length, diameter and average number by unit surface area for the myofilaments; the percentage of cell volume for the myoplasm and mitochondria; and the average number of peripheral elements of the sarcoplasmic reticulum (dyads) by unit surface area. These characteristics agree with a phasic fibre type for the CRM. The muscular insertions are described, as are the junctions between the two cells of each CRM: hemidesmosomes, desmosomes and gap junctions. The two symmetrical CRM are coupled by a large gap junction between two subcerebral sarcoplasmic processes. Some of these processes partially enter the cerebral neuropile; each CRM is innervated at this level. The functions of the cytological specializations are discussed in the light of the behaviour assumed by the CRM: the retraction of the rotatory apparatus in the trunk of T. rattus.     

Fine structure of fast-twitch and slow-twitch guinea pig muscle fibers

The guinea pig soleus muscle is a convenient model for the study of slow-twitch intermediate (STI) fiber ultrastructure because it is composed entirely of fibers of this class. Such fibers were compared with fast-twitch red (FTR) and fast-twitch white (FTW) fibers from the vastus lateralis muscle. FTW fibers are characterized by small, sparse mitochondria, a narrow Z line and, an extensive sarcoplasmic reticulum arranged primarily in longitudinal profiles at the A band and with numerous expansions at the I band. Abundant mitochondria with a dense matrix and subsarcolemmal and perinuclear aggregations are typical of FTR fibers. These fibers contain a plexus of sarcoplasmic reticulum at the A band and a less extensive network at the I band. The Z lines are wider (890 ± 74 Å) than those of FTW fibers (582 ± 62 Å). STI intermediate fibers are distinguished from other types by wide Z lines (1205 ± 58 Å), a faint M band, and a less extensive sarcoplasmic reticulum. Compared to FTR fibers, STI fiber mitochondria are usually smaller with less notable subsarcolemmal accumulations. FTW fibers have a more limited capillary supply, rarely contain lipid inclusions, and thus may be restricted to phasic activity. Extensive capillarity, mitochondrial and lipid context, and fast contraction times indicate possible phasic and tonic roles for FTR fibers. STI fibers, characterized by numerous lipid inclusions, extensive capillarity, relatively numerous mitochondria, but slow contraction-relaxation cycles, are morphologically suited for tonic muscle activity.     

THE MYOFILAMENT ARRANGEMENT IN THE FEMORAL MUSCLE OF THE COCKROACH, LEUCOPHAEA MADERAE FABRICIUS

The structure of the femoral muscle of the cockroach, Leucophaea maderae, was investigated by light and electron microscopy. The several hundred fibers of either the extensor or flexor muscle are 20 to 40 µ in diameter in transverse sections and are subdivided into closely packed myofibrils. In glutaraldehyde-fixed and epoxy resin-embedded material of stretched fibers, the A band is about 4.5 µ long, the thin filaments are about 2.3 µ in length, the H zone and I band vary with the amount of stretch, and the M band is absent. The transverse sections of the filaments reveal in the area of a single overlap of thick and thin filaments an array of 10 to 12 thin filaments encircling each thick filament; whereas, in the area of double overlap in which the thin filaments interdigitate from opposite ends of the A band, the thin filaments show a twofold increase in number. The thick filament is approximately 205 to 185 A in diameter along most of its length, but at about 0.2 µ from the end it tapers to a point. Furthermore, some well oriented, very thin transverse sections show these filaments to have electron-transparent cores. The diameter of the thin filament is about 70 A. Transverse sections exhibit the sarcolemma invaginating clearly at regular intervals into the lateral regions of the A band. Three distinct types of mitochondria are associated with the muscle: an oval, an elongate, and a type with three processes. It is evident, in this muscle, that the sliding filament hypothesis is valid, and that perhaps the function of the extra thin filaments is to increase the tensile strength of the fiber and to create additional reactive sites between the thick and thin filaments. These sites are probably required for the functioning of the long sarcomeres.     

The Striated Musculature of Blood Vessels : I. General Cell Morphology

The musculature of small lung veins, of the thoracic portion of the inferior vena cava, and of other thoracic veins of the mouse have been studied in the electron microscope. Tissues were fixed in 1 per cent osmium tetroxide buffered with veronal, to which either sodium chloride or sucrose had been added. Methacrylate or araldite served as embedding matrices. Phosphotungstic acid or uranyl acetate was used to stain some of the preparations. Thin sections were examined in a Siemens and Halske Elmiskop Ib electron microscope. The entire musculature of the veins examined was of the striated type. It represents a variety of cardiac muscle, characterized by centrally located nuclei, typical mitochondria, and narrow I bands. Many I bands cannot be recognized at all. H and M bands are likewise indistinct. There is a double array of primary and secondary myofilaments. Mitochondria are large and numerous and contain many cristae. The endoplasmic reticulum consists of longitudinal tubules which run through the whole sarcomeres and bypass Z bands, and of transverse tubules which accompany Z bands. Some "triads," located at Z levels, consist of flattened vacuoles flanked by such transverse tubules. Small vesicles located at Z bands, close to the nucleus, and beneath the plasma membrane may represent still other portions of the reticulum.     

Laser Confocal scanning microscopy of the surface membrane/T-tubular system and the sarcoplasmic reticulum in insect striated muscle stained with DiIC18(3)

The structure of the surface membrane/transverse tubular (T-tubular) system and of the sarcoplasmic reticular (SR) of the labial adductor muscle of the honey bee (Apis mellifera) was examined by laser confocal scanning microscopy, after staining with the fluorescent membrane probe DiIC₁₈(3). The following components of the surface membrane/T-tubular system were visualized: transverse tubular networks that are located in the A-band close to the A–I junction and form dyads with the SR, longitudinal tubules that link the T-tubular networks within and between sarcomeres, and surface invaginations of larger diameter that contain tracheoles. The well developed SR forms a dense network of branching and anastomosing tubules in the A-band. A few tubular elements in the interfibrillar space in the 1-band link the SR of adjacent sarcomeres. This study demonstrates the advantages of the laser confocal microscope and lipophilic fluorescent dyes for studying the 3-D structure of cellular membrane systems.     

THE OCCURRENCE OF A SUBUNIT PATTERN IN THE UNIT MEMBRANES OF CLUB ENDINGS IN MAUTHNER CELL SYNAPSES IN GOLDFISH BRAINS

Observations additional to those previously reported (34) on boutons terminaux and club endings on Mauthner cell lateral dendrites, primarily as seen in sections of permanganate-fixed material, are described. Certain new findings on OSO₄-fixed endings are also included. The boutons terminaux are closely packed in the synaptic bed with ~ 100 to 150 A gaps between their contiguous unit membranes and a few interspersed glial extensions. Their synaptic membrane complexes (SMC) appear as pairs of unit membranes separated by ~ 100 to 150A clefts. They contain many vesicles and unoriented mitochondria, but no neurofilaments. The club endings after KMnO₄ fixation are, as after OSO₄ fixation (34), again seen surrounded by a layer of extracellular matrix material. These endings contain relatively few synaptic vesicles, a few unit membrane limited tubules ~ 300 A in diameter, and mitochondria oriented perpendicular to the SMC. Neurotubules and neurofilaments are not clearly seen. These components are also virtually absent in the Mauthner cytoplasm. No ribosomes are seen in the KMnO₄-fixed material. The unit membranes of the SMC of club endings show up clearly in essentially the same junctional relations described after formalin-OSO₄ fixation (34). In addition, the synaptic discs in transverse section show a central beading repeating at a period of ~ A associated with scalloping of the cytoplasmic surfaces. In oblique views, dense lines are seen repeating at a period of ~ 90 A. In frontal views a hexagonal array of close-packed polygonal facets is seen. These repeat at a period of ~ 95 A. Each has a central dense spot <25 A in diameter. Similar subunits are seen in the unit membranes of synaptic vesicles.     

The fine structure of the drum muscles of the Tigerfish,Therapon jarbua,as compared with the trunk musculature

Summary The fibers of drum and trunk muscles of the Tigerfish, Therapon jarbua, differ greatly in diameter. The myofibrils of the trunk muscles are irregularly oriented, while those of the drum muscles are rolled into spiral or concentric bands. Both muscle types possess the sarcomere structure typical of cross-striated musculature. However, the myofibrils of the drum muscles differ greatly in sarcomere length and width from those in the trunk musculature. The trunk muscles contain few mitochondria, whereas in the drum muscles mitochondria are abundant. The sarcoplasmic reticulum (SR) of the drum muscles takes the form of elongated tubes in both the A and the I region; that of the trunk musculature consists of small vesicles. Of the two muscle types, the drum muscle contains more SR. With respect to the form of the T system, the trunk musculature is of the Z type and the drum muscles of the A-I type. The drum muscle displays a considerably greater number of motor endplates; these lack typical junctional folds and have mitochondria with very few cristae. No fat could be demonstrated in either the drum or the trunk muscles. However, the concentration of glycogen is higher in the drum muscle than in the musculature of the trunk.This work was accomplished with support from the Deutsche Forschungsgemeinschaft and is gratefully dedicated to Prof. R. Danneel on the occasion of his 75th birthday.     

Ultrastructure of the muscles of the dorsal diaphragm in Locusta migratoria

Summary The alary muscles of Locusta migratoria adults make up the major tissue of the dorsal diaphragm which separates pericardial and perivisceral sinuses in the abdomen. The alary muscles are striated with a sarcomere at rest measuring about 9 m. The Z-line has a staggered-beaded arrangement with A-bands and I-bands readily discernable. Thick myofilaments are surrounded by 10 or more thin filaments. The sarcoplasm has few mitochondria near the area of the Z-line, dyads are present and sarcoplasmic reticulum is poorly developed. Axons which innervate the alary muscle are either contained within invaginated folds of the sarcolemma of the muscle cells or the muscle cells send finger-like projections to envelop the axons. The synaptic terminals contain synaptic vesicles between 40 and 45 nm in diameter and a few electron-dense granules near or less than 170 nm in diameter. Away from synaptic terminals the axon profiles show few or no granules. The axons are accompanied everywhere by well-developed glial cells. This then is not typical neurosecretomotor innervation, however, the presence of electron-dense granules suggests the possibility of peptidergic neurotransmission.     

Optical diffraction of the Z lattice in canine cardiac muscle

Optical diffraction patterns from electron micrographs of both longitudinal and cross sections of normal and anomalous canine cardiac Z bands have been compared. The data indicate that anomalous cardiac Z bands resembling nemaline rods are structurally related to Z bands in showing a repeating lattice common to both. In thin sections transverse to the myofibril axis, both electron micrographs and optical diffraction patterns of the Z structure reveal a square lattice of 24 nm. This lattice is simple at the edge of each I band and centered in the interior of the Z band, where two distinct lattice forms have been observed. In longitudinal sections, oblique filaments visible in the electron micrographs correspond to a 38-nm axial periodicity in diffraction patterns of both Z band and Z rod. We conclude that the Z rods will be useful for further analysis and reconstruction of the Z lattice by optical diffraction techniques.     

Ultrastructural Studies of the Development of Nerves in Hydra

Three types of mature epidermal neurons and several of theirdifferentiating stages aie presented in this ultrastructuralstudy. Each of the three types, neurosensory, neurosecretory,and ganglionic cells, is derived from interstitial cells, (i)Mature neurosensory cells contain elongated nuclei, a well-developedcilium in each cell, and membrane-bounded neurosecretory droplets(700–1300 A in diameter). There may be two or more neuritesin which are numerous microtubules, glycogen particles, ribosomesand many neurosecretory droplets, (ii) Mature neurosecretorycells closely resemble neurosensory cells, except that no ciliumis present. The perikarya contain small, dense nuclei, neurosecretorydroplets (850–1300 A in diameter), mitochondria, glycogenparticles, and microtubules. Active Golgi complexes are presentin both cell types. The nemites are similar to those describedfor neurosensory cells, (iii) Mature ganglionic cells are bipolaror multipolar. The small, dense nuclei are surrounded by a smallamount of cytoplasm. The neurites contain mostly microtubules;a few mitochondria, ribosomes, and glycogen particles are alsopresent, but there are no secretory droplets. To date, only neurosensory and neurosecretory cells have beenobserved in the gastiodermis. They are structurally indistinguishablefrom their epideimal counterparts. A significant finding is that three types of synapses—neuromuscular,neuronematocyte, and interneuronal—are identified in boththe epidermal and gastrodermal neurons.     

Ultrastructure and potential taxonomic importance of euspermatozoa and paraspermatozoa in the volutid gastropods Zidona dufresnei and Provocator mirabilis (Caenogastropoda, Mollusca)

The ultrastructure of mature spermatozoa is investigated for the first time in the Volutidae, based on the commercially significant South American species Zidona dufresnei (Donovan, 1823) (fresh material) and supplemented with observations on testicular (museum) material of the deep sea New Zealand species Provocator mirabilis (Finlay, 1926). Euspermatozoa of Z. dufresnei (ex sperm duct) consist of: (1) a tall-conical acrosomal vesicle (with short basal invagination, constricted anteriorly) which is flattened anteriorly and associated with an axial rod, centrally perforate basal plate and short accessory membrane; (2) a rod-shaped, solid and highly electron-dense nucleus (with short basal fossa containing centriolar complex and initial portion of a 9 + 2 axoneme); (3) an elongate midpiece consisting of the axoneme sheathed by 5–6 helical mitochondrial elements, each exhibiting a dense U-shaped outer layer; (4) an elongate glycogen piece (axoneme sheathed by nine tracts of putative glycogen granules); (5) a dense annulus at the junction of the midpiece and glycogen piece and (6) a short free tail region (axoneme surrounded only by plasma membrane). Paraspermatozoa of Z. dufresnei are vermiform and dimorphic: the first type contains approximately 14–20 axonemes (arranged peripherally and interspersed with microtubules) and numerous oblong dense vesicles, numerous less dense (round) vesicles, occasional, large lipid-like vesicles, and scattered mitochondria; the second type contains 25–31 axonemes (peripherally arranged, interspersed with microtubules), occasional mitochondria and extensive cytoplasm. Results obtained for P. mirabilis from testis material are essentially as observed in Z. dufresnei, although the euspermatozoan acrosome still has to achieve its compressed transverse profile. Observations on paraspermatozoa were limited by fixation quality of available (testis) tissues, but these cells are similar to the first type of Zidona paraspermatozoa. Although most of the euspermatozoal features are also observed in many neotaenioglossans and neogastropods, the U-shaped outer layer of each mitochondrial element has not previously been reported and may prove a diagnostic feature of the Volutidae, the subfamily Zidoniinae or possibly only the Zidonini (in which Z. dufresnei and P. mirabilis are currently placed).     

THE FINE STRUCTURE OF SHEEP MYOCARDIAL CELLS; SARCOLEMMAL INVAGINATIONS AND THE TRANSVERSE TUBULAR SYSTEM

An electron microscope study of sheep myocardial cells has demonstrated the presence of a transverse tubular system, apparently forming a network across the cell at each Z band level. The walls of these tubules resemble the sarcolemma in consisting of two dense layers—plasma membrane and basement menbrane; continuity of the tubule walls with the sarcolemma can be seen when longitudinal sections of a cell are obtained between two subsarcolemmal myofibrils and at the same time perpendicular to the cell surface. The demonstration of communication between the lumen of the transverse tubular system and the extracellular space appears to be more definite in this study than in any work hitherto published. It provides anatomical evidence of a possible direct pathway for transmission of the activating impulse from the sarcolemma to the myofibril Z bands.     

Ultrastructure of minor veins inCucurbita pepo leaves

Summary The minor veins ofCucurbita pepo leaves were examined as part of a continuing study of leaf development and phloem transport in this species. The minor veins are bicollateral along their entire length. Mature sieve elements are enucleate and lack ribosomes. There is no tonoplast. The sieve elements, which are joined to each other by sieve plates, contain mitochondria, plastids and endoplasmic reticulum as well as fibrillar and tubular (190–195 diameter) P-protein. Fibrillar P-protein is dispersed in mature abaxial sieve elements but remains aggregated as discrete bodies in mature adaxial sieve elements. In both abaxial and adaxial mature sieve elements tubular P-protein remains undispersed. Sieve pores in abaxial sieve elements are narrow, lined with callose and are filled with P-protein. In adaxial sieve elements they are wide, contain little callose and are unobstructed. The intermediary cells (companion cells) of the abaxial phloem are large and dwarf the diminutive sieve elements. Intermediary cells are densely filled with ribosomes and contain numerous small vacuoles and many mitochondria which lie close to the plasmalemma. An unusually large number of plasmodesmata traverse the common wall between intermediary cells and bundle sheath cells suggesting that the pathway for the transport of photosynthate from the mesophyll to the sieve elements is at least partially symplastic. Adaxial companion cells are of approximately the same diameter as the adaxial sieve elements. They are densely packed with ribosomes and have a large central vacuole. They are not conspicuously connected by plasmodesmata to the bundle sheath.     

PHYSIOLOGICAL IMPLICATIONS OF VICIA FABA AND NICOTIANA TABACUM GUARD-CELL ULTRASTRUCTURE

The guard cells of Vicia faba and Nicotiana tabacum contain numerous mitochondria, elements of endoplasmic reticulum, spherosomes, and peroxisome-like microbodies. A full ribosomal complement appears in young but not in fully mature guard cells. Numerous small lipid droplets external to the plasmalemma were noted in mature Vicia guard cells. Chloroplasts were found in both epidermal and guard cells of both species. Full photosynthetic capacity was indicated by the grana fretwork of guard-cell chloroplasts. A specialized peripheral reticulum was observed in the guard-cell chloroplasts of Vicia. Plasmodesmata were observed in both walls between sister guard cells and between guard and epidermal cells. In the latter case plasmodesmata were found primarily in pit fields of transverse walls. It is postulated that the small volume of guard cells allows them an osmotic advantage over larger neighboring cells in generating turgor.     

Morphology,ultrastructure, and development of the parasitic larva and its surrounding trophamnion of Polypodium hydriforme Ussov (Coelenterata)

Summary The larval stage of Polypodium hydriforme is planuliform and parasitic inside the growing oocytes of acipenserid fishes. The larva has inverted germ layers and a special envelope, the trophamnion, surrounding it within the host oocyte. The trophamnion is a giant unicellular provisory structure derived from the second polar body and performing both protective and digestive functions, clearly a result of adaptation to parasitism. The trophamnion displays microvilli on its inner surface, and irregular protrusions anchoring it to the yolk on its outer surface. Its cytoplasm contains long nuclear fragments, ribosomes, mitochondria, microtubules, microfilaments, prominent Golgi bodies, primary lysosomes, and secondary lysosomes with partially digested inclusions.The cells of the larva proper are poorly differentiated. No muscular, glandular, neural, interstitial, or nematocyst-forming cells have been found. The entodermal (outer layer) cells bear flagella and contain rough endoplasmic reticulum; the ectodermal (inner layer) cells lack cilia and contain an apical layer of acid mucopolysaccharid granules. The cells of both layers contain mitochondria, microtubules, and Golgi bodies; their nuclei display large nucleoli with nucleolonema-like structure, decondensed chromatin, and some perichromatin granules. At their apical rims, the ectodermal cells form septate junctions; laterally, the cells of both layers form simple contacts and occasional interdigitations. The lateral surfaces of entodermal cells are strengthened by microtubules.     

Fine structure of the corpus Luteum of the raccoon during pregnancy

Summary Ultrastructure of the granulosa lutein cells of the raccoon from throughout pregnancy has been described. The lutein cells often from epithelial cords which are separated by the connective tissues, capillaries and lymphatics. Based on the arrangements and modifications of the cytoplasmic organelles and inclusions, three types of lutein cells have been recognized. The type I lutein cells predominantly contain tubular, agranular endoplasmic reticulum, juxtanuclear Golgi complexes, a few round to rod-shaped mitochondria, some free ribosomes, and occasional lipid droplets. Occasionally the tubular cristae of mitochondria and tubular smooth endoplasmic reticulum appear contiguous. The type II cells contain abundant lace-like and/or stacked fenestrated endoplasmic reticulum cisternae that frequently form membranous whorls, some tubular, agranular endoplasmic reticulum, mitochondria, and lipid droplets. Mitochondria are usually small, but unusual large ones also occur. The small, rod-to round-shaped mitochondria usually have tubular cristae; but the large, oval, elongate, and cup shaped mitochondria possess tubular, lamellar, plate like, and whorl-like cristae. The plasma membranes of the cells are complexly elaborated and folded, especially when apposing each other. In favorable sections, strands of fenestrated cisternae appose the folds of the plasma membranes. In general, the amount of cytoplasmic organelles and inclusions vary greatly in the cells. The type III cells predominantly contain lipid droplets and sparse cytoplasmic organelles. The type I and II cells are found throughout pregnancy, but the type III cells are observed from mid gestation to term. The cytological features of type I and II cells suggest that they probably secrete most of the steroids, whereas the type III cells primarily store lipids.This research was supported by UPSHS grant AM-11376 and NIH contract 69-2136.     

Formation of Flat Lamellar Intramembrane Lipid Structures in Microorganisms

Analysis of freeze-fracture replicas and thin sections of cells of the bacteria Sulfobacillus thermosulfidooxidans and Anaerobacter polyendosporus showed that their cytoplasmic membranes contain some regions in the form of flat lamellar inverted lipid membranes a few tenths of nanometers to a few microns in size. The specific features of these membrane structures are as follows: (i) they contain no familiar intramembrane particles commonly present on freeze-fracture replicas; (ii) in cross thin sections, intramembrane structures are bifurcate on the periphery and look like thylakoids; and (iii) the leaflets of intramembrane structures in S. thermosulfidooxidans cells are corrugated. These structures were revealed in bacterial cells cultivated under normal growth conditions. The data obtained suggest the occurrence of a complex type of compartmentalization in biological membranes. Received: 17 July 2000/Revised: 22 November 2000