Scanning electron microscopy of the muscle system of Hydra magnipapillata

The fine structure of the ectodermal and endodermal muscle layers of Hydra magnipapillata has been analyzed by scanning electron microscopy after hydrolytic removal of the mesoglea with NaOH and subsequent exposure of the basal and lateral aspects of the layers by mechanical dissection. The ectodermal muscle layer consists of fibrous processes of epithelial cells extending longitudinally to the body axis, whereas the endodermal muscle layer comprises cells with hexagonal bases and several strands of myonemes oriented circularly. In each layer, the muscular elements tightly interdigitate, extending a continuous muscle sheet along the mesoglea. The ectodermal and endodermal muscle sheets communicate with each other via foliate microprojections penetrating the mesoglea. On the lateral aspect of the ectodermal epithelium, spiny nerve fibers run along the upper surface of the muscle processes. The spines are often attached to muscle processes, suggesting that the former monitor muscle contraction. Nerve fibers occasionally come into contact with the mesoglea through narrow gaps between the muscle processes. In the hypostomal ectoderm, a small spindle-shaped cell, probably sensory in nature, extends an apical cilium and a long basal process.     

Somatic tissue–male germ cell barrier in Hydra viridis (hydrozoa,coelenterata)

In Hydra viridis, cordons of male germ cells lie in gonadal compartments, which are enlarged spaces between the elongated and “spongy” epidermal cells. The germ cells are surrounded by these cells, except for small areas where the interstitial cells and spermatogonia are in direct contact with the mesoglea. Cells from both epidermis and gastrodermis project cytoplasm into the mesoglea, where they contact each other and form trans-mesogleal bridges. The latter exhibit gap junctions, which are particularly abundant at the spermary region. Here, the mesoglea is thinner then elsewhere in the body. Both epithelia are joined by septate junctions toward their apical ends, which are totally impermeable to horseradish peroxidase (HRP). HRP gained entry to the cells of both epithelia by pinocytosis. Incorporation into the cells was high at the basal disk, in the tentacles, and in the mesoglea in the lower part of the body stalk. The tracer was never found within the gonadal space of the testis during spermatogenesis. In mature spermaries during spermiation, tracer-filled intracellular vacuoles fused with the gonadal spaces as the thin cytoplasmic columns of the epidermal cells ruptured; HRP thus gained access to the germ cells. During spermatogenesis, germ cells of Hydra viridis are in a closed compartment. The barrier that controls the access of metabolites to the germ cells is formed by epidermal cells, thinned-out mesoglea, and numerous transmesogleal interepithelial bridges. The presumed role of the barrier is the control of the environment (1) where interstitial cells are differentiating into spermatogonia and meiosis occurs and (2) in which ripe spermatozoa are kept immotile until spermiation.     

Functional organization of battery cell complexes in tentacles of Hydra attenuata

Ultrastructural and light microscopic observations on the organization of thick and thin regions of hydra's tentacles, made on serial sections and on whole fixed, plastic-embedded tentacles, reveal the existence of two levels of anatomical order in the tentacle ectoderm: (1) The battery-cell complex (BCC), composed of a single epitheliomuscular cell (EMC) and its content of enclosed nematocytes and neurons; and (2) the battery cell complex ring (BCC ring), an arrangement of 4 or more BCCs into larger units organized as rings around the circumference of the tentacle. All EMCs of the distal tentacle appear to contain batteries of nematocytes, and are, therefore, called “battery cells.” Apart from battery cell complexes and migrating nematocytes, there are no other cell types in the tentacle ectoderm. Battery cells are composed of three distinct regions: the cell body, peripheral attenuated extensions and myonemes. Thick tentacle bands are composed of cell bodies, whereas thin bands are made up of attenuated extensions. Myonemes contribute to both thick and thin regions. It was confirmed that each battery cell has several myonemes, which appear to interdigitate with myonemes of other more proximal and distal battery cells, but not with battery cells of the same BCC ring. Nematocytes have several basal processes. Some processes insert between myonemes and contact the mesoglea; other processes insert into cuplike extensions of myonemes, and are connected to myonemal cups by desmosomal junctions. These observations are discussed in relation to mechanical and electrical aspects of tentacular contraction and bending.     

Tissue healing and septate desmosome formation in hydra

Tissue healing was studied in hydra tissue grafts by means of light and electron microscopy. Healing is begun by the gastrodermis: subsequently the epidermis fuses and the mesoglea is repaired. Epidermis fusion is first brought about by long processes from the basal portions of the epithelial cells bridging the wound gap and adhering to opposing cells. Irregular septate desmosomes form early in this process and continuously become more neatly organized. Concommitant with the healing process at the graft site, neighboring cells are also rearranging, their septate desmosomes undergoing transient disorganizations. We conclude that the organization of septate junctions is dynamic, and may be undergoing a balanced but continuous, steady state turnover. During the healing process the forces acting on the desmosomes, and other aspects of the cells' architecture, are not balanced, and the junctions grow and become more highly organized.     

Fine structural study of interstitial cells associated with the deep muscular plexus of the rat small intestine,with special reference to the intestinal pacemaker cells

Two types of interstitial cells have been demonstrated in close association in the deep muscular plexus of rat small intestine, by electron microscopy. Cells of the first type are characterized by a fibroblastic ultrastructure, i.e. a well-developed granular endoplasmic reticulum, Golgi apparatus and absence of the basal lamina. They form a few small gap junctions with the circular muscle cells and show close contact with axon terminals containing many synaptic vesicles. They may play a role in conducting electrical signals in the muscle tissue. Cells of the second type are characterized by many large gap junctions that interconnect with each other and with the circular muscle cells. Their cytoplasm is rich in cell organells, including mitochondria, granular endoplasmic reticulum and Golgi apparatus. They show some resemblance to the smooth muscle cells and have an incomplete basal lamina, caveolae and subsurface cisterns. However, they do not contain an organized contractile apparatus, although many intermediate filaments are present in their processes. They also show close contacts with axon terminals containing synaptic vesicles. These gap-junction-rich cells may be regular components of the intestinal tract and may be involved in the pacemaking activity of intestinal movement.     

A freeze-fracture study of the papillary layer of the rat incisor enamel organ

After tooth enamel has been secreted it undergoes maturation or hardening. This process is mediated by ruffled and smooth-ended ameloblasts and associated papillary layer cells. The cells of the papillary layer are characterized by large numbers of mitochondria, coated vesicles, microvilli, and gap junctions. These features have led numerous investigators to speculate that the papillary layer is an ion-transporting epithelium. We have conducted freeze-fracture studies of the rat papillary layer in order to better characterize the surface features of these cells. The cell membranes of the papillary cells contained large numbers of intramembrane particles of various sizes ranging from 4 to 9 nm in diameter. Gap junctions were present at the cell surface and in the cytoplasm in the form of annular gap junctions. The intramembrane particles or connexons of both types of gap junctions were about 8-9 nm wide and were either packed randomly or present in the so-called 'crystallized' state. At the interface between smooth-ended ameloblasts and papillary layer cells, a well-developed zonula occludens was present along the basal surfaces of the ameloblasts and several large gap junctions were formed between the two cell types. The capillary network associated with the papillary layer was characterized by a thin endothelium containing large numbers of fenestrations.     

The fine structure of the area postrema of the sheep

The ultrastructural features of the area postrema (AP) were investigated in the suckling lamb, weaned lamb and adult sheep. No morphological differences were observed between lambs and sheep. Unciliated ependymal cells, linked by zonulae adherentes-type junctions and gap junctions, cover the AP ventricular surface. Clusters of pyriform neurons, glial cells, and axons are present in the parenchyma. The blood vessels are surrounded by wide perivascular spaces, which present an inner and outer basal lamina. The capillaries are of the fenestrated type. Perivascular glial cells rest on the outer basal lamina of the perivascular space and form a continuous ensheathment with their cell bodies or with flattened interdigitating processes. Along adjacent perivascular glial processes gap junctions are present. From our ultrastructural observations it appears that the overall cellular morphology of AP of the sheep does not differ substantially from that of monogastric mammals.     

Le système nerveux des cténaires

Summary Ultrastructural evidence is given of the occurrence of nervous elements in the mesoglea of Ctenophores based on the presence of the typical synapses of this phylum.In Beroids, nervous fibers from the ectodermal nerve-net cross the epithelial basal membrane and run through the mesoglea; they are devoid of any ensheathing cell. These neurites build highly differentiated synapses upon the muscles and upon peculiar cells, tentatively named mesenchymal cells.In Cydippids, nerve fibers and nerve cell-bodies have been observed in the mesoglea of the tentacles. The mesogleal core of each tentacle contains mesenchymal cells and a thick strand of neurons and neurites, forming a kind of elongated ganglion. Neurites of either the axial neurones or the epithelial nerve-net neurones form numerous radial nerve strands across the tentacular muscles. Interneural, neuro-muscular and neuro-mesenchymal junctions are very frequent in the tentacle.As far as the organization of the mesoglea is concerned, the Ctenophora thus appear closer to Turbellaria than to Cnidaria.

Ce travail a bénéficié de la collaboration technique de Madame J. Amsellem que nous remercions vivement.     

Ultrastructural Evidence for Neuromuscular Systems in Coelenterates

Cellular interrelationships and synaptic connections in tentaclesof several species of coelenterates were examined by means ofelectron microscopy to determine if neuromuscular pathways werepresent. The presence of sensory cells, ganglion cells, epitheliomuscularcells, interneuronal synapses, and neuromuscular junctions suggeststhat neuromuscular pathways are present in coelenterates. Nakedaxons without sheath cells form several synapses en passantwith the same and with different epitheliomuscular cells aswell as with nematocytes and other neurons. Interneuronal synapsesand neuromuscular and neuronematocyte junctions have clear ordense-cored vesicles (700–1500 Å in diameter) associatedwith a dense cytoplasmic coat on the presynaptic membrane, acleft (100–300 Å in width) with intracleft filaments,and a subsynaptic membrane with a dense cytoplasmic coat. Atscyphozoan neuromuscular junctions there is a subsurface cisternaof endoplasmic reticulum, which is separated from the epitheliomuscularcell membrane by a narrow cytoplasmic gap (100–300 Åin width) . Neuromuscular junctions in coelenterates resembleen passant axonal junctions with smooth muscle in higher animals. Morphological evidence is presented for a simple reflex involvinga two-cell (sensory or ganglion-epitheliomuscular cell) or three-cell(sensory-ganglion-epitheliomuscular cell) pathway that may resultin the coordinated contraction of the longitudinal muscle intentacles of coelenterates.     

Cellular structure and ultrastructure of the black coral Antipathes aperta: 1. Organization of the tentacular epidermis and nervous system

The tentacular epidermis of the black coral Antipathes aperta is organized into three distinct regions, containing at least nine different types of cells. The outermost region is dominated by spirocytes along with two types of nematocytes, organized into discrete wart-like batteries. The two nematocyte types both contain microbasic b-mastigophore nematocysts. The outer boundary of the wart is marked by the presence of both spumous and vesicular mucus cells. The ciliation of the wart is contributed principally by the spirocytes. Warts are enveloped and separated from one another by an unusual neurosensory cell complex that extends from the tentacular surface to the mesogleal connective tissue foundation. Funnel-like, flagellated cells composing the complex connect with ganglion cells composing the dominant portion of the nerve net system. Branches of this complex also penetrate the central portion of the wart, making direct contact with the cnidae. The tentacular mid-region is composed of nematocytes and spirocytes in various stages of maturation, along with epitheliomuscular cell (EMC) somata. The EMC's narrow apically extend toward the tentacle surface, forming contacts with the cnidae. The basal end of the EMC expands to form the larger portion of the tentacular musculature. The inner region of tentacular epidermis is marked by a neuromuscular complex sheathed by extensions of mesoglea. The ganglion cells occur as a plexus deep within the tentacle and form polarized junctions with the EMC's, but neuromuscular synapses are not well enough defined for documentation. Polarized synapses lacking well-defined membrane thickenings characterize the interneuronal junctions. Granular cells lining the mesogleal surface appear to be responsible for mesogleal fibrillogenesis.     

Cell junctions in the endocrine dorsal body cells of Helisoma duryi (Mollusca: Pulmonata): an in vitro study

The endocrine dorsal bodies of gastropod molluscs regulate reproduction and are closely associated with the central nervous system. Previous studies on Helisoma duryi have shown that the dorsal body cells of reproducing snails contain more gap junctions than those of non-reproducing virgin snails. More dorsal body cells were isolated from virgin snails than reproducing snails in Ca(2+) and Mg(2+) free experimental saline. The isolated spherical cells attached to the culture dish, spread and formed filopodium-like processes within a few hours in culture medium containing Ca(2+) and Mg(2+). Many isolated cells reaggregated after 4-6 h in culture forming septate junction-like and gap junction-like cell contacts, as revealed by thin section and freeze-fracture studies. Following 10 min incubation in carboxyfluorescein diacetate the isolated cells fluoresced and, after aggregation, these cells transferred fluorescent dye to unlabelled cells. Cell aggregation was inhibited by cytochalasin D. Staining by NBD-phallacidin revealed the presence of actin in the filopodium-like processes of spread cells and in the perinuclear cytoplasm. It is likely that the septate junction-like contacts provide sites of cell attachment between aggregating cells; gap junctions are involved in intercellular communication, and actin is required in this process.     

Spermiogenesis in Seison nebaliae (Rotifera, Seisonidea): further evidence of a rotifer-acanthocephalan relationship

The spermatozoa of Seison nebaliae are filiform cells about 70 mum long with a diameter of 0.6 mum. They have a slightly enlarged head, 2.5 mum long, followed by a long cell body. The flagellum starts from the head, and runs parallel to the cell body, contained in a groove along it. The head contains an acrosome, two large, paired para-acrosomal bodies, the basal body of the flagellum and the anterior thin extremity of the nucleus. The cell body contains the main portion of the nucleus, a single mitochondrion located in its distal portion, and many accessory bodies with different shapes. The flagellum has a 9 + 2 axoneme. The study of spermiogenesis shows the Golgian origin of the acrosome and the para-acrosomal bodies and reveals some peculiarities: a folding of the perinuclear cisterna is present between the proacrosome and the basal body of the flagellum in early spermatids and the flagellum runs in a canal inside the spermatid cytoplasm. The basal body migrates anteriorly. These characters are shared partly by the Rotifera Monogononta and, to a large extent, by the Acanthocephala studied so far. Many details of the spermiogenetic process are identical to those of Acanthocephala, thus suggesting that the processes in the two taxa are homologous.     

Ultrastructural features of the epidermis in turbellaria from the Lake Baikal Geocentrophora wagini (Lecithoepitheliata, Plathelminthes)

The epidermis of Geocentrophora wagini was studied using transmission electron microscopy. The turbellarian body was entirely covered by cilia, whose density was higher on the ventral surface compared with the dorsal one. In all regions examined, the epidermis was made up of a one-layered insunk epithelium. The basal matrix, underlying the epidermis, was a well developed basement membrane (BM) with bilayered structure, overlying the muscle network of circular and longitudinal fibers. The double plasma membranes, extending from the apical surface of epidermis to BM, were linked by specialized cell junctions. This suggested that epidermis had a cellular rather than a cyncytial arrangement. Each insunk epidermal cell was made of two unequal parts: a comparatively thin surface plate attached to BM by hemiadherens junctions, and a massive nucleated portion located below the body wall musculature in the parenchyma. A thin cytoplasmic bridge connected the epidermal plate with the nucleated cell body. The epidermal plates were joined by belt-like junctions along their adjacent surfaces. Inconspicuous zonula adherens (ZA) had a most apical position, and prominent septate junction was arrayed proximally to this zonula. Except ZA, cell boundaries in epidermis were frequently flanked by rows of light tubules and vesicles. In the basal half of the epithelial sheet, they were occassionally accompanied by single cisternae of rough endoplasmic reticulum (RER). The ultrastructure of the insunk cell body and that of the surface plate showed a considerable similarity. The common features were distinctive profiles of RER and GA, the presence of epitheliosomes, light tubules and vesicles, centrioles and fibrous granules. Thus, ultrastructural features allow a rather reliable identification of epidermal cells in the parenchyma, despite the absence of any visible morphological association between cell body and its epidermal plate.     

Electron microscopic study of intestinal epithelium of <Emphasis Type="Italic">Saccoglossus mereschkowskii</Emphasis> (Enteropneusta,Hemichordata)

The epithelium of the hepatic region of the intestine in Saccoglossus mereschkowskii, a representative of enteropneusts (Enteropneusta, Hemichordata), a group located at the base of Chordata, has been studied by using electron microscopy. The ultrastructure of ciliated and granular epithelial cells, elements of the intraepithelial nerve layer, and intercellular junctions are characterized. The data on the details of the structure of the ciliary apparatus and the system of ciliary rootlets are presented. Justification is provided for the presence of a complicated construction in the ciliated cells, a supportive carcass of cilia that performs a mechanical stabilizing function, and possibly the synchronization of the ciliary movement. The existence of cilia with two centrioles is considered as adaptation to the high functional load on the ciliary apparatus. Well-developed bundles of myofilaments have been revealed in the cytoplasm of the basal parts of ciliated cells, which characterizes these cells as epitheliomuscular. Peculiarities indicating the role of ciliated cells in absorption are described, as well as the capability of these cells for balloon-like secretion. Data are presented on the accumulation of reserved nutritional substances in the cell cytoplasm in the form of lipids and glycogen. With respect to their function, ciliated cells are determined as the ciliated secretory-absorptive epitheliomuscular cells. The location of secretory granules in both apical and basal parts of granular cells indicates the exocrine-endocrine function of these cells. There are no typical endocrine cells in the intestinal epithelium of S. mereschkowskii. Several types of granules are described in the cytoplasm of nerve fibers. Junctions between nerve fibers and basal parts of ciliated and granular epithelial cells have been revealed; the neural regulation of the contractile and secretory functions of epithelial cells is assumed. The intestinal epithelium of enteropneusts is presumed to contain a regulatory neuroendocrine system composed of receptor cells of the open type, secretory endocrine-like cells, and of nerve elements of the nervous layer.     

Ultrastructural features of the adult hermaphrodite gonad of Caenorhabditis elegans: relations between the germ line and soma.

Genetic and embryological experiments have established the Caenorhabditis elegans adult hermaphrodite gonad as a paradigm for studying the control of germline development and the role of soma-germline interactions. We describe ultrastructural features relating to essential germline events and the soma-germline interactions upon which they depend, as revealed by electron and fluorescence microscopy. Gap junctions were observed between oocytes and proximal gonadal sheath cells that contract to ovulate the oocyte. These gap junctions must be evanescent since individual oocytes lose contact with sheath cells when they are ovulated. In addition, proximal sheath cells are coupled to each other by gap junctions. Within proximal sheath cells, actin/myosin bundles are anchored to the plasma membrane at plaque-like structures we have termed hemi-adherens junctions, which in turn are closely associated with the gonadal basal lamina. Gap junctions and hemi-adherens junctions are likely to function in the coordinated series of contractions required to ovulate the mature oocyte. Proximal sheath cells are fenestrated with multiple small pores forming conduits from the gonadal basal lamina to the surface of the oocyte, passing through the sheath cell. In most instances where pores occur, extracellular yolk particles penetrate the gonadal basal lamina to directly touch the underlying oocytes. Membrane-bounded yolk granules were generally not found in the sheath cytoplasm by either electron microscopy or fluorescence microscopy. Electron microscopic immunocytochemistry was used to confirm and characterize the appearance of yolk protein in cytoplasmic organelles within the oocyte and in free particles in the pseudocoelom. The primary route of yolk transport apparently proceeds from the intestine into the pseudocoelom, then through sheath pores to the surface of the oocyte, where endocytosis occurs. Scanning electron microscopy was used to directly visualize the distal tip cell which extends tentacle-like processes that directly contact distal germ cells. These distal tip cell processes are likely to play a critical role in promoting germline mitosis. Scanning electron microscopy also revealed thin filopodia extending from the distal sheath cells. Distal sheath filopodia were also visualized using a green fluorescent protein reporter gene fusion and confocal microscopy. Distal sheath filopodia may function to stretch the sheath over the distal arm.     

Fate of annular gap junctions in the papillary cells of the enamel organ in the rat incisor

Summary To investigate the mechanisms whereby annular gap junctions in the papillary cells of the enamel organ are degraded intracellularly, continuously growing rat incisors were examined by electron microscopy of routine thin sections as well as for the cytochemical localization of inorganic trimetaphosphatase activity. Routine thin-section analysis revealed small flat or undulated gap junctions, hemi-annular gap junctions between an invaginated cell process and a cell body, and fully internalized cytoplasmic annular gap junctions. Both hemi-annular and annular gap junctions usually contain various organelles and/or inclusions, such as mitochondria, endoplasmic reticulum, ribosomes, vesicles, and lysosomes in the cytoplasm confined by the junctional membranes. Annular gap junctions are sometimes fused with vesicular or tubulovesicular structures. Cytochemistry of inorganic trimetaphosphatase activity revealed an intense enzymatic reaction within a system of tubular structures and round or oval dense bodies. Both structures are believed to correspond to primary lysosomes. A part of the Golgi apparatus also shows a weak reaction. Although hemi-annular gap junctions never show enzymatic reaction, annular gap junctions sometimes contain reaction products throughout their interior cytoplasm and inclusions. Fusion of annular gap-junctional membranes with reaction-positive tubular structures is also observed. In one instance, revealed in serial sections, an annular gap junction was encircled entirely by a reaction-positive structure. These results suggest that cytoplasmic annular gap junctions are formed by endocytosis of hemi-annular gap junctional membranes from the cell surface and then degraded intracellularly by lysosomal enzymes.     

Germ line stem cell differentiation in Drosophila requires gap junctions and proceeds via an intermediate state

Gap junctions coordinate processes ranging from muscle contraction to ovarian follicle development. Here we show that the gap junction protein Zero population growth (Zpg) is required for germ cell differentiation in the Drosophila ovary. In the absence of Zpg the stem cell daughter destined to differentiate dies. The zpg phenotype is novel, and we used this phenotype to genetically dissect the process of stem cell maintenance and differentiation. Our findings suggest that germ line stem cells differentiate upon losing contact with their niche, that gap junction mediated cell-cell interactions are required for germ cell differentiation, and that in Drosophila germ line stem cell differentiation to a cystoblast is gradual.     

Entry of algal symbionts into oocytes of the coral Litophyton arboreum

On the Red Sea coral reefs Litophyton arboreum is a common octocoral whose endodermal cells are associated with endosymbiotic dinoflagellates (zooxanthellae). Colonies of this species are gonochoric and brood planulae which, upon release, are already associated with the algal symbionts. Algal cells within membrane-bound vacuoles are observed within the gastrovascular cavity of the polyps, adjacent to the oocytes and are gradually phagocytized by the follicular cells which surround the oocytes. During oogenesis, temporary gaps open in the mesoglea underlying the follicular cells. Symbionts within vacuoles, along with cytoplasm and various organelles derived from the follicular cells, are translocated through these gaps. Subsequently, groups of zooxanthellae accumulate at the perioocytic zone, flanked between the mesoglea and oocytic microvilli. At a later stage, prior to the commencement of the breeding season, symbionts pass through the oolemma and rest inside the periphery of the oocytes. It is proposed that early uptake of zooxanthellae by sexual progeny at the oocyte stage, indicates a highly specialized mode of interaction between this algal symbiont and its host.     

Fine structure of the rectal pads in the desert locust Schistocerca gregaria with reference to the mechanism of water uptake

The rectal pads of Schistocerca gregaria are composed of three different cell types: epithelial, secondary and junctional cells. The rectal pads are interconnected by simple rectal cells and both are lined internally by a articular intima. The epithelial cells exhibit extensive infoldings of the apical plasma membranes that are closely associated with mitochondria. Their lateral plasma membranes are highly folded around large mitochondria and enclose intercellular channels and spaces. They are united by belt and spot desmosomes, septate junctions, gap junctions and scalariform junctions, but terminate in a basal syncytium without contacting the basal plasma membranes. The apical and basal cytoplasm contain coated vesicles, dense tubular elements, multivesicular bodies and lysosomes, suggesting receptor-mediated endocytosis of small peptide molecules into the epithelial cells. The apical membrane infoldings of the secondary cells are also associated with large mitochondria. Their basal plasma membranes are covered by connective cell processes and connected with them by spot desmosomes which may be involved in solute recycling. The presence of neurosecretory-like axons near the secondary cells suggests that they exert local control on the function of these cells. The ultrastructural details are examined in relation to their role in solute and water transport.     

Unusual features of intercellular junctions in the larvacean Oikopleura dioica

Summary In the pelagic larvacean Oikopleura dioica, the epithelium lining the alimentary tract consists of ciliated and unciliated cell types. The ciliated cells also exhibit an apical border of long microvilli. Between the microvilli, the cellular membrane often projects deeply down into the cytoplasm; the membranes of these invaginations and those of apicolateral interdigitations may be associated with one another by tight junctions. Some of these junctions may be autocellular. The tight junctions are seen by freeze-fracture to be very simple in construction, composed of a single row of intramembranous particles, which may be fused into a P-face ridge. There is a dense cytoplasmic fuzz associated with these tight junctions which may extend into adjoining zonula adhaerens-like regions. The invaginations of the apical membranes are, in addition, associated by gap junctions which may also be autocellular. More conventional homocellular and heterocellular tight and gap junctions occur along the lateral borders of ciliated cells and between ciliated and unciliated cells. These gap junctions possess a reduced intercellular cleft and typical P-face connexons arranged in macular plaques, with complementary E-face pits. Both cell types exhibit extensive stacks of basal and lateral interdigitations. The tight junctions found here are unusual in that they are associated with a dense cytoplasmic fuzz which is normally more characteristic of zonulae adhaerentes.