Anatomy,histology, histochemistry and fine structure of the Harderian gland in the South American armadillo <Emphasis Type="Italic">Chaetophractus villosus</Emphasis> (Xenarthra,Mammalia)

The anatomical, histological, histochemical and ultrastructural characteristics of the Harderian gland of the armadillo Chaetophractus villosus were described. The gland is the largest structure in the bony orbit. It is situated in the anteroventral region of the orbit. Obvious structural differences are not observed between males and females. The gland is compound-branched tubulo-alveolar, being characterized by a single layer of columnar cells surrounded by myoepithelial cells. It possesses a single excretory duct opened into the inner canthus. All glandular cells show yellow-green autofluorescence and additionally some glandular lumen may contain dense autofluorescent solid accretions. There are two peculiar and outstanding cytoplasmic features. One is represented by the smooth endoplasmic reticulum (SER), forming a closely woven meshwork. The other one is represented by “membranous bodies” apparently derived from the SER, RER and cytoskeleton with a “Star of David” configuration situated in the supranuclear region. Three types of vesicles are detected in the cytoplasm. Histochemical staining methods reveal lipids, proteins, neutral and acidic containing glycoconjugates in secretory vesicles. The mechanism of secretion appears either merocrine or apocrine. The epithelium of the intra- and inter-lobular excretory ducts suggests secretory activity. Tubulo-acinar glands similar to those seen in the lacrimal gland and nictitans glands are found related to the intralobular and main excretory ducts. The capillary network is characterized by fenestrated endothelium. The stroma possesses unmyelinated axons and plasma cells. The normal secretion of the secretory endpieces, particularly lipids, proteins and glycoconjugates, is complemented by mucous and serous secretions released by ductal cells and glands associated to the ducts.     

The vomeronasal organ of the South American armadillo Chaetophractus villosus (Xenarthra, Mammalia): anatomy, histology and ultrastructure

The vomeronasal organ (VNO) is a chemoreceptive structure that has not been extensively studied in the Xenarthran order. Tissue samples from the VNO of the armadillo Chaetophractus villosus were prepared for light and electron microscopy. The VNO is located in the anterior part of the base of the nasal septum. It is tubular in shape, ～ 18 mm in length and opens in the rostral region of the nasal cavity and with a blind caudal end. Its lumen is lined by sensory (SE) and nonsensory (NSE) epithelium. The SE shows sensory, supporting and basal cells whereas the NSE contains ciliated and nonciliated secretory cells and basal cells. At the ultrastructural level, the sensory cells appear as bipolar neurons with conspicuous microvilli on their free surface. The supporting cells of the SE contain numerous membrane-bound vesicles in their apical regions. A peculiar feature not found in other mammals, is the presence of concentric whorls of RER cisterns frequently observed in their basal expansions. Infiltrating plasma cells can be detected in the SE basal region close to the dorsal junctional area. This region also exhibits an unusual type of basal cell, probably responsible for the generation of new vomeronasal receptor neurons. The ciliated NSE cells exhibit numerous ovoids or irregularly shaped membranous protrusions projecting from the plasma membrane of the cilia. As far as we know, this is the first study reporting the presence of this feature in ciliated NSE cells. The nonciliated cells are characterised by scarce large secretory granules and apical microvilli. The vomeronasal glands are compound-branched tubuloacinar glands with serous acinar cells. Four types of secretory granules are present. The ducts of these glands reach the lumen in the dorsolateral region between the NSE and SE. Hypolemmal nerve terminals were observed contacting secretory cells. Fenestrated and nonfenestrated capillaries constitute the vascular supply to these glands. Plasma cells, intimately associated with acinar cells, were frequently observed.     

Principal coagulation factors and natural anticoagulants in the armadillo Chaetophractus villosus (Mammalia, Xenarthra, Dasypodidae)

Blood coagulation is an important process regulated by a large number of proteins, which transform fibrinogen into fibrin. This mechanism was identified in several species other than humans. In this work we study the coagulation factors and principal natural anticoagulants in armadillos (Chaetophractus villosus). Coagulation factors FVII, FXII, FIX, FXI, FX, FVIII, FV, antithrombin (AT), protein C (PC), and protein S (PS) were identified. The activities of all factors were homogeneous for both sexes. Correlation studies showed associations between FVII and FX, FVII and FIX, FVII and FXI, FIX and FXI, and FIX and FX. Sex-related differences were observed for PS but not for AT or PC. There was positive correlation between PS and FV in females. These data constitute the first contribution to the knowledge of the proteins participating in the coagulation in this species and they demonstrate that the hemostatic process is similar in the armadillos and man.     

Haemostatic mechanisms of the armadillo Chaetophractus villosus (Xenarthra, Dasypodidae)

Haemostatic mechanisms in humans are formed by two activation pathways, named tissular factor pathway and contact system pathway, and a common pathway, that promotes the formation of the fibrin clot. These mechanisms have also been identified in other mammals. In this work we evaluate their presence in the armadillo Chaetophractus villosus, a mammal of the neotropical Xenarthra Order. Twenty animals (ten males and ten females), previously adapted to captivity, were used. Fibrinogen (Fbg) was measured at the beginning and at the end of the experimental period; prothrombin time (PT), activated partial thromboplastin time (APTT) and thrombin time (TT) were measured at the same time during the experience. Neither Fbg nor the other parameters evaluated showed sex-related differences, but Fbg diminished during the experimental period. TT related to PT and APTT showed longer times and larger dispersion of the individual values. Compared to the human beings, PT and TT were longer and APTT was shorter. Considering those results, we conclude that C. villosus has the same basic coagulation system as human beings. However, in this species the contact system pathway seems to play a greater role in the activation of the coagulation system than it does in man, and the common pathway appears to limit the velocity of the system.     

Olfactory mucosa of the South American armadillo Chaetophractus villosus: An ultrastructural study

The sense of olfaction in armadillos plays an important role, suggested by the great development of the nasal structures, olfactory bulbs, and related brain regions. The mammalian olfactory mucosa is a privileged site of neuronal death and regeneration during the whole life span. A detailed knowledge of its ultrastructure is convenient for gaining insight into the factors controlling those phenomena. We performed this work in species not previously studied in order to provide a firm basis for further research on those factors. No information is available on the histology and ultrastructure of the olfactory mucosa in the order Xenarthra to which armadillos belong. Samples from the endoturbinals of the armadillo Chaetophractus villosus were prepared for light and electron microscopic examination by the usual conventional means. The olfactory epithelium of Chaetophractus villosus shows the classical three types of cells: supporting cells, olfactory receptor neurons, and basal cells. The olfactory neurons and the basal cells were similar to that described in other species. Two different types of supporting cells are described. An outstanding characteristic of the supporting cells is the normal presence of abundant phagosomes, apical secretory granules, apocrine-like protrusions, and highly developed smooth endoplasmic reticulum. Apoptotic bodies are frequently found in the infranuclear cytoplasm of supporting cells. The ductular epithelium of Bowman's glands reveals secretory activity. The lamina propria shows mixed Bowman's glands. Great development of smooth endoplasmic reticulum is observed in the mucous acinar cells. Evidence for merocrine and apocrine mechanisms in the Bowman's glands is presented. The presence of apoptotic bodies and phagosomes in supporting cells suggests a participation in the cellular events induced by cell death and proliferation of the olfactory epithelium. The variety of characteristics exhibited by the supporting cells of the olfactory mucosa may contribute to a deeper understanding of their scarcely known functions. Anat. Rec. 252: 325–339, 1998. © 1998 Wiley-Liss, Inc.     

Characterisation of glycoconjugate sugar residues in the vomeronasal organ of the armadillo Chaetophractus villosus (Mammalia, xenarthra)

Conventional carbohydrate histochemistry and the binding patterns of 21 lectins were analysed to characterise the glycoconjugate content in the components of the vomeronasal organ of the armadillo Chaetophractus villosus . The mucomicrovillous complex of the sensory epithelium bound most of the lectins studied. No reaction was observed with Con A, PSA, S-Con A and SBA, and the sustentacular cells were stained with UEA-I, DSL, LEL, STL and Con A. The vomeronasal receptor neurons were labelled with S-WGA, WGA, PNA, UEA-I, STL, Con A, S-Con A, ECL and RCA₁₂₀. The basal cell layer reacted with S-WGA, WGA, LCA, UEA-I, DSL, LEL, STL, Con A, JAC and VVA. The nonsensory epithelium exhibited a differential staining in relation to the different components. The mucociliary complex stained with ECL, DBA, JAC, RCA₁₂₀, STL, LCA, PHA-E, PHA-L, LEL, BSL-I and VVA. However, SJA and UEA-I stained the mucus complex lining a subpopulation of columnar cells. The cytoplasm and cell membranes of columnar cells was labelled with DBA, DSL and LCA. The apical region of these cells exhibited moderate reactivity with LEL and SJA. None of the lectins bound specifically to secretory granules of the nonsecretory cells. Basal cells of the nonsensory epithelium were labelled with DSL, LEL, LCA, BSL-I and STL. The vomeronasal glands showed a positive reaction with WGA, DSL, LEL, LCA, DBA, PNA, RCA₁₂₀ and SBA. Subpopulations of acinar cells were observed with ECL, S-WGA, Con A, S-Con A and DBA. PNA and RCA₁₂₀ stained the cells lining the glandular ducts. In comparison with previous results obtained in the olfactory mucosa of the same group of armadillos, the carbohydrate composition of the vomeronasal organ sensory epithelium differed from the olfactory sensory epithelium. This is probably related to the different nature of molecules involved in the perireceptor processes.     

Identification and localisation of glycoconjugates in the olfactory mucosa of the armadillo Chaetophractus villosus

Conventional histochemistry and the binding patterns of 22 biotinylated lectins were examined for characterisation of glycoconjugates in the components of the olfactory mucosa of the armadillo Chaetophractus villosus . The mucous lining the olfactory epithelium showed binding sites for DSL, WGA, STL, LEL, PHA-E and JAC. Only the basilar processes of the supporting cells stained for Con-A and S-Con A. The olfactory receptor neurons stained with LEL, LCA, Con A, S-Con A, JAC and PNA. The layer of basal cells did not react with any of the lectins studied. Bowman's glands in the lamina propria showed subpopulations of acinar cells reacting with SBA, S-WGA, WGA, STL, Con A, PSA, PNA, SJA, VVA, JAC and S-Con A, but in our optical studies with lectins we were unable to differentiate between mucous and serous cells in the way that is possible on electron microscopy. The ducts of Bowman's glands were labelled with S-WGA, STL, LEL, PHA-E, BSL-I and JAC. This histochemical study on the glycoconjugates of the olfactory mucosa in the order Xenarthra provides a basis for further experimental investigations.     

Harderian gland and the lacrimal gland of the lizard Podarcis s. sicula: histology, histochemistry, and ultrastructure

Histology, histochemistry, and ultrastructure of the Harderian gland and lacrimal gland of the lizard Podarcis s. sicula were investigated. The Harderian gland, located at the medial corner of the orbit, can be divided into three zones showing different tinctorial features either with Mallory or hematoxylineosin stains. The glandular cells of the acinar medial zone secrete predominantly acidic sulphated mucosubstances. The acinar cells of the intermediate zone contain secretory granules that show a weak reaction to the histochemical tests for mucosubstances. The lateral zone has a tubulo-acinar type of structure and tests strongly for proteins, whereas Alcian-PAS staining is very weak. The lacrimal gland is smaller than the Harderian gland and lies in the region of the posterior commissure of the eyelids. it shows the same histological and histochemical characteristics of the medial zone of the Harderian gland, i.e., it is mucous secreting. At the ultrastructural level the zonation is well defined, especially when the secretory granules are examined. Granules of the mucoid type are found in the lacrimal gland and the medial zone of the Harderian gland. The secretory granules of the lateral part of the Harderian gland show a composite structure never described before. Therefore, they have been called "special secretory granules." Each of these granules is composed of three sharply separated components. It is not known whether the three components correspond to different secretions. Histochemical tests suggest that they are of the serous type. Both mucous and serous granules are secreted by the same glandular cells of the intermediate zone of the Harderian gland. The two types of granules usually occupy different cell compartments. The mechanism of secretion appears either merocrine or apocrine in both the Harderian gland and the lacrimal gland.     

Morphological observations on the Harderian gland of the North American opossum (Didelphis virginiana)

Summary The Harderian gland of the North American opossum (Didelphis virginiana) is large and well developed, despite the absence of a nictitating membrane in the adult of this species. The elongate glands are surrounded by a delicate connective tissue capsule from which thin septae extend, subdividing the gland into numerous lobules. The secretory units of the opossum Harderian gland are drained by a well defined but not extensive intralobular and interlobular duct system. Most of the secretory end pieces consist of tubuloalveolar units with widely dilated lumina filled with secretory product. Numerous intact lipid vesicles suspended within an amorphous material constitute the luminal contents. Cells lining the tubuloalveolar secretory endpieces are usually columnar in shape, and characterized by numerous lipid-containing secretory vesicles and aggregations of polytubular complexes 40–60 nm in diameter. In addition, these cells contain numerous large irregularly shaped mitochondria, whose matrix is of considerable electron density. Intralobular and interlobular ducts are lined by electron-lucent epithelial cells that lack both the lipid-containing vesicles and the large mitochondria, although typical smaller mitochondria are found scattered within the cytoplasm. Both secretory endpieces and ductal elements are invested by an abundance of myoepithelial cells. A second, smaller serous type of secretory unit may occur near the centre of some Harderian gland lobules. In these units secretory tubules and acini are compactly arranged surrounding a narrow lumen. Serous cells are pyramidal in shape and the cytoplasm is characterized by numerous electron-dense secretory granules and scattered profiles of rough endoplasmic reticulum. Basolateral cell membranes show extensive infoldings and intercellular canaliculi are present. The overall size of cells forming the serous secretory units is much less than that comprising the tubuloalveolar secretory endpieces.     

Fibrinolytic system of the armadillo <Emphasis Type="Italic">Chaetophractus villosus</Emphasis> (Xenarthra,Dasypodidae)

The fibrinolytic mechanism in the armadillo Chaetophractus villosus (Mammalia, Xenarthra, Dasypodidae) quite unknown until now was studied. Results were compared with those corresponding to healthy adult human beings. Whole blood lysis time and diluted blood lysis time were not detectable in armadillos. Euglobulin clot lysis time and plasminogen activity (Plg) were lower than in healthy humans. We established the presence of the fibrinolytic system in Ch. villosus through the measurement of fibrin fibrinogen degradation products. The activity of the plasminogen activator inhibitor was two to four times greater than in healthy humans. The activity of the alpha 2 anti-plasmin (α2AP) was similar and displaced toward lower values. The Plg/α2AP relation was lower. The results obtained suggest that Ch. villosus has a hypercoagulable and hypofibrinolytic profile. Our findings are not only the first contribution to elucidate the physiology of the fibrinolytic system in Xenarthra but also contribute to develop an animal model for studies in haemostasis and thrombosis.     

The fine structure of ducts and subareolar ducts in the resting gland of the female breast

Summary The ultrastructure of the ducts and subareolar ducts of the resting female breast have been described. In transverse section ducts have longitudinal folds, some of which are solid ridges of cells. Four distinctive cell types were distinguished; epithelial cells, lymphocyte type cells, basal clear cells and myoepithelial cells. The epithelium is single layered, multiple at folds, and similar in general morphology to the terminal ductal-lobular unit. Well developed terminal bars may prevent cellular disruption during contraction, and apical cytoplasmic vesicles represent possible secretory material. Basal cytoplasmic bodies may represent transport of secretory products into or from the periductal stroma. Intranuclear vesicles may also be linked to secretory activity. The myoepithelium is well differentiated with numerous cytoplasmic filaments and 9+0 cilia, forming a discontinuous layer around the epithelium. The basal lamina is generally multilaminate. Capillaries are 1–5 m in diameter and exterior to the delimiting fibroblasts of the epithelial-stromal junction.     

Axonal projections and conduction properties of olfactory peduncle neurons in the armadillo (Chaetophractus vellerosus)

Summary Extracellular unit recording was employed to study the axonal properties and efferent projections of antidromically identified neurons in the olfactory peduncle (OP) region of a primitive eutherian macrosmatic mammal, the south american armadillo (Chaetophractus vellerosus). Of 72 cells which satisfied the criteria for antidromic invasion, 55 (76%) and 17 (24%) responded to ipsi- and contralateral olfactory bulb (IOB; COB) stimulation, respectively. The absolute refractory period (3.25±0.3 ms; mean±SE) and the conduction velocity (CV; 1.94±0.2 m/s; mean±SE) of IOB and COB driven neurons were negatively correlated (r=-0.52; p < 0.001). In paired-shock tests (8–1950 ms interval), and early supernormal period (SPN) of increased CV and excitability was found following the relative refractory period in 82% of tested cells (N=50); this period was followed by a late subnormal phase (SBN) of decreased CV and increased threshold in 58% of neurons (N=50). Significant correlations were found to exist between: CV and absolute magnitude of latency variation (r=-0.55; p < 0.001; n=43), CV and duration of SPN and SBN periods (r=-0.60; p < 0.002; n=24 and r=0.58; p < 0.02; n=19, respectively) and between duration of SPN and SBN phases (r=0.79; p < 0.001; n=30). Maximum latency variation during the SPN and SBN periods was attained in a gradual, additive manner. 90% of long-latency units (15–30 ms; n=22) showed progressive increases in initial antidromic latency (16–22% of control) when stimulated with pulse frequencies of 2.5, 10,15, 20 or 40 Hz; initial antidromic latency and magnitude of latency increase were positively correlated (r=0.92). Supported by grants from CONICET, CONICOR and FPPM, Buenos Aires and Cordoba, Argentina     

Ultrastructural characterisation of the olfactory mucosa of the armadillo Dasypus hybridus (Dasypodidae, Xenarthra)

The ultrastructure of the olfactory mucosa of the armadillo Dasypus hybridus was studied. A comparison with the olfactory mucosa of another armadillo ( Chaetophractus villosus ) was made. The olfactory mucosa of D. hybridus shows many features which are similar to those of other mammals. Interestingly, it differs from the olfactory mucosa of the armadillo C. villosus . A suggestion is made that these differences may be due to differences in the digging habits of these species. In Dasypus , the supporting cells (SCs) showed dense vacuoles, multivesicular bodies and lysosome-like bodies probably related with the endocytotic system. The SCs show a dense network of SER presumably associated with xenobiotic mechanisms. The olfactory receptor neurons exhibit lysosome-like bodies and multivesicular bodies in their perikarya. These organelles suggest the presence of an endocytotic system. Duct cells of Bowman's glands exhibit secretory activities. Bowman's glands are compound-branched tubulo-acinar mixed glands with merocrine secretory mechanisms.     

The middle ear of the pink fairy armadillo Chlamyphorus truncatus (Xenarthra,Cingulata, Chlamyphoridae): comparison with armadillo relatives using computed tomography

The pink fairy armadillo Chlamyphorus truncatus is the smallest extant armadillo and one of the least-known fossorial mammals. The aim of this study was to establish if its middle ear is specially adapted to the subterranean environment, through comparison with more epigeic relatives of the groups Euphractinae (Chaetophractus villosus, Chaetophractus vellerosus, Zaedyus pichiy) and Dasypodinae (Dasypus hybridus). We examined the middle ears using micro-computed tomography and subsequent three-dimensional reconstructions. D. hybridus has a relatively small middle ear cavity, an incomplete bulla and ‘ancestral’ ossicular morphology. The other species, including Chlamyphorus, have fully ossified bullae and middle ear ossicles, with a morphology between ‘transitional’ and ‘freely mobile’, but in all armadillos the malleus retains a long anterior process. Unusual features of armadillo ears include the lack of a pedicellate lenticular apophysis and the presence, in some species, of an element of Paaw within the stapedius muscle. In common with many subterranean mammals, Chlamyphorus has a relatively flattened malleo-incudal articulation and appears to lack a functional tensor tympani muscle. Its middle ear cavity is not unusually enlarged, and its middle ear ossicles seem less robust than those of the other armadillos studied. In comparison with the euphractines, there is no reason to believe that the middle ear of this species is specially adapted to the subterranean environment; some aspects may even be indicative of degeneration. The screaming hairy armadillo, Chaetophractus vellerosus, has the most voluminous middle ear in both relative and absolute terms. Its hypertrophied middle ear cavity likely represents an adaptation to low-frequency hearing in arid rather than subterranean conditions.     

Observations on the fine structure of the lateral vestibular nucleus (Deiters' nucleus) in the cat

Summary A study has been made of the ultrastructure of the lateral vestibular nucleus of the normal cat. The study includes light microscopical observations made in Golgi material. The internal structure of the various types of cells is described. The soma of the larger nerve cells is surrounded by a protoplasmic layer, constituted by astroglial sheets, dendrites and boutons; glial cell bodies are usually located outside the layer. The smaller nerve cells display few axosomatic synapses and may be in direct contact with myelinated fibres and glial perikarya. Spines are present on the soma of large and small nerve cells and on all parts of the dendrites. The proximal dendrites are usually the part of the neurons which is most amply supplied with boutons.Various types of boutons and cell junctions are described and an attempt is made to correlate the findings in electron micrographs with those made in Golgi sections.The study serves as a basis for observations made in experimental material where afferent fibres to the nucleus have been damaged.This investigation has been supported in part by Grant NB 02215-07 from the National Institute of Neurological Diseases and Blindness, US-Public Health Service. This aid is gratefully acknowledged.     

Lectin histochemistry of gastrointestinal glycoconjugates in the greater horseshoe bat, Rhinolophus ferrumequinum (Schreber, 1774)

Mucins in the gastrointestinal tract of Rhinolophus ferrumequinum were investigated by histochemistry and lectin histochemistry to evaluate morphofunctional variations of different regions and their possible physiological and evolutionary implications. Histochemical methods included periodic acid-Schiff (PAS), Alcian blue (AB) at pH 2.5 and 1.0 and high-iron-diamine AB pH 2.5. Binding of lectins Con A, DBA, WGA, LTA, LFA, PNA and SBA; LFA, PNA and SBA with prior sialidase treatment; and paradoxical Con A were evaluated. The oesophagus lacked glands. The stomach was divided into a short cardias, a wide fundus and a brief pylorus. The surface muciparous cells secreted sulpho- and sialomucins with N-acetylgalactosamine (GalNAc) residues, N-acetyllactosamine and (beta1,4 N-acetylglucosamine)(n) chains. Towards the pylorus, N-acetylgalactosamine residues disappeared and acidity decreased. Cardiac glands, neck cells in the fundic glands, pyloric and duodenal Brunner's glands all shared neutral, stable class-III mucins, mainly with N-acetylgalactosamine sequences. The intestine was divided into a duodenum, a jejuno-ileum and a short rectum. The goblet cells produced sulpho- and sialomucins with sialylated N-acetylgalactosamine sequences, (beta1,4 N-acetylglucosamine)(n) and N-acetyllactosamine, whose sialylation increased towards the rectum. The main features of the mucins are probably associated with the requirements of fast absorption and food passage and in protection against mechanical and pathogenic injuries.     

On the Olfactory Anatomy in an Archaic Whale (Protocetidae,Cetacea) and the Minke Whale Balaenoptera acutorostrata (Balaenopteridae,Cetacea)

The structure of the olfactory apparatus is not well known in both archaic and extant whales; the result of poor preservation in most fossils and locational isolation deep within the skulls in both fossil and Recent taxa. Several specimens now shed additional light on the subject. A partial skull of an archaic cetacean is reported from the Pamunkey River, Virginia, USA. The specimen probably derives from the upper middle Eocene (Piney Point Formation) and is tentatively assigned to the Protocetidae. Uncrushed cranial cavities associated with the olfactory apparatus were devoid of sediment. CT scans clearly reveal the dorsal nasal meatus, ethmoturbinates within the olfactory recess, the cribriform plate, the area occupied by the olfactory bulbs, and the olfactory nerve tract. Several sectioned skulls of the minke whale (Balaenoptera acutorostrata) were also examined, and olfactory structures are remarkably similar to those observed in the fossil skull from the Pamunkey River. One important difference between the two is that the fossil specimen has an elongate olfactory nerve tract. The more forward position of the external nares in extant balaenopterids when compared with those of extant odontocetes is interpreted to be the result of the need to retain a functional olfactory apparatus and the forward position of the supraoccipital/cranial vertex. An increase in the distance between the occipital condyles and the vertex in balaenopterids enhances the mechanical advantage of the epaxial musculature that inserts on the occiput, a specialization that likely stabilizes the head of these enormous mammals during lunge feeding. Anat Rec, 2013. © 2012 Wiley Periodicals, Inc.     

The fine structure of ampullary and tuberous electroreceptors in the South American blind catfish Pseudocetopsis spec

Summary Two types of electroreceptors, the ampullary and the tuberous electroreceptor (silurid knollenorgan) in the epidermis of the catfish, Pseudocetopsis spec., were investigated with semithin and ultrathin serial sections. The ampullary organ contains one or two sensory cells which are embedded in supporting cells at the base of open epithelial canals. They bear some slender microvilli on their apical surface and form several synaptic bars. The afferent myelinated nerve fiber arborizes in the connective tissue papilla and looses its myelin sheath about 30 m below the supporting cell layer. A second thin myelinated axon occur up to the supporting cell layer. The tuberous electroreceptor organ contains one large receptor cell. Most of the cell body is exposed to the lumen of a specialized proximal canal segment and is closely covered with microvilli. A single myelinated axon looses its myelin sheath within the supporting cell layer about 1 m before terminating as a flat calyx at the base of the sensory cell. A functional significance of the two types of receptors will be discussed.