Studies on the structure,ultrastructure and function of the subcommissural organ-reissner's fibre complex of the European eel Anguilla anguilla L.

Summary The Subcommissural organ (SCO) of the European eel consists of tall columnar cells which secrete Reissner's fibre (RF) into the third ventricle and store colloid-like materials of differing electron densities in their hypendymal regions. The cells of the SCO carry large cilia and numerous microvilli.Function of the SCO-RF complex has been investigated by an in situ staining technique using aldehyde fuchsin and by the intracranial injection of ³⁵S-cysteine combined with autoradiography. Osmotic stimuli had no effect on stainability of the complex, though indications were obtained from the autoradiographic studies that it was rendered more active by transferring eels from sea water to fresh water. The effects of changes in background colouration and total illumination were investigated using the in situ technique and it was found that a change from a black background to a white one and from a black background to darkness appeared to stimulate the complex. It is shown that activity of the SCO-RF complex, unlike that of the hypothalamo-neurohypophysial system does not appear to be affected by stimuli that may be regarded as merely stressful.The findings are evaluated and discussed in the light of previous work and it is suggested that the use of radioactive tracers offers the best chance of success in establishing the function of the complex.     

Über den Sekretionsvorgang im Subcommissuralorgan eines Knochenfisches (Pristella riddlei Meek)

Zusammenfassung Die licht- und elektronenmikroskopische Untersuchung des Subcommissuralorgans von Pristella riddlei (Meek), eines Knochenfisches aus der Familie der Characidae, ergab weder sichere Hinweise auf eine Innervation durch den Tractus pinealis, noch konnte ein unmittelbarer Kontakt der sekretorischen Zellen mit dem Blutgefäßsystem festgestellt werden. Das Sekret wird in den 3. Ventrikel abgegeben, wo es den Reissnerschen Faden bildet. An der intracellulären Sekretbereitung sind das rauhe endoplasmatische Reticulum und der Golgi-Apparat beteiligt.

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Summary A light and electron microscopic study of the subcommissural organ of Pristella riddlei Meek (Osteichthys, Characidae) furnished neither evidence for an innervation by the pineal tract, nor for the direct contact of secretory cells with the blood vessel system. The secretory material is released into the third ventricle where it forms Reissner's fibre. The rough endoplasmic reticulum and the Golgi apparatus take part in the intracellular production of the secretory material.

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Ausgeführt mit dankenswerter Unterstützung durch die Deutsche Forschungsgemeinschaft.     

Cytochemical localization and function of the 3-linked glucan callose in the developing cotton fibre cell wall

Summary Several recent biochemical studies concerning the hemicellulosic content of the developing cotton fibre wall have pointed to an important increase of 1,3-linked glucans at the onset of the secondary wall formation and their slow decrease until the end of fibre development (Meinert andDelmer 1977,Huwyler et al. 1978, 1979,Maltby et al. 1979). These almost insoluble glucans are extra-cellular and possibly associated with the S₁ or winding layer, but no other data on their exact localization were given.By means of a specific fluorescence method, using a 0.05% decolourized aniline blue solution, we show that one of these 3-linked glucans,callose, is always localized, independently of the fibre age, in the innermost wall layer bordering the cell lumen, from the onset of the secondary thickening up to the end of fibre development. Some possible roles assumed by these callose deposits are suggested and discussed. They may be involved in the normal mechanism of cellulose biosynthesis, as being effectively consumed by turnover or, more probably, as forming a permanently restored interface or matrix where cellulose microfibrils undergo a sort of maturation and are oriented before their definitive incorporation in the organized cell wall. They are not to be confused with the wound callose deposits which characterize damaged or immature fibres.     

The sensory innervation of the pineal organ in the lizard,Lacerta viridis,with remarks on its position in the trend of pineal phylogenetic structural and functional evolution

Summary The sensory innervation of the pineal organ of adult Lacerta viridis has been investigated. Some specimens of Lacerta muralis lillfordi were also used. In the pineal epithelium, a small number of nerve cell pericarya of a sensory type are present. They lie either solitary or in small clusters close to the basement membrane. The axons originating from the nerve cell bodies, i. e. the pineal sensory nerve fibers, first course in the intraepithelial nerve fiber layer which is only locally present and contains a restricted number of unmyelinated fibers. In Lacerta viridis, the pineal fibers generally leave the epithelium at the proximal part of the organ proper. They then form small bundles which run along the outer surface of the basement membrane in the leptomeningeal connective tissue covering. At the proximal end of the pineal stalk the single bundles assemble constituting the pineal nerve. In Lacerta muralis the fibers leave the pineal epithelium at the proximal end of the stalk running farther down within the epithelium. Many fibers become myelinated after leaving the pineal epithelium. The pineal nerve runs ventralward in the midplane just caudal to the habenular commissure to which no fibers are given off. Continuing their ventralward course between the habenular commissure and the rostral end of the posterior commissure which is traversed by some of them, the pineal fibers reach the dorsal border of the subcommissural organ. Small separate aberrant pineal bundles traverse the posterior commissure at various more caudal levels. Having reached the dorsal border of the subcommissural organ, part of the pineal fibers continue their ventralward course directly running along the lateral sides of this organ to reach the periventricular nerve fiber layer lateral and ventral to it. A restricted number of fibers first turns in a caudal direction running between the base of the posterior commissure and the base of the subcommissural organ before turning ventralward to reach the periventricular layer. Most probably, pineal fibers do neither join the posterior commissural system nor innervate the subcommissural organ. Once having reached the periventricular layer, some pineal fibers curve in a rostral direction while others, before doing so, send a collateral in a caudal direction. Both, the main fibers and the collaterals, contribute to the formation of the periventricular layer. The sites of termination of the pineal fibers could not be ascertained.From the presence of intraepithelial sensory nerve cell bodies and from literature data on the ultrastructure of pineal neurosensory cells it is concluded that the adult pineal organ of Lacerta has a, although rudimentary, (photo)sensory function. The demonstration by our guest-worker Dr. W. B. Quay, of the intraepithelial presence of a tryptamine compound, probably serotonin, points, moreover, to a secretory function of this organ.In adult Lacerta a well-developed parietal nerve connects the parietal eye with the left lateral habenular nucleus. It traverses the habenular commissure.In gratitude and with admiration this paper is dedicated to Prof. Berta Scharrer and to the memory of Prof. Ernst Scharrer.     

Hypothalamic control of the pars intermedia in Xenopus laevis tadpoles

Summary In Xenopus laevis tadpoles the relation between a paired nucleus of bio-amine producing neurons in the caudal hypothalamus and the pars intermedia of the hypophysis was studied.Treatment of the animals (stage 49 to 50 of Nieuwkoop and Faber's normal table) with reserpine caused aggregation of the skin melanophores within one hour, followed by redispersion five to six hours after the beginning of the experiment. This was at exactly the same time as the bio-amines in the caudal hypothalamus disappeared. However, the drug was ineffective if the nuclei had been removed. This indicates that reserpine acts via these nuclei and does not influence the skin melanophores directly.It was concluded that the initial aggregation of the melanophores may be the result of a reduced extrusion of MSH from the pars intermedia, caused by an increased output of a MIF by the bio-amine producing nuclei. The redispersion was explained by assuming that the bio-amines were depleted and the nuclei stopped with the extrusion of the MIF. This does not mean that the production of a MIF is the only function of the paired bio-amine producing nucleus in the caudal hypothalamus.The author thanks Prof. Dr. P. G. W. J. van Oordt for his helpful comments and criticism. Mr. J. H. I. J. M. ten Berge and Mr. E. W. A. Kamperdijk provided great assistance during the course of the experiments. Mr. H. van Kooten made the diagram and the photograph.     

The cytochemistry of glycoconjugates in the planum nasolabial gland of the goat as studied by electron microscopic methods

Summary In the planum nasolabial glands of the goat, glycoconjugates of glandular and duct cells have been studied by means of a series of electron microscopic cytochemical methods. In the glandular cells glycoconjugates with vicinal diol groupings were present in secretory granules, certain elements of the Golgi complex, lysosome-like dense bodies, the surface coat of the plasma membrane, the majority of intracellular cytomembranes, glycogen particles and the basal lamina. In duct cells, glycoconjugates with the same properties were localized in similar ultrastructures, except for secretory granules, which were not detected in these cells. By lectin cytochemistry, glycoconjugates in glandular cell secretory granules contained a variety of saccharide residues such as -d-mannose, -d-glucose,N-acetyl-d-glucosamine and -l-fucose. The cytochemical properties of the secretory glycoconjugates are discussed in relation to the physiological functions performed by the planum nasolabial glands in the goat.     

Immunohistochemical localization of urotensin I and other neuropeptides in the caudal neurosecretory system of three species of teleosts and two species of elasmobranchs

Summary In three species of teleosts — carp Cyprinus carpio; grass carp Ctenopharyngodon idella; and crucian carp Carassius auratus — the caudal neurosecretory system displays small, medium-sized and large neurons. Urotensin I (UI)-immunoreactive and UI-nonreactive neurons were found in all three groups; in general, the number of the latter neurons exceeded that of the former. Noteworthy are: (i) UI-immunoreactive fibers in the caudal spinal cord and (ii) dense accumulations of UI-immunoreactive product around the capillaries of the urophysis. In two species of elasmobranchs — cat shark Heterodontus japonicas and swell shark Cephaloscyllium umbratile — neurosecretory neurons decreased in size in rostro-caudal direction. Most of the neurosecretory perikarya, their axons and the corresponding neurohemal areas were UI-immunoreactive, but a small number of secretory neurons was devoid of immunoreaction. Oxytocin, arginine vasopressin, substance P, somatostatin, neurotensin, vasoactive intestinal polypeptide and gastrin-releasing peptide were not detected in the caudal neurosecretory system of the carp.     

Neural and Glial Secretion in the Spinal Cord of the Lamprey (Lampetra planeri)

In the spinal cord of Lampetra planeri neurosecretion and glial secretion could be demonstrated with histochemical methods and the aid of the electron microscope. Neurosecretion is present in either all or some “dorsal cells” which are scattered along the whole spinal cord. These cells have the characteristic fine structure of highly active secretory neurons. The cells produce three types of secretory material. One not bound to vesicles in form of a finely granulated substance, a second one in spherical, uniformely electron-dense elementary vesicles with a diameter of 100–140 nm, and a third one in vesicles of the monoamine type. In the most caudal part of the spinal cord glial secretion is present. Groups of glial cells produce a fine, granulated material and release it into the adjacent intercellular space. From there the material extends into other intercellular spaces of the spinal cord. We are of the opinion that the “dorsal cells” are forerunners of the cells of Dahlgren in higher vertebrates and that the neurosecretion and glial secretion in the spinal cord of lampreys demonstrate very primitive conditions.     

Secretory activity of the subcommissural organ in Rana temporaria under osmotic stimulation

Summary The secretory activity of the subcommissural organ (SCO) in the frog Rana temporaria was studied under conditions of dehydration. After injection of a radioactive precursor the amount and concentration of radioactively labelled material in the SCO are smaller in dehydrated than in control animals. Concomitantly, the growth rate of the CSF-fibre (Reissner's fibre) increases in dehydrated animals. It follows that water deprivation enhances the secretory activity of the SCO.To investigate whether the SCO may be responsible for the secretion of an aldosteronotropic factor as suggested in the literature, brains were incubated in vitro with a radioactive precursor and with or without aldosterone. The SCO of the aldosterone-treated brains contains more radioactively labelled material than the SCO of the control brains. It is argued that this is indicative of a lower secretory activity. It means that aldosterone inhibits the secretory activity of the SCO, possibly by a process of negative feed-back regulation. The results of the present experiments can be interpreted in favour of an involvement of the SCO-Reissner's fibre complex in osmoregulation.     

Heterogeneity of astrocytic membranes in the optic nerve and spinal cord of the lizard,Anolis carolinensis

Summary The architecture of astrocytic membranes in the optic nerve and the spinal cord of the lizard, Anolis carolinensis, was investigated by use of the freeze-fracturing technique. Whereas astrocytes in mammals reveal so-called orthogonal arrays of particles (OAPs) in their membranes, astrocytes in lower vertebrates lack these structures. This study demonstrates for the first time OAPs in astrocytes from a submammalian species. They were found commonly in the optic nerve and less frequently in the spinal cord. However, the OAPs in astrocytes of spinal cord were confined to midtrunk levels; the astrocytes in the caudal spinal cord failed to reveal OAPs.Additionally, the ependymal cells around the central canal did not show any OAPs, either in the thoracic or in the caudal spinal cord. They were interconnected by gap and tight junctions, which were not intercalated with each other.The findings support our current working hypothesis according to which the presence and absence of OAPs in astrocytes may be correlated with regenerative incapability or capability of CNS-structures; i.e., whereas the thoracic spinal cord in Anolis carolinensis is known to be incapable of regeneration after injury, the caudal spinal cord is regenerative.     

Contributions to the morphology,anatomy, and karyology ofRhabdodendron,and a reconsideration of the systematic position of theRhabdodendronaceae

Rhabdodendron macrophyllum (Spruce ex Benth.)Huber andR. amazonicum (Spruce exBenth.)Huber differ in several anatomical and morphological characters (secretory cavities, hypoderm, peltate hairs, internodal region and petiole). A position of the monotypicRhabdodendronaceae in theCentrospermae as recently suggested is hardly supported: Peltate hairs, lysigenous secretory cavities and spicular cells in the leaves, multilacunar nodes, chromosome number (R. macrophyllum: n = 10; first count for the genus resp. family), ± simultaneous (or slightly centripetal) development of the androeceum, anacrostyly and two ovules in the unicarpellate gynoeceum, apparent disc, monotelic racemes, and data available from literature (pollen, sieve tube plastids) clearly indicate a close affinity toRutaceae, and even make the family rank ofRhabdodendron questionable.     

Differential distribution of lectin-binding glycoconjugates in the secretory granules of hamster oviductal ampulla during the estrous cycle: a quantitative cytochemical analysis

 High resolution lectin-gold cytochemistry was used to quantitatively analyze the distribution of glycoconjugates in the hamster oviductal ampulla during the five stages of the estrous cycle. Lectins binding to N-acetyl-d-galactosamine-, d-galactose-, and sialic acid-associated glycoconjugates in the secretory granules of ampullary epithelial secretory cells showed staining of equal intensity throughout the five different stages of the estrous cycle. In contrast, the labeling intensity of glycoconjugates which contain N-acetylglucosamine as terminal sugar residues reached its maximum around the time of ovulation, i.e., at proestrus. Glycoconjugates which carry fucose and mannose as terminal sugar residues appeared to be totally absent from the secretory granules of the oviductal ampulla during the estrous cycle. Together, electron microscopic observations combined with quantitative results indicate that N-acetyl-d-galactosamine-, d-galactose-, and sialic acid-associated glycoconjugates may be secreted into the ampullary lumen irrespective of the stage of the estrous cycle, whereas the secretion of certain N-acetylglucosamine-associated glycoconjugates is stage specific and reaches its peak at the time of ovulation. These findings suggest that, at the time of ovulation, the ampullary epithelium changes its secretory activity and contributes its secretory products to the zona pellucida of oocytes freshly released from the ovary. Accepted: 10 August 1998     

The caudal spinal cord of coho salmon (Oncorhynchus kisutch): Immunocytochemical evidence of a “caudal serotoninergic system”

Summary The caudal spinal cord of the coho salmon was investigated by means of immunocytochemistry using antisera against serotonin, urotensin I, urotensin II, somatostatin and a urea-extract of bovine Reissner's fiber (AFRU). Populations of serotonin-immunoreactive (IR) neurons were found rostral and dorsal to the urophysis in close spatial association with caudal secretory neurons. Thick, smooth serotonin-IR processes extended toward the external surface of the spinal cord where they displayed conspicuous terminal dilatations. Thin, beaded serotonin-IR fibers appeared to innervate populations of caudal secretory and somatostatin-IR cerebrospinal fluid-contacting neurons. Most caudal neurosecretory cells displayed both urotensin I and urotensin II immunoreactivities; only a minority reacted exclusively with either urotensin I or urotensin II antisera. Urotensin II-IR and somatostatin-IR cerebrospinal fluid (CSF)-contacting neurons were found as an integral component of the central canal wall in the caudal spinal cord and filum terminale; their dendritic processes appeared to contact Reissner's fiber, which displayed a weak AFRU-immunoreactivity while inside the central canal, but became strongly reactive in the interior of the terminal ventricle as it formed the massa caudalis. The distribution of serotoninergic processes points to a regulatory role in the function of caudal secretory and CSF-contacting neurons and to a putative serotonin release into the subarachnoid space and/or meningeal vasculature. It is also suggested that the CSF-contacting neurons of the central canal may participate in a feedback mechanism controlling the secretory activity of the subcommissural organ.Supported by Grant A/1095-1 from the International Foundation for Science, Sweden, to C.Y.; Grant I/63-476 from Volkswagen-Stiftung to E.R.; and Grant S-85-39 from the Dirección de Investigaciones, Universidad Austral de Chile     

Neuropharmacological analysis of synaptic transmission in the Lorenzinian ampulla of the skate Raja clavata

Summary Dissected ampullae of Lorenzini of the skate (Raja clavata) were studied with the aim of determining the synaptic transmitter between electroreceptor cell and afferent fibre. Resting activity and stimulus-evoked activity in response to electrical pulses were recorded in single afferent units at constant perfusion with normal and test solutions containing different putative neurotransmitters. Presynaptic transmitter release was blocked by Mg²⁺ (up to 50 mM) to investigate the effects of the test substances upon the postsynaptic membrane. l-Glutamate (l-GLU) and l-aspartate (l-ASP), both at concentrations between 10^-7 and 10^-3 M, enlarged strongly resting and stimulus-evoked discharge frequency in the afferent fibre. If transmission was blocked by high Mg²⁺, resting discharge frequency could be restored by l-GLU or l-ASP. The glutamate agonists quisqualate (10^-8–10⁵ M) and N-methyl-D-aspartate (10^-5–10^-3 M) enlarged spontaneous activity in the afferent fiber. The same was found for kainic acid (10^-9–10^-5 M). Taurine at concentrations between 10^-5 and 10^-3 M caused a concentration-dependent decrease in afferent activity. The same was found for gammaaminobutyric acid (GABA; 10^-5–10^-4 M), and for the catecholamines adrenaline and noradrenaline, both in concentrations between 10^-5 and 10^-3 M. Serotonine (10^-5–10^-3 M) and dopamine (10^-5-10^-3 M) had no effect on resting or evoked activity in the Lorenzinian ampulla afferents. Acetylcholine (ACh; 10^-4 M) enlarged discharge frequency in those units with initial rates lower than 22–25 Hz, but diminished discharge frequency in fibres with initial activity higher than 25 Hz. When synaptic transmission was blocked by high Mg²⁺ solution, perfusion with additional ACh did not restore resting activity in the afferent fibre. The results suggest that the most probable transmitter in the afferent synapse of the ampullae of Lorenzini is l-GLU or l-ASP, or a substance of similar nature.Abbreviations ACh acetylcholine - GABA gamma aminobutyric acid - KA kainic acid - l-ASP l-aspartate - l-GLU l-glutamate - NMDA N-methyl-D-aspartate - Q quisqualate - n.s. normal solution     

The neurosecretory system of the adult Calliphora erythrocephala

Summary Neurosecretory cells in the brain and suboesophageal ganglion of the adult female Calliphora erythrocephala have been studied in the light microscope. The paraldehydefuchsin stain (PAF) gave by far the clearest pictures.The medial neurosecretory cells of the protocerebrum (m.n.c.) show definite cyclic changes as to the size of the nuclei and the content of secretory material. The cytological changes depend on the age of the fly and the diet given and are correlated with ovarian development.The nuclear size is held to express the metabolic activity of the cells. Cells with large nuclei, as found in young sugar-flies (S1D) and meat-fed flies with developing ovaries (S4D/M2D), contain less secretory material which is released through the axons, while the m.n.c. of old sugar-flies (S6D) have small nuclei and are stuffed with secretory material which is stored in the perikarya.These results confirm those obtained by darkfield microscopy of living m.n.c. by Lea and E.Thomsen (1962 and unpublished).No really convincing evidence for the existence of more than one type of m.n.c. was found.Two small groups of lateral cells were observed. Possibly neurosecretory are further: 1-(2) cells at the base of each optic lobe, two groups of 2–3 cells on the caudal side of the brain, and 2 cells ventrally in the suboesophageal ganglion.Giant neurons of unknown function are situated very near the m.n.c. Their axons join those from the m.n.c., but end in the suboesophageal ganglion.The same region comprises a number of peculiar cells, each containing a large, fluid-filled vacuole (the vacuolated cells). Similar cells are associated with the possibly neurosecretory cells on the caudal side of the brain.My sincere thanks are due to my wife Dr. Ellen Thomsen, who made all the excisions of the brains and the measurements of the nuclei, to T.C.Normann for valuable assistance with the photographic work, and to Mrs. K. Bahnert for technical help with the gallocyanin method. The Carlsberg Foundation has supported the work with grants.     

The in vitro neonatal rat spinal cord preparation: a new insight into mammalian locomotor mechanisms

The in vitro neonatal rat spinal cord preparation is the first mammalian nervous system isolated from the brainstem to the caudal end of the spinal cord. It permits the study of the cellular properties of mammalian locomotor networks and is unique in containing all the nervous structures related to locomotion. Although being a very immature system, this model has been considered as an adult preparation in which mammalian locomotor central pattern generators can be studied in detail. Nevertheless, one can also follow the development of locomotor functions during the perinatal period. Contrary to the adult, all neuroactive substances can directly reach the cellular structures in the brainstem-spinal cord preparation. When a neuroactive substance is applied to the bath, a single rhythmic activity is recorded along the cord. In fact, three rhythms can be isolated: one at the cervical level for the forelimbs, one at the lumbar level for the hind limbs and one in the sacrococcygeal region for the tail. Studies carried out on this preparation deal with three major areas: (1) relations between spontaneous activity and maturation of spinal network, (2) organisation of the different spinal networks, (3) key role of the descending pathways.Abbreviations 5-HT serotonin - ADP after-depolarization - AHP after-hyperpolarization - CPG central pattern generator - E0-E21 embryonic day 0–21 - INs interneurones - MLR mesencephalic locomotor region - MNs motoneurones - NMA N-methyl-d,l-aspartic acid - P0-P21 postnatal day 0–21 - PCPA p-chloro phenylalanin     

Glycoconjugates in the secretory epithelium of the chicken mandibular gland

Summary In the secretory epithelium of the chicken mandibular gland, glycoconjugates have been studied by means of histochemical methods of light and electron microscopy. In light microscopy, a series of histochemical procedures have been employed which included lectin—peroxidase—diaminobenzidine methods and a digestion technique with neuraminidase or-amylase. In electron microscopy, a battery of methods were used that corresponded to those employed in light microscopy. In the secretory cells of the chicken mandibular gland, vicinal diol- and sulphate-containing glycoconjugates with sialic acid,-d-mannose,-d-glucose and-d-galactose residues were visualized and the possible histophysiological significances of such glycoconjugates were discussed with special reference to the functions of the salivary gland.     

Morphologische Studie über den Reissnerschen Faden bei niederen Wirbeltieren

Zusammenfassung Die licht- und elektronenmikroskopische Untersuchung des Reissnerschen Fadens bei Pristella riddlei Meek und Larven von Salamandra salamandra terrestris zeigt im gesamten Verlauf des Reissnerschen Fadens Anlagerungen von Zelldetritus. Durch diese Beobachtung wird Olssons Theorie einer Transportfunktion des Reissnerschen Fadens gestützt.

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Summary In a light and electron microscope study on Reissner's fibre of Pristella riddlei Meek and larvae of Salamandra salamandra terrestris it is shown, that there are appositions of cell detritus in the whole length of Reissner's fibre. This observation supports Olsson's theory of a transport function of Reissner's fibre.

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Großenteils im Institut für Histologie und experimentelle Biologie der Universität München (Direktor: Professor Dr. med. R. Bachmann) und mit Unterstützung durch die Deutsche Forschungsgemeinschaft durchgeführt.     

The genusIschnea (Asteraceae,Senecioneae) in New Guinea

The endemic New Guinean genusIschnea F. Muell. (Asteraceae, Senecioneae, Blennospermatinae) is revised and four species are recognized. Characters of special interest are tubeless ray florets, male disc florets, and secretory spaces in leaves. A principal component analysis is made on theIschnea elachoglossa F. Muell. complex which shows great variation. One new species,I. capellana Swenson, from the Star Mountains, is described. A key, illustrations, and distribution maps to all species are supplied.     

The ultrastructural basis for the identification of cell types in the pancreatic islets I. Guinea pig

Summary The pancreatic islets of the guinea pig have been studied by light and electron microscopy. The B granules in glutaraldehyde-fixed tissue often are cup-shaped with an indentation visible on one side of the granule. Phosphotungstic acid hematoxylin (PTAH) positive cells have been characterized by electron microscopy as three subtypes based on the size of the secretory granules. A_a cells are the most common and have secretory granules around 200 m in diameter. A_b cells have large secretory granules around 300 m in diameter and are relatively infrequent. A_c cells are the least common and have small (160 m) granules. Characteristic D cells are identifiable by electron microscopy and, on the basis of the subsequent study (Munger, Caramia, and Lacy, 1965), are identified as the silver positive cells observed by light microscopy.This investigation was supported in part by United States Public Health Service research grants GM-10102 and GM-03784 from the Institute of General Medical Sciences, and AM-01226 from the Institute of Arthritis and Metabolic Diseases.-The authors wish to acknowledge the valuable technical assistance of Mrs. Aileen Sevier and Mrs. Lidia Donohue.