Functional morphology of the pars intermedia of the rat hypophysis as revealed with the electron microscope

Summary The pars intermedia of the hypophysis of normal and experimental rats was studied by electron microscopy. Observations of the hypophysis at various intervals following formalin induced stress or adrenalectomy indicate the existence of a functional relationship between the posterior lobe, the pars intermedia, and the adrenal cortex.Glandular cells of the normal pars intermedia are divided into two types, i. e., the light and dark cells. The former type dominates in number and is characterized by a large amount of cytoplasm filled with clear vesicles 250–350 m in diameter. Dark secretory granules smaller than 300 m are few in number and restricted to the Golgi region.After a single injection of formalin, the clear vesicles of the light cell dimmish and dark secretory granules varying in opacity increase in number. Transition from dark granules to clear vesicles is suggested. Three to five days after adrenalectomy, the light cells contain an abundance of moderately dense vesicles which are smaller than the larger more electron lucent vesicles of the normal light cells. The moderately dense vesicles are about 200 m in diameter and are extremely abundant filling the entire cytoplasm of the light cells 7 days after adrenalectomy.Bundles of unmyelinated nerve fibers are often observed in the pars intermedia, and a typical neuroglandular synapse was found in the pars intermedia of a sham-operated animal suggesting neural control of the secretion process of pars intermedia cells.The author wishes to express his hearty thanks to Dr. K. Kurosumi for his guidance throughout this work.     

Locations and central projections of neurons associated with the retrocerebral neuroendocrine complex of the cockroach Periplaneta americana (L.)

Summary Retrograde diffusion and precipitation of Co²⁺ reveals in the ipsilateral pars lateralis (PL) and contralateral pars intercerebralis (PI) of the brain neurons that enter the corpus cardiacum (CC), and, possibly, the corpus allatum (CA) on each side. The PL group consists of 29.6±8.4 somata that fill. Of these, 5.6±0.6 exceed 25 m in diameter, 14.3±2.7 range from 15–25 m, and 9.6±7.6 are smaller than 15 m. After CoCl₂ was applied to the right CC-CA of two males, 239 and 265 somata in the left PI stained. Except for 16 ranging from 30–45 m and chiefly located anteriorly, a majority of these somata measured 10–25 m.The only somata revealed by staining whole brains with the performic acid-resorcin fuchsin method are neurosecretory cells 10–20 m in diameter located within the PI. In starved adult males there are 92.4±8.1 on the right, and 93.2±6.9 on the left. The largest somata in the PL group contain numerous granules that stain with paraldehyde fuchsin. These somata also fill with Co²⁺, and belong to neurosecretory cells that extend into the CC-CA.The cerebral distribution of branches from the PL group, and the relationship of these to the corpora pedunculata, central body, and arborizations from the PI decussation are described.     

Ultrastructure of the pituitary gland (pars distalis) in sockeye salmon (Oncorhynchus nerka) during gonad maturation

Summary The fine structure of the various hormone-producing cell types (with the exclusion of the prolactin cells) in the pituitary gland (pars distalis) of migratory sockeye salmon is described. All fish were in an advanced stage of sexual maturation. In the proximal pars distalis five cell types were distinguished: growth hormone cells, ACTH cells, gonadotrops, vesicular cells, and chromophobe cells. Gonadotrops were also found throughout the rostral pars distalis. A conspicuous feature of the gonadotrops was the presence of two kinds of secretory inclusions: small electron-dense granules (200–375 m) and large, relatively electron-translucent globules (400–2 000 m). The large vesicular cells, so called because of their conspicuous vesicular endoplasmic reticulum, were numerous and often appeared to contain some small granules. It is argued that they may represent a second type of gonadotropic cell, which, in earlier stages of gonad development, contains many granules but becomes largely degranulated near the time of reproduction when the other gonadotrops (globular gonadotrops) abound. The chromophobes, which were smaller and far less abundant than the vesicular cells, also appeared to contain small granules (120–280 m). They are probably thyrotrops.The assistance of Mr. S. Killick, of the International Pacific Salmon Fisheries Commission, who helped in the collection of salmon, is gratefully acknowledged.     

The electromotor system of the electric catfish (Malapterurus electricus): A fine-structural analysis

Summary The electromotor system of the electric catfish (Malapterurus electricus) consists of two large ganglion cells situated in the spinal cord, two single axons containing electric nerves and two large electric organs with several million electroplaque cells. The small, irregularly stacked electroplaque cells possess at their center a crater-like indentation from which a stalk like protrusion arises. Many synaptic contacts derived from a single axon collateral are carried on lobe-like protrusions at the terminal knob of this stalk. The electric nerve consists of a large myelinated axon (diameter: 25 m) surrounded by many layers of connective tissue cells. The two ganglion cells (200 m in diameter) are rich in elements of the rough endoplasmic reticulum, Golgi apparatus and lysosomal structures. The cytoplasm of the soma changes its appearance towards the voluminous axon hillock (50 m in diameter) which these organelles do not enter. The cell soma is perforated in a tunnel-like manner by blood capillaries, axons and processes of glial cells. The cell soma and dendrites are covered with two types of synapse. One type forms mixed chemical and electrical (gap junctions) contacts with intermediate attachment plaques. The other type is only chemical in nature. This system may be useful in the study of an identified vertebrate giant neuron.     

Identification of the LH and TSH-secreting cells in the pituitary gland of the rhesus monkey

Summary Pituitary glands from juvenile (pre-pubertal) and adult male and female rhesus monkeys were examined following immunocytochemical staining with antisera to the beta subunits of ovine luteinizing hormone (LH) and of human thyroid stimulating hormone (TSH). The LH antiserum reacts with a cell that is PAS-positive, occurs singly and is randomly distributed throughout the pars distalis. The diameter of these cells is approximately 11.5 m. They do not seem to vary in number in either juveniles (pre-pubertals) or adults, or in males or females. There appears to be fewer LH cells in the pituitary glands of pregnant and lactating females. In addition to staining cells in the pars distalis, the antiserum also reacts with a population of cells located in the pars tuberalis.The cells that stain with the anti-TSH serum are confined primarily to the pars distalis. They are approximately 15.8 m in diameter and are generally found in groups or clusters located in the anterior and medial regions of the gland. The TSH cells vary in number from one animal to another; however, this variability is unrelated to the age or the sex of the animals. No demonstrable changes occur in the number of TSH cells during pregnancy or lactation.Supported by NIH General Research Support Grant RR05654The author wishes to express appreciation to the Hormone Distribution Program of NIAMDD for the preparations of ovine FSH, TSH and human TSH, and to Drs. H. Papkoff for the ovine LH and LH, L. Reichert for the human FSH, J. Vaitukaitis for the anti-human TSH, and L.A. Sternberger for the PAP complex     

Nuclear uptake and retention of androgen by the pituitary gland of the hamster and the rat

Summary Castrated adult male hamsters and castrated adult female rats were injected with either 0.2 g (hamsters) or 0.5 g (rats) of ³H-dihydrotestosterone (107 Ci/mmole)/100 grams body weight and killed 11/2 h later. The pituitary glands were removed and processed for both autoradiography and immunocytochemistry (hamster) or only autoradiography (rats). Localization of the androgen was found in 10–15% of the cells of the pars distalis in both species. Only cells that stained for luteinizing hormone (LH) in the hamster's pars distalis concentrated the androgen. Also cells in both the pars intermedia and pars nervosa (1–5%) concentrated the androgen in both species. Although the number of cells that concentrated the androgen in the pars intermedia and pars nervosa was small, this finding may be related to recent physiologic data that suggest that the gonadal steroids may play a role in regulating water retention and natriuresis.This study was supported by USPHS Research Career Development Award KO4NS0000164 (P.J. Sheridan) and USPHS Grants No. 1 RO1 NS12933, P30 HD10202 and HD 10914     

Control of the pars intermedia of the lizard,Anolis carolinensis

Summary The ultrastructure of the intermediate lobe of the hypophysis was studied in Anolis carolinensis with the use of a threefold aldehyde fixative. Lizards with a brown skin were selected. The possibility of two types of secretory cells is discussed; neither cell type is innervated. Type I cells are rarely found and contain dense granules approximately 0.3 m in diameter; Type II cells vary widely in secretory activity. Most of the Type II cells contain a large number of dense secretory granules (up to about 1.3 m in diameter) almost filling the cytoplasm. Rough endoplasmic reticulum (RER), Golgi apparatus and mitochondria are poorly developed. Only some of these cells show signs suggesting a high secretory activity, namely a well developed RER, Golgi apparatus and numerous mitochondria. In these cells the RER sometimes forms large intracisternal droplets (up to 7 m in diameter). Two of the animals exhibited a more uniform, high secretory activity. Large (about 2 m in diameter), pale vacuoles, probably of extracellular character, were found mostly in the vicinity of the perivascular septum. Their role in the release of MSH is discussed. The present data, which are discussed with reference to earlier findings (Forbes, 1972), form the morphological basis for an experimental study on regulation of MSH release (Larsson et al., 1979).Supported by grants from the Swedish Natural Science Research Council (to P. Meurling) and the Royal Physiographic Society of LundThe authors are indebted to Mrs. Ingrid Hallberg, Mrs. Kirsten Thörneby and Mrs. Lena Sandell for valuable technical assistance and to Miss Inger Norling for photographic aid     

Plant regeneration from embryo-derived callus of oil palm – the effect of exogenous polyamines

Regeneration in oil palm was achieved through somatic embryogenesis/organogenesis from embryo-derived callus. Callus was induced from mature embryos of the cross 281 (D)×18 (P) on modified MS medium supplemented with 2,4-D (113.12 M) and 2-iP (14.76 M). The embryogenic calluses obtained were transferred to Blaydes medium supplemented with 2,4-D (0.045 M) and one of the following growth regulators: TDZ (4.54 M), zeatin riboside (2.85 M), putrescine (1 mM) and spermine (100 M). Secondary somatic embryogenesis was found to occur in media supplemented with polyamines. The efficiency of formation of somatic embryos, secondary somatic embryos and shoot meristemoids were significantly higher in putrescine containing medium. Histological studies were also undertaken.     

Polarized distribution of γ interferon-stimulated MHC antigens and transferrin receptors in a clonal cell line isolated from Fisher rat thyroid (FRT cells)

Summary The fine structure of single identified muscle fibers and their nerve terminals in the limb closer muscle of the shore crab Eriphia spinifrons was examined, using a previous classification based on histochemical evidence which recognizes a slow (Type-I) fiber and three fast (Type-II, Type-III, Type-IV) fibers. All four fiber types have a fine structure characteristic of crustacean slow muscle, with 10–12 thin filaments surrounding each thick filament and sarcomere lengths of 6–13 m. Type-IV fibers have sarcomere lengths of 6 m while the other three types have substantially longer sarcomeres (10–13 m). Structural features of nerve terminals revealed excitatory innervation in all four fiber types but inhibitory innervation in Type-I, Type-II, and Type-III fibers only. Thus fibers with longer sarcomeres receive the inhibitor axon but those with shorter sarcomeres do not. Amongst the former, synaptic contact from an inhibitory nerve terminal onto an excitatory one, denoting presynaptic inhibition, was seen in Type-I and Type-II fibers but not in Type-III and Type-IV fibers. Inhibitory innervation of the walking leg closer muscle is therefore highly differentiated: some fibers lack inhibitory nerve terminals, some possess postsynaptic inhibition, and some possess both postsynaptic and presynaptic inhibition.     

Phenylacetic acid-induced somatic embryogenesis in cultured hypocotyl explants of geranium (Pelargonium x hortorum Bailey)

Summary The effect of a non-indole compound, phenylacetic acid (PAA), on the induction of somatic embryogenesis in tissue cultures of geranium (Pelargonium x hortorum Bailey cv. Scarlet Orbit Improved) was investigated. Hypocotyl explants derived from young, dark-grown seedlings were cultured on Murashige and Skoog (1962) medium (MS) supplemented with PAA or IAA (0.01–120 M) alone or in combination with BAP (8 M). Somatic embryogenesis was induced by both PAA and IAA at 0.01–20 M with 8 M BAP, however, the optima differed considerably for the two compounds. Maximal activity of IAA for somatic embryogenesis was found at 0.1–2.5 M, whereas PAA gave best results at 10 and 20 M under identical culture conditions. Higher concentrations (30–120 M) of IAA or PAA in the medium induced callusing in the explants, but the callus was neither embryogenic nor morphogenic.Abbreviations BAP N⁶-benzylaminopurine - IAA indole-3-acetic acid - MS Murashige and Skoog (1962) - PAA phenylacetic acid     

Ontogenesis of cells producing polypeptide hormones (ACTH,MSH, LPH,GH, Prolactin) in the fetal hypophysis of the rat: Influence of the hypothalamus

Summary The ontogenesis of cells containing polypeptide hormones (ACTH, MSH, LPH, GH and Prolactin) was investigated in the fetal rat hypophysis by immunohistochemistry using the peroxidase-antiperoxidase complex.Corticotrophs, melanotrophs and lipotropic cells were revealed earlier in the pars distalis than in the pars intermedia. In the pars distalis, cells producing LPH were found in the morning of day 15 of gestation using anti-- or anti--LPH sera, and in the afternoon using anti-- or -endorphin sera. Cells containing -MSH were observed from the afternoon of day 15. The cells stainable with the anti--MSH, anti--(17-39)ACTH and anti--(l-24)ACTH sera appeared on day 16. In the pars intermedia, the cells producing -MSH, MSH, - and -endorphin, and -LPH were observed in the morning of day 17, while cells containing ACTH were only revealed in the afternoon of the same day of gestation. Based on the treatment of serial paraffin sections with various antisera, it was clearly shown that MSH, ACTH, and LPH occur in the same cells located in the pars distalis as in the pars intermedia.The development of the corticotrophs, melanotrophs and lipotropic cells does not require the presence of the fetal hypothalamus or other central nervous structures. The pituitary glands of 21 day-old fetuses encephalectomized on day 16 showed as many reactive cells as those of the littermate controls.The somatotrophs were first revealed in the pars distalis in the afternoon of day 19. The cells producing prolactin were not observed before day 21 of gestation. On some cases GH and prolactin were found together in one cell. The cytodifferentiation of GH and prolactin cells is apparently not under hypothalamic control.     

Occurrence of colloid-containing follicles in the pars distalis of pituitary glands from aging guinea pigs

Summary Colloid-containing follicles in the pars distalis of pituitary glands from guinea pigs at various ages ranging from 5 days to 36 months were examined by the periodic acid-Schiff (PAS) reaction, immunohistochemistry, and electron microscopy. The follicles storing PAS-positive colloid were first detected in 6-month-old animals, in which only a few follicles were present and mean diameters of colloid deposits were small: 4.3±1.0 m in males and 4.1±0.4 m in females. Thereafter, the follicles gradually increased in number and size with age. The largest number of follicles was observed in the senile groups: 410.5±92.3 in males, 454.7±84.7 in females. Mean diameters of colloid masses in the senile groups were more than 2 times larger than those in 6-month-old animals: 10.0±0.1 m in males, 9.7±0.1 m in females. These findings suggest that the formation of colloidcontaining follicles in the guinea-pig pars distalis is an aging phenomenon. The follicular lumina were mainly surrounded by thin cytoplasmic processes or cell bodies of folliculo-stellate cells immunoreactive for S-100 protein. The lining folliculo-stellate cells showed aggregations of intermediate-sized filaments, numerous lysosomes and colloid-like inclusions. Granulated cells in contact with colloid were occasionally encountered. Intracellular cavities storing colloid-like and fibrous materials were detected in the syncytial formation of GH cells.     

Plant regeneration via somatic embryogenesis in Styrian pumpkin: cytological and biochemical investigations

Somatic embryo formation was induced from cotyledon explants of Styrian pumpkin (Cucurbita pepo L. subsp. pepo var. styriaca Greb.) by using a solid MS medium supplemented with 16.11M NAA and 4.44M BA or 26.85M NAA and 13.32M BA. The callus proliferation was more efficient on medium supplemented with 26.85M NAA and 13.32M BA. In contrast, the embryogenic response was higher on medium with lower concentrations of growth regulators (16.11M NAA and 4.44M BA). The time needed for embryo induction did not depend on medium composition. Embryos in globular stage were transferred to three different maturation media, containing 2.89M GA₃ in combination with 0.54M NAA, 11.42M IAA and growth regulator-free medium. The germination rate was the highest when embryos were cultured on medium with 11.42M IAA. Plantlets grown on this medium achieved maturity suitable for transplantation into soil within 9 to 10weeks. The regenerated plants were successfully transferred into field and developed fertile flowers and set fruits. Biochemical analysis showed significant lower total glutathione levels among in vitro grown plantlets compared to seedlings grown in soil. When the plantlets were transferred into soil, they reached a normal size within a month and the glutathione concentration was comparable to seed-derived plants at the same developmental stage. Transmission electron microscopy was used to investigate possible differences in the ultrastructure of cells from callus cultures, and leaf cells of regenerated and seed-derived plants. Differences in the ultrastructure were found within chloroplasts which contained only single thylakoids, large starch grains and small plastoglobuli in callus cells in comparison to leaf cells, which possessed a well developed thylakoid system, small starch grains and large plastoglobuli.     

Sulfur amino acid metabolism in the developing rhesus monkey brain: Interrelationship of taurine and glutamate

The relationship of taurine to glutamate, and to other amino acids, has been examined in the occipital lobe of the developing rhesus monkey. During development taurine decreases in concentration (4.96 mol/g in fetus to 1.52 mol/g in adult) while glutamate increases (7.92 mol/g in fetus to 11.26 mol/g in adult). When the concentration of taurine is plotted against that of glutamate in fetal, neonatal and adult animals there is a significant correlation in the fetal (p<0.01) and adult (p<0.01) but not in the neonatal occipital lobe samples. This correlation in both fetal and adult brain is specific for these two amino acids. Subcellular fractionation studies further indicate that this relationship may be of special importance in nerve endings.This paper is dedicated to Dr. Derek Richter on his seventy-fifth birthday.     

<Emphasis Type="Italic">In vitro</Emphasis> propagation of the genus <Emphasis Type="Italic">Clivia</Emphasis>

We have developed a protocol for the in vitro propagation of the genus Clivia. Shoots were regenerated when fragments of the peduncle-pedicel junction (PP junction) from young inflorescences were used as explants. The optimal media for PP junction were Murashige and Skoog (MS)-based medium containing 10 M of 6-benzyladenine (BA) and 10 M of 2,4-dichlorophenoxyacetic acid (2,4-D) or MS supplemented with 5 M BA, 10 M -naphthaleneacetic acid (NAA), 250 mg l^-1 glutamine and 500 mg l^–1 casein hydrolysate and their usage depended on the breeding lines. Multiplication from initiations and in vitro seedlings was the best when the explants were cut longitudinally through the meristem and placed on MS plus 44 M BA. Plantlets were transferred on to hormone -free MS medium with charcoal for rooting.     

Fine structure,origin, and distribution density of the autonomic nerve endings in the tarsal muscle in the eyelid of the mouse

Summary The fine structure, origin, and distribution density of the autonomic nerve endings in the tarsal muscle of the mouse were studied by histochemistry and electron microscopy. With histochemical methods, the fine nerve plexus in the normal muscle shows both catecholamine-positive varicose fibers and acetylcholinesterase-active varicose fibers. The former are distributed more densely than the latter. After superior cervical ganglionectomy, the catecholamine-positive fibers disappear, while after pterygopalatine ganglionectomy, the acetylcholinesterase-active fibers vanish. In electron micrographs, the varicosities appear as expansions containing many synaptic vesicles. The axonal expansions partly lack a Schwann sheath and directly face the pinocytotic vesicle-rich zones of the smooth muscle cells. A relatively wide space, 0.1 to 1.0 m in width, lies between nerve expansion and muscle cell. The expansions can be classified into two types: Type I having small granular synaptic vesicles, and Type II having agranular vesicles instead of small granular synaptic vesicles. Type I undergoes degeneration after superior cervical ganglionectomy, while Type II degenerates after pterygopalatine ganglionectomy. This indicates that Type I corresponds to the synaptic ending of the adrenergic fiber originating from the superior cervical ganglion, and Type II to the synaptic ending of the cholinergic nerve fiber derived from the pterygopalatine ganglion. Type I is more frequent (88/10⁴ m² area of muscle) than Type II (17/10⁴ m²).     

Tumor necrosis factor-α in macrophages of heart,liver, kidney,and in the pituitary gland

The relationship between myonuclear number, cellular size, succinate dehydrogenase activity, and myosin type was examined in single fiber segments (n=54; 9±3 mm long) mechanically dissected from soleus and plantaris muscles of adult rats. One end of each fiber segment was stained for DNA before quantitative photometric analysis of succinate dehydrogenase activity; the other end was double immunolabelled with fast and slow myosin heavy chain monoclonal antibodies. Mean±S.D. cytoplasmic volume/myonucleus ratio was higher in fast and slow plantaris fibers (112±69 vs. 34±21x10³ m³) than fast and slow soleus fibers (40±20 vs. 30±14x10³ m³), respectively. Slow fibers always had small volumes/myonucleus, regardless of fiber diameter, succinate dehydrogenase activity, or muscle of origin. In contrast, smaller diameter (<70 m) fast soleus and plantaris fibers with high succinate dehydrogenase activity appeared to have low volumes/myonucleus while larger diameter (>70 m) fast fibers with low succinate dehydrogenase activity always had large volume/myonucleus. Slow soleus fibers had significantly greater numbers of myonuclei/mm than did either fast soleus or fast plantaris fibers (116±51 vs. 55±22 and 44±23), respectively. These data suggest that the myonuclear domain is more limited in slow than fast fibers and in the fibers with a high, compared to a low, oxidative metabolic capability.     

Somatic embryogenesis and plant regeneration in wild cotton (Gossypium klotzschianum)

A simple and efficient method for high frequency somatic embryogenesis and plant regeneration from hypocotyl-derived cultures and suspension cultures of Gossypium klotzschianum Anderss, a wild, diploid species of cotton is described here. Embryogenic cultures were induced from hypocotyl sections on MSB medium with 0.9 M 2,4-D and 2.32 M kinetin. MSB medium containing 0.045 M 2,4-D, 0.93 M kinetin, 2.46 M IBA promoted embryogenic culture proliferation and embryo development. Suspension cultures with 0.23 M 2,4-D and 0.93 M kinetin also produced many embryos. Somatic embryos cultured on MSB medium with PGRs produced secondary embryos, and embryos developed into normal plantlets on PGR-free MSB medium. Regenerated plantlets were transferred onto the quarter-strength MSB medium with 0.5% active charcoal to avoid recallusing. Hypocotyls were better than cotyledons for culture induction and plant regeneration. 2,4-D and kinetin were essential for culture induction and maintenance.     

Histochemie und Elektronenmikroskopie des Nucleus ruber des Huhnes (Gallus domesticus)

Zusammenfassung Der Nucleus ruber (N.r.) des Huhnes gliedert sich in eine größere Pars magnocellularis und eine kleinere Pars parvocellularis. Innerhalb des N.r. kommen drei Arten von Neuronen vor, nämlich große (70–100 m), mittelgroße (30–70 m) und kleine (unter 30 m). Die großen Nervenzellen beschränken sich auf die Pars magnocellularis, die mittelgroßen und kleinen sind auch in der Pars parvocellularis anzutreffen. Histochemisch zeichnen sich die großen und mittelgroßen Neurone durch hohe Enzymaktivitäten aus, wobei in den großen die der Glykolyse, in den mittelgroßen die des oxidativen Stoffwechsels im Vordergrund stehen. Elektronenmikroskopisch werden in den Perikarya der großen und mittelgroßen Neurone zahlreiche, in den kleinen nur wenige Zellorganellen gefunden. —Die Innervation des N.r. ist beim Huhn überwiegend cholinerg. — Eine Blut-Hirn-Schranke ist deutlich ausgebildet.

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Histochemistry and ultrastructure of the red nucleus of hens (Gallus domesticus)

Summary The red nucleus (r.n.) of hens is divided into a big magnocellular and a small parvocellular part. Within the r.n. three types of neurons can be distinguished — big (70–100 m), medium-sized (30–70 m) and small ones (below 30 m). The big nerve cells are confined to the pars magnocellularis, whereas the medium-sized and small neurons can also be observed in the parvocellular part. Histochemically the big and medium-sized nerve cells are characterized by high enzyme activities. Furthermore in the big neurons the glycolysis is more active in comparison with other metabolic pathways; in the medium-sized nerve cells the oxidative metabolism predominates. By means of electron microscopy many organelles (lysosomes, mitochondria, ergastoplasm, Golgi apparatus) exist in the big and medium-sized neurons; the pericarya of the small nerve cells are poorly organized. — As to the innervation the r.n. of hens is rich in cholinergic nerve fibres.-A blood brain barrier is well developed.

     

Neuronal connections in the ocellus of the wasp (Paravespula vulgaris L.)

Summary Studies of the dorsal ocelli of the wasp Paravespula vulgaris (L.) led to the following results: Under a biconvex corneal lens, 150 m in thickness, about 600 receptor cells are located. The rhabdomeres of two adjacent cells form a closed plate-like rhabdom (0.5–1.0 m in thickness, 6 m in width and 10–25 m in depth or length).In the lateral ocellus the receptor cells synapse up to 8 ocellar nerve fibers, and in the median ocellus they synapse up to 16 (20–30 m thick) ocellar nerve fibers.The ocellar synaptic plexus may display three types of synapses between the two types of neurons: (i) Receptor-cell axons are presynaptic to dendrites of the first-order interneurons. (ii) Dendrites of the first-order interneurons are presynaptic to receptor-cell axons. (iii) The subunits of a dendrite of first-order interneurons form synapses with each other.The present work was partially supported by the Stiftung Volkswagenwerk