Localization of gamma-tubulin to the basal foot associated with the basal body extending a cilium

The human oviduct epithelium primarily consists of ciliated cells and secretory cells. Solitary cilia usually extend from the apical surface of the secretory cells. We investigated the localization of -tubulin in the ciliary basal apparatus of both cell types by fluorescence immunohistochemistry and immunoelectron microscopy. In addition to basal bodies, -tubulin was identified in the lateral basal foot, especially the basal foot cap. This observation is consistent with previous observations that microtubules radiate from the basal foot and the basal foot serves as the microtubule organizing centre.     

The larval morphology of the marine bryozoan Bowerbankia gracilis (Ctenostomata: Vesicularioidea)

Summary The larval morphology of the marine bryozoan Bowerbankia gracilis has been investigated by light and electron microscopy. The barrel-shaped larva (200 m long and 150 m in diameter) is light yellow without any apparent eyespots, although it is positively phototactic during its brief free-swimming existence. The primary morphological characteristics of the larva are: (1) a large corona that forms most of the larval surface, (2) a small apical disc without blastemas, (3) a deep pallial sinus lined by an extensive pallial epithelium, (4) an internal sac without regional specializations, and (5) a polypide rudiment in the oral hemisphere. This organization is characteristic of larvae of the ctenostome superfamily Vesicularioidea, and differs radically from the organization of all other bryozoan larvae examined. The major morphological differences occur in the size and organization of the apical disc, the pallial epithelium, and the internal sac. In most bryozoans, these regions of the larval epithelium represent rudiments of the polypide and the body wall epidermis of the ancestrula. The oral polypide rudiment, the extensive pallial epithelium, and the reduced internal sac in vesicularioid larvae indicate that their pattern of metamorphosis also differs radically from the metamorphoses of other bryozoans.Figure Abbreviations AB aboral - acr axial ciliary rootlet - ad apical disc - anc aboral nerve cord - ANT anterior - arm apical retractor muscle - b basal body - bf basal foot process - c corona - cc ciliated cleft - ce centriole - ci cilium - cl cupiform layer of the polypide rudiment - cp ciliary pit - cr ciliary rootlet - enr equatorial neural ring - g glandular cells of the pyriform organ - gl glycocalyx - go Golgi complex - gr granule - hcr horizontal ciliary rootlet - ic intercoronal cell - igf inferior glandular field - ip infrapallial cells - is internal sac - jp juxtapapillary cells - l lipid droplets - L lateral - m mesenchyme - m Type I mesenchyme cell - m Type II mesenchyme cell - m Type III mesenchyme cell - mb median band of the polypide rudiment - mc marginal cells of the apical disc - mi mitochondria - mr microridge - mv microvilli - nn nerve nodule - np neural plate - nu nucleus - O oral - oce oral ciliated epithelium - op opening to the internal sac - ovc oral vesicular collarette - p papilla of the pyriform organ - pa pallial cell - pe pallial epithelium - po pyriform organ - POS posterior - pp parasagittal patches of undifferentiated cells - pr polypide rudiment - rer rough endoplasmic reticulum - sc supracoronal cells - sg secretory granules - sgf superior glandular field - sp suprapallial cells - tc terminal cone - tf transitional filaments - u undifferentiated cells - va vacuole - vc vesicular cell - wc wedge-shaped cells of the apical disc - y yolk granule - za zonula adhaerens Caption Abbreviations Gp Glutaraldehyde-phosphate - Os Osmium     

Particle motion in single acinar cells observed by microscope laser light scattering spectroscopy

We report here the direct observation of the motion of cytoplasmic subcellular organelles and macromolecules within single, metabolically active, pancreatic acinar cells using microscope laser-light scattering spectroscopy (MLLSS). The relative amount and the effective diffusion coefficient of the mobile particles shows a marked difference in magnitude between the apical and basal regions of the cell. Secretory stimulation evoked by the cholinergic agonist bethanechol induces changes in the relative motion in the cytoplasm of approximately one fifth of the acinar cells. This study demonstrates the feasibility and utility of a novel technique, MLLSS, for the analysis of intracellular events in regions as small as (2 m)³ in single, granule secreting cells. It also shows MLLSS to be a powerful tool for the detection and measurement of altered motion in disrete subcellular regions of small mammalian cells after biochemical and pharmacological manipulations.     

Ultrastructure of serotonin-containing cells in the pineal organ of Lampetra planeri (Petromyzontidae)

Summary The ultrastructure of the cells containing residual bodies (Collin, 1969) was investigated in the pineal organ of Lampetra planeri. These cells are characterized by their indoleamine metabolism (Meiniel, 1978; Meiniel and Hartwig, 1980). Morphologically, they belong mainly to two types: (1) a photoreceptor cell type, and (2) a pinealocyte cell type. The first type is present in the pineal sensory epithelium and in the atrium, while the second is observed in the deep part of the atrium. Intermediate cell types are rare. All these cells are characterized by the presence of voluminous dense bodies, the 5-HT-storing structures, in their cytoplasm.The elongated cone-type photoreceptor cells show a segmental organization and well-developed outer segments consisting of short disks (2–3 m), while their basal pedicles form synapses with the dendritic processes of neurons. The pinealocytes are spherical or oval in shape, their receptor poles being regressed to cilia of the 9+0 type. In these cells, no synaptic ribbons have to date been observed. In both cell types a Golgi apparatus is present producing dense granules 130 nm in diameter and a polymorphous dense material.The photoreceptor cells most probably respond to light and transmit a sensory (i.e., nervous) message. In addition, they produce and metabolize indoleamines, probably including, melatonin (Meiniel, 1978; Meiniel and Hartwig, 1980). The pinealocytes, in spite of their loss of direct photosensitivity, retain their capacity to metabolize indoleamines (Meiniel, 1978; Meiniel and Hartwig, 1980).The presence, in the same pineal organ, of another photoreceptor cell type (cf. Collin, 1969–1971) differing morphologically as well as biochemically (no detectable indoleamine metabolism) from the photoreceptor cell type described in the present investigation, points to the existence of two different sensory cell lines: (1) a pure photoreceptor line, and (2) a photoneuroendocrine line. The phylogenetic evolution of these two cell lines is discussed in terms of functional analogy.     

Endocrine cells in the human fetal small intestine

Summary In this report we describe the time of appearance and ultrastructural features of enteroendocrine cells (EECs) in the human fetal small intestine (SB) between 9 and 22 weeks gestation. Thirteen distinctive EECs were identified in fetal SB. Two of these, not found in normal adult SB, appeared within the stratified epithelium of the proximal SB at 9–10 weeks. They were arbitrarily termed primitive and precursor cells. As in all fetal EECs, the pale cytoplasm of the primitive cell contains a distinctive population of secretory granules (SGs). Primitive cell SGs average 200–330 nm; some have dense cores with lucent halos while others are filled with a homogeneous dense or flocculent material. The SGs of the precursor cells are larger, averaging up to 1 m in diameter and their contents vary in electron density. A third group of cells not described in normal adult SB was arbitrarily termed transitional cells. These have two populations of SGs; one resembles the SGs of the precursor cells, and the other resembles the SGs of some of the specific adult type EECs. Transitional EC, S, I and G cells are seen. In addition, mature appearing EC, S, G, I, L, D, and D₁ cells were identified by 12 weeks of gestation. The primitive, precursor, and transitional cells may represent sequential developmental precursors of adult type EECs.Supported by Research Grant AM-17537 from the National Institutes of Health, Besthesda, MarylandThe authors would like to thank Ms. Linda Barstein for her excellent technical assistance     

The filter apparatus of Rana temporaria and Bufo bufo larvae (Amphibia,Anura)

Summary In larvae of Rana temporaria and Bufo bufo the location of filter apparatus within the larval organization, the arrangement of the morphological parts as branchial food trap, ventral velum, and filter rows, as well as their surface anatomy, are similar to that of other species of Orton's larval type IV. The means by which mucous with its entrapped food particles is transported from the filter rows to the esophagus is finally resolved. The dorsally positioned ciliary cushion extends far ventrally between the filter plates. From their contact with the filter rows, the cilia transport the mucous to Kratochwill's caudally positioned Flimmerrinne and from there to the esophagus. The original chordate principle of mucous entrapment and ciliary transport is thus retained by these anuran larvae. The only modification specific to the latter is the division into a ventral filter apparatus, whose epithelia serve for mucus entrapment, and a dorsal ciliary area.Six different types of cell may be distinguished ultrastructurally: (1) The ubiquitous squamous epithelium with merocrine extrusions; (2) the large supporting cells of the filter rows and of the ventral velum; (3) the ciliary cells of the ciliary cushion; (4) three different types of mucous producing secretory cells: (a) A type of cell similar to the goblet cell is found in the ciliary cushion (merocrine extrusion); (b) The secretory pits of the ventral velum and the secretory ridges have similar bottle-shaped merocrine secretory cells; (c) The merocrine apical cells of the filter rows are the final kind. It is evident that the ciliary cushion epithelium resembles that of both the manicotto glandulare of anuran larvae and the trachea and bronchus of Mammalia.Supported by the Deutsche Forschungsgemeinschaft-DFG     

Ultrastructure of the main excretory duct epithelium of the female mouse submandibular gland with special reference to sexual dimorphism

The fine structure of the main excretory duct epithelium (MEDE) of female mouse submandibular gland was investigated by scanning and transmission electron microscopy and the results compared with the previously established structure of male mouse MEDE. A comparative analysis of the subepithelial capillaries of both sexes was also performed. In this pseudostratified epithelium, principal cell-types were observed: types-I,-II,-III and basal cells. This differed significantly from male MEDE, where type-II and-III are absent and type-I cells are the most numerous. The latter cell-type had abundant mitochondria, a few lipid-containing granules, lysosomes in the infra-nuclear cytoplasm and well-developed basal infoldings. These cells were also characterized by abundant glycogen granules throughout the cytoplasm, many profiles of strands of smooth endoplasmic reticulum in the apical region, and lysosomes in the infra-nuclear region. Type-II cells were the second most numerous. Their most characteristic features were the presence of tubular vesicles which appeared to be invaginated from the plasma membrane, RER, SER, free ribosomes, a few peroxisomes with nucleoids, and primary lysosomes in extremely light cytoplasm. They had many mitochondria throughout the cytoplasm, except in the apical region, a few lipid-containing granules and no basal infoldings. Type-III cells were very few and were characterized by well developed basal infoldings, abundant free ribosomes, RER, SER, vesicles containing moderately dense material, and many lipid-containing granules. They also had many mitochondria throughout the cytoplasm, except apically. Basal cells had a large nucleus and the cytoplasm had few organelles. In the male continuous capillaries predominated in the subepithelial network, and capillary density per 200 m of epithelium (3.76±1.54) was lower than in the female, as was the number of fenestrae per 10 m of available endothelium (4.46±1.71). In the female, fenestrated capillaries predominated, and the capillary density per 200 m of epithelium was 6.76 (±1.54), and the number of fenestrae per 10 m of available endothelium was 4.91 (±1.77).     

Electron microscope observations on the early development of the rat

Summary The early development of the rat, from the mature oocyte through fertilization until the 8-cell stage, has been studied with the electron microscope. The fine structure is described and discussed, with particular reference to the following topics. The middle piece of the spermatozoon is found in every stage studied, within the ovum cytoplasm; it is not significantly altered by this situation. The nucleoli are numerous during the 1-cell stage and often present in positions that suggest their extrusion into the cytoplasm; in subsequent stages a branching structure develops around them. The dividing cell presents the mitotic apparatus with its centrioles, hollow looking fibers, chromosomes, but without centromeres; in the cytoplasm the small granules align in rows. Mitochondria are evenly distributed during the 1-cell stage and can be found in the 8-cell stage constricted as if dividing. The multivesicular bodies constitute an abundant population of cytoplasmic elements that may be related to the endoplasmic reticulum or the Golgi complex, neither of which is clearly recognizable. In the 8-cell stage the cytoplasm segregates into two zones, one of which contains all the multivesicular bodies, while the mitochondria are found in both of them; this distinction provides some basis to adscribe to the multivesicular bodies the properties of the so called metachromatic particles.The support of the Gildemeister Foundation is gratefully acknowledged.     

Structural and enzymatic studies on the plasma membrane domains and sodium pump enzymes of absorptive epithelial cells in the avian lower intestine

Summary The coprodaeum of the domestic hen maintained on a low-NaCl diet adapts by enhanced sodium transport. This study examines the adaptive response at the single cell and whole organ levels. Surface areas of apical (microvillous) and basolateral plasma membranes of columnar absorptive epithelial cells were estimated by use of ultrastructural stereology. The activities of succinic dehydrogenase (a mitochondrial enzyme) and ouabain-sensitive, potassium-dependent paranitrophenyl phosphatase (a sodium pump enzyme) were determined in tissue homogenates. Sodium, potassium-ATPase (pump enzyme) activity in cell membranes was localized by ultrastructural cytochemistry. Apical and basolateral membranes responded differently. In high-NaCl hens, the membrane signature of the average cell was 32 m² (apical), 932 m² (lateral) and 17 m² (basal). Cells from low-NaCl hens had more apical membrane (49 m² per cell) but essentially the same area of basolateral membrane. However, total surfaces per organ were greater for all membranes. Sodium pump enzymes were localized in basolateral membranes. Enzyme activities per unit mitochondrial volume and per unit basolateral membrane surface were higher in low-NaCl birds. These findings are discussed in the context of known mechanisms of transcellular sodium transport via apical ion channels and basolateral pumps.     

The cellular organization and nervous supply of the basilar papilla in the lizard,Calotes versicolor

Summary The basilar papilla of the lizard Calotes versicolor contains about 225 sensory cells. These are of two types: the short-haired type A cells in the ventral (apical) part of the organ, and the type B cells with long hair bundles, in the dorsal (basal) part of the organ. The type A cells are unidirectionally oriented and are covered by a tectorial membrane while the type B cells lack a covering structure and their hair bundles are oriented bidirectionally. Apart from those differences, the type A and type B cells are similar. They are columnar, and display the features common to most sensory cells in inner ear epithelia. The sensory cells are separated by supporting cells, which have long slender processes that keep the sensory cells apart. Close to the surface of the basilar papilla a terminal bar of specialized junctions interlocks adjacent cells. Below this, adjacent supporting cells are linked by an occluding junction.The cochlear nerve enters from the medial (neural) aspect. The fibres of the nerve lose their myelin sheaths as they enter the basilar papilla. Each sensory cell is associated with several nerve endings. All the nerves identified were afferent. Marked variations were seen between nerve endings in the basilar papilla, but no morphological equivalents of any functional differences were observed.This work is supported by grant no. B76-12X-00720-11A from the Swedish Medical Research Council, and by funds from the Karolinska Institute, Stockholm, Sweden.     

Distribution of a galactose-specific lectin in endoderm cells from early chick embryos

Summary Cells from the endoderm of the area opaca of gastrulating chick embryos were maintained in stationary cultures, stained with antibodies against the endogenous -D-galactoside-binding lectin and examined by immuno-fluorescence. In the majority of cells fluorescence was present as an irregular circular web in the central cytoplasm. In cells that appeared to be migrating increased fluorescence was observed in the peripheral cytoplasm and retraction fibers. In regions where a portion of the cell was detaching from the substratum high fluorescence was observed in the extracellular footprints deposited by the cell.     

On the fine structure of the aglomerular renal tubule in Lophius piscatorius

Summary The fine structure of the secretory tubules in the kidney of the aglomerular goose-fish (Lophius piscatorius) is described. The cells have a pyramidal shape, are joined together by multiple desmosomes, and share as main characteristics: abundant and deep inflections of the basal and lateral cell membranes; coated luminal plasma membranes forming multiple microvilli or a genuine brush border; moderate numbers of comparatively small mitochondria, usually unassociated with the basal and lateral plasma membrane specializations; numerous multivesicular bodies occuring in the apical cytoplasm; abundant large lysosome-like bodies in the intermediate regions of the cytoplasm; and comparatively poor development of endoplasmic reticulum and Golgi apparatus.The observations suggest that the cells perform both absorptive and secretory functions and are metabolically unusually active in autolytic and heterolytic work. Comparisons with other aglomerular species indicate that the ability for active secretory function is not necessarily dependent on a close association between plasma membrane and mitochondria; however, this ability does appear to require a markedly increased basal and/or lateral cell surface created by multiple invaginations of the plasma membrane. The abundance of desmosomes and associated structures appears to represent a unique structural specialization of the goosefish tubule, and indicates that the cells must be firmly anchored to one another to supply a rigid and mechanically continuous lining of the tubule. The multivesicular bodies probably represent endocytic vacuoles which fuse with apical vesicles and invaginate their outer membrane to form the internal vesicles; they appear to transform to ambilysosomes via a function as heterophagosomes and — later — combined hetero- and autophagosomes.Supported by grants from Karolinska Institutet, Fonden til Videnskabens Fremme and Konsul Johannes Fogh-Nielsen og fru Ella Fogh-Nielsens Légat. Part of the study was performed at the Zoological Station at Naples, Italy. The assistance of Mrs. Britt-Marie Karlsson is gratefully acknowledged.     

The villous stroma of the human placenta

Summary In human placental villi the connective tissue is constructed by mesenchymal cells, small and large reticulum cells and fibroblasts. During early pregnancy mesenchymal cells dominate; starting with the third month of gestation the reticulum cells are in the majority within the terminal villi, the fibroblasts within the stem villi. Ultrastructurally intermediary types of cells can be differentiated. Together with reticular and collagenous fibres the reticulum cells form the basic architecture of the villous stroma during the first 2/3 of gestation: the reticular type of stroma. This consists of a network of cells and fibres with fetal vessels fitted in between. The remaining interspaces form a fluid system of compartments in which Hofbauer cells are suspended. They are called stromal channels. During the last trimester these channels and the Hofbauer cells as well are progressively replaced either by voluminous masses of fibres (fibrous type of stroma, mainly in the stem villi) or by sinusoidal enlargements of fetal capillaries (sinusoidal type of stroma, mainly in the terminal villi).Supported by grants from the Deutsche ForschungsgemeinschaftThe authors are indebted to Mrs. E. Böhm and Mrs. E. Schäfer for skilful technical assistance     

Elektronenmikroskopische Beobachtungen an den Sehzellen des Blutegels,Hirudo medicinalis L.

Summary The visual cell of the leech (Hirudo medicinalis) contains a big vacuole filled with a moderate dense substance (vitreous body). The wall of the vacuole consists of a system of microvilli (brush border) merging into the substance of the vitreous body. The cytoplasmic zone underneath the brush border has a special structure consisting of flattened sacs and vesicles giving a radial striation to this cell region.The cytoplasm is filled with a great number of mitochondria and small vesicles. Bundles of fine filaments are striking features of the cytoplasm; in the periphery of the cell they are in close contact with the cell membrane (half desmosomes). Elements of the endoplasmic reticulum and free ribosomes may also be present. The cells regularly contain membrane-bounded inclusions having a dense granular or cristalline content.The visual fibers (processes of the visual cells) show a certain similarity to the unmyelinated nerve fibers of higher animals. They are ensheathed into sheath cells possibly of glial nature. Processes of these cells often surround the visual cells and are sometimes embedded in their cytoplasm. Both visual and sheath cells are covered with a homogeneous basement membrane.The possible role of the structures and the problems of conduction from the brush border onto the cell surface are discussed.     

Glycoproteins in rabbit uterus during implantation

Summary The synthesis of glycoproteins in rabbit uterine epithelium during the late preimplantation period was studied using tritiated N-acetylglucosamine. In vivo labelling was achieved by the intra-uterine implantation of agar gel columns containing the precursor. Autoradiography showed the radioactivity to be predominantly localized in the apical cell surfaces, with single cells exhibiting an accumulation of silver grains in their supranuclear cytoplasm. After gel electrophoresis of uterine flushings, activity was mainly found in the -glycoprotein fraction. Fluorescein isothiocyanate (FITC)-conjugated wheat-germ agglutinin reacted with the apical cytoplasm and surfaces of the endometrial cells. However, FITC-conjugated concanavalin A exhibited a different binding pattern, reacting first with the basal cytoplasm, and later with the apical cytoplasm. After concanavalin-A staining, single cells exhibited positive vesicles in their lateral and apical parts. These cells may be released into the uterine lumen until 210 h post coitum. Neither of the lectins reacted with ciliated cells. Concanavalin A showed an affinity for the -glycoprotein fraction of the uterine secretion. The results indicate that, although all endometrial cells contain glycoproteins, only a few of these seem to be involved in the synthesis of secretory products.Supported by grants Ki 154/9-3 and 154/10-1 from the Deutsche Forschungsgemeinschaft     

Fine structure of the ocellus of Sarsia tubulosa (Hydrozoa,Anthomedusae)

Summary The fine structure of the ocellus of Sarsia tubulosa is described. The ocellar cup is formed of pigment cells and receptor cells. The receptor cells outnumber the pigment cells in almost a 2:1 ratio. Lateral extensions of neighbouring pigment cells enclose a distal region of 2 to 10 receptor cells. The receptor cell body is 5–7 m in diameter with an apical extension (20–60 m long) that reaches the ocellar cavity. A cilium (9+2 microtubules) arises from the distal part of the receptor cell. The ciliary membrane forms lateral microvilli. The tips of a number of cilia are swollen into large vesicles forming a cornea. The central region of the ocellar cavity contains extracellular electron dense homogeneous material surrounded by swollen ciliary tips and small vesicles. The close apposition between the plasma membrane covering the distal part of adjacent receptor cells as well as the adjacent ciliary shafts suggests the presence of gap junctions. The basal part of each receptor cell forms an axon. The axons of receptor cells form 3 to 4 nerve bundles that join to form the optic nerve. Synapses occur between receptor cell bodies, between axons and receptor cell bodies and among axons.     

Histochemical studies on the distribution of β-glucuronidase and succinic dehydrogenase in young and old spinal ganglion cells of the rat

Summary The present study compares the distribution of -glucuronidase and succinic dehydrogenase in young and old spinal ganglion cells of rat. In young cells there are indications of cyclic activity of these enzymes, i.e., in some stages there are perinuclear concentrations of the enzymes, at other times -glucuronidase and succinic dehydrogenase are uniformly distributed throughout the cytoplasm. These stages have been discussed with the identical distribution of mitochondria. However, in old spinal ganglion cells both -glucuronidase and succinic dehydrogenase become mainly concentrated in the pigment areas, suggesting thereby their possible role in the production of pigment, through the medium of the mitochondria.     

Observations on the fine structure of the cercaria of Notocotylus attenuatus and formation of the cyst wall of the metacercaria

Summary The dorsal tegument of the mature cercaria of Notocotylus attenuatus is a syncytial, cytoplasmic layer, containing two types of secretory granule which are identifiable ultrastructurally. The type 1 secretory bodies are electron lucid, whereas most type 2 granules have a banded appearance. The ventral tegument contains granules which are secreted from the type 3 cells; the type 3 granules are membrane bound, electron dense, and consist of both an amorphous and a finely striated zone. The type 4 cells mainly contain cigar-shaped granules consisting of an amorphous core surrounded by concentric striations. The granules exhibit structural variability in shape and content. The type 4 cells undergo a cellular migration to the tegument during encystment. The structure of the posterior-lateral glands and mode of secretion of the granules are described. Possible functions of microtubules are discussed for each cell type. Details of some secretory processes involved in the formation of the hemispherical cyst wall are described. The layers of the cyst wall may be related to the granular contents of the various parenchymal cells of the cercaria. The tegument of the metacercaria originates primarily from the cytoplasm of the type 1, type 2, type 3 and type 4 cells.     

Morphology of the Respiratory Trees in the Holothurians Apostichopus japonicus and Cucumaria japonica

The morphology of the respiratory trees in the holothurians Apostichopus japonicus and Cucumaria japonica was studied using histochemical and electron microscopic techniques. The epithelium of the respiratory tree cavity in A. japonicus consists of columnar cells about 17–20 m high. In C. japonica, this epithelium is composed of two cell types: bulbous cells embedded in the connective tissue layer and secretory cells; cells are 1–12 m high. A characteristic feature of cells of the respiratory tree cavity epithelium in these species is the presence of numerous phagosomes and coated vesicles in the apical cytoplasm. The cell surface has many microvilli and a single cilium. Cells of the coelomic epithelium contain vacuoles in the apical part and myofibrils in the basal region; the thickness of this epithelium in A. japonicus and C. japonica is 9–16 and 10–30 m, respectively. Based on the different structure of the respiratory trees and the absence of hemocytes in A. japonicus, it is suggested that the holothurian species studied have different routes of oxygen transport from the environment to the internal organs.     

Surface morphology of the subfornical organ in the rabbit's brain

Summary The ventricular surface of the subfornical organ of the rabbit's brain was studied with scanning and transmission electron microscopic techniques. The ependymal covering was found to consist of hexagonal cells with convex apical surfaces. From the center of each cellular surface a single kinocilium up to 6 m in length protrudes into the liquor. It is usually covered with secretory material having the shape of pearlstrings. The surface aspect of the subfornical organ suggests secretion into the liquor by emptying of giant vacuoles which originate below the ependyma in nerve cells, move towards the surface, develop pressure while flattening their ependymal cover and finally erupt, leaving collapsed ependyma- and/or nerve cells bag on the surface of the organ. A second mechanism of more granular secretion by ependymal cells appears possible.We are indebted to Fräulein E. Östermann, Frau L. Schulze and Frau H. Zuther-Witzsch for excellent technical assistance.