The ultrastructure of Blastocystis galli from chickens

The ultrastructure stages of Blastocystis galli were studied in chicken's intestine and in laboratory cultures. There were found morphological structures: surface coat (cell from chickens' intestine showed a very thick surface coat); cell membrane--there were some small electron-opaque deepening "pockets" on the membrane; inner membrane; endoplasmic reticulum with attached ribosomes, which present in the cytoplasm; all cells contained numerous of small vacuoles and large glycogen inclusions in cytoplasm; mitochondria with tubular cristae; nucleus with granules condensed chromatin; central vacuole; Golgi complex was represented by number of plates grouped in a pite; the cyst-like forms were surrounded by multilayered wall.     

Ultrastructural studies of the bovine Graafian Follicle and corpus luteum

Summary Ovaries were obtained from normal adult dairy cows at all days of the estrous cycle. The largest Graafian follicle and corpus luteum were excised and prepared for electron microscopic study.In the follicle wall, membrana granulosa cells contained granular endoplasmic reticulum and mitochondria with villous or lamellar cristae. The theca interna cells during proestrus and estrus contained ribosomes separated from endoplasmic reticulum. The latter during these periods assumed tubular and tortuous shapes. Mitochondria during these periods assumed rounded shapes, were occasionally cup-shaped, and developed tubular cristae.In the corpus luteum, the large luteal cells during metestrus and diestrus contained an abundance of agranular, tubular, branching membranes of endoplasmic reticulum and Golgi apparatus. Mitochondria were large, with tubular cristae, but smaller mitochondria, with irregular or villous cristae, were also present. Transitional bodies of the latter mitochondria to another form were observed. Cup-shaped and annular mitochondria were present during diestrus. In the small luteal cells, large vesicular membrane formations were present and often associated with lipid bodies. The cells were lipid-laden. Lysosomes and granular bodies were present during luteal regression.The observed features of the granulosa cells are related to protein synthesis, those of the pre-ovulatory theca interna cells and metestrus-diestrus large luteal cells to steroid synthesis, and those of the small luteal cells to lipid storage.This investigation was supported by a General Research Support Grant to the College of Veterinary Medicine, University of Minnesota, and Research Grant No. GM-07009, of the United States Public Health Service. Approved for publication as Scientific Journal Series Paper No. 6344, Minnesota Agricultural Experiment Station. The work reported is taken from the senior author's Ph. D. thesis.     

On the ultrastructure of the ovary of the liver fluke (Fasciola hepatica L.)

Summary The ovary of the liver fluke has been studied with light and electron microscopy. The organ consisted of germ cells and a layer of peripheral cells suggested to be nurse cells, and was surrounded by a capsule containing muscular tissue. The peripheral cells rested on a thick basement membrane and were irregular in outline. Their nuclei were of irregular shape, the mitochondria were dark with few cristae and the endoplasmic reticulum was tubular or vesicular and partly studded with ribosomes. The germ cells were rounded or polyhedral except in the outer part of the ovary where some of them showed irregular processes. The germ cells of the outer region (oogonia) were relatively small and in close contact with the cells suggested to be nurse cells. The inner germ cells (oocytes) were large and loosely packed. Their nuclei were irregular and contained round distinct nucleoli. The nuclear envelopes showed numerous pores. The endoplasmic reticulum was very sparse, but free ribosomes were abundant in the cytoplasm. This corresponded with a strong basophilia removable with RNase. In addition round basophilic bodies formed by densely packed ribosomes and membraneous material occurred in close spatial relation to mitochondria. The latter contained dense granules and few cristae. Groups of vesicles and membraneous lamellae were found in the cytoplasm, but they were considerably smaller than vertebrate Golgi complexes. Numerous dense spherical granules were found mainly in the periphery of the large germ cells. The granules were strongly osmiophilic except in the terminal part of the ovary. They were PAS-positive, but negative to Sudan dyes.Supported by a grant from Jordbrukets Forskningsråd, Stockholm.     

Structure of taste buds in foliate papillae of the rhesus monkey, Macaca mulatta

Taste buds in foliate papillae of the rhesus monkey were examined by electron microscopy. Three distinct cell types were identified. Type I cells were narrow elongated cells containing an oval nucleus, bundles of intermediate filaments, several Golgi bodies, and characteristic apical membrane-bounded dense granules. These cells exhibited morphological variations: some had a moderately dense cytoplasm, perinuclear free ribosomes, and flattened sacs of rough endoplasmic reticulum; others had a more lucent cytoplasm, dilated irregular rough endoplasmic reticulum, lysosome-like dense bodies, and lipid droplets. Type II cells typically contained a spherical, pale nucleus, a prominent nucleolus, supranuclear and infranuclear Golgi bodies, mitochondria with tubular cristae, and one or two centrioles. This cell type, too, showed some variation in the relative amounts of ribosomes and smooth endoplasmic reticulum, which varied inversely with each other. Type III cells were characterized by a clear apical cytoplasm essentially devoid of ribosomes and containing microtubules. In a few type III cells, the peri- and infranuclear regions contained many ribosomes and some rough endoplasmic reticulum. In most Type III cells, there were large numbers of dense and clear vesicles in the peri- and infranuclear regions; some of the vesicles were grouped in synapse-like arrangements with adjacent nerves. The morphological variations exhibited by all three cell types could be accounted for by age differences in each of the cells. This would be consistent with the notion that cell renewal occurs in each of the three cell populations.     

Special secretory cells in the soybean seed coat

Summary The seed coat of soybean (Glycine max L. Merr.) is of physiological interest for synthesis and transport of amino acids and photosynthates during embryo development. A transmission and scanning electron microscopic study to elucidate the structure of the seed coat disclosed a specialized convex area (antipit) appressed to a concave pit in the center of the abaxial surface of the cotyledon. The antipit, which lies on the inner surface of the seed coat at a medial point in the anterior to posterior direction of the seed, contained specialized secretory cells bounded by loose multi-layered cell walls. These cells were rectangular in the developing seed, varied in length, and contributed directly to the convex morphology of the antipit seen on the ventral surface of the seed coat. At maturity these cells assumed the shape of a cone, extending from the aleurone layer in a perpendicular array. The aleurone and cone cells contained numerous Golgi apparatus, laminated rough endoplasmic reticulum, secretory vesicles, and amyloplasts. Secretory vesicles arose directly from tubules of fenestrated trans cisternae of the Golgi apparatus. Mitochondria were clustered with the amyloplasts; stacks of lamellar cisternae of rough endoplasmic reticulum were associated with the nucleus and Golgi apparatus. The cellular contents, the interconnections by plasmodesmata, and the close physical association with the cotyledon suggested that the aleurone and cone cells may be involved in symplastic transport of nutrients for use by the developing embryo.This paper is dedicated to the memory of my parents, Joseph and Theresa Yaklich, who by their example taught me the value of work and the enjoyment of simple things.     

Ultrastructure of the mouse tracheal epithelium

The ultrastructure of mouse tracheal epithelium was examined. The three cell types, basal cells, ciliated cells and goblet cells, described for other mammalian trachea were found to be present although goblet cells occurred only rarely. A cell type, termed the nonciliated cell, not described in other mammalian trachea was frequently found in mouse tracheal epithelium. These cells contained abundant smooth and rough endoplasmic reticulum, free ribosomes, a large Golgi complex, and many mitochondria. There were many vesciles containing an electron dense material near the luminal surface of these cells; these cells were positive for PAS. These features suggested a secretory function for the cells. This, along with the scarcity of goblet cells, suggested that the nonciliated cells of mouse tracheal epithelium fulfill the function of the goblet cells found in other mammalian trachea.     

Differentiation of the golden hamster oviduct epithelial cells during postnatal development: an electron microscopic study

The ultrastructural changes in the process of differentiation of the epithelial cells of the golden hamster oviduct during postnatal development were investigated by means of electron microscopy. In the epithelium of the ampulla of the neonatal oviducts, no differentiated ciliated cells or secretory cells were identified. In these undifferentiated cells, free ribosomes were well developed, but rough endoplasmic reticulum (RER) and the Golgi apparatus were undeveloped. Cells undergoing ciliogenesis were first identified at 3.5 days after birth, and some ciliated cells appeared at 4.5 days. In the nonciliated cells, marked changes in the organelles were observed at this time. Subsequently, some nonciliated cells containing well-developed RER and Golgi apparatus were observed at 9.5 to 10.5 days after birth, and a few mature secretory cells were observed at 10.5 days. An increase in secretory granules occurred in the secretory cells at 12.5-15.5 days after birth. Many fully mature ciliated and secretory cells were observed at 15.5 days after birth. After 20.5 days after birth, the epithelium was identical with that of the adult golden hamster. Quantitative data indicated that the differentiation of ciliated cells began earlier and took place over a more extended period of time than did that of the secretory cells in the golden hamster oviduct during postnatal development.     

The Highly Conserved COPII Coat Complex Sorts Cargo from the Endoplasmic Reticulum and Targets It to the Golgi

Protein egress from the endoplasmic reticulum (ER) is driven by a conserved cytoplasmic coat complex called the COPII coat. The COPII coat complex contains an inner shell (Sec23/Sec24) that sorts cargo into ER-derived vesicles and an outer cage (Sec13/Sec31) that leads to coat polymerization. Once released from the ER, vesicles must tether to and fuse with the target membrane to deliver their protein and lipid contents. This delivery step also depends on the COPII coat, with coat proteins binding directly to tethering and regulatory factors. Recent findings have yielded new insight into how COPII-mediated vesicle traffic is regulated. Here we discuss the molecular basis of COPII-mediated ER–Golgi traffic, focusing on the surprising complexity of how ER-derived vesicles form, package diverse cargoes, and correctly target these cargoes to their destination.The port of entry into the secretory pathway is the endoplasmic reticulum (ER). Approximately one-third of the eukaryotic proteome traffics from this multifunctional organelle (Huh et al. 2003). This diverse set of cargo is translocated into the ER, folded, and modified before it travels to the Golgi, where further modifications occur. From the Golgi, cargo is sorted to other subcellular compartments to perform a variety of cellular functions. The highly conserved machinery required for these transport events was initially identified through genetic screens in the yeast Saccharomyces cerevisiae, and insights into the function of this machinery were provided through the use of in vitro transport assays. Advances in microscopy, in particular, the use of GFP fusion proteins and live cell imaging, have also played a critical role in understanding the dynamics of membrane traffic. In this article, we describe the mechanistic advances that have helped us to understand how diverse cargo correctly traffics from the ER to the Golgi complex in lower and higher eukaryotes. Even though these mechanisms are largely conserved, they are more complex at the molecular and organizational levels in metazoans.     

Membrane traffic from the endoplasmic reticulum to the Golgi apparatus is disturbed by an inhibitor of the Ca2+-ATPase in the ER

Summary An electron microscopic study of cress (Lepidium sativum L.) roots treated with cyclopiazonic acid (CPA), an inhibitor of the Ca²⁺-ATPase in the endoplasmic reticulum (ER) has been carried out. Drastic changes in the endomembrane system of the secretory root cap cells were observed. After treatment with CPA dense spherical or elliptoidal aggregates of ER (diameter 2–4 m) were formed in addition to the randomly distributed ER cisternae characteristic for control cells. The formation of ER aggregates indicates that in spite of an inhibition of the Ca²⁺ -ATPase in the ER by CPA, membrane synthesis in the ER continued. The ER aggregates are interpreted as a reservoir of ER membrane material newly synthesized during the 2 h CPA-treatment. Hypertrophied Golgi cisternae and secretory vesicles, which are characteristic for secretory cells under control conditions, were completely absent. Additionally the shape of the Golgi stacks was flat and the diameter of the cisternae was shortened by about one third. These phenomena are indicative of an inactive state of the Golgi apparatus. The cellular organization of both other cell types of the root cap, meristematic cells and statocytes, was not visibly affected by CPA, both having a relatively low secretory activity. The formation of ER aggregates as well as the reduction of Golgi compartments are indications for the existence of a unidirectional transport of membrane material from the ER to the Golgi. It is suggested that the membrane traffic from the ER to the Golgi apparatus is regulated by the cytosolic and/or luminal calcium concentration in secretory cells of the root cap.Abbreviations CPA cyclopiazonic acid - ER endoplasmic reticulum     

Seasonal ultrastructural variations in pinealocytes of the woodchuck,Marmota monax

The ultrastructure of the pinealocyte in the woodchuck, Marmota monax, was studied during the four seasons of the year. Fall cells have a fairly uniform cytoplasmic density, organelles consistent with synthetic and/or secretory activity and rather extensive pericapillary and intercellular spaces. Many winter pinealocytes are nearly devoid of ribosomes and granular endoplasmic reticulum but contain lipid droplets associated with mitochondria. Pericapillary and intercellular spaces are minimal. Spring glands have the greatest variation in cytoplasmic density with intercellular and pericapillary spaces similar to that seen in fall glands. Cells containing electron dense cytoplasm have Golgi zone associated, secretory granules, free ribosomes, short sections of granular endoplasmic reticulum and dense bodies. Cells with a more electron lucent cytoplasm are similar to the most frequently observed summer pinealocytes which have numerous Golgi zones but few associated secretory granules. Microtubules are prominent in the cytoplasm of these cells, the plasma membranes are smooth and intercellular and pericapillary spaces are minimal. A yearly rhythm or cyclic activity of the pinealocyte is suggested.     

Effect of postganglionic sympathectomy on the ultrastructure of the rat parotid gland

Summary Following two weeks of superior cervical ganglionectomy, the parotid glands of adult rats were removed and studied by electron microscopy. Sympathectomy induced striking alterations of acini, resulting in a heterogeneous population of acinar cells, but it had no obvious effect on the duct system. Most of the altered cells could be classified on a cytological basis as dark cells or light cells. Dark cells predominated and contained more secretory granules, less granular endoplasmic reticulum, fewer Golgi membranes, and smaller lumina and intercellular canaliculi than normal acinar cells. The synthesis and extrusion of secretory products appeared to be minimal in these cells. Light cells possessed ultrastructural features, such as dilated cisternae of granular endoplasmic reticulum and prominent Golgi membranes, which were opposite to those of dark cells and indicative of a high degree of secretory activity.The heterogeneous population of cells following sympathectomy indicates that the sympathetic nervous system may play an important role in regulating the secretory synchrony of acinar cells.Supported by U.S.P.H.S. Grant DE 02110.     

Fine Structure of the Adrenocortical Homolog and the Corpuscles of Stannius of Amia calva L.

A comparison is made between the fine structure of yellow corpuscles and white corpuscles located within the kidneys of the holostean fish, Amia calva L. The yellow corpuscles are composed of epithelial cells possessing all the features of steroid-producing tissues, namely an abundance of vacuoles, tubular smooth endoplasmic reticulum, and mitochondria with tubular cristae. The Golgi apparatus is also a conspicuous component of their cytoplasm. These cells are homologous to adrenocortical cells of higher vertebrates and they have cytoplasmic projections which extend into the lumina of surrounding sinusoids. The white corpuscles possess epithelial cells of variable appearance but all cells contain secretory granules and an extensive rough endoplasmic reticulum. The secretory granules appear to originate at the Golgi apparatus and occasionally are observed intact in the intercellular space. However the method of release of these granules was not clearly defined. These corpuscles are similar to the corpuscles of Stannius which have been described in modern bony fish. The presence of multivesicular bodies and smooth endoplasmic reticulum in some cells may reflect the origin of the corpuscles of Stannius from the tubular nephron. A. calva appears to be a suitable organism for comparative studies into the function of the adrenocortical homolog and corpuscles of Stannius in “primitive” fish.     

Fine structure of the corpuscles of stannius in the toadfish.

The micro-anatomy of the corpuscles of Stannius of the toadfish, Opsanus tau, an aglomerular marine teleost, has been studied by light and electron microscopy. The corpuscles are composed of extensively anastomosed cords of epithelial cells which maintain intimate contact with blood capillaries. Most of the epithelial cells contain acidophilic granules which also show a positive reaction with the periodic acid-Schiff technique and aldehyde fuchsin. On the basis of fine structural criteria, three cell types can be recognized. The granular cells contain abundant quantities of granular endoplasmic reticulum, ribosomes, Golgi apparatus with prosecretory granules, coated vesicles, polymorphic mitochondria with lamellar cristae, filaments, microtubules, a cilium, a variety of lysosome-like dense bodies, glycogen particles, lipid droplets, secretory granules and intranuclear lipid-like inclusions. One variety of agranular cell (type I) is characterized by the total absence of secretory granules, but it contains large amounts of granular endoplasmic reticulum and ribosomes, conspicuous profiles of Golgi apparatus, coated vesicles and sometimes an abundance of glycogen. Another variety of agranular cell (type II) has poorly developed cytoplasmic organelles. The perivascular space between the capillary and parenchyma contains connective tissue cells and abundant nerve fibers. The different types of epithelial cells observed in the corpuscles of Stannius of this fish may represent functional stages of the secretory cycle in a single cell type.     

The endomembrane system of differentiating carposporangia in the red algaChondria tenuissima: Occurrence and participation in secretion of polysaccharidic and proteinaceous substances

Summary The endomembrane system during carposporogenesis inChondria tenuissima was studied using electron microscopy and histochemistry. Profiles of the nucleus are convoluted, resulting in a highly increased surface area. Stacked cisternae are found within the peripheral part of the nucleus. Vesicles, tubules and membrane bound fibrillar bodies occur within the nucleoplasm. The endoplasmic reticulum surrounds the nuclear envelope.The endoplasmic reticulum and the Golgi apparatus, together with small transition vesicles, represent a functional unit. They form two different secretory substances during carposporogenesis. In young stages, carbohydrates are produced by normal dictyosomes within large, normal exocytotic Golgi vesicles. They do not react positively with PAS or Thiéry method and are believed to represent cell wall material. In later stages, the central area of the Golgi cisternae becomes filled with electron dense material. The individual cisternae are transformed into cored vesicles at the trans-face of the dictyosomes. The dense core of the vesicles is proteinaceous and stains with coomassie brilliant blue R. The peripheral fibrillar material is polysaccharidic and reacts positively using the Thiéry method. The contents of the cored vesicles are believed to participate in carpospore attachment. The ER gives rise to cytolysosomes in which starch grains are sequestrated and digested. Mucilaginous sacs seem to be similarly formed.     

Electron microscopic observations on a new cell type in the fundus mucosa of the mouse stomach

Summary In the course of electron microscopic investigations of the fundus mucosa of the mouse stomach, a few cells of an unknown type were found by chance in the deep portions of the glands. These cells are characterized by two different kinds of specific granules in their cytoplasm, one of which being large and less dense, and the other one being small and dense. The large less dense granules resemble zymogen granules of the chief cell, which are formed by the rough endoplasmic reticulum and Golgi system. The small dense granules are quite similar to the secretory granules of the basal granulated cell, and are considered to be formed in the Golgi complex. Release figures of the small dense granule were not observed, numerous granules, however, were observed in close contact with the basal cell membrane. The occurrence of these two kinds of granules in one cell suggests that the basal granulated cell and the zymogenic cell originate from the same entodermal stem element.The author cordially thanks Professor Dr. Hisao Fujita, Department of Anatomy, Hiroshima University School of Medicine, for his kind advices and criticisms.     

Protein Translocation across the Rough Endoplasmic Reticulum

The rough endoplasmic reticulum is a major site of protein biosynthesis in all eukaryotic cells, serving as the entry point for the secretory pathway and as the initial integration site for the majority of cellular integral membrane proteins. The core components of the protein translocation machinery have been identified, and high-resolution structures of the targeting components and the transport channel have been obtained. Research in this area is now focused on obtaining a better understanding of the molecular mechanism of protein translocation and membrane protein integration.Protein translocation across the rough endoplasmic reticulum (RER) is an ancient and evolutionarily conserved process that is analogous to protein export across the cytoplasmic membranes of eubacterial and archaebacterial cells both with respect to the mechanism and core components. The RER membrane of eukaryotic cells is contiguous with the nuclear envelope and is morphologically composed of interconnected cisternae and tubules. Electron microscope images of mammalian cells and tissues revealed that the cisternal regions of the cytoplasmic surface of the endoplasmic reticulum are densely studded by membrane-bound ribosomes (Palade 1955a,b), giving rise to the term “rough ER.” The RER-bound ribosomes in en face images are often arranged in spirals or hairpins (Palade 1955a; Christensen and Bourne 1999), indicative of polyribosomes that are actively engaged in protein translation.Consistent with this high density of membrane-bound ribosomes, the RER is a major site of protein biosynthesis in eukaryotic cells. The nuclear envelope, the Golgi, lysosome, peroxisome, plasma membrane, and endosomes are biosynthetically derived from the rough ER. The three major groups of proteins that are synthesized by RER-bound ribosomes include secretory proteins, integral membrane proteins destined for ER-derived membranes, and the lumenal-resident proteins of the ER, Golgi, nuclear envelope, and lysosome. For those membranes that are not physically linked to the ER (e.g., the lysosome), integral membrane and lumenal proteins are delivered to their destination by vesicular transport pathways. Bioinformatics analysis of fully sequenced eukaryotic genomes indicates that roughly 30% of open reading frames encode integral membrane proteins (Wallin and von Heijne 1998); hence, a major role of the RER is the biosynthesis of membrane proteins. An important class of membrane proteins that are integrated into the RER has single carboxy-terminal TM spans and are known as tail-anchored (TA) membrane proteins. The posttranslational integration pathway for TA proteins has been a subject of several recent reviews (Borgese and Fasana 2011; Shao and Hegde 2011), thus we will not address the TA pathway in this article.     

Fine structure of the corpus Luteum of the raccoon during pregnancy

Summary Ultrastructure of the granulosa lutein cells of the raccoon from throughout pregnancy has been described. The lutein cells often from epithelial cords which are separated by the connective tissues, capillaries and lymphatics. Based on the arrangements and modifications of the cytoplasmic organelles and inclusions, three types of lutein cells have been recognized. The type I lutein cells predominantly contain tubular, agranular endoplasmic reticulum, juxtanuclear Golgi complexes, a few round to rod-shaped mitochondria, some free ribosomes, and occasional lipid droplets. Occasionally the tubular cristae of mitochondria and tubular smooth endoplasmic reticulum appear contiguous. The type II cells contain abundant lace-like and/or stacked fenestrated endoplasmic reticulum cisternae that frequently form membranous whorls, some tubular, agranular endoplasmic reticulum, mitochondria, and lipid droplets. Mitochondria are usually small, but unusual large ones also occur. The small, rod-to round-shaped mitochondria usually have tubular cristae; but the large, oval, elongate, and cup shaped mitochondria possess tubular, lamellar, plate like, and whorl-like cristae. The plasma membranes of the cells are complexly elaborated and folded, especially when apposing each other. In favorable sections, strands of fenestrated cisternae appose the folds of the plasma membranes. In general, the amount of cytoplasmic organelles and inclusions vary greatly in the cells. The type III cells predominantly contain lipid droplets and sparse cytoplasmic organelles. The type I and II cells are found throughout pregnancy, but the type III cells are observed from mid gestation to term. The cytological features of type I and II cells suggest that they probably secrete most of the steroids, whereas the type III cells primarily store lipids.This research was supported by UPSHS grant AM-11376 and NIH contract 69-2136.     

Immunocytochemical and morphometrical study of thyrotropic cells of Rana ridibunda

Summary Indirect immunoflorescence and PAP techniques for light microscopy as well as the immunogold complex technique for electron microscopy were used to localize and identify thyrotropic (TSH) producing cells in the pars distalis of Rana ridibunda. A double immunostaining procedure was used to distinguish TSH cells from other glycoprotein hormone producing cells. Rabbit anti-human--TSH was used as the primary antiserum and revealed a basophil, PAS and alcian blue positive cell type in the ventro-central zone of the gland. Under the electron microscope, TSH cells show irregular morphology, polymorphic secretory granules with diameters ranging between 120 and 375 nm and poor development of the endoplasmic reticulum and Golgi complex; they are usually polarized towards capillaries. Ultrastructural morphometry (point-counting method) was used to evaluate stereological parameters of rough endoplasmic reticulum, Golgi complex, secretory granules and mitochondria.This work has been supported by grant 2184-83 from the Comisión Asesora (CAICYT) of Spain     

Ultrastructural immunolocalization of basic fibroblast growth factor to lipid bodies and secretory granules in human mast cells

Isolated human lung mast cells were used to identify subcellular sites of basic fibroblast growth factor using a postembedding immunogold method. The factor was present in quantity in secretory granules and cytoplasmic lipid bodies. Cisterns of smooth endoplasmic reticulum and ribosome clusters, closely associated with lipid bodies, contained the factor as did the nuclear matrix. Factor-positive lipid bodies were adjacent to nuclear pores and often indented perinuclear cisternae. Altered secretory granules with reduced density, characteristic of secretion by piecemeal degranulation in mast cells, showed reduced gold label for basic fibroblast growth factor; small, electron-lucent (80–100nm) transport vesicles near altered granules were labelled for the factor. Since these mature mast cells do not display extensive arrays of classical secretory organelles, such as rough endoplasmic reticulum and Golgi structures, these new subcellular localizations for basic fibroblast growth factor suggest several possible alternative release routes for a cytokine devoid of a signal sequence characteristic of regulated secretory proteins.     

Endocytic routes to the Golgi apparatus

 The endocytic routes of labelled lectins as well as cationic ferritin were studied in cells with a regulated secretion, i.e. pancreatic beta cells, and in constitutively secreting cells, i.e. fibroblasts and HepG2 hepatoma cells, paying particular attention to routes into the Golgi apparatus. Considerable amounts of internalised molecules were taken up into the trans Golgi network (TGN) and into Golgi subcompartments in all three cell types as well as in secretory granules of the pancreatic beta cells. The internalised materials did not pass rapidly the TGN and Golgi stacks, but were still present hours after internalisation, being then particularly concentrated in TGN-elements and in the transmost Golgi cisterna. Endocytosed materials reached forming secretory granules present in the TGN. Further, direct fusion between endocytotic vesicles and mature secretory granules was observed. Golgi subcompartments as well as endocytic TGN containing endocytosed materials were in close apposition to specialised regions of the endoplasmic reticulum. The Golgi apparatus including its parts containing endocytosed materials were transformed into a tubular reticulum upon treatment with the fungal metabolite Brefeldin A. Rarely, internalised material was observed in the lumen of the endoplasmic reticulum, thus providing evidence for an endocytic plasma membrane to endoplasmic reticulum route. Accepted: 9 March 1998