Distinct Granule Types Research Articles

Starch granules and their size distribution in wheat: Biosynthesis, physicochemical properties and their effect on flour-based food systems

Starch is a vital component of wheat grain and flour, characterized by two distinct granule types: A-type starch (AS) with granules larger than 10 µm in diameter, and B-type starch (BS) with granules measuring no more than 10 µm in diameter. This review comprehensively evaluates the isolation, purification, and biosynthesis processes of these types of granules. In addition, a comparative analysis of the structure and properties of AS and BS is presented, encompassing chemical composition, molecular, crystalline and morphological structures, gelatinization, pasting and digestive properties. The variation in size distribution of granules leads to differences in physicochemical properties of starch, influencing the formation of polymeric proteins, secondary and micro-structures of gluten, chemical and physical interactions between gluten and starch, and water absorption and water status in dough system. Thus, starch size distribution affects the quality of dough and final products. In this review, we summarize the up-to-date knowledge of AS and BS, and propose the possible strategies to enhance wheat yield and quality through coordinated breeding efforts. This review serves as a valuable reference for future advancements in wheat breeding.

Defective acid hydrolase secretion in RUNX1 haplodeficiency: Evidence for a global platelet secretory defect.

RUNX1 haplodeficiency is associated with thrombocytopenia, platelet dysfunction and a predisposition to acute leukaemia. Platelets possess three distinct types of granules and secretory processes involving dense granules (DG), α-granules and vesicles or lysosomes containing acid hydrolases (AH). Dense granules and granule deficiencies have been reported in patients with RUNX1 mutations. Little is known regarding the secretion from AH-containing vesicles. We studied two related patients with a RUNX1 mutation, easy bruising, and mild thrombocytopenia. Platelet aggregation and 14 C serotonin in platelet-rich plasma (PRP) were impaired in response to ADP, epinephrine, collagen and arachidonic acid. Contents of DG (ATP, ADP), α-granules (β-thromboglobulin) and AH-containing vesicles (β-glucuronidase, β-hexosaminidase, α-mannosidase) were normal or minimally decreased. Dense granules secretion on stimulation of gel-filtered platelets with thrombin and divalent ionophore A23187 (4-12μmol L-1 ) were diminished. β-thromboglobulin and AH secretion was impaired in response to thrombin or A23187. We studied thromboxane-related pathways. The incorporation of 14 C -arachidonic acid into phospholipids and subsequent arachidonic acid release on thrombin activation was normal. Platelet thromboxane A2 production in whole blood serum and on thrombin stimulation of PRP was normal, suggesting that the defective secretion was not due to impaired thromboxane production. These studies provide the first evidence in patients with a RUNX1 mutation for a defect in AH (lysosomal) secretion, and for a global defect in secretion involving all three types of platelet granules that is unrelated to a granule content deficiency. They highlight the pleiotropic effects and multiple platelet defects associated with RUNX1 mutations.

The process of granule exocytosis in non-stimulated atrial granular cells of the snail, Achatina achatina: An ultrastructural, histochemical and immunocytochemical study

Abundant secretory granular cells (GCs) in the Giant African land snail atrium harbor a range of bioactive substances and undergo rapid total degranulation in response to stimulation of the cardiac nerve or stressful influences. Here we have analyzed exocytotic events in the non-stimulated GCs. It was shown that the GCs contain three major distinct types of granules that differ histochemically, immunocytochemically and ultrastructurally, each performing specific functions. The type I granules characteristically filled with electron-lucent homogeneous materials exhibit intense immunoreactivity for bioactive proteins and therefore are considered to be storage granules. Histochemistry using vital staining with Acridine Orange and Gomori acid phosphatase technique has revealed lysosomal-related nature of the electron-dense type II granules. Digestion remnants appearing as fine filamentous materials fill the type III granules. Only the type III granules fuse together and with the plasma membrane form degranulation channels and surface pores, through which the debris is removed from the cell. The finding of granules exhibiting intermediate ultrastructural, histochemical and immunocytochemical features suggests that the major granule types represent most stable states along a granule empting continuum. Thus, under physiological conditions, the GCs continuously produce secretory proteins and so maintain readiness for stress-response, but use protein degradation machinery to prevent massive release of these bioactive substances into hemolymph.

EGFP-FMRP Granules Are Proto-Granules That Can Be Shunted to Stress Granules During a Stress Response

AbstractOverexpressed EGFP-FMRP has been used as a substitute for endogenous fragile X mental retardation protein (FMRP) in protein-protein interaction studies, and in studies concerning the composition, the formation and the localization of neuronal granules. However, the question of whether this tool truly recapitulates the properties of the endogenous protein has not been addressed. Here we demonstrate that overexpressed EGFP-FMRP forms three distinct granule types based on colocalization with various marker proteins. The majority of EGFP-FMRP-containing granules are larger and more amorphous than known granule types. Consistent with this, there is only partial colocalization with stress granule or P-body markers and no colocalization with a putative Drosophila ortholog neuronal granule marker. Nevertheless, agents such as arsenite and hippurstanol, which create endogenous stress granules and P-bodies, drive EGFP-FMRP into such granules. Additionally, whereas inhibiting cellular methyl-protein formation alters the composition of endogenous FMRP-containing stress granules, we found that such treatment had little effect on the formation of EGFP-FMRP granules, or their composition. Altogether these data suggest that many overexpressed EGFP-FMRP granules represent proto-stress granules requiring external stimuli for their conversion. More importantly, the inherent heterogeneity of these granules suggests that caution should be used in extrapolating results obtained with EGFP-tagged surrogates of FMRP to endogenous FMRP granules.

EGFP-FMRP Granules Are Proto-Granules That Can Be Shunted to Stress Granules During a Stress Response

Overexpressed EGFP-FMRP has been used as a substitute for endogenous fragile X mental retardation protein (FMRP) in protein-protein interaction studies, and in studies concerning the composition, the formation and the localization of neuronal granules. However, the question of whether this tool truly recapitulates the properties of the endogenous protein has not been addressed. Here we demonstrate that overexpressed EGFP-FMRP forms three distinct granule types based on colocalization with various marker proteins. The majority of EGFP-FMRP-containing granules are larger and more amorphous than known granule types. Consistent with this, there is only partial colocalization with stress granule or P-body markers and no colocalization with a putative Drosophila ortholog neuronal granule marker. Nevertheless, agents such as arsenite and hippurstanol, which create endogenous stress granules and P-bodies, drive EGFP-FMRP into such granules. Additionally, whereas inhibiting cellular methyl-protein formation alters the composition of endogenous FMRP-containing stress granules, we found that such treatment had little effect on the formation of EGFP-FMRP granules, or their composition. Altogether these data suggest that many overexpressed EGFP-FMRP granules represent proto-stress granules requiring external stimuli for their conversion. More importantly, the inherent heterogeneity of these granules suggests that caution should be used in extrapolating results obtained with EGFP-tagged surrogates of FMRP to endogenous FMRP granules.

Granule formation mechanisms within an aerobic wastewater system for phosphorus removal.

Granular sludge is a novel alternative for the treatment of wastewater and offers numerous operational and economic advantages over conventional floccular-sludge systems. The majority of research on granular sludge has focused on optimization of engineering aspects relating to reactor operation with little emphasis on the fundamental microbiology. In this study, we hypothesize two novel mechanisms for granule formation as observed in three laboratory scale sequencing batch reactors operating for biological phosphorus removal and treating two different types of wastewater. During the initial stages of granulation, two distinct granule types (white and yellow) were distinguished within the mixed microbial population. White granules appeared as compact, smooth, dense aggregates dominated by 97.5% "Candidatus Accumulibacter phosphatis," and yellow granules appeared as loose, rough, irregular aggregates with a mixed microbial population of 12.3% "Candidatus Accumulibacter phosphatis" and 57.9% "Candidatus Competibacter phosphatis," among other bacteria. Microscopy showed white granules as homogeneous microbial aggregates and yellow granules as segregated, microcolony-like aggregates, with phylogenetic analysis suggesting that the granule types are likely not a result of strain-associated differences. The microbial community composition and arrangement suggest different formation mechanisms occur for each granule type. White granules are hypothesized to form by outgrowth from a single microcolony into a granule dominated by one bacterial type, while yellow granules are hypothesized to form via multiple microcolony aggregation into a microcolony-segregated granule with a mixed microbial population. Further understanding and application of these mechanisms and the associated microbial ecology may provide conceptual information benefiting start-up procedures for full-scale granular-sludge reactors.

Secretion of heparin-binding protein from human neutrophils is determined by its localization in azurophilic granules and secretory vesicles

Human neutrophils have an important role in host defense against microbial infection. At different stages of an infectious process, neutrophils progressively up-regulate receptors and release various effector molecules. These are stored in several distinct types of granules with varying propensity to be secreted. Heparin-binding protein (HBP), also known as CAP37 or azurocidin, is a multifunctional, inactive serine-protease homologue. The present work shows that HBP is released from neutrophils on stimulation with secretagogues that do not trigger the secretion of azurophilic granule content. Therefore, the subcellular localization of HBP was investigated in more detail. Immunofluorescence microscopy revealed that HBP was localized close to the plasma membrane. Further analysis by fractionation of postnuclear supernatants from cavitated neutrophils showed that HBP is stored in azurophilic granules and secretory vesicles but that it is also detected to a minor extent in the plasma membrane. These findings were confirmed by immunoelectron microscopy showing that HBP colocalized with marker proteins of azurophilic granules and secretory vesicles. The presence of HBP in secretory vesicles possibly depends on the stage of cell differentiation, since the promyelocytic cell line HL-60 contains less HBP than mature neutrophils, stored exclusively in the less easily mobilized azurophilic granules. Our findings suggest that HBP can be synthesized or targeted to easily mobilized compartments at a late stage of neutrophil maturation. The ability of neutrophils to secrete HBP from secretory vesicles may be important for proinflammatory functions of this protein, such as the alteration of vascular permeability.

Fine structure of the neurohemal sinus gland of the shore crab, Carcinus maenas, and immuno-electron-microscopic identification of neurosecretory endings according to their neuropeptide contents.

The sinus gland of the shore crab, Carcinus maenas, is a compact assembly of interdigitating neurosecretory axon endings abutting upon the thin basal lamina of a central hemolymph lacuna. Four types of axon endings are distinguishable by the size distribution, shape, electron density and core structure of their neurosecretory granules. One additional type of axon ending is characterized by electron-lucent vacuoles and vesicles. The axon profiles are surrounded by astrocyte-like glial cells. Various fixations followed by epoxy- or Lowicryl-embedding were compared in order to optimize the preservation of the fine structure of the granule types and the antigenicity of their peptide hormone contents. By use of specific rabbit antisera, the crustacean hyperglycemic, molt-inhibiting, pigment-dispersing, and red-pigment-concentrating hormones were assigned to the four distinct granule types which showed no overlap of immunostaining. Epi-polarization microscopy and ultrathin section analysis of immunogold-stained Lowicryl-embedded specimens revealed that immunoreactivity to Leu-enkephalin and proctolin is co-localized with molt-inhibiting hormone immunoreactivity in the same type of granule. The size and core structure of the immunocytochemically identified granule types vary little with the different pretreatments but, in some cases, to a statistically significant extent. The present results are compared with those from earlier studies of sinus glands in different crustaceans. The methods of granule identification used in this study supplement the classical approach in granule typing; they are easier to perform and more reliable for the analysis of release phenomena in identified secretory neurons supplying the neurohemal sinus gland.

The characteristics of granules from upflow anaerobic sludge blanket reactors

Granules from an industrial UASB reactor were maintained in bench scale (201.) UASB reactors and fed with a synthetic medium based on volatile fatty acids. After 60 days of reactor operation, three distinct granule types were found to have evolved within the reactors. These types were defined by their colour: black, grey and white. These granules also differed in structure (texture) and elemental composition. The proportion of black, grey and white granules in the bed changed when the organic loading rate was changed. At the low loading rate, a large number of “hollow” granules appeared which were less dense than the liquid and were washed out at the top of the reactor.

The lytic granules of natural killer cells are dual-function organelles combining secretory and pre-lysosomal compartments.

Cytolytic lymphocytes contain specialized lytic granules whose secretion during cell-mediated cytolysis results in target cell death. Using serial section EM of RNK-16, a natural killer cell line, we show that there are structurally distinct types of granules. Each type is composed of varying proportions of a dense core domain and a multivesicular cortical domain. The dense core domains contain secretory proteins thought to play a role in cytolysis, including cytolysin and chondroitin sulfate proteoglycan. In contrast, the multivesicular domains contain lysosomal proteins, including acid phosphatase, alpha-glucosidase, cathepsin D, and LGP-120. In addition to their protein content, the lytic granules have other properties in common with lysosomes. The multivesicular regions of the granules have an acidic pH, comparable to that of endosomes and lysosomes. The granules take up exogenous cationized ferritin with lysosome-like kinetics, and this uptake is blocked by weak bases and low temperature. The multivesicular domains of the granules are rich in the 270-kD mannose-6-phosphate receptor, a marker which is absent from mature lysosomes but present in earlier endocytic compartments. Thus, the natural killer granules represent an unusual dual-function organelle, where a regulated secretory compartment, the dense core, is contained within a pre-lysosomal compartment, the multivesicular domain.

Sorting of three secretory proteins to distinct secretory granules in acidophilic cells of cow anterior pituitary.

The distribution of three proteins discharged by regulated exocytosis--growth hormone (GH), prolactin (PRL), and secretogranin II (SgII)--was investigated by double immunolabeling of ultrathin frozen sections in the acidophilic cells of the bovine pituitary. In mammotrophs, heavy PRL labeling was observed over secretory granule matrices (including the immature matrices at the trans Golgi surface) and also over Golgi cisternae. In contrast, in somatotrophs heavy GH labeling was restricted to the granule matrices; vesicles and tubules at the trans Golgi region showed some and the Golgi cisternae only sparse labeling. All somatotrophs and mammotrophs were heavily positive for GH and PRL, respectively, and were found to contain small amounts of the other hormone as well, which, however, was almost completely absent from granules, and was more concentrated in the Golgi complex, admixed with the predominant hormone. Mixed somatomammotrophs (approximately 26% of the acidophilic cells) were heavily positive for both GH and PRL. Although admixed within Golgi cisternae, the two hormones were stored separately within distinct granule types. A third type of granule was found to contain SgII. Spillage of small amounts of each of the three secretory proteins into granules containing predominantly another protein was common, but true intermixing (i.e., coexistence within single granules of comparable amounts of two proteins) was very rare. It is concluded that in the regulated pathway of acidophilic pituitary, cell mechanisms exist that cause sorting of the three secretory proteins investigated. Such mechanisms operate beyond the Golgi cisternae, possibly at the sites where condensation of secretion products into granule matrices takes place.

Pernicious anaemia and mucosal endocrine cell proliferation of the non-antral stomach.

There is a recognised association between pernicious anaemia and the development of gastric carcinoma, endocrine cell hyperplasia, and carcinoid tumour. Multiple endoscopic biopsies from the body mucosa of seven patients with pernicious anaemia showed small intestinal metaplasia with varying degrees of inflammation, fibrosis, and expansion of the lamina propria. Using conventional silver and lead stains, endocrine cells were inconspicuous. Staining for the general neural and neuroendocrine markers NSE and PGP 9.5 revealed a proliferation of endocrine cells in the epithelium and isolated clumps of endocrine cells in the lamina propria. The clumps were composed of two cell types, either small or large. Some of these endocrine cells showed gastrin, 5HT, VIP and substance P immunoreactivity of varying intensity. Ultrastructurally nine morphologically distinct types of granules were found some of which correlated with the immunohistochemistry. Some separate islands were composed solely of endocrine cells while others had a definite neural component, suggesting that the former arise from 'budding off' of enteroendocrine cells and the latter originate from the neuroendocrine cells of the lamina propria plexus. Thus there may be a dual origin of carcinoid tumours. Carcinoid tumours associated with pernicious anaemia tend to be multifocal and are infrequent. Less than 50 such cases have hitherto been reported. Our findings of endocrine cells proliferations in seven cases of pernicious anaemia indicate that this may be an adaptive change that occurs frequently and provides the basis on which carcinoids, less frequently, develop.

Pulmonary small cell carcinoma showing tripartite differentiation in individual cells

A pulmonary small cell carcinoma is described that was composed of small cells containing argyrophilic dense core granules. Foci of epidermoid differentiation and foci of glandular differentiation associated with intra- and extracellular secretion of acidic mucosubstances were also present. In some areas glandular mucous filled hollows were surrounded by cells that were joined apically by junctional complexes and laterally by very large, well developed desmosomes. These cells contained large and numerous tonofilament bundles as well as two morphologically distinct types of granules, dense core endocrine type granules and mucous granules. The tumor clearly arose from terminal bronchi, and foci of basally situated hyperplastic or neoplastic cells were seen beneath otherwise normal columnar ciliated epithelium. The histogenesis and logical explanations for the tripartite differentiation patterns seen within single cells of this tumor are discussed.

Cytochemical aspects of Mercenaria mercenaria hemocytes.

The hemocytes of the hard clam M. mercenaria were of three types: an agranulocyte, a small, and a large granulocyte. The agranulocyte, with only a thin periphery of cytoplasm surrounding the nucleus, had no visible cytoplasmic granules in living preparations but did exhibit a few centers of nonspecific esterase activity. This cell type represented 2% of the hemocyte population. The small granulocyte possessed four distinct granule types and comprised 61% of the total cell population. Large granulocytes accounted fro 37% of all hemocytes. While they contained the same four granule types identified in the small granulocyte, only one-third the total number were present. The nucleus of all three hemocyte types appeared morphologically similar. The four types of granules observed were a blunt, dot-like, a refractile and a filamentous granule. Blunt granules were identified as mitochondria, based on their ability to reduce Janus Green B to diethyl safranin, the presence of NADH dehydrogenase activity and boundary staining with Sudan black B. Dot-like granules were identified as lysosomes on the basis of neutral red staining, localization of acid phosphatase and nonspecific esterase activity and staining with Sudan black B. Refractile granules were demonstrated to be membrane-bound, lipid-filled structures that reacted positively with Sudan black B and Oil red O, respectively; these granules act as lipid storage centers. Nuclear similarity of the three cell types suggest that these cells might represent different stages of maturity, rather than three distinct cell lines. This was also indicated by the similar yet graded cytochemical reactions and the varying degree of motility and phagocytic activity demonstrated by hemocyte types.

Neutrophil leukocytes and inflammation of the bovine mammary gland

Observations on the production, distribution, structure, and function of neutrophils is briefly reviewed and related to the role of neutrophils in inflammation of the bovine mammary gland. The mature neutrophil is an end stage cell possessing an armament of two distinct types of granules containing a variety of enzymatic and non-enzymatic constituents. Neutrophils are produced in the bone marrow by an orderly process of mitosis and maturation. The developing neutrophils form a morphologic continuum with functional compartmentalization into mitotic, maturative and reserve pools. Within the blood vasculature, neutrophils are either freely circulating or remain marginated along vessel walls in capillary beds, particularly in the lungs, liver and spleen. However, neutrophils so distributed are in a dynamic equilibrium, and after a brief sojourn in blood, they migrate into body cavities and tissues constituting a physiologic barrier to microbial invasion. Neutrophils from the marrow reserve and marginal pools rapidly and preferentially mobilize into areas of acute inflammation in response to chemotactic factors generated locally. There, they phagocytize, kill, and digest bacteria in a complex but sophisticated manner utilizing energy and granular constituents. During this process neutrophils also release certain agents which by themselves are inflammatory. Studies in developing calves have shown that (a), there is a sizable marrow reserve of neutrophils, (b) newly formed neutrophils appear in blood in about 7 days, (c) the circulating and marginal pools of neutrophils are about equal in size, and (d) neutrophils disappear from blood randomly with a half-time of about 9 hours. Such information on mature cows is not available. Experimental studies on mastitis in normal and neutropenic cows have demonstrated the dual role of neutrophils also in this species viz. , in protection against mammary infections and as incitors of the inflammatory reaction. Neutrophils rapidly mobilize into the mammary parenchyma and subsequently into the milk in glands experiencing acute inflammatory reaction and continue to emigrate for a variable period after disappearance of clinical signs of mastitis. While much has become known about neutrophil chemotaxis, phagocytosis, and bacterial killing in other species, little is known about bovine neutrophils in this regard and, particularly, in relation to mastitis. The bovine mammary gland is an excellent model to study inflammation and the cow has a remarkable ability to adapt to a heavy daily loss of neutrophils in milk.

Ultrastructure of the human submaxillary gland. IV. Serous granules.

AbstractAcinar serous granules in specimens of human submaxillary gland fixed with aldehydes followed by osmium were examined in the electron microscope. Typical mature granules had a tripartite substructure: (1) an eccentrically placed dense spherule; (2) material of intermediate density adherent to the inner aspect of the granule membrane in the form of a crescent; and (3) a less dense, homogeneously fibrillo‐granular matrix. The dense spherules appeared within expanded Golgi saccules; other granule constituents apparently were of secondary acquisition. A few cells contained atypical serous granules in which material of intermediate density was disposed in random loops, whorls, and striae. Such granules usually lacked dense spherules. It is postulated that the two morphologically distinct types of granules represent separate classes of serous secretory products.

Changes in fine structure during silk protein production in the ampullate gland of the spider Araneus sericatus.

The ampullate silk gland of the spider, Araneus sericatus, produces the silk fiber for the scaffolding of the web. The fine structure of the various parts of the gland is described. The distal portion of the duct consist of a tube of epithelial cells which appear to secrete a substance which forms the tunica intima of the duct wall. At the proximal end of the duct there is a region of secretory cells. The epithelium of the sac portion contains five morphologically distinct types of granules. The bulk of the synthesis of silk occurs in the tail of the gland, and in this region only a single type of secretory droplet is seen in the epithelium. Protein synthesis can be stimulated by the injection of 1 mg/kg acetylcholine into the body fluids. 10 min after injection, much of the protein stored in the cytoplasm of the epithelial cells has been secreted into the lumen. 20 min after stimulation, the ergastoplasmic sacs form large whorls in the cytoplasm. Protein, similar in electron-opacity to protein found in the lumen, begins to form in that portion of the cytoplasm which is enclosed by the whorls. The limiting membrane of these droplets is formed by ergastoplasmic membranes which lose their ribosomes. No Golgi material has been found in these cells. Protein appears to be manufactured in the cytoplasm of the tail cells in a form which is ready for secretion.

Origin of granules in polymorphonuclear leukocytes. Two types derived from opposite faces of the Golgi complex in developing granulocytes.

The origin, nature, and distribution of polymorphonuclear leukocyte (PMN) granules were investigated by examining developing granulocytes from normal rabbit bone marrow which had been fixed in glutaraldehyde and postfixed in OsO(4). Two distinct types of granules, azurophil and specific, were distinguished on the basis of their differences in size, density, and time and mode of origin. Both types are produced by the Golgi complex, but they are formed at different stages of maturation and originate from different faces of the Golgi complex. Azurophil granules are larger ( approximately 800 mmicro) and more dense. They are formed only during the progranulocyte stage and arise from the proximal or concave face of the Golgi complex by budding and subsequent aggregation of vacuoles with a dense core. Smaller ( approximately 500 mmicro), less dense specific granules are formed during the myelocyte stage; they arise from the distal or convex face of the Golgi complex by pinching-off and confluence of vesicles which have a finely granular content. Only azurophil granules are found in progranulocytes, but in mature PMN relatively few (10 to 20%) azurophils are seen and most (80 to 90%) of the granules present are of the specific type. The results indicate that inversion of the azurophil/specific granule ratio occurs during the myelocyte stage and is due to: (a) reduction of azurophil granules by multiple mitoses; (b) lack of new azurophil granule formation after the progranulocyte stage; and (c) continuing specific granule production. The findings demonstrate the existence of two distinct granule types in normal rabbit PMN and their separate origins from the Golgi complex. The implications of the observations are discussed in relationship to previous morphological and cytochemical studies on PMN granules and to such questions as the source of primary lysosomes and the concept of polarity within the Golgi complex.

The ultrastructure of normal human first trimester placenta

First trimester human placentae were studied with the electron microscope. The syncytial trophoblast has microvilli within which are structures similar to those seen in stereocilia. The syncytial trophoblast cytoplasm has four distinct types of granules. One type is seen not only in the syncytial trophoblast, but is apparently in transit across the basement membrane, connective tissue, and upon the walls of blood vessels. These granules are felt to be nutritive lipid. A second type of granule, arising from Golgi membranes, is identical to secretory granules of human term placenta. The prior literature on granules in the syncytial trophoblast is discussed. A hypothesis is presented that the secretory granules may be the steroid hormones estrogen and progesterone. Glycogen accumulations occur in both epithelial cell types. Cells are seen representing transition forms between the cytotrophoblast and syncytial trophoblast. Hofbauer and Langhans cells are more numerous as compared with term. Capillaries occur showing a marked constriction, an extensive endothelial lining, and no smooth muscle coat. A hypothesis is presented to explain the marked capillary constriction.