Cell biology, clinicopathological profile, and classification of gastro-enteropancreatic endocrine tumors

 Recent developments in the field of endocrine cell biology and pathology at both morphological and molecular levels are briefly outlined and discussed as a basis for endocrine tumor characterization. The main tools available for identifying the endocrine nature of the tumors, their pathogenetic interpretation, and experimental reproduction with special emphasis on tumor antecedents are reported. Based on this, classifications of endocrine tumors of the pancreas and gastrointestinal tract are developed, covering most clinical (hyperfunctional syndromes included), pathological, and biological patterns, with special emphasis on tumor prognosis. Received: 20 June 1997 / Accepted: 15 September 1997     

Hyperplastic and neoplastic endocrine cells of the pancreas in aspiration biopsy

Aspiration biopsy of four primary endocrine tumors of the pancreas showed single and large sheets of tumor cells. Tumor cells were polygonal in shape with ill-defined, filmy or granular cytoplasm and regular round nuclei containing a finely granular chromatin pattern and inconspicuous or prominent nucleoli. Focal gland-like arrangement was seen in several sheets of tumor cells. Aspirate from liver metastases of an endocrine tumor of the pancreas showed single and clustered oval cells with granular cytoplasm and eccentrically located round and hyperchromatic nuclei. Smears prepared from aspirates of three fibrotic pancreata containing nodules of hyperplastic endocrine cells showed smaller fragments of endocrine-cell epithelium with focal gland-like arrangement. Individual hyperplastic endocrine cells displayed granular, filmy, and ill-defined cytoplasm and round to oval hyperchromatic nuclei showing a finely granular chromatin pattern. Nuclear pleomorphism was noted in some cell groups.     

The endocrine pancreas in chronic pancreatitis

Summary The endocrine pancreatic tissue from patients with severe primary chronic pancreatitis (n=6), secondary chronic pancreatitis due to duct obstruction by carcinoma (n=6) and non-diabetic, non-pancreatitic controls (n=4) was studied qualitatively and quantitatively using specific immunocytochemistry and electron microscopy. Grouping of variously sized islets in the sclerotic tissue (sclerosis islets), islet neoformation by ductuloinsular proliferation, and intrainsular fibrosis were the main qualitative findings. Immunocytochemical quantitation of the distribution of insulin (B), glucagon (A), somatostatin (D) and pancreatic polypeptide (PP) producing cells revealed a significant relative increase in the number of A cells and a decrease in the number of B cells of the sclerosis islets in primary chronic pancreatitis (B-44.1±9.3%:A-38.3±2.4%:D-8.6±5.1%:PP-4.6±4.1%) as well as in secondary chronic pancreatitis (B-38.0±14.3%:A-38.4±19.0%:D-9.1±5.8%:PP-14.5±23.4%) compared with controls (B-71.1±8.1%:A-24.3±5.5%:D-8.0±2.8%:PP-0.5±0.4%). The number of PP cells was significantly increased in primary chronic pancreatitis only. It is suggested that scarring of the exocrine pancreas affects islet composition, probably by impairment of the local circulation and of glucose diffusion, thus leading to reduction of the number and glucose sensitivity of B cells. The hyperplasia of A and PP cells appears to be a secondary phenomenon due to the loss of B cells.     

Duct changes and K-ras mutations in the disease-free pancreas: analysis of type, age relation and spatial distribution

Recent molecular studies have suggested that hyperplastic duct lesions of the pancreas are potential precursors of pancreatic ductal carcinoma. This study examines the type, distribution, age-related incidence and K-ras codon 12 mutation rate of duct lesions in the normal pancreas. Postmortem pancreases from 140 patients were screened for the presence of mucinous cell hypertrophy (MHT), ductal papillary hyperplasia (DPH), adenomatoid ductal hyperplasia (ADH), and squamous metaplasia (SQM). Microdissected cell samples were analyzed for K-ras codon 12 mutations by polymerase chain reaction amplification of exon 1 of the K-ras gene, combined with constant denaturing gel electrophoresis, and analyzed by sequencing. Of the 140 specimens 114 showed duct lesions. The lesions were evenly distributed throughout the pancreas. They were more common beyond the age of 40. MHT was present in 68%, DPH in 36%, ADH in 40%, and SQM in 36% of the cases. K-ras mutations were found in 19 samples from 15 out of 79 pancreases (18%), including all types of duct lesions and a variant of ADH with dense stroma. 67% of the K-ras-positive specimens showed the transition GGT to GAT (8) or GTT (5). Hyperplastic/metaplastic duct changes of the pancreas increase with age, but their distribution pattern in the pancreas differs from that of ductal carcinomas. Received: 16 April 1999/Accepted: 26 May 1999     

Development of the adult endocrine pancreas during metamorphosis in the sea lamprey,Petromyzon marinus L. II. Electron microscopy and immunocytochemistry

The development of the adult endocrine pancreas was followed throughout metamorphosis in the sea lamprey using electron microscopy and immunocytochemistry. It was discovered that the caudal pancreas develops from the larval extrahepatic common bile duct through the process of transdifferentiation (dedifferentiation/redifferentiation). Early in metamorphosis the bile duct epithelial cells possess large vacuoles, resembling autophagic vacuoles, containing recognizable cell material. There is a loss of the large bundles of intermediate filaments characteristic of the larval bile duct epithelium. These same cells are then seen to contain granules immunoreactive for insulin. Pancreatic islets develop within the base of the bile duct epithelium from these transdifferentiated cells and migrate into the surrounding connective tissue to form the caudal pancreas. The cranial pancreas was found to develop from the epithelia lining the developing adult diverticulum and anterior intestine in a similar fashion as those in the larva. The second cell type to appear in either portion of the developing pancreas is similar to the third cell type of the adult: cells immunoreactive for somatostatin do not appear until late in metamorphosis in either region. © 1993 Wiley-Liss Inc.     

Morphogenetic concepts of normal and abnormal growth in the human prostate

 Benign prostatic hyperplasia (BPH) and prostate cancer are multifactorial disease processes, involving a growing number of biochemical, genetic and epigenetic factors. Their pathogenesis, however, remains poorly understood. The present review examines current morphogenetic concepts of normal and abnormal growth in the human prostate. This includes the role of basal cells in organogenesis and cancerogenesis, the impact of cell–matrix interactions, and the importance of cellular heterogeneity in tumour progression and hormone-insensitive growth. Knowledge of morphogenesis and morphology is required in any scientific approach to BPH and prostate cancer. Received: 21 March 1998 / Accepted: 25 May 1998     

Immunohistochemical studies of endocrine cells in heterotopic pancreas

Summary Twenty-one specimens of heterotopic pancreas were investigated using the indirect immunoperoxidase method for insulin, somatostatin, glucagon, pancreatic polypeptide (PP) and gastrin. Ten specimens showed ducts, acini and islets, seven showed ducts and acini, and four showed a ductal component alone. Pyloric gland-like mucous glands were occasionally identified in association with the ductal component. In eight of ten lesions containing islets, the islets were round and had a clearly defined outline with many glucagon cells and either none or a modest number of PP cells (dorsal type). In the remaining two lesions, the islets showed varying sizes and irregular outline with many PP cells and a few or no glucagon cells (ventral type). In either type of islets, insulin and somatostatin were detected, but gastrin cells were absent. Some isolated endocrine cells were also present among the acinar and ductal components. Their occurrence in ducts was more frequent in lesions or areas mainly composed of the ductal compoment than in those with less prominent ductal tissue. In eight lesions a few gastrin cells were found in the ductal component which showed goblet cell metaplasia and pyloric gland metaplasia. An intimate relationship between goblet cell metaplasia and appearance of G cells is noteworthy.     

Unusual prostatic adenocarcinoma with endocrine basophilic FSH-immunoreactive cells

We report an unusual variant of prostatic adenocarcinoma with marked endocrine differentiation (mixed endocrine-exocrine adenocarcinoma). Endocrine cells accounted for 60% of the tumour cells, were positive with silver impregnation and for chromogranin A, synaptophysin, and neuron-specific enolase, and coexpressed the exocrine antigens prostatic acid phosphatase and prostatic-specific antigen. Most of the endocrine cells were basophilic with haematoxylin-eosin and proved immunoreactive for alpha subunit of human chorionic gonadotropin and follicle-stimulating hormone. The remaining endocrine cells were represented by eosinophilic cells positive for serotonin, and by calcitonin and serotonin-immunoreactive cells not identifiable in haematoxylin-eosin-stained sections. On ultrastructural analysis, two types of endocrine cells were identified. The most frequent cell type showed abundant cytoplasmic round, electron-dense neurosecretory granules, either small (212±44 nm) or large (471±114 nm), resembling those of gonadotropic pituitary cells. The second type of endocrine cells contained irregular electron-dense granules similar to those of serotonin-storing enterochromaffin cells. Received: 6 October 1999 / Accepted: 16 November 1999     

Apoptosis and occurrence of Bcl-2, Bak, Bax, Fas and FasL in the developing and adult rat endocrine pancreas

Apoptotic cell death is thought to play a crucial role in the manifestation of insulin- and non-insulin dependent diabetes mellitus. Therefore, apoptosis and apoptotic markers were studied in the rat endocrine pancreas to get insight into the possible life cycle of Langerhans islets.The islets were investigated at 13 time points between day E19 and 18 months. At each time point, histologic sections were treated with the direct fluorescein-labelled TUNEL method and immunostained for pancreatic hormones (glucagon, insulin), apoptotic promoters (Bak, Bax, Fas, Fas Ligand) as well as for the anti-apoptotic peptide Bcl-2. All tissue sections were investigated using confocal laser scanning microscopy under identical settings for semiquantitative estimation of staining intensity. TUNEL-positive cells occurred in all pre- or postnatal stages.The findings indicated a biphasic apoptotic activity in the endocrine pancreas during the lifetime of rats. The first phase began at E19 and peaked at P5 accompanied by a considerable increase in Bak fluorescence staining intensity, while the second phase began at P30 and peaked at 18 months with increasing amounts of Fas and FasL staining intensities in the islet cells. The presented in situ data may be important for understanding the increased age-related vulnerability of islet cells and for studies of isolated and cultivated rat islets. Accepted: 9 May 2000     

Ontogeny of immunoreactive somatolactin, prolactin and growth hormone secretory cells in the developing pituitary gland of Cichlasoma dimerus (Teleostei, Perciformes)

Prolactin, growth hormone and somatolactin constitute a hormone family because they are structurally related and are secreted by acidophilic cells of different regions of the adenohypohyisis. In this work, we report the ontogeny of ir-prolactin, ir-growth hormone and ir-somatolactin cells in the developing pituitary gland of the cichlid fish Cichlasoma dimerus (Teleostei; Perciformes). Antisera raised against fish pituitary hormones were used. In this species hatching occurs 54 hs after fertilization and the three different cell types were recognized two days later. The neurohypophysis was recognized on day 14 after hatching and in later stages it began to show the characteristic deep interdigitations of the adults. On day 42 (juvenile stage) the distribution of ir-PRL, ir-GH and ir-SL showed the pattern described for adults of this species. The ir-SL cells were not PAS-positive in larvae as they are in adults. This would suggest the presence of a nonglycosilated form of somatolactin in early stages of development which may coexist in adults with a glycosilated form. The appearence of these hormones so early in development suggest their importance in the survival of fish larvae but further studies focused on the ontogeny of hypothalamic factors that regulate their synthesis and secretion must be performed. Accepted: 30 January 2001     

Growth hormone as an early embryonic growth and differentiation factor

In this review we consider the evidence that growth hormone (GH) acts in the embryo as a local growth, differentiation, and cell survival factor. Because both GH and its receptors are present in the early embryo before the functional differentiation of pituitary somatotrophs and before the establishment of a functioning circulatory system, the conditions are such that GH may be a member of the large battery of autocrine/paracrine growth factors that control embryonic development. It has been clearly established that GH is able to exert direct effects, independent of insulin-like growth factor-I (IGF-I), on the differentiation, proliferation, and survival of cells in a wide variety of tissues in the embryo, fetus, and adult. The signaling pathways behind these effects of GH are now beginning to be determined, establishing early extrapituitary GH as a bona fide developmental growth factor.     

Strict glycemic control in diabetic dogs with closed-loop intraperitoneal insulin infusion algorithm designed for an artificial endocrine pancreas

 The ultimate goal of the development of an artificial endocrine pancreas is to achieve long-term strict glycemic regulation. To establish the physiological insulin delivery route of the artificial endocrine pancreas, intraperitoneal insulin infusion may be important. For this purpose, we tried to develop a closed-loop intraperitoneal insulin infusion algorithm by analyzing the pharmacokinetics of intraperitoneal regular insulin absorption using a mathematical model. The parameters for this algorithm were calculated to simulate the plasma insulin profile after intraperitoneal insulin injection as closely as possible. To evaluate the appropriateness of this algorithm, we tried glycemic control after an oral glucose load of 2 g/kg or a meal load of 80 kcal/kg in diabetic dogs by applying the algorithm. With the use of the subcutaneous insulin lispro infusion algorithm, which we have previously reported, alloxan-induced diabetic dogs exhibited postprandial hyperglycemia and delayed hyperinsulinemia, followed by hypoglycemia after an oral glucose load of 2 g/kg. However, by using the intraperitoneal insulin infusion algorithm, excellent glycemic control (postprandial blood glucose levels of 9.1 ± 0.8 mmol/l at 70 min and 3.8 ± 0.3 mmol/l at 240 min, respectively) could be achieved without any associated delayed hyperinsulinemia or hypoglycemia. Glycemic excursion after a meal load of 80 kcal/kg was also controlled from 3.9 to 10.1 mmol/l. Our results confirm that the intraperitoneal insulin infusion algorithm in vivo is feasible and that this algorithm can be superior to the subcutaneous insulin lispro infusion algorithm in the regulation of blood glucose. Received: September 11, 2002 / Accepted: October 16, 2002 Acknowledgments This work was supported by research grants from the Japanese Society for Artificial Organs and the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant-in-Aid for Scientific Research 14580825) to Dr. M. Sakakida et al. Correspondence to:M. Sakakida     

Dysregulation of the cell cycle and chromosomal imbalances in juxtaglomerular cell tumors - a comparative study with endocrine tumors of the pancreas

Two juxtaglomerular cell tumors (JGCTs) were investigated in comparison with 14 endocrine tumors of the pancreas (ETPs), focusing on the cell cycle, apoptosis, and cytogenetic changes. JGCTs revealed nuclear accumulation of Cyclin D₁, together with the cyclin-dependent kinase inhibitors p21^Cip1/Waf1 and p27^Kip1. In contrast, no accumulation of Cyclin D₃, p53, p16^INK4a, or Mdm-2 was seen. Bcl-2 protein was intensively, but Rb only moderately, expressed. This immunoreactive profile was not found in the ETPs, which were negative for Bcl-2, p27^Kip1, p21^Cip1/Waf1, and - with one exception - for Cyclin D₁ (1/14) but expressed Cyclin D₃ in 7/14 cases. JGCTs displayed characteristic genetic alterations with combined losses of chromosomes 9, 11, 15, and 21 and gains of chromosome 18. In contrast, no characteristic pattern of genetic alterations was found in ETPs. In both, the amount of chromosomal aberrations correlated with tumor size. In small ETPs and JGCTs, genetic losses dominated over gains of chromosomes, whereas in large/malignant ETPs, gains and losses were equally affected. Thus, JGCTs represent a special type of renal endocrine neoplasm characterized by deregulation of cell cycle components and a typical profile of chromosomal aberrations. Since only two JCTs were investigated, further studies for validation of these results are, however, necessary.     

Effects of tri-iodothyronine (T3), insulin, insulin-like growth factor I (IGF-I) and transforming growth factor beta1 (TGFβ1) on the proportion of insulin cells in cultured embryonic chick pancreas

 With a view to ultimately identifying factors involved in the development of pancreatic insulin cells, we have cultured dorsal pancreatic buds from 5-day chick embryos on a basement membrane matrix (Matrigel) in a serum-free medium supplemented with selected factors. The endodermal components of the buds were freed of almost all the mesenchyme so as to eradicate as much as possible of this source of some such factors. In 7-day cultures, insulin and glucagon cells were demonstrated immunocytochemically; numbers of insulin cells were expressed as a percentage of insulin plus glucagon cell counts. Our standard medium contained insulin. Addition of tri-iodothyronine to this medium did not increase the proportion of insulin cells, but in combination with raised concentrations of glucose and essential amino acids it improved somewhat the marked increase previously recorded for these nutrient conditions. Omission of insulin from the standard medium greatly reduced the proportion of these cells; substitution of insulin by insulin-like growth factor I increased the proportion considerably more than did insulin. To test for an overall effect of growth factors, explants were cultured in standard medium on Matrigel containing reduced amounts of growth factors: the proportion of insulin cells proved to be increased over that reached on normal Matrigel. The suspicion that transforming growth factor β1, a component of Matrigel, might act to reduce the proportion of insulin cells was tested and found to be correct. It is suggested that the different factors studied here may affect either or both of proliferation and determination in the differentiation pathway of insulin vis-à-vis glucagon cells. Accepted: 13 March 1998     

Oncocytic non-functioning endocrine tumor of the pancreas

Herein is presented the case of a malignant non-functioning endocrine tumor of the pancreas with oncocytic features, and a discussion on the high incidence of malignancy in oncocytic endocrine pancreatic tumors. The patient was a 65-year-old woman who showed no paraneoplastic symptoms produced by functioning pancreatic endocrine tumors. The primary tumor was located in the body and tail of the pancreas, and had metastasized to the liver. Tumor cells were arranged in a ribbon-like or trabecular pattern and had an abundant eosinophilic cytoplasm containing numerous mitochondria and neurosecretory granules. The cytoplasm of the tumor cells was intensely stained with an antimitochondrial antigen antibody. Most tumor cells stained positively with Grimelius stain and for chromogranin A. Some tumor cells also stained for synaptophysin. However, the tumor cells negatively stained for hormones such as insulin, glucagon, somatostatin, gastrin, vasoactive intestinal peptide and pancreatic polypeptide, for serotonin, and for pancreatic enzymes such as amylase and trypsin. Analysis of 18 oncocytic pancreatic endocrine tumors, consisting of those reported previously and that in the present case, suggests that the high incidence of malignancy in oncocytic endocrine tumors is associated with the high incidence of non-functioning endocrine tumors among them, most of which are malignant.     

The patterns of cell death and of macrophages in the developing forebrain of the tree shrew Tupaia belangeri

The patterns of cell death and of macrophages were investigated in the forebrain and eyes of the tree shrew Tupaia belangeri during five phases of optic cup formation. Seventeen embryos were studied. Three- dimensional reconstructions were made from one embryo of each phase. In phase 1 (V-shaped optic evagination) a midline band of cell death passes through the closing anterior neuroporus. From phases 2 (optic vesicle) to 5 (far-advanced invagination) the midline band of cell death extends in the dorsal wall of the forebrain to its rostral pole and, further, into its ventral wall. At the approximate future position of the optic chiasm this ventral pycnotic area, predicted but so far unidentified by others, is connected to a previously described second band of cell death passing through the optic anlagen. Recently, evidence has been presented that chicken embryos develop holoprosencephaly and cyclopia when ventral forebrain structures are lost secondary to experimentally induced apoptosis. Our findings in Tupaia suggest that, in cases of spontaneous malformations of this kind, such an atypical pycnotic area in the ventral telencephalon might result from the defective regulation of cell death processes during optic cup formation. In the forebrain and eyes of Tupaia, the occurrence of bands of cell death precedes the appearance of the earliest intraepithelial macrophages. From phase 3 (onset of invagination) onwards almost all of them are concentrated along the band of cell death. Accepted: 28 September 1999     

Epidermal growth factor, vascular endothelial growth factor and progesterone promote placental development in rat whole-embryo culture

 The development and survival of rat embryos in whole-embryo culture is limited by the lack of any maternal blood circulation in a purely fetal placenta. If the resulting placental insufficiency could be overcome for some time by an increase of the placental exchange area, a prolonged culture period would result and facilitate the development of embryos. In the present study, several attempts to stimulate proliferation and growth of the fetal placenta were made by the addition of vascular endothelial growth factor (VEGF), epidermal growth factor (EGF) and progesterone to the culture medium. Rat embryos were routinely explanted with their embryonic membranes at 10.5 days of gestation. Decidua, parietal yolk sac, Reichert’s membrane and the layer of superficial trophoblastic giant cells were removed. The explants were cultured and gassed continously for 24 h in rotating plastic tubes containing rat serum, diluted to 50% with modified COON’s F12 medium. Either of the two growth factors or progesterone were added to each culture tube and a control group was cultured without any factor. After the addition of each of these factors the stimulatory effect on placental growth was assessed by morphometric evaluation of several placental parameters from semithin cross-sections: On adding each of the factors the whole cross-sectional area of the placenta significantly increased, as did the area of the fetal placental mesenchyme. VEGF also increased the area of the trophoblast, and the area of the blood vessels enclosed within the trophoblast, by an average of 9.4% and 23.6%, respectively. Thus, VEGF treatment resulted in a measurable extension of the exchange area of the fetal placenta. Accepted: 4 February 1998     

Ontogeny of NADPH-diaphorase in rat forebrain and midbrain

 This study characterizes the developmental expression of NADPH-diaphorase from embryo to adulthood in the forebrain, midbrain and cerebellum of rat brain via histochemical staining. On embryonic day 12 no neurons stained. Labeling was observed in certain nuclei from E15 through the postnatal period to adulthood. Labeling in neurons increased or maintained a constant level with increased age. The embryo demonstrated substantial labeling in neurons of the caudate putamen, bed nucleus of the stria terminalis, preoptic area, lateral hypothalamic area, paraventricular thalamic nucleus, ventromedial hypothalamic nucleus, magnocellular nucleus posterior commissure, and periaqueductal central gray. Additional neuronal labeling was observed postnatally in the olfactory bulb, cerebral cortex, amygdala, various nuclei of the thalamus, interpeduncular nucleus, linear nucleus of the raphe, pretectal area and superior colliculus. In the cerebellum, labeling appeared only after P14 in cells of the molecular cell layer and granular cell layer. The sizes of labeled neurons developed significantly from P4 to P14 in several nuclei. The distinctive temporal and spatial expression pattern of NADPH-diaphorase implies that the NO/cGMP system may play an important role in physiological and developmental functions. Accepted: 8 September 1997     

Ontogeny, postnatal development and ageing of endocrine pancreas in Bubalus bubalis

The ontogenesis, postnatal development and ageing of the endocrine pancreas in mammals have not been extensively studied. In order to improve understanding of this organ, we studied the buffalo pancreas during fetal and postnatal development. Glucagon, insulin and somatostatin immunoreactive cells (i.c.) were first seen in 2-mo-old embryos. Pancreatic polypeptide (PP) i.c. were observed during the 3rd month of gestation. The early embryo pancreas was almost totally composed of endocrine tissue. The endocrine portion only slightly increased in mass with animal growth, whereas the exocrine portion noticeably increased in mass during the late fetal and postnatal periods. In adults, therefore, the exocrine portion was more evident than the endocrine portion. Three types of islet were observed in fetal and young buffalos: small, large and PP-islets. The small islets were composed of insulin, glucagon, somatostatin and PP i.c. The large islets were primarily composed of insulin i.c. and a few glucagon, somatostatin and PP i.c. The PP islets were mostly composed of PP i.c. with a few somatostatin, insulin and glucagon i.c. The number of large islets greatly diminished by adulthood. Glucagon, insulin, somatostatin and PP i.c. were also seen scattered in the exocrine parenchyma and along the duct epithelium. In the duct epithelium, these cells were either single or grouped, and they sometimes formed a protrusion projecting towards the connective tissue. These morphological features were primarily observed in fetuses and young buffalos.     

Intussusceptive microvascular growth in the lung of larval Xenopus laevis Daudin: a light microscope, transmission electron microscope and SEM study of microvascular corrosion casts

The remodeling of the uniform wide, plexus-like capillary bed of the lung of metamorphosing tadpoles of the South African clawed toad Xenopus laevis (Daudin) is studied from developmental stages 54 to 65 by scanning electron microscopy (SEM) of microvascular corrosion casts (VCCs), light microscopy (LM) and transmission electron microscopy (TEM). VCCs reveal that the remodeling of the existing uniform, plexus-like lung capillary bed into well-defined alveolar capillary meshworks starts in the caudal lung and then gradually proceeds cranially. Vascular remodeling is entirely by intussusceptive microvascular growth through insertion and enlargement of new and fusion of pre-existing capillary meshes. Analyses of lung tissue serial sections at the LM and TEM level confirm the presence of intracapillary cushions and tissue posts and correlate these structures in respect of size and location to the round to slit-like imprints and tiny ”holes” found in VCCs. Additionally, SEM of VCCs give clear evidence that intussusceptive microvascular growth is also involved in the remodeling and maturation of alveolar arterioles and venules. Accepted: 8 March 2000