Engineering a soluble high-affinity receptor domain that neutralizes staphylococcal enterotoxin C in rabbit models of disease

Superantigens (SAgs) are a class of immunostimulatory exotoxins that activate large numbers of T cells, leading to overproduction of cytokines and subsequent inflammatory reactions and systemic toxicity. Staphylococcal enterotoxin C (SEC), a SAg secreted by Staphylococcus aureus, has been implicated in various illnesses including non-menstrual toxic shock syndrome (TSS) and necrotizing pneumonia. SEC has been shown to cause TSS illness in rabbits and the toxin contributes to lethality associated with methicillin-resistant S.aureus (MRSA) in a rabbit model of pneumonia. With the goal of reducing morbidity and mortality associated with SEC, a high-affinity variant of the extracellular variable domain of the T-cell receptor beta-chain for SEC (~14 kDa) was generated by directed evolution using yeast display. This protein was characterized biochemically and shown to cross-react with the homologous (65% identical) SAg staphylococcal enterotoxin B (SEB). The soluble, high-affinity T-cell receptor protein neutralized SEC and SEB in vitro and also significantly reduced the bacterial burden of an SEC-positive strain of MRSA (USA400 MW2) in an infective endocarditis model. The neutralizing agent also prevented lethality due to MW2 in a necrotizing pneumonia rabbit model. These studies characterize a soluble high-affinity neutralizing agent against SEC, which is cross-reactive with SEB, and that has potential to be used intravenously with antibiotics to manage staphylococcal diseases that involve these SAgs.

Organ preservation with EC, HTK, and UW, solution in orthotopic rat liver transplantation. Part II. Morphological study

The quality of organ preservation is of major importance in minimizing the incidence of primary graft nonfunction and organ rejection. For this study a new semiquantitative score was developed that grades morphologic tissue alterations in the liver according to their frequency and severity. It was applied to assess commonly used perfusion solutions for their efficacy in preventing early and late tissue damage after rat liver transplantation. For transplantation the livers were stored in Euro-Collins (EC, group I; n = 11), histidine-tryptophan-alpha-ketoglutarate (HTK, group II; n = 11), or University of Wisconsin solution (UW, group III; n = 11). Rat liver transplantation was performed with graft arterialization by the method of Engemann. Biopsies were taken for morphological examination and semiquantitative scoring during the donor operation, after 4 h of cold storage, 1 h after reperfusion, and 4 weeks postoperatively. An immunohistological bromodeoxyuridine (BrdU) assay was also performed on the day of dissection to assess the rate of hepatic proliferation. Semiquantitative morphological analysis gave widely differing results in all experimental groups after 4 h of ischemia. There was less intracellular and interstitial edema, fatty degeneration, intralobular necrosis, and hepatocellular proliferation in the HTK group than in the other groups. Neither after cold ischemia nor 1 h after reperfusion did Kupffer-cell activation occur; this is known to play a major role in the development ofischemia and reperfusion injury. Furthermore, late changes such as bile-duct proliferation and vascular and sinusoidal alterations appeared less frequently in this group. The hepato-protective powers of HTK solution might therefore be due to decreased Kupffer-cell activation.

Organ preservation with EC, HTK, and UW solutions in orthotopic liver transplantation in syngeneic rats. Part I: Functional parameters

Ischemic injury to the liver is known to influence the outcome of liver transplantation. In this study the efficacy of Euro-Collins (EC), histidine-tryptophan-ketoglutarate (HTK), and University of Wisconsin (UW) preservation solution was analyzed in the model of orthotopic liver transplantation in syngeneic rats. The study design was as follows: Group I, Euro-Collins solution (n = 11); Group II, Histidine-Tryptophan-Ketoglutarate solution (n = 11); Group III, University of Wisconsin solution (n = 11). The rat liver transplantation was performed with arterialization of the graft as described by Engemann. The postoperative follow-up was 28 days. The perfusion flow rate of the preservation solution measured during organ perfusion revealed lowest levels in the UW group and comparable levels in Groups I and II. Postoperative graft function was monitored by measuring liver enzymes (aspartate amino-transferase, ASAT, alanine aminotransferase, ALAT), bilirubin and bile production. The survival rate was 10/11 in each group. Liver enzymes and bilirubin increased postoperatively and went back to normal within 2 or 3 weeks. In contrast to bilirubin, the liver enzymes showed a biphasic increase with maxima on the 1st and 5th days (range: ALAT, 220-264 U/L; ASAT, 145-177 U/L). Bile production was observed in all groups, but was significantly higher after UW-preservation (P < .005). Analysis of inflammatory cells revealed high concentrations of intrasinusoidal leukocytes and lymphocytes in the graft with a maximum on the 5th day.

Orthotopic rat liver transplantation and bile duct reconstruction by a splint technique

The aim of the present study is to investigate the impact of bile duct reconstruction by a splint technique, a method which has not been sufficiently researched in animals after liver transplantation. Three experimental groups were set up: I = control, sham operation; II = bile duct reconstruction; III = orthotopic rat liver transplantation (ORLT). After bile duct reconstruction, serum levels of ASAT and ALAT in group II revealed a peak on the first postoperative day. The transplanted animals (group III) showed a second peak in liver enzyme levels on the fifth postoperative day; it was significantly higher than in group II. Serum bilirubin was more elevated in the transplant group, with a peak on day 7. Morphological investigations at the end of surgery revealed only intralobular necrosis and reactive changes in the liver capsule (group II); after transplantation (group III), there was also interstitial and intracellular edema, fatty degeneration and disintegration of the sinusoidal lining. One month later, necrosis, bile duct proliferation, cholestasis, cholangitis and vascular alterations were found in groups II and III. Furthermore, an increased rate of hepatocellular and bile duct proliferation was observed. These findings are partly due to the bile duct reconstruction. We recommend that a bile duct reconstruction control group should be included in ORLT experiments.

Partial orthotopic liver transplantation in rats

A partial orthotopic liver transplantation technique (70% POLT) for use in rats and comparable with the corresponding recipient operation in the 'splitting transplantation' in man was developed. Body weight, liver function, histological and electron-microscopic findings were studied in comparison with whole rat liver transplantation with rearterialization, 30% POLT and corresponding liver resections. After 70 and 30% POLT typical signs of hepatic regeneration were found, but no pathological alterations in the electron-microscopic picture. This POLT model might be helpful for the investigation of unresolved questions in 'splitting transplantation'.

Developing home care standards you can count on ... and cost on!

Cost management is a six-step process that begins with identifying important activities of patient care, identifying desired outcomes of patient care, and the most predictable and efficient process to be used to ensure outcome accomplishment. The more caregivers and managers understand the cost-benefit connection between patient care and resource costs, the easier it will be for them to manage (rather than be a victim of) quality, productivity, and profitability.

Effect of aging on cold tolerance and thyroid activity in CBA/Ca inbred mice

Relationships between cold tolerance, serum levels of thyroxine (T4), thyronine (T3), and thyrotropine (TSH), and thyroid morphometry have been investigated in male CBA/Ca inbred mice at various ages through their life span. From the data obtained it appeared that there was an age-related decrease in cold tolerance up to 18 months of age which was followed by an increase, the age effect being most apparent in relation to cold resistance and cold tolerance during the recovery period following cold exposure. The age-related changes in cold tolerance appeared to be associated with changes in the serum concentrations of T3, T4, and TSH. In contrast to the T3 serum levels which showed a decrease at 36 months, the thyroxine contents showed a perceptible decrease from the age of 12-18 months onwards. A similar pattern was observed for the TSH levels, with a peak at 21 months, followed by a decline at 30 months. A relationship with age between serum T4 level and thyroid weight was indicated together with structural changes in the thyroid gland, particularly during senescence, for example the size and number of thyroid epithelial cells had become enlarged by 30 months of age.

Percutaneous biopsy of the liver

Retrospective evaluation of 510 percutaneous CT-guided biopsies of the liver mainly fine-needle aspiration biopsies for cytology (89% of cases), yielded an overall accuracy rate of 92% and a sensitivity of 94%. The relatively high percentage of false-positive diagnoses (7% of all benign tumors) may be reduced by more consistent consideration of possible errors in cytology and a more consistent use of large bore biopsies.

[Use of the stable nitrogen isotope 15N in assessing liver metabolism in hormonal contraception]

In 37 women with long-term use of oral contraceptives (average use 13.4 years) and 12 women who had not taken oral contraceptives for at least 5 years we determined the serum enzymes ALAT, ASAT, Gamma-GT and conducted the 15N-ammonium test as a marker of partial metabolic performance of the liver. In addition we studied total protein, protein fractions in the serum and haematologic parameter. 18 out of 37 women showed a morbid result in the 15N-ammonium test. In 4 of these 18 women morphological changes representing hepatic damage caused by contraceptives could be detected. On the average ALAT, Gamma-GT and 15N-ammonium test in both groups are clearly different. Because the enzymic levels are being in the normal range, only the 15N ammonium test enables to give a hepatotoxic evidence. It is possible to conclude that long-term use of oral contraceptives influences the liver metabolism.

Vanadate mimics effects of fungal cell wall in eliciting gene activation in plant cell cultures

Cell-suspension cultures of peanut (Arachis hypogaea L.) can be used as a very sensitive and rapidly responding physiological system for monitoring extracellular signals. Elicitors effect the activation of the genes that code for a set of enzymes synthesizing stilbenes. Within 2-6 h after administering micromolar, concentrations of orthovanadate to the suspended cells, the enzyme activities of phenylalanine ammonia-lyase, stilbene synthase, and cinnamate 4-hydroxylase increased 10-to 100-fold. The transient time course of induction, and the quality and quantity of gene expression found with vanadate as artificial elicitor were very similar to those observed after biotic stress generated by fungal cell walls. The dose-response of vanadate as an elicitor of gene expression in intact cells matched precisely its inhibitory effect on the ATPase activity of isolated plasma membrane. By concentrating, on the profiles of cinnamate 4-hydroxylase activity, we observed differences between the effects elicited by fungal cell wall or vanadate when different stages of cell development were analyzed. Unlike the fungal elicitor, vanadate did not induce the hydroxylase activity when cells at the stationary phase of the cell cycle were used. This lack of response was not the result of a decrease in membrane biosynthesis. The finding, that the effects of vanadate and fungal elicitor are additive indicates that vanadate does not interfere negatively with the perception of the biotic signal but rather addresses the same intracellular intermediate of the signalling process. We hypothesize that membrane potentials created or modulated by ATPases may be intermediates in the signal chain, starting with the recognition process at the plasma membrane and eventually leading to the production of stilbenes as low-molecular-weight plant-defence products.

Alterations in the intestinal microvillous surface area during the whole life span of the female rat ileum

The present EM morphometric analysis is a continuation of a series of studies in which the changes of the microvillous surface area of the small intestine was investigated in the course of the entire life span of the female rat. It was concluded from the present data that the alterations of the ileal microvillous surface area are more balanced in the young, pregnant, lactating, adult, old and senile animals than that was observed in the case of the duodenum and jejunum. Positive correlations were found between the parameters height and surface area and, density and surface area measures of the microvilli. Microvillous width and density revealed an inverse relationship mainly in the case of the young and adult animals.

[Value of histopathologic findings following liver transplantation based on biopsy course studies]

A total of forty-four biopsies from nine liver transplantation patients, who had developed complications postoperatively, were studied histologically and, in part, electron-microscopically. Morphologic signs of acute rejection were seen in three patients, while there were indications of chronic rejection in one patient. In combined lesions, such as drug-induced hepatosis, cholangitis, hepatitis, and rejections, the interpretation of biopsy findings may be difficult, and special entities cannot in every case be separated unequivocally. Changes of serum bilirubin and of serum enzyme activity do not run entirely parallel to the microscopically detectable structural lesions. For diagnosis of complications after liver transplantation both the morphologic and the clinical findings must be considered.

Stimulation of wound healing, using brain extract with fibroblast growth factor (FGF) activity

A bovine brain fraction with FGF activity (fibroblast growth factor) increased the formation of granulation tissue of rats in an experimental wound healing model. It was characterised by intensified formation of new capillaries, stimulation of the synthesis function of fibroblasts and myofibroblasts, and proliferation of freshly developed granulation tissue. The results obtained from light microscopy and autoradiographic electron microscopy with 3H-thymidine suggested significant increase in the labelling of activated, undifferentiated endothelial cells in vascular sprouts and of differentiated endothelial cells in developed capillaries. FGF S3 appeared to speed up the differentiation of freshly formed endothelial cells. Also labelled were pericytes and other fibroblastoid cells, undifferentiated mesenchymal cells, juvenile undifferentiated fibroblasts, mature fibroblasts, myofibroblasts, and macrophages. Pericytes are presumed to be preferential precursors of myofibroblasts, on account of their occurrence in time and similar ultrastructural parameters. Close neighbourhood of lymphocytes and labelled fibroblasts gave rise to the assumption of specific effects, such as stimulation by lymphokines.

Inhibition of SV40 DNA synthesis by vesicular stomatitis virus in doubly infected monkey kidney cells

Vesicular stomatitis virus (VSV) inhibited SV40 DNA synthesis in doubly infected synchronized Vero cells. Gel-electrophoretic profiles demonstrated that SV40 DNA monomers accumulated in all stages of supercoiling, regardless of whether cells were superinfected with VSV early or late in the S phase. These gel profiles were indistinguishable from ones obtained from SV40-infected, cycloheximide-treated cells in the absence of VSV infection. Radiolabel in the partial supercoils could be chased into supercoils, but only by restoring protein synthesis. The relative rates of SV40 DNA chain elongation were determined in VSV-superinfected and nonsuperinfected cells. The gradients of 3H incorporation as a function of distance from the origin of replication in pulse-labeled form I DNA were unaffected by VSV. It is concluded that VSV inhibition of SV40 DNA synthesis is an indirect result of inhibition of host cell protein synthesis and it is suggested that incompletely supercoiled SV40 chromatin is not a good template for DNA synthesis.

[Simulation of the effect of biologically active substances in models]

The results obtained from historadioautographic examinations of the epithelium of the small intestine of rats under the action of procaine served as a basis for examining the proliferative system by using a model. The following results have been obtained through computerized simulation: 1. The maximum life-time of the enterocytes is prolonged at constant villous height, which means that on average the villous is studded with older cells than normally. 2. The prolonged time of generation of the proliferatively active cryptic epithelial cells calls for a pronounced reduction in the pool of reserve cells (cells in the G0 phase, non-growth fraction), i.e. for their activation in order to realize the system characteristics described under 1. 3. The condition of the system induced by procaine may be compared, in its changed tendency, neither to the changing tendencies during aging nor to those under pathological conditions. It is rather a new condition that is being produced and which is characterized, inter alia, by the characteristics described under 1 and 2.

The influence of biologically active substances on the regenerative processes of enterocytes in the small intestine of the rat

The influence of a prolonged treatment with procaine or Turigeran on the cell population kinetics of the mucous membrane of the small intestine was studied by means of 3H-thymidine histoautoradiography in 240 12-month-old male Wistar rats. The investigations produced the following findings: 1. There are no differences in the cell population kinetics of the mucous membrane of the small intestine between procaine- and Turigeran-treated animals. But as, on the other hand, significant differences are found between treated and untreated animals in terms of all cell-kinetic characteristics, this effect has to be attributed to procaine alone. 2. In treated animals the partial stages of the cell cycle are prolonged as a result of which the generation time is delayed. 3. Due to the fact that the transit time of the cells in the crypts and on the villi is reduced, a greater number of labelled cells can finally be observed despite reduced proliferation rates in the crypts (labelling index) of treated animals. 4. Procaine will continue to act with undiminished effectiveness for at least two weeks after discontinuation of treatment.

The effect of exercise on cell proliferation in the non-ischemic heart after myocardial infarction

The influence of physical training (180 hours, 3 hours per day) on the 3H-thymidine incorporation into the connective tissue cell nuclei and into the myocardial cell nuclei of the non-ischemic heart after myocardial infarction was studied in 38 untrained and trained male Wistar rats. In the connective tissue cells physical training caused a clearly reduced proliferative activity which could be demonstrated until the seventh day after infarction. Besides, in trained animals there was a decreased DNA turnover of the heart muscle cells during the first few days following myocardial infarction. Probably, the observed changes are an expression of the different adaptability of the untrained and trained non-ischemic myocardium.

[Experimental autoradiographic study on the proliferation kinetics of the ischemically injured rat small intestine]

The kinetics of proliferation of the ischemically injured small intestine in rats were tested after recovery of the circulation in dependence on the age by means of the autoradiography. The marking index and the middle density of silver grain were applied. The time of the anoxia amounted to 3 hours, since in that case a regeneration can hardly be expected after this time. With a complete regeneration both the number of the synthesizing cells and the synthesized DNA quantity can be calculated after the recovery of circulation. Differences dependent on the age become clear, in a similar way to the physiological kinetics of proliferation. The values of comparison with the control group are reached in rats at an age of 6 weeks already after nearly 18 to 20 hours. On the other hand the time of the regeneration amounted to nearly 36 to 48 hours in the rats at the age of 12 months. The demand of an early diagnosis and recovery of the circulation within 3 hours is emphasized.

[Autoradiographic investigations on the age dependent regeneration of parenchymal liver cells (author's transl)]

It is possible to represent the age dependent change of the ability of reproduction on the model of liver regeneration. Because of the reduced growth fraction with increasing age there is a delay of the beginning and a decrease of the maximum ascent of regeneration. In old rats the proliferating activity decreases to consistently low levels earlier than in young and adult rats. There are no age dependent differences in the total amount of regeneration 21 days after 1/3 hepatectomy. In contrast to the literature we are able to show, that already by means of 1/3 hepatectomy highest values of liver regeneration are achievable.

[The aging of intermitotic cells (author's transl)]

The aging of the cells depends on their mitotic activity, which is particularly evident in multicellular organisms. By way of histoautoradiography we studied the cell kinetics on a total of 244 Wistar rats aged 6 days, 6 weeks, 6, 12, 23 and 28 months respectively, with these studies being centred on the mucosa of the small intestine since this mucosa is one of those tissues with the highest cell metabolism. We succeeded in demonstrating that the regeneration rate of cells per hour in the crypts of the small intestine and the migration speed of the enterocytes differ in young and old individuals and that the intermitotic cells have age-dependent properties as well. In addition, we could prove that intermitotic cells too have a non-growth fraction which, at an advanced age, decreases only a little, although significantly in terms of statistics. For the easily vulnerable crypt epithelium it is a reserve capacity and can be included in the proliferating pool, if necessary.

[Quantitative morphological investigations in livers and liver tumors after taking contraceptives (author's transl)]

Since the introduction of the orally applicable contraceptive steroids repeated reports have been published on the occurrence of side effects largely manifesting in the liver. By means of morphometrical and densitometrical evaluation of histological slide preparations of livers damaged by contraceptives (biopsies, operation material), qualitative and quantitative transitions between the individual degrees of the lesion up to malign cell transformation were registered by automated microscope picture analysis. It appeared that this cell transformation, like similar as in experimental hepatocarcinogenesis, is gradual process. Among the most remarkable cytopathological features are changes of nuclear and nucleolar size and of the sinusoid area and of the number of Kupffer's cells. The latter are clearly increased in hepatosis caused by contraceptives as compared with normal livers, they are reduced in the hepatocellular adenoma, and reach a value near zero in the carcinoma.

[Adenyl cyclase activity and the stimulation of adenosine 3',5'-cyclic monophosphate in blood vessel walls during ageing and under stress (author's transl)]

It is well-known that the cAMP-adenylate cyclase system is important in mediating the effects of numerous hormones. We investigated the age-dependent behaviour of this system in aortas and femoral arteries of male Wistar rats (at the age of 10 days, 1, 4, 8, 12, and 22 months). It was found that: The basic adenylate cyclase activity had considerably been decreased beyond the first month of life, thereafter it was almost constant. The response of adenylate cyclase to guanylyl-imidodi-phosphate and NaF had essentially been elevated since the 4th month of age. The possibility of stimulating the cAMP generation due to epinephrine and histamine had substantially been increased since the 12th month of age. Besides the ability of adrenaline and histamine to stimulate the formation of cyclic AMP was investigated in broken cell preparation and intact cells of smooth muscle of the aorta and femoral artery of rats which had been subjected to daily intermittend immobilization of 1, 3, and 17 weeks. It was found that this type of stress led to an instability of the blood pressure which was associated with an increase in the sensitivity of adenylate cyclase in the broken cell preparations from the arteries to adrenaline and histamine and with a heightened cyclic AMP response to the two hormones in the intact arterial smooth muscle cells. The sensitivity of cardiac adenylate cyclase for adrenaline remained unchanged.

[Hair cell noise damage after improved hypoxia tolerance (author's transl)]

Guinea pigs were gradually adapted to a simulated altitude of 10,000 m. The round window microphone potential (RMP) frequency response was measured 24 hours after pure tone exposition (2.7 kHz, 130 dB, 1 hr). The electrophysiologically demonstrable sound induced damage of the organ of Corti of the adapted guinea pigs was significantly smaller than those of the nonadapted (normal) group. Altitude adapted animals: m = 18, Hb: 18.5 +/- 1.8 g%, Hc: 57.3 +/- 4.4+ RMP mean loss: 1.7 +/- 0.8 dB.

CONTROLS

n = 18, Hb: 12.7 +/- 1.8 g%, Hc: 40.5 +/- 1.6%. RMP mean loss: 5.8 +/- 2.4 dB. All differences significant p less than 0.001. The evident possibility to reduce noise induced hair cell damage by means of previous adaptation to high altitude might be explained by the greater hypoxia tolerance and perhaps additional better oxygen supply to the receptor cells.

The influence of diabetes mellitus and hypercorticism on the wound healing of experimental myocardial infarction in rats

In diabetic and glucocorticoid-treated rats myocardial infarction is produced by coronary artery ligation. After the ligation was performed the animals were given 3H-thymidine or 3H-proline at different times. The following parameters were determined: number of DNA- and tropocollagen-synthesizing connective-tissue cells both at the infarction border and infarction site; mean silver-grain density above the nuclei or cells; number of mitoses. The labelling and mitotic indices as well as the standard deviation (in percent) from the mean values were calculated. The following results were obtained: 1. The retarded formation of collagen fibres in diabetic animals is caused by a reduced number of tropocollagen-synthesizing fibroblasts and by a diminished synthesizing performance of the individual cells. 2. Glucocorticoids have a pronounced inhibitory effect on granulation-tissue formation, The 3H--thymidine indices are strikingly low. The synthesis of collagen precursors in the fibroblasts is reduced. The release of tropocollagen from the connective-tissue cells is slowed down.

The influence of hyperthyroidism and hypothyroidism on the wound healing of experimental myocardial infarction in the rat

In hyperthyroid and hypothyroid rats myocardial infarction was produced by coronary artery ligature. At different times following the ligature the animals were given 3H-thymidine or 3H-proline. The following parameters were determined: the number of DNA- and tropocollagen-synthesizing connective-tissue cells at the infarction border and at the infarction site; the mean silver-grain density above the nuclei or cells; the duration of one cell cycle; and the number of mitoses. The labelling and mitotic indices as well as the percentage of the standard deviation from the mean values were estimated. The following results were obtained: 1. In hyperthyroidism wound healing is accelerated. The number of DNA-synthesizing connective-tissue cells is increased. The cell-cycles are shorter. The release of tropocollagen by the fibroblasts takes place at a strikingly fast rate. 2. In hypothyroidism wound healing is delayed. The 3H-thymidine and mitotic indices are lowered. The release of tropocollagen is slowed down. The granulation tissue contains localized accumulations of mucopolysaccharides.

The influence of age on the wound healing of experimental myocardial infarction in rats

In 6-weeks-old, 5-months-old and 21-months-old rats myocardial infarction was induced by coronary artery ligature. After performing the ligature the animals were administered 3H-thymidine, 3H-proline or 35S-sulphate at different times. The following parameters were determined: number of DNA- and tropocollagen-synthesizing connective tissue cells at the infarction border and at the infarction site; mean silver grain density above the nuclei or cells; duration of a cycle; number of mitoses, and incorporation of radioactive sulphate at the infarction site. In addition, the labelling and mitotic indices as well as the percentage of the standard deviation from the mean values were estimated. The following results were obtained: 1. The rate of granulation tissue formation in the necrotic zone is determined by the mitotic activity of the cells. With advancing age the cell cycles are being prolonged which results in retardation of wound healing. 2. The disturbed DNA-replication in old age is not associated with a time shift in the occurrence of the mitotic and labelling peaks. 3. With advancing age the number of fibroblasts synthesizing collagen precursors decreases. There exists no age-dependence of the 3H-proline incorporation rate, of the intracellular transport, of the synthesis of collagen precursor and of the release of labelled tropocollagen. In all age-groups under study these processes last approximately 4 hours. 4. Collagen fibre formation is accompanied by an increased synthesis of acid mucopolysaccharides. In infarction callosities the content of acid mucopolysaccharides mostly is constant. 5. The proliferating endothelial cells have a pronounced metabolic activity and a markedly short generation time.