A QUANTITATIVE DESCRIPTION OF CORTISONE-INDUCED ALTERATIONS IN THE ULTRASTRUCTURE OF RAT LIVER PARENCHYMAL CELLS

A stereological comparison of the hepatic parenchymal cells from 125-g male rats given a daily injection for 6 days of either 5 mg of cortisone acetate or saline (controls) was carried out with both light and electron microscopy. Cortisone treatment results in an increase in average parenchymal cell cytoplasmic volume from 5100 to 5800 µ³ and a decrease in average nuclear diameter from 7.1 to 6.5 µ. The volume of the average mitochondrion is increased fourfold in midzonal and peripheral regions of hepatic lobules, and there is a decrease in the number of mitochondria per cell such that the total mitochondrial volume per cell remains approximately unchanged. The numbers of peroxisomes are reduced, while the numbers of lysosomes and lipid droplets are increased in all parts of the lobules. The average volume of glycogen is doubled in all cells. The areas of membranes of the smooth- and rough-surfaced endoplasmic reticulum are decreased to one-half and two-thirds of their control values, respectively. The effects of cortisone on these various structural elements is discussed with respect to steroid-related alterations in biochemical processes.     

ANALYSIS OF TIME DEPENDENT CHANGES OF LYMPHOPOIETIC ACTIVITY IN ORGAN CULTURES OF EMBRYONIC CHICKEN THYMUS

The time dependent development of lymphocytes in organ cultures of the thymus obtained from 10-day-old chick embryos was characterized by an initial phase of exponential increase of the number of lymphocytes per thymus followed by a plateau phase with no further increase in cell number. The proportion of cells in DNA synthesis dropped rapidly during the first 10 days of culture. Simultaneously the lymphocytes turned progressively smaller, as evidenced by both cell diameter and dry mass and constituted a homogeneous population of small cells at the end of the culture period. Thymic anlagen partially depleted of lymphoid precursor cells by a short hot pulse with ³H-TdR showed a prolonged exponential phase and reached normal plateau cell numbers 2–4 days later than usual. Furthermore, at least in the first part of the plateau phase, a reduction in the number of lymphoid cells per thymus resulted in a recovery in terms of the cell number which was associated with increased DNA synthesis. These results are compatible with the regulation of thymic lymphopoiesis in organ culture through a mechanism operating via cell density.     

OSMOTIC PROPERTIES OF THE EGG CELLS OF THE OYSTER (OSTREA VIRGINICA)

Investigations of the osmotic properties of oyster eggs by a diffraction method for measuring volumes have led to the following conclusions: 1. The product of cell volume and osmotic pressure is approximately constant, if allowance is made for osmotically inactive cell contents (law of Boyle-van''t Hoff). The space occupied by osmotically inactive averages 44 per cent of cell volume. 2. Volume changes over a wide range of pressures are reversible, indicating that the semipermeability of the cell during such changes remains intact. 3. The kinetics of endosmosis and of exosmosis are described by the equation, See PDF for Equation, where dV is rate of volume change; S, surface area of cell, (P-P_e), the difference in osmotic pressure between cell interior and medium, and K, the permeability of the cell to water. 4. Permeability to water during endosmosis is 0.6µ³ of water per minute, per square micron of cell surface, per atmosphere of pressure. The value of permeability for exosmosis is closely the same; in this respect the egg cell of the oyster appears to be a more perfect osmometer than the other marine cells which have been studied. Permeability to water computed by the equation given above is in good agreement with computations by the entirely different method devised by Jacobs. 5. Permeability to diethylene glycol averages 27.2, and to glycerol 20.7. These values express the number of mols x 10^–15 which enter per minute through each square micron of cell surface at a concentration difference of 1 mol per liter and a temperature of 22.5°C. 6. Values for permeability to water and to the solutes tested are considerably higher for the oyster egg than for other forms of marine eggs previously examined. 7. The oyster egg because of its high degree of permeability is a natural osmometer particularly suitable for the study of the less readily penetrating solutes.     

STABILITY OF DNA IN PURKINJE CELL NUCLEI OF THE MOUSE : An Autoradiographic Study

Neurons of the mouse were labeled with [³H]thymidine during their prenatal period of proliferation. The ³H activity of the Purkinje cell nuclei was then studied autoradiographically 8, 25, 55, and 90 days after birth. The measured grain number per nucleus decreased by about 14% between the 8th and 25th postnatal days and then remained constant up to 90 days. There was no significant decrease of the ³H activity of the Purkinje cell nuclei after correction of the measured grain number per nucleus for increasing nuclear volume of the growing Purkinje cells and for the influence of [³H]β self-absorption in the material of the sections. Injection of a high dose of [³H]thymidine into young adult mice did not result in ³H labeling of either Purkinje or other neurons in other brain regions. The results agree with the concept of metabolic stability of nuclear DNA. "Metabolic" DNA could not be observed in these experiments.     

Quantitative changes in rat renin secretory granules after acute and chronic stimulation of the renin system

In order to study the cellular mode of renin secretion, stereological methods were used to estimate number and volume of rat renin secretory granules during stimulation of the renin system. An acute decrease in renal perfusion pressure to 40 mmHg for 5 min increased plasma renin concentration (PRC) twofold, but did not significantly change the number of renin granules per arteriole or the renin-containing volume of the arteriole. Chronic stimulation was achieved by a combination of low-salt diet and inhibition of angiotensin-converting enzyme (ACE) for 14 days, and resulted in a 36-fold increase in PRC, a 20-fold increase in the number of granules per arteriole, and a 17-fold increase in the arteriolar volume that contained renin. An acute decrease in renal perfusion pressure to 40 mmHg for 5 min in the chronically stimulated rats increased PRC further (1.6-fold), and significantly reduced the number of granules per arteriole by 4000 (45% reduction), but did not change the renin-containing arteriolar volume significantly. The average renin granule size was 0.35 μm³ with no significant differences among the groups. We conclude that recruited granular cells contribute significantly to renin release, and that all granular cells along the arteriole participate in secretory responses. The reduced number of renin granules after acute stimulation is compatible with exocytosis as the dominating mechanism of renin release.     

隔沟交替灌溉条件下玉米根系形态性状及结构分布

为揭示根系对土壤环境的适应机制,研究了隔沟交替灌溉条件下玉米根系形态性状及结构分布。以垄位和坡位的玉米根系为研究对象,利用Minirhizotrons法研究了根系（活/死根）的长度、直径、体积、表面积、根尖数和径级变化及其与土壤水分、土温和水分利用效率（WUE）的相关关系。结果表明,对于活根,在坡位非灌水区域复水后根系平均直径减小,而根系日均生长速率、单位面积土壤根系体积密度、根尖数和表面积均增大,并随灌水区域土壤水分的消退逐渐减小;对于死根,在坡位非灌水区域复水后根系日均死亡速率、根系体积密度、根尖数和表面积变化均减小,其中根系死亡速率和死根直径随土壤水分的消退逐渐降低,而死根体积密度、根尖数和表面积分布随土壤水分降低呈增大趋势;在垄位,根系形态分布趋势与坡位一致,除根系直径与与坡位比较接近外,其他根系形态值均小于坡位。将根系分成4个径级区间分析根系的形态特征,结果表明在根系长度和体积密度分布中以2.5－4.5 mm径级的根系所占比例最大,在根尖数和根系表面积分布中以0.0－2.5 mm径级的根系为主。通过显著性相关分析,死根直径、体积密度、活根表面积等根系形态与土壤含水率、土壤温度和WUE间均存在显著或极显著的正相关关系,部分根系形态指标（如根系的生长速率、活根体积密度）只与坡位土壤含水量、土壤温度具有明显的相关性,表明隔沟交替灌溉对坡位根系形态的调控作用比垄位显著。     

HYDROSTATIC PRESSURE AND TEMPERATURE IN RELATION TO STIMULATION AND CYCLOSIS IN NITELLA FLEXILIS

Nitella flexilis cells are not stimulated to "shock stoppage" of cyclosis by suddenly evacuating the air over the water or on sudden readmission of air, or on suddenly striking a piston in the water-filled chamber in which they are kept with a ball whose energy is 7.6 joules, provided the Nitella cell is not moved by currents against the side of the chamber. Sudden increases in hydrostatic pressure from zero to 1000 lbs. or 0 to 5000 lbs. per square inch or 5000 to 9000 lbs. per square inch usually do not stimulate to "shock stoppage" of cyclosis, but some cells are stimulated. Sudden decreases of pressure are more likely to stimulate, again with variation depending on the cell. In the absence of stimulation, the cyclosis velocity at 23°C. slows as the pressure is increased in steps of 1000 lbs. per square inch. In some cells a regular slowing is observed, in others there is little slowing until 4000 to 6000 lbs. per square inch, when a rapid slowing appears, with only 50 per cent to 30 per cent of the original velocity at 9000 lbs. per square inch. The cyclosis does not completely stop at 10000 lbs. per square inch. The pressure effect is reversible unless the cells have been kept too long at the high pressure. At low temperatures (10°C.) and at temperatures near and above (32°–38°C.) the optimum temperature for maximum cyclosis (35–36°C.) pressures of 3000 to 6000 lbs. per square inch cause only further slowing of cyclosis, with no reversal of the temperature effect, such as has been observed in pressure-temperature studies on the luminescence of luminous bacteria. Sudden increase in temperature may cause shock stoppage of cyclosis as well as sudden decrease in temperature.     

Brassinolide Effect on Growth of Apical Meristems, Ethylene Production, and Abscisic Acid Content in Potato Tubers

Treatment of intact potato (Solanum tuberosum L., cv. Nevskii) tubers with 24-epibrassinolide (EB) resulted in prolonged deep dormancy, increased production of ethylene and higher contents of free and bound abscisic acid (ABA) in buds. EB at the most efficient concentration 0.021 mg dm^–3, applied immediately after tuber harvest, inhibited sprouting by 36 – 38 d, increased ethylene formation after 1 and 7 d of storage by almost 300 and 150%, respectively, and increased the content of both free and bound ABA during the whole period of storage (on average by about 80%). Electron microscopic and morphometric studies showed that EB brings about a decrease in cell volume in tunica and all types of meristems and an increase in the number of vacuoles, accompanied by a decrease in their volume.     

Growth Dynamics of Mitochondria in Synchronized Chinese Hamster Cells

The increase in cell volume (from electronic cell sizing) and the apportionment of this volume amongst the nuclear, cytoplasmic, and mitochondrial subcellular compartments (from electron microscopy) were studied throughout the cell division cycle in partially synchronized cultures of Chinese hamster V79-S171 cells. Average whole cell volume was found to increase smoothly, consistent with the doubling in one generation of individual cell volume. Nuclear size increased in like fashion. Mean total mitochondrial volume and number of mitochondria per cell both showed a different kind of variation, most notably a significant decrease in G₁ and G₂ as compared with mid S. These results are therefore counter to a model of simple doubling of mitochondria either synchronously with the cell division cycle or asynchronously. Absolute mean values per cell for log phase Chinese hamster cells were also determined, as follows: whole cell volume, 710 μ³; nuclear volume, 190 μ³; total mitochondrial volume, 37.5 μ³; number of mitochondria per cell, 90.     

Waveform dispersion, not reflection, may be the major determinant of aortic pressure wave morphology

The objective of this study was to investigate the determinants of aortic pressure waveform morphology in the thoracoabdominal aorta with specific reference to features of potential prognostic value for cardiovascular disease. In particular, we aimed to determine the location of major pressure wave reflection sites within the aorta. Aortic pressure waveforms were acquired with 2-Fr Millar Mikro-tip catheter transducers in 40 subjects (26 men, 14 women), and repeated in 10 subjects, at five predetermined points within the aorta: aortic root, transverse arch, and at the levels of the diaphragm, renal arteries, and aortic bifurcation. Waveforms were analyzed for augmentation index (AI), time to inflection point (Ti), and pressure parameters. AI decreased progressively between the aortic root and bifurcation (P < 0.001), and Ti increased (P < 0.01). There was the expected progressive peripheral amplification of systolic and pulse pressures and fall in time to peak pressure (all P < 0.001). There was no difference on repeat pullback or between sexes. These data are at variance with the concept that central AI results solely from pressure wave reflection, when Ti would be expected to decrease and AI increase with distal progression. Pressure wave propagation phenomena may contribute, and the potential role of frequency dispersion merits investigation.     

Effect of acute heat stress on rat adrenal cortex — a morphological and ultrastructural study

The stereological structure of rat adrenal gland was analysed by light and electron microscopy after an acute (60 min) exposure to high ambient temperature (38°C). Under these conditions a significant increase in plasma corticotrophin (ACTH), serum corticosterone and aldosterone levels were observed. Histological and stereological investigation at light microscopy showed significant decrease in volume density of capsule and zona glomerulosa, increase in volume of fasciculata cells, and decrease of numerical density of zona fasciculata cells and mean diameter of blood vessels. At the ultrastructural level, volume density of nuclei and mitochondria of zona glomerulosa cells were significantly increased and that of lipid droplets decreased. Volume density of mitochondria of fasciculata cells was significantly increased, while number of lipid droplets per μm² of cell was reduced. In the cells of zona reticularis significant increase in the number of lipid droplets was found. The response of zona glomerulosa may be interpreted as immediate reaction to dehydration, while alterations detected in zona fasciculata, which were less extensive, were related to purely stressogenic effects of high ambiental temperature.     

Water-relation parameters of epidermal and cortical cells in the primary root ofTriticum aestivum L.

Water-relation parameters of root hair cells, hairless epidermal cells, and cortical cells in the primary root of wheat have been measured using the pressure-probe technique. Under well-watered conditions the mean cell turgor of cortical cells was 6.8±1.9 (30) bar (mean±SD; the number of observations in brackets). In hairless epidermal and root hair cells the mean cell turgor was 5.5±1.9 (22) and 4.4±1.5 (15) bar, respectively. Despite the large variability, turgor pressure was significantly lower (confidence interval=0.95) in epidermal cells relative to cortical cells. This may be a consequence of the ultrafiltration of ions by the external cell wall and-or plasmalemma of epidermal cells. The volumetric elastic modulus of the cells ranged from 10 to 150 bar. This parameter was dependent on cell volume, but within experimental accuracy, was independent of cell type. No pressure dependence of the volumetric elastic modulus was observed in these cells. The half-times for water exchange ranged from 1.8 to 48.8 s. The mean value increased in the order root hair < hairless epidermal < cortical cells and was directly related to volume to surface area ratio. Thus the hydraulic conductivities of the three cell types were similar and averaged 1.2±0.9·10^-6 (170) cm s^-1 bar^-1. No polarity was observed between inwardly and outwardly directed water flow. The similarity of the hydraulic conductivities of root hairs to those of other cells indicates that the membranes of root hairs are not particularly specialized for water transport. The overall hydraulic conductivity for radial water flow across the root was estimated from the pressure-probe data using a simple model and was compared with that measured directly on whole roots using an osmotic backflow technique. It was tentatively concluded that upon sudden osmotic perturbation, the major pathway for water transfer across the root may be through the symplasm and involve net flow from vacuole to vacuole.     

Effect of atmospheric pressure on maize root growth and ethylene production

Maize (Zea mays) seedlings were exposed to elevated atmospheric pressures while growing in moist sand in open plastic envelopes to evaluate the effects of directly applied atmospheric pressure on ethylene production and root growth. Effects were evaluated after 24 h. The threshold pressures necessary to promote ethylene production and decrease root elongation were about 600 and 400 kPa, respectively. Direct atmospheric pressure, at levels up to 300 kPa, mimicked the control decrease in root diameter and increased diameter only slightly at 500 to 1200 kPa. In contrast, in previous work it was shown that physical impedance resulting from compression of the growth medium by external application of 100 kPa increased the ethylene production rate 4-fold and the root diameter 7-fold while reducing elongation 75% in 10 h. The relative insensitivity of roots to direct atmospheric pressure suggests that they perceive physical impedance, achieved experimentally by compressing the growth medium, via a surface mechanism rather than via a pressure-sensing mechanism.     

Viscoelastic Properties of the Human Red Blood Cell Membrane: I. Deformation, Volume Loss, and Rupture of Red Cells in Micropipettes

Single human red blood cells suspended in buffered Ringer's solution were rapidly drawn, at recorded pressures, into glass micropipettes of diameter 0.6-3.2 μm. Cells could enter micropipettes of diameter ≥ 2.9 μm with minimal pressure. In micropipettes of 0.9-2.9 μm, the pressure required increased linearly with decreasing diameter. For diameters 2.5-2.9 μm, pressures ranged up to 7 cm Hg, and the cells returned to normal biconcave shape on release. For diameters 1.9-2.5 μm, the required pressures ranged from 7 to 17 cm Hg. The released cells were crenated. In micropipettes 0.9-1.9 μm, the pressures required ranged from 17 to 34 cm Hg. The cells hemolyzed on entry. As diameter decreased from 0.9 to 0.6 μm, cells were drawn into dumbbell shapes and parts of the cells were pinched off without complete hemolysis of the cell. Using an accepted value of 138 μm² for the mean cell area, the mean volume of the human red cell was calculated to be 94 μm³. Under mechanical stress, about 12% of this volume is rapidly exchangeable with the external medium. The cell volume may further decrease by 20% which is not reversible.     

Permeabilizing soybean protoplasts to macromolecules using electroporation and hypotonic shock

The percentage of soybean cell culture protoplasts permeabilized by electroporation was dependent on the voltage and the number of successive pulses that were applied. Best results were obtained with two 50 milliseconds, 400 volts per centimeter pulses after which 78% of the surviving protoplasts had been permeabilized to the fluorescent dye calcein. Quantitation of the volume of extracellular fluid taken up was performed using radioactive inulin (molecular weight 5000-5500). Typically between 20 and 40 nanoliters of fluid was taken up by 10⁶ protoplasts. Electroporation and hypotonic shock treatments (M Saleem, AJ Cutler 1986 J Plant Physiol 124: 11-21) were compared with respect to the volume of fluid taken up under optimum conditions. Electroporation produced 10 times more uptake than hypotonic shock treatment. In all experiments there was a direct relationship between the number of protoplasts lysed and both the amount of fluid taken up and the percentage of surviving protoplasts that were permeabilized.     

Effects of Inhibitory Concentrations of 3-Indolylacetic Acid and 3-Indolylacetonitrile on Cell Division and Tissue 2

Inhibitory concentrations of 3-indolylacetonitrile (IAN) cause,in cultured excised tomato roots, a marked decrease in the rateof cell division at the apical meristem but only a slight reductionin the lengths of mature exodermal and cortical cells. The reducedrate of cell division is associated with a decrease in the.number of meristematic cells at the root apex. By contrast,3-indolylacetic acid (IAA) causes marked reduction in the lengthsof mature cortical cells but does not markedly reduce cell-divisionrate at the apical meristem. Various lines of evidence indicate that both IAA and IAN causea relative increase in the number of longitudinal and a decreasein the number of transverse division walls in the meristematiczone of the root apex. Partial inhibition of the linear growth of excised tomato rootsby IAA and IAN is accompanied by increases in root and stelardiameters. These increases result from radial enlargement ofthe cortical cells and increase in the number of stelar cellsin the transverse section. The enlarged steles contain an increasednumber of lignified xylem elements, but only with the most inhibitoryconcentration of IAN (10^–4g./ml.) is there evidence ofthe development of secondary xylem. Both auxins increase significantlythe xylem vessel unit length.     

CORTISONE-INDUCED ALTERATIONS IN MITOCHONDRIAL FUNCTION AND STRUCTURE

The effects of cortisone treatment on oxygen consumption, oxidative phosphorylation, and fine structure of rat liver mitochondria have been studied. Male rats weighing 125 g were treated for 6 days with 5 mg of cortisone acetate or isotonic saline. On the 7th day, sections of liver were excised and processed for light and electron microscopy. Mitochondrial respiration and oxidative phosphorylation were studied with mitochondria isolated from these livers. Cortisone treatment is responsible for a 14–40% decrease in the amount of oxygen consumed per mg of mitochondrial protein when succinate, α-ketoglutarate, or β-hydroxybutyrate are used as substrates, or with ascorbate and N,N,N¹,N¹-tetramethyl p-phenylenediamine as electron donors. In addition, oxidative phosphorylation is uncoupled with a lowering of the P:O ratios. Randomly selected liver cells have been analyzed by quantitative morphometric techniques. The average mitochondrial volume is increased fourfold in the peripheral and midzonal regions with a commensurate decrease in the number of mitochondria per cell. These alterations are present throughout the hepatic lobule, but are most marked in midzonal cells. The total mitochondrial volume per cell and the per cent of the total cytoplasmic volume occupied by mitochondria remains relatively unaltered, as does the total amount of cristae surface per cell. While the mitochondria are enlarged, they are not "swollen." The relationships between the steroid hormone treatment and the alterations in mitochondrial function and structure are discussed.     

Ionic Osmoregulation during Salt Adaptation of the Cyanobacterium Synechococcus 6311

The mechanisms of salt adaptation were studied in the cyanobacterium Synechococcus 6311. Intracellular volumes and ion concentrations were measured before and after abrupt increases of external NaCl concentrations up to 0.6 molar NaCl. Equilibrium volumes, measured with a rapid and accurate electron spin resonance spin probe method, showed that at low NaCl concentrations the cells did not shrink as expected for an impermeable solute. However, when the NaCl concentration exceeded a critical value, volume losses occurred. These losses were not fully reversed by hypoosmotic treatment, suggesting membrane damage. The critical value of irreversible volume loss paralleled the increase in salinity during cell growth. Rapid mixing experiments showed that exposure of Synechococcus 6311 to non-damaging NaCl concentrations caused water extrusion from the cells; the volume decreases were time resolved to about 200 milliseconds. Subsequently, volumes increased rapidly as NaCl moved into the cells. Controls recovered their volumes within 15 seconds, while salt-adapted cells grown at 0.6 molar NaCl required 1 minute for volume equilibration. This decrease in the rate of cell volume recovery indicates that salt adaptation is accompanied by changes in cell membrane properties. Subsequent to these initial rapid volume changes, a more gradual sequence of ion movement and sugar accumulation was observed. Under conditions for photoautotrophic growth, significant Na⁺ extrusion was observed 30 min after salt shock. Sucrose accumulation reached a maximum value after 16 hours and K⁺ accumulation reached equilibrium after 40 hours. The final concentrations of K⁺ and Na⁺ and sucrose and glucose inside the 0.6 molar NaCl-grown cells indicate that the inorganic ions and organic `compatible' solutes are the major osmotic species which account for the adaptation of Synechococcus 6311 to salt.     

Thyroid C cells of middle-aged rats treated with estradiol or calcium

 The structure and function of C cells of middle-aged female rats (14-months old) treated with estradiol dipropionate (EDP), calcium (Ca) or a combination of EDP+Ca were studied. A stereological method was used to determine the volume of calcitonin (CT)-immunoreactive C cells and their nuclei, and the relative volume density and mean number of the C cells per section were calculated. Serum levels of CT, osteocalcin, parathyroid hormone (PTH), and β-estradiol were also measured. A significant decrease in body weight of the rats treated with EDP or EDP+Ca was observed. These treatments led to a significant decrease in cellular and nuclear volumes, relative volume density, and mean number of C cells per section, in comparison with the corresponding controls. A reduction of the serum level of CT, PTH, and osteocalcin was also recorded in EDP- and EDP+Ca-treated animals. No statistically significant differences between Ca- and vehicle-injected rats, with regard to all morphometric C cell parameters and biochemical values determined, were seen. However, a conspicuous degranulation of the C cells and decreased immunoreactivity for CT in the Ca-receiving group, which could be interpreted as the signs of increased activity of these cells, were noticed. This effect of Ca was also observed in rats injected with EDP and Ca in combination, when the inhibitory effect of EDP on C cell function was less noticeable than in the group treated with EDP alone. Accepted: 29 July 1997     

STUDIES ON THE DIVISION CYCLE OF MAMMALIAN CELLS : V. Modifications of Time Parameters by Different Steady-State Culture Conditions

In cultures of murine neoplastic mast cells, the duration of different phases of the division cycle (G₁, S, G₂, and mitosis [M]) was determined under optimal and several well-defined suboptimal growth conditions. Two methods of evaluation were applied to the same culture system: first, the relative number of G₁, S, G₂, and M cells was determined by pulse labeling of samples with thymidine-³H and subsequent radioautography in conjunction with a microfluorometric technique permitting rapid measurements of cellular DNA content; second, after pulse labeling with thymidine-³H, the variations with time of the mitotic labeling index were analyzed. Suboptimal culture conditions were obtained by reducing the concentration of single essential medium components (leucine, glucose, or serum) or by the addition of specific metabolic inhibitors (actinomycin D, amethopterin). Growth-limiting culture conditions resulted in increased generation times. Even under control conditions, the cell number doubling time exceeded the generation time, and this difference was more pronounced in suboptimal media. Under most of the suboptimal conditions tested, the increase in generation time was attributable primarily to an extended duration of the G₁ phase. Under certain growth-limiting conditions, however, other phases were also prolonged. In addition, the variabilities of the generation time and of certain cell cycle phases were increased under suboptimal culture conditions. Results obtained by the two methods of evaluation were, in general, in good agreement with each other. Some differences were, however, observed and interpreted in terms of cell death and/or asymmetric frequency distributions of cell cycle parameters.