Cell cycle-dependent protein secretion by Saccharomyces cerevisiae.

Synchronized Saccharomyces cerevisiae cell populations were used to examine secretion rates of a heterologous protein as a function of cell cycle position. The synchronization procedure had a profound effect on the type and quality of data obtained. When cell synchrony was induced by cell cycle-arresting drugs, a significant physiological perturbation of cells was observed that obscured representative secretion data. In contrast, synchronization with centrifugal elutriation resulted in synchronized first-generation daughter cells with undetectable perturbation of the physiological state. The synchronized cells did not secrete significant amounts of protein until they reached cell division, suggesting that the secretion process in these cells is strongly cell cycle dependent. However, the maximum secretion rate of the synchronized culture (7-14 molecules/cell/second) was significantly lower than that of an asynchronous culture (29-51 molecules/cell/second). This result indicates that young daughter cells isolated in the synchronization process exhibit different protein secretion behavior than older mother cells that are absent in the synchronized cell population but present in the asynchronous culture.     

Hormone-stimulated secretion of luteinization factor in porcine granulosa cells.

Luteinization stimulator (LS) is an intrafollicular compound which was shown to be released by granulosa cells under in vitro conditions with stimulatory effects on immature granulosa cell differentiation. This study was undertaken to determine the effects of various endocrine agents which are involved in the regulation of ovarian function on LS secretion by porcine granulosa cells isolated from 5-8-mm follicles (LGC). Cell conditioned media (CM) obtained after the 4-day culture of LGC were tested in the culture of immature (small) granulosa cells (SGC). The activity of LS released into the LGC conditioned medium was estimated by measuring progesterone (P4) produced by SGC in the presence of CM. Stimulation of P4 secretion was observed after addition of media from cultures treated by LHRH (10(-4) mol.l-1), epinephrine (10(-5) mol.l-1), LH (1 microgram.ml-1), dbcAMP (0.5 and 2.0 micrograms.ml-1) or insulin (1.0-5.0 micrograms.ml-1). Norepinephrine (10(-5) and 10(-7) mol.l-1), estradiol (0.1 and 1.0 microgram.ml-1) and prolactin (0.1 and 1.0 microgram.ml-1) did not change steroidogenic activity of CM. Epinephrine and norepinephrine (10(-5) and 10(-7) mol.l-1), LH (1 microgram.ml-1), dbcAMP (2.0 microgram.ml-1) and estradiol (1 microgram.ml-1) alone enhanced P4 production by SGC, whereas LHRH (10(-3) and 10(-4) mol.l-1), insulin (1.0-5.0 microgram.ml-1) and prolactin (0.1 and 1.0 microgram.ml.-1) did not have any effect. These observations suggest that the process of LS secretion in developing follicles is subject to a specific hormonal control.     

Pattern of chromosomal replication in synchronized lymphocytes

Summary The BrdUrd-Hoechst 33258-Giemsa technique was employed to study patterns of chromosomal replication in human lymphocytes synchronized by Methotrexate (MTX). It is proposed that in the presence of MTX, a major portion of the cell population is blocked in an advanced stage of the S-phase and not in the G₁/S border of the cell cycle. At this point, the replication of the chromosomal segments corresponding to the R-bands is terminated, and the replication of the G-bands and the inactive X-chromosomes is being initiated. The use of this method in the study of higher resolution patterns of chromosomal replications is proposed.     

Cell Synchrony Techniques. I. A Comparison of Methods

Abstract Selected cell synchrony techniques, as applied to asynchronous populations of Chinese hamster ovary (CHO) cells, have been compared. Aliquots from the same culture of exponentially growing cells were synchronized using mitotic selection, mitotic selection and hydroxyurea block, centrifugal elutriation, or an EPICS V cell sorter. Sorting of cells was achieved after staining cells with Hoechst 33258. After synchronization by the various methods the relative distribution of cells in G₁ S, or G₂+ M phases of the cell cycle was determined by flow cytometry. Fractions of synchronized cells obtained from each method were replated and allowed to progress through a second cell cycle. Mitotic selection gave rise to relatively pure and unperturbed early G₁ phase cells. While cell synchrony rapidly dispersed with time, cells progressed through the cell cycle in 12 hr. Sorting with the EPICS V on the modal G₁ peak yielded a relatively pure but heterogeneous G₁ population (i.e. early to late G₁). Again, synchrony dispersed with time, but cell-cycle progression required 14 hr. With centrifugal elutriation, several different cell populations synchronized throughout the cell cycle could be rapidly obtained with a purity comparable to mitotic selection and cell sorting. It was concluded that, either alone or in combination with blocking agents such as hydroxyurea, elutriation and mitotic selection were both excellent methods for synchronizing CHO cells. Cell sorting exhibited limitations in sample size and time required for synchronizing CHO cells. Its major advantage would be its ability to isolate cell populations unique with respect to selected cellular parameters.     

Modulation of fibroblast response to maitotoxin along the cell division cycle

Maitotoxin (MTX) induces an increase of [Ca²⁺]_i and of phosphoinositide breakdown in various cell types. The [Ca²⁺]_i increase followed with fluorescent probes on cell suspensions has been described as slow and lasting, in contrast to the signal induced by calcium ionophores such as ionomycin. MTX effects have been studied on two fibroblastic cell lines, BHK21 C13 and FR 3T3, synchronized by serum deprivation treatment performed in an isoleucine-free medium for BHK21 C13 cells. In BHK21 C13 cells, flow cytometry analysis showed that two stages, G1/S and G2/M, were particularly susceptible to MTX treatment. Scanning laser cytometry demonstrated that calcium response of FR 3T3 fibroblasts followed with Indo-1 varied during the cell division cycle. The [Ca²⁺]_i increase was almost always vertical, but its delay after MTX addition lasted from zero (S and G2/M transition) to 10–20 min (G1) or more (G2). No [Ca²⁺]_i change could be detected during mitosis. The [Ca²⁺]_i response at the S phase was biphasic. These observations suggest that (1) the lasting response described in the literature represents a global cell population effect, and (2) cells are more sensitive to MTX at specific stages of the cell division cycle, which could correspond to periods when calcium signals have been detected in different cell types.Abbreviations MTX maitotoxin - [Ca²⁺]_i intracellular calcium concentration - IP₃ inositol triphosphate     

Transient inhibition of DNA synthesis by 5-fluorodeoxyuridine leads to overexpression of dihydrofolate reductase with increased frequency of methotrexate resistance

Transient but incomplete suppression of DNA synthesis by a single exposure of an asynchronous population of cells to 5-fluoro-2'-deoxyuridine (FdUrd) increases the frequency of appearance of methotrexate (MTX)-resistant colonies. This increase was greater than 10-fold following a 6-h incubation of cells with 3 microM FdUrd prior to selection in MTX, an interval one-half the normal L1210 cell cycle time. During this period of exposure to FdUrd, DNA synthesis decreased to 25% of control rates and cells accumulated at the G1/S interface. The 6-h incubation with FdUrd resulted in greater than a 2.5-fold increase in the dihydrofolate reductase protein level in the treated cell population, which was accounted for, at least in part, by increased de novo synthesis of the enzyme as assessed by [35S]methionine labeling. This increase in dihydrofolate reductase was associated with a decrease in growth inhibition by MTX. A brief reversal (2 h) of FdUrd-induced DNA synthesis inhibition by the addition of thymidine eliminated the amplification of dihydrofolate reductase and the enhanced emergence of MTX-resistant clones. Beyond this, an analysis of clones that survive MTX selection indicates that the dihydrofolate reductase gene copy in cells spontaneously resistant to 50 nM MTX and those which resulted after the additional pretreatment with FdUrd for 6 h are comparable with a 2-4-fold amplification of enzyme in most clones. These studies demonstrate that FdUrd enhancement of dihydrofolate reductase expression can have a profound effect upon the incidence and expression of MTX resistance and that dihydrofolate reductase gene amplification may be another basis for antagonism between these agents.     

Limitations to the amplification and stability of human tissue-type plasminogen activator expression by Chinese hamster ovary cells

Chinese hamster ovary cell production of recombinant tissue-type plasminogen activator (t-PA) was increased by amplification of cotransfected dihydrofolate reductase cDNA using stepwise adaptation to increasing methotrexate (MTX) concentrations. The highest producing clones were isolated at 5 microM MTX and yielded 26,000 U/10(6) cells/day t-PA (43 microgram/10(6) cells/day). Above 25 microM MTX, cell specific t-PA production rates became increasingly variable and the cDNA copynumbers decreased. No apparent correlation between the cell specific t-PA production rate and the growth rate was observed upon subcloning of the amplified cells. When MTX selection was removed, the t-PA production rate decreased up to tenfold within 40 days; this was accompanied by an up to 60% drop in cDNA copynumber. Subclones isolated after 108 days of culture in the absence of MTX were, on average, sixfold more stable than their parental cells. In culture without MTX, the maximum stable t-PA production rate obtained (over 250 days) was 7000 +/- 750 U/10(6) cells/day (approximately 12 microgram/10(6) cells/day), approximately threefold lower than the maximum unstable levels of production reached under selective pressure. Taken together, these results define a wide range of the highest t-PA expression rates obtained under MTX selection, for which stable expression without selection has not been reported.     

Bursal anti-steroidogenic peptide (BASP): modulation of mitogen-stimulated bursal-lymphocyte DNA synthesis

The present study examined the effects of bursal anti-steroidogenic peptide (BASP) on mitogen-induced DNA synthesis in bursa-derived B-lymphocytes in short-term culture. Partially purified extracts of chicken bursa of Fabricius tissue, containing BASP, significantly (P < 0.05) reduced DNA synthesis in bursal-lymphocytes exposed to increasing concentrations of phorbol 12,13-dibutyrate (PDB). Following these initial observations, BASP, further purified from bursal extracts using sequential rpHPLC fractionation, was observed to reduce (P < 0.05) both B-lymphocyte PDB-stimulated DNA synthesis and ovarian granulosa cell progesterone biosynthesis with bioactivity observed at similar retention times in each assay, suggesting that each bioactivity may be due to the same or similar molecules. A similar BASP-enriched fraction was not effective in altering basal levels of DNA synthesis in chick embryonic kidney cells. Subsequently, BASP was further purified by several sequential chromatographic methods including: C-18 rpHPLC (preparative rpHPLC followed by a semi-preparative rpHPLC column), cation exchange chromatography, molecular sieve HPLC chromatography, and SDS-PAGE. Biologically active material was observed at approximately 29 or 34 kDa. Protein concentration was determined and bioactivity was evaluated. Anti-proliferative effects of this partially purified BASP on bursal-lymphocytes was observed at concentrations as low as 1.6 micrograms ml-1, with complete suppression of mitogen-stimulated DNA synthesis observed at approximately 25 micrograms ml-1. This partially purified BASP was also efficacious for attenuation of ovarian granulosa cell progesterone biosynthesis at concentrations as low as 0.4 microgram ml-1, with complete suppression of gonadotrophin-stimulated progesterone biosynthesis observed at approximately 0.8 microgram ml-1. While BASP is efficacious for attenuation of both granulosa cell steroidogenesis and bursal-lymphocyte proliferation, these data suggest that BASP is much more potent with regard to anti-steroidogenic activity.     

Biodegradation of trichloroethylene and toluene by indigenous microbial populations in soil.

The biodegradation of trichloroethylene (TCE) and toluene, incubated separately and in combination, by indigenous microbial populations was measured in three unsaturated soils incubated under aerobic conditions. Sorption and desorption of TCE (0.1 to 10 micrograms ml-1) and toluene (1.0 to 20 micrograms ml-1) were measured in two soils and followed a reversible linear isotherm. At a concentration of 1 micrograms ml-1, TCE was not degraded in the absence of toluene in any of the soils. In combination, both 1 microgram of TCE ml-1 and 20 micrograms of toluene ml-1 were degraded simultaneously after a lag period of approximately 60 to 80 h, and the period of degradation lasted from 70 to 90 h. Usually 60 to 75% of the initial 1 microgram of TCE ml-1 was degraded, whereas 100% of the toluene disappeared. A second addition of 20 micrograms of toluene ml-1 to a flask with residual TCE resulted in another 10 to 20% removal of the chemical. Initial rates of degradation of toluene and TCE were similar at 32, 25, and 18 degrees C; however, the lag period increased with decreasing temperature. There was little difference in degradation of toluene and TCE at soil moisture contents of 16, 25, and 30%, whereas there was no detectable degradation at 5 and 2.5% moisture. The addition of phenol, but not benzoate, stimulated the degradation of TCE in Rindge and Yolo silt loam soils, methanol and ethylene slightly stimulated TCE degradation in Rindge soil, glucose had no effect in either soil, and dissolved organic carbon extracted from soil strongly sorbed TCE but did not affect its rate of biodegradation.     

Population-dependent requirements of vitamin B12 and metabolically related substances of several mouse cell types in serum-free, albumin-fortified medium

Mouse FM3A cells propagated well in serum-free medium containing 0.5% serum albumin and 1 microgram of insulin/ml. The vitamin B12 (B12) requirement of the cells depended on the population density. This requirement disappeared when a sufficiently large cell population was present. A combination of 1-100 ng of B12/ml and 4 micrograms of hypoxanthine/ml resulted in a synergistic increase in cell growth at low cell densities. A similar growth response was obtained when the B12 plus hypoxanthine was replaced by 4 micrograms of hypoxanthine/ml in combination with 100 ng of thymidine/ml, 1 microgram of folic acid/ml or 1 microgram of folinic acid/ml, even though 1 microgram of folic acid/ml already was present in the medium. Experiments on single cell inoculation showed that colony size and the yield of cells grown in B12-supplemented medium were much larger than those for cells grown in B12-free medium. A more critical population-dependent B12 requirement was demonstrated in mouse Ehrlich and L cells and their hybrids. At less than 100 cells there was no propagation in serum-free medium lacking B12, folinic acid and thymidine; whereas, a satisfactory growth response was obtained in medium supplemented with these substances.     

Synchronization of human leukemic cells: Relevance for high-resolution chromosome banding

Summary The cell-cycle kinetics of synchronized K562 human leukemic cells and bone marrow cells from adults with acute leukemia were studied in order to develop more reliable methods for producing increased numbers of mitoses, particularly those with elongated chromosomes suitable for high-resolution banding. Parameters examined included DNA content, mitotic index (MI), and chromosome preparations. K562 cells synchronized with methotrexate (MTX), thymidine (Tdr), or hydroxyurea (HU) showed two-fold increases in peak MI. Optimal harvesting times after release from block were approximately 10.5, 12.5, and 14.5 h for MTX, HU, and Tdr, respectively. MTX was selected for studies with cells from patients. Cells from 7 of the 10 patients studied showed 4.4-fold increases in peak MI. The optimal harvesting time was 9.5 to 11.5 h after release from block, considerably later than the 6 h time previously assumed in studies using stimulated lymphocytes. Cells from the three remaining patients showed no increase in MI after synchronization: and the lack of response may have been related to the high proportion of cells in G₀+G₁ prior to MTX exposure. For both the K562 cell line and most patient specimens, the combination of synchronization with appropriate release times and short Colcemid exposure (10 min) resulted in substantially improved chromosome preparations.     

Cell synchrony techniques. I. A comparison of methods

Selected cell synchrony techniques, as applied to asynchronous populations of Chinese hamster ovary (CHO) cells, have been compared. Aliquots from the same culture of exponentially growing cells were synchronized using mitotic selection, mitotic selection and hydroxyurea block, centrifugal elutriation, or an EPICS V cell sorter. Sorting of cells was achieved after staining cells with Hoechst 33258. After synchronization by the various methods the relative distribution of cells in G1, S, or G2 + M phases of the cell cycle was determined by flow cytometry. Fractions of synchronized cells obtained from each method were replated and allowed to progress through a second cell cycle. Mitotic selection gave rise to relatively pure and unperturbed early G1 phase cells. While cell synchrony rapidly dispersed with time, cells progressed through the cell cycle in 12 hr. Sorting with the EPICS V on the modal G1 peak yielded a relatively pure but heterogeneous G1 population (i.e. early to late G1). Again, synchrony dispersed with time, but cell-cycle progression required 14 hr. With centrifugal elutriation, several different cell populations synchronized throughout the cell cycle could be rapidly obtained with a purity comparable to mitotic selection and cell sorting. It was concluded that, either alone or in combination with blocking agents such as hydroxyurea, elutriation and mitotic selection were both excellent methods for synchronizing CHO cells. Cell sorting exhibited limitations in sample size and time required for synchronizing CHO cells. Its major advantage would be its ability to isolate cell populations unique with respect to selected cellular parameters.     

Changes in DNA capacity for actinomycin-D binding in nuclei of interphase cells

3H-AMD binding to DNA in interphase nuclei was tested on asynchronous and synchronous LS/BL cell populations under physiological conditions and after exposure to gamma rays (60Co). 3H-AMD binding to DNA in an asynchronous cell population appeared to be nearly constant and independent of 3H-AMD concentration. However, in comparing individual cells, a great variability could be observed. In synchronized cells the DNA accessibility for 3H-AMD binding changed in the course of the cell cycle, with a maximum occurring at the late G1-phase (13.95 X 10(-12) mumol/nucleus) and a minimum at the late G2-phase (2.63 X 10(-12) mumol/nucleus). In irradiated cells the DNA capacity for 3H-AMD binding was growing with the increasing dose (5-80 Gy) from 4.9 to 11.2 X 10(-12) mumol 3H-AMD/nucleus.     

Immunocytochemical study of the action of cis-dichlorodiamino platinum on the human metaphase chromosome

The reaction of cis-dichlorodiamino platinum (cis-Pt) with the human metaphase chromosome was studied using both Giemsa staining and the action of anti-cis-Pt antibody. Two kinds of experiment were performed: (1) cis-Pt was placed in contact with cells in culture (in vivo tests), (2) cis-Pt was placed in contact with chromosomes fixed on slides (in vitro tests). In vivo, the presence of cis-Pt during phase G1 of the cell cycle is important for the reaction; in vitro, competition between cis-Pt and Giemsa for fixation on chromosomal DNA was shown to occur.     

Dna-end binding activity of Ku in synchronized cells

Three different types of cells were synchronized by various methods and DNA-end binding (DEB) activities of Ku were compared with asynchronous controls. In CHO K1 cells synchronized in G1 phase by serum starvation and in S phase by serum refeeding, DEB activity was reduced in S cells but remained unchanged in G1 cells. However, the same type of cells synchronized in G1/S phase by double thymidine block and in S phase by releasing the blockage, have the same DEB activity as asynchronous controls. A similar result was found in RKO and HeLa cells synchronized by the latter method. Arresting cells in mitosis with nocodazole also generated different cell cycle effects. Ku activity was reduced in CHO K1 and RKO cells, but not in HeLa cells after treatment with nocodazole. In phase-enriched cells separated by centrifugal elutriation, DEB activities were similar at different stages of the cell cycle in all three types of cells. Thus, different synchronization procedures can give very different values of Ku activity in a cell type-dependent manner. Results from elutriated cells are consistent, and suggest DEB activity of Ku does not change with the cell cycle.     

Inhibition of Bacillus licheniformis spore growth in milk by nisin, monolaurin, and pH combinations

The effects of nisin and monolaurin, alone and in combination, were investigated on Bacillus licheniformis spores in milk at 37 degrees C. In the absence of inhibitors, germinated spores developed into growing vegetative cells and started sporulation at the end of the exponential phase. In the presence of nisin (25 IU ml-1), spore outgrowth was inhibited (4 log10 reduction at 10 h). Regrowth appeared between 10 and 24 h and reached a high population level (1.25 x 10(8) cfu ml-1) after 7 d. Monolaurin (250 micrograms ml-1) had a bacteriostatic effect during the first 10 h but thereafter, regrowth occurred slowly with a population level after 7 d (4 x 10(5) cfu ml-1) lower than that of nisin. Different combined effects of nisin (between 0 and 42 IU ml-1), monolaurin (ranging from 0 to 300 micrograms ml-1), pH values (between 5.0 and 7.0) and spore loads (10(3), 10(4), 10(5) spores ml-1) were investigated using a Doehlert matrix in order to study the main effects of these factors and the different interactions. Results were analysed using the Response Surface Methodology (RSM) and indicated that nisin and monolaurin had no action on spores before germination; only pH values had a significant effect (P < or = 0.001), i.e. spore count decreased as the pH value increased in relation to germination. Sublethal concentrations of nisin (30 IU ml-1) and monolaurin (100 micrograms ml-1) in combination acted synergistically on outgrown spores and vegetative cells, showing total inhibition at pH 6.0, without regrowth, within 7 d at 37 degrees C.     

Can Nocodazole,an Inhibitor of Microtubule Formation,Be Used to Synchronize Mammalian Cells?

Abstract Nocodazole, a temporary inhibitor of microtubule formation, has been used to partly synchronize Ehrlich ascites tumour cells growing in suspension. the gradual entry of cells into mitosis and into the next cell cycle without division during drug treatment has been studied by flow cytometric determination of mitotic cells, analysing red and green fluorescence after low pH treatment and acridine orange staining. Determination of the mitotic index (MI) by this method has been combined with DNA distribution analysis to measure cell-cycle phase durations in asynchronous populations growing in the presence of the drug. With synchronized cells, it was shown that in the concentration range 0.4–4.0 μg/l, cells could only be arrested in mitosis for about 7 hr and at 0.04 μg/ml, for about 5 hr. After these time intervals, the DNA content in nocodazole-blocked cells was found to be increased, and, in parallel, the ratio of red and green fluorescence was found to have changed, showing entry of cells into a next cell cycle without division (polyploidization). It was therefore only possible to partially synchronize an asynchronous population by nocodazole. However, a presynchronized population, e.g. selected G₁ cells or metabolically blocked G₁/S cells, were readily and without harmful effect resynchronized in M phase by a short treatment (0.4 μg/ml, 3–4 hr) with nocodazole; after removal of the drug, cells divided and progressed in a highly synchronized fashion through the next cell cycle.     

Use of a stationary bed reactor and serum-free medium for the production of recombinant proteins in insect cells.

Insect cells (Spodoptera frugiperda) have been cultured in a stationary bed reactor, packed with a fibrous polyester carrier. When the bioreactor was perfused with serum-supplemented medium, a cell density of 6 x 10(6) cells ml-1 packed carrier was reached. Scanning electron microscopy investigations have shown that the insect cells grew along the three-dimensionally oriented fibers of the Fibra-cel carrier. After infection of the logarithmically growing cells with a recombinant baculovirus (Autographa californica) containing the gene coding for beta-galactosidase, the medium in the bioreactor was changed to serum-free medium. At day 13 postinfection (p.i.), a beta-galactosidase level of 320 microgram ml-1 and, at day 17 p.i., a virus titer of 2.1 x 10(8) TCID50 units ml-1 (day 17 p.i.) were reached. In another bioreactor, operated in a similar way but with serum-containing medium, a beta-galactosidase concentration of 360 microgram ml-1 and a virus titer of 2.3 x 10(8) TCID50 units ml-1 were obtained. These results indicate the potential use of this production system for the production of recombinant protein and baculovirus in insect cells.     

Hydrocortisone stimulation of human mammary epithelial cells

Rapid proliferation of mammary epithelial cells derived from biopsy specimens of human fibroadenomas was observed when medium was supplemented with ten percent fetal bovine serum and hydrocortisone (5 microgram per ml-1). Hydrocortisone in combination with FBS also led to a 2.5-fold increase in cell cluster attachment and subsequent colony formation. A similar effect was not observed with human serum. In contrast to fibroblast cell systems, insulin did not significantly alter cell growth. The results show that a mitogenic response to glucocorticoids by mammary epithelium may depend on the presence of factors in sera.     

Activation of plasminogen by the early bovine embryo

Activation of the plasma zymogen plasminogen to the enzyme plasmin by the early bovine embryo was evaluated. Sixteen-cell embryos to early morulae were collected at death from handmated synchronized and superovulated crossbred beef cows. Embryos were cultured in Ham's F-12 medium supplemented with 15 mg/ml bovine serum albumin containing 0, 15, 30, 60 or 120 micrograms/ml plasminogen in a humidified atmosphere of 5% CO2 in air at 37 degrees C. Cultures were observed every day, and stage of development was recorded. Medium was collected at 24-h intervals, starting at initiation and continuing through 288 h of culture. Plasminogen activator and plasmin levels in the culture media were determined, using a caseinolytic assay. The percentages of embryos developing to the initiating hatching blastocyst, hatched blastocyst, attached blastocyst, and attached blastocyst with trophoblastic outgrowth stages were not significantly different between the five levels of plasminogen. Initiation and completion of hatching, however, accelerated as plasminogen concentration increased in the culture media. Plasminogen activator production, expressed as milliunits X ml-1 X h-1 X viable embryo-1, was low for the first 48 h of culture, increased between 48-120 h, and tended to plateau thereafter. Plasminogen activation, measured indirectly as the plasmin concentration in a microdrop of medium and expressed as microgram plasmin X ml-1 X h-1 X viable embryo-1, followed plasminogen activator production, and was consistently low for the first 48-72 h of culture. Embryonic activation of plasminogen increased sharply thereafter, and also plateaued after 120 h.