In vitro mesenchymal stem cell differentiation after mechanical stimulation

OBJECTIVES

Mesenchymal stem cells (MSC) are multipotent cells capable of differentiating into adipocytic, chondrocytic and osteocytic lineages on suitable stimulation. We have hypothesized that mechanical loading may influence MSC differentiation and alter their phenotype accordingly.

MATERIALS AND METHODS

Mouse bone marrow-derived MSC were established in vitro by differential adherence to plastic culture plates and grown in low glucose medium with 10% foetal calf serum and growth factors. Cells grew out and were subcultured up to 20 times. Differentiation protocols were followed for several cell lineages. Clones with trilineage potential were seeded in type I collagen gels and incubated in a tensioning force bioreactor and real-time cell-derived forces were recorded. Gels were fixed and sectioned for light and electron microscopy.

RESULTS

Cell monolayers of parent and cloned mouse bone marrow-derived MSC differentiated into adipocytes, osteocytes and chondrocytes, but not into cardiomyocytes, myotubes or neuronal cells. When cast into type I collagen gels and placed in tensioning bioreactors, MSC differentiated into fibroblast-like cells typical of tissue stroma, and upregulated α-smooth muscle actin, but rarely upregulated desmin. Electron microscopy showed collagen and elastin fibre synthesis into the matrix.

CONCLUSIONS

These experiments confirmed that MSC cell fate choice depends on minute, cell-derived forces. Applied force could assist in commercial manufacture of cultured bio-engineered prostheses for regenerative medicine as it mimics tissue stresses and constitutes a good model for development of tissue substitutes.

Intestinal metaplasia in liver of rats after partial hepatectomy and treatment with acetylaminofluorene

OBJECTIVES

The liver is widely recognized for its ability to self-regenerate after damage. Hepatocyte replication is the primary source of liver restoration, although hepatic stem cells (of one kind or another) may be a secondary font, only brought into effect when primary regeneration is severely compromised.

MATERIALS AND METHODS

In experiments using small rodents, such an injury can be inflicted by surgically removing a large portion of the liver followed by treatment with hepatotoxin 2-acetylaminofluorene. Regeneration by hepatocyte replication is blocked and thus, stem cell involvement is promoted. However, other responses may be stimulated and this study describes the presence of mucinous glandular structures in the healing liver after two-thirds of its volume was removed via hepatectomy followed by treatment with 2-acetylaminofluorene.

RESULTS

Unique observation of intestinal metaplastic cells was seen under alcian blue/periodic acid-Schiff staining.

CONCLUSION

The existence of this phenotype (along with oval cells and small hepatocyte-like cells) is evidence of multipotency of progenitors involved in the hepatic healing response.

Cell proliferation rates in an artificial tissue-engineered environment

Worldwide, and particularly in Europe, Japan and the USA, cardiovascular disease is a major killer. It can be treated using tissue or organ transplant surgery, but donor organs may be scarce. Tissue engineering is the integration of engineering principles and biology to produce satisfactory synthetic replacement body parts, using viable cells in a suitable matrix, for regenerative medicine. The aim of this study was to measure and compare cell proliferation kinetics after different time intervals of myofibroblasts in a synthetic matrix, thus to be able to deduce the period that a transplanted-cell population can be expected to survive in a tissue-engineered environment. Porcine aortic wall cells were grown in a porous sponge scaffold, that later could be fashioned into aortic or heart valve substitutes. Freshly acquired cells were seeded on identical sponges and were grown under normal culture conditions for a period of 4 weeks. Seeding concentration was a million cells per sponge. Cells progressively populated the sponges, both covering the surface and infiltrating the depth of the matrix, via sponge pores. Samples were taken at 1 week and at 4 weeks, and the rate of cell proliferation was determined by the metaphase arrest technique. Specimens were also taken for light and electron microscopy to determine whether these transplanted cells were capable of synthesizing their own extracellular matrix.

Heart valve and arterial tissue engineering

In the industrialized world, cardiovascular disease alone is responsible for almost half of all deaths. Many of the conditions can be treated successfully with surgery, often using transplantation techniques; however, autologous vessels or human-donated organs are in short supply. Tissue engineering aims to create specific, matching grafts by growing cells on appropriate matrices, but there are many steps between the research laboratory and the operating theatre. Neo-tissues must be effective, durable, non-thrombogenic and non-immunogenic. Scaffolds should be bio-compatible, porous (to allow cell/cell communication) and amenable to surgery. In the early days of cardiovascular tissue engineering, autologous or allogenic cells were grown on inert matrices, but patency and thrombogenicity of grafts were disappointing. The current ethos is toward appropriate cell types grown in (most often) a polymeric matrix that degrades at a rate compatible with the cells' production of their own extracellular matrical proteins, thus gradually replacing the graft with a living counterpart. The geometry is crucial. Computer models have been made of valves, and these are used as three-dimensional patterns for mass-production of implant scaffolds. Vessel walls have integral connective tissue architecture, and application of physiological level mechanical forces conditions bio-engineered components to align in precise orientation. This article reviews the concepts involved and successes achieved to date.

E1B-deleted adenovirus (dl1520) gene therapy for patients with primary and secondary liver tumors

Clinical studies were performed with a recombinant mutant adenovirus with an E1B 55-kDa deletion, dl1520, to assess its toxicity and efficacy in patients with irresectable primary and secondary liver tumors. A phase I study showed that dl1520 was well tolerated when administered directly intratumorally, intraarterially, or intravenously up to a dose of 3 x 10(11) PFU. Ultrastructural examination of tissue showed the presence of adenovirus in cell cytoplasm around the nucleus and revealed two dissimilar end points of cell death after virus infection: a preapoptotic sequence and necrosis. A phase II study showed that the combination of dl1520 and 5-fluorouracil (5-FU), when infused into the hepatic artery, was well tolerated. Further improvement in the recombinant vector design will be needed in order to achieve better clinical response.

Detection of adenovirus and initiation of apoptosis in hepatocellular carcinoma cells after Ad-p53 treatment

Transcription of the p53 gene can regulate progression of apoptosis in a wide variety of tissues. Three categories of human hepatocyte culture have been used to show the initiation of apoptosis after treatment with p53-bearing adenovirus. Chang liver cells are derived from normal liver tissue and express native p53, whereas hepatocellular carcinoma (HCC)-derived cell lines were Hep3B (p53-deleted) and PLC/PRF/5 (p53-mutant). Cultures were infected with Ad-p53 (15 particles per cell; 36 hours), and after treatment, morphological changes in all cell categories were observed by electron microscopy. Infection was evident in the cytoplasm of all treated cell types: after entry across the plasma membrane viruses translocated and came to rest surrounding and adjacent to nuclei, cytoplasm proximal to nuclear membranes became dense with virus- and membrane-derived debris, but intact viruses did not enter nuclei. Apoptosis, recognized morphologically by characteristic chromatin and cytoplasmic condensation, occurred more frequently in HCC-derived cells, and the ultimate fate of apoptotic bodies was phagocytosis and degradation by neighboring cells.

Immunolabeling for electron microscopy

The protocols in this chapter concern postembedding immunolabeling for transmission electron microscopy; other schedules, such as pre-embedding methods, frozen tissue processes, and procedures for scanning electron microscopy, can be found elsewhere (1). In principle, immunolabeling at the electron microscope (EM) level follows the same precepts as immunolabeling at the light microscope level; in tissues or cells, the location of an antigen of interest is identified by a specific antibody, and must be visualized appropriately for investigation. Electron microscopy permits us to distinguish subcellu-lar organelles, and therefore ultrastructural localization of antigen position. At the EM level, however, the "visualizing step" needs to be provided by an electron-dense entity, most often a heavy metal, which reflects incident electrons; this is in contrast to the final step of light microscope level techniques, in which the final reaction product is sought to be colored (and where there is an element of choice of which color to use). Tissue processing for EM is considerably more severe than that for light microscopy, and thus maintenance of antigenicity in tissue is more taxing. Before immunolabeling for electron microscopy can be fruitful, the first step is to ensure that the antigen of interest is present (or is still present) in the tissue; this is done by performing a thorough procedure at the light microscope level on wax-embedded sections. Once a positive result has been obtained, studies can progress to the ultrastructural level. If the presence of an antigen cannot be demonstrated in a wax-embedded block, it will not be demonstrable in a resin-embedded EM block of the same tissue. In such a case, pre-embedding and frozen tissue techniques can be of use at both light and electron microscope levels.

Electron microscopic assessment of adenovirus-mediated transfer

This chapter describes the method of preparing and observing hepatocellular carcinoma (HCC) and surrounding normal liver cells infected with therapeutically p (53)-transfected adenovirus (Ad-p (53)), so that morphology of the cells and viruses, and crucially their relationships to each other, are revealed. In standard practice, ultrastructural analysis of viruses carried in body fluids (e.g., stool or mucus) is sufficient for diagnosis, using the technique of phosphotungstate - dark field staining-of the aqueous extract. That method, however, is not suitable when one needs to examine precise subcellular location of viruses in situ, with tissue and cells intact, for complete pathological assessment; here, we describe our method (1) for transmission electron microscopy of the ultrastructure of virus-infected tumors. Tissue fixation, osmication, embedding, section cutting, and observation of Ad-p (53) infection will be included.

Hepatic extramedullary granulopoiesis after treatment with liver-stimulatory xenobiotics, in lpr mice

This study demonstrates that extramedullary hematopoiesis occurs in livers of adult lpr mice and, after treatment with each of three xenobiotic compounds--phenobarbital, cyproterone acetate, and nafenopin--it includes granulopoiesis. lpr mice are used as a model of the human disease systemic lupus erythematosus (SLE). The develop a syndrome very similar to that of human sufferers. In untreated lpr mice, mononuclear white blood cells were discernible in hepatic sinusoidal foci; T and B lymphocytes were distinguished from each other by immunocytochemistry at light microscope level. After treatment with any of the xenobiotic compounds, immunolabeling demonstrated the additional presence of granulocytes in foci, and, at electron microscope level neutrophils, eosinophils and their precursors were clearly recognizable.

Pericardial substitution after cardiopulmonary bypass surgery: a trial of an absorbable patch

Primary closure of the pericardium affords some protection against adhesion formation and the consequent hazards of resternotomy. However, its completion may be impractical and hazardous, and therefore the pursuit of an ideal pericardial substitute has prompted much research. Twenty calves were divided into 3 groups for the study. All animals underwent right posterolateral thoracotomy. The test group (group X), consisting of 6 animals, received a poly-beta-hydroxybutyrate patch (PHB) to close the pericardium following cardiopulmonary bypass (CPB). In group Y (9 animals) the pericardium was left open following CPB. Group Z (5 animals) also had their pericardium left open but did not undergo CPB (non-CPB). The plasminogen activating activity (PAA) of homogenates of pericardial tissue samples were measured in 5 animals in group X, and 5 in group Z. Samples were taken at three time points from the time of pericardiotomy, and at reoperation 4 weeks later. In group X (CPB) there was a significant reduction in the PAA during the operation with some recovery at reoperation. The reduction in the pericardial PAA of group Z (non-CPB) animals did not reach significance. For both group X and group Z the progress of mesothelial damage, compared with that at zero time, showed a significant increase. In addition, their pericardial inflammatory features became more apparent in the later samples but more significantly in group Z. This study demonstrated no significant short-term differences in adhesion formation or postoperative coronary anatomy visibility between any of the groups. At reoperation the patch material contained pronounced macrophage activity but no regenerative mesothelium. There were no infective episodes in any of the animals studied. Furthermore, this study suggests that CPB in comparison to non-CPB has a significant affect on pericardial PAA.

Liver stem cells: when the going gets tough they get going

The ability of the liver to regenerate is widely acknowledged, and this is usually accomplished by the entry of normally proliferatively quiescent hepatocytes into the cell cycle. However, when hepatocyte regeneration is impaired, small bile ducts proliferate and invade into the adjacent hepatocyte parenchyma. In humans and experimental animals these ductal cells are referred to as oval cells, and their association with defective regeneration has led to the belief that they are the progeny of facultative stem cells. Oval cells are of great biological interest since they may represent a target population for hepatic carcinogens, and they may also be useful vehicles for ex vivo gene therapy for the correction of inborn errors of metabolism. The ability of oval cells to differentiate into hepatocytes has been demonstrated unequivocally. However, this process only occurs when the regenerative capacity of hepatocytes is overwhelmed, and thus, unlike the intestinal epithelium, the liver is not behaving as a classical continually renewing stem cell-fed lineage.

Wild-type p53 induces apoptosis in Hep3B through up-regulation of bax expression

We demonstrated that introduction and expression of wild-type p53 gene in the human hepatocellular carcinoma cell line, Hep3B, resulted in up-regulation of both p21WAF1/CIP1 and bax gene expression and apoptosis. This cell line contains integrated hepatitis B virus sequences and lacks the expression of both p53 and retinoblastoma tumor suppressor genes because of deletions. Our results suggest that whereas an increased level of bax expression mediates apoptosis, an increased level of p21WAF1/CIP1 expression does not induce arrest of cell growth, presumably because of the deletion of the retinoblastoma gene. This study also confirms reported observations that p53 is a tumor suppressor gene, which induces apoptosis in malignant cells that lack normal p53 activity because of mutation, deletion, or inactivation of the gene by the presence of oncogenic viral proteins.

Reversal of phenobarbital-induced hyperplasia and hypertrophy in the livers of lpr mice

Fas is a cell surface receptor that mediates apoptosis, and Fas mRNA has been demonstrated in hepatocytes. MRL/MP-lpr/lpr mice carry the mutated lymphoproliferation-associated gene, lpr, that codes for truncated Fas protein, resulting in reduced apoptotic potential in some circumstances. Phenobarbital treatment of experimental animals induces cytochrome P450 enzymes, and thus acts as a growth stimulus to the liver with both hyperplasia and hypertrophy; cessation results in reversion of liver to normal size with apoptosis playing a role. This study has determined the respective contributions of atrophy and apoptosis to this involution in Fas-defective and normal-FAs bearing animals. Between the first day and the fifth day after phenobarbital cessation, the weights of both Fas-defective (lpr/lpr) livers and control (lpr/+) livers reduced. Hepatocyte hypertrophy gradually reverted in both categories of mouse and this was the greater contribution to reduction in liver size. In lpr/lpr animals, there was a consistent level of apoptosis which remained relatively constant, while numbers of apoptotic cells in control livers increased over the period. This investigation has shown that in liver, a mechanism to execute apoptosis is operative even in Fas-defective mice, but it is not sensitive to signals activated by the removal of the growth stimulus. This is in contrast to mice which can mount a Fas-mediated response; thus a separate apoptotic pathway is indicated.

Effect of total hepatic vascular exclusion during liver resection on hepatic ultrastructure

The aim of this investigation was to observe ultrastructural changes in the liver in response to warm ischemia during liver surgery. In 11 noncirrhotic patients, hepatic resection was performed under total vascular exclusion (TVE). The mean duration of warm ischemia was 28 minutes (range 16-48 minutes). Three specimens were taken from each patient: before clamping, at the end of TVE, and after reperfusion. Biopsy specimens were studied by light microscopy and by transmission electron microscopy (EM). At the end of the ischemic phase sinusoids were collapsed with resultant loss of normal hepatic architecture. Morphological changes to hepatocytes included focal chromatin condensation at the nuclear margins, distended nuclear envelope, and swelling of both mitochondria and endoplasmic reticulum. After reperfusion these changes reversed. The phenomenon of sinusoidal and hepatocellular recovery after TVE was seen in all the cases of this study, irrespective of age, sex, disease, type and severity of surgical intervention, and duration of TVE. It can be concluded that TVE over a period of 48 minutes has no irreversible deleterious effects on the ultrastructure of the noncirrhotic liver.

An analysis of apoptosis in lymphoid organs and lupus disease in murine systemic lupus erythematosus (SLE)

Apoptosis is a programmed cell death process that helps to regulate both T cell and B cell development. In this study, we have investigated the levels of apoptotic death in cells of the thymuses and spleens (white matter) of autoimmune MRL-lpr/lpr mice with progressive lymphadenopathy and SLE disease activity; we also examined the renal pathology in these animals. Fas is a cell surface receptor, which when activated initiates the sequence of events that lead to apoptosis. In MRL-lpr/lpr mice Fas is defective, so the competency for apoptosis may be reduced. In young animals of advancing age the thymuses enlarged until in 5-month-old females the average weight was three times that at 1 month, and spleen and kidney weights also increased in size disproportionately. At light microscope level apoptotic cells in tissue sections were counted using both routine eosin and haematoxylin staining (to identify them by their morphology) and in situ end-labelling of cells with DNA strand breaks; their presence was further confirmed by electron microscopy. As the mice aged, the numbers of apoptotic cells in thymic cortex, thymic medulla and spleen white pulp areas reduced significantly (P < 0.01-0.001), whereas in BALB/c normal controls they increased significantly (P < 0.05). These changes were coincident with the development of severe lupus, whose activity was assessed by measuring serum anti-ssDNA and anti-dsDNA antibody titres and urinary protein (albumin) level which were elevated significantly by 5 months of age (P < 0.001 for both ssDNA and dsDNA and P < 0.01 for urine albumin) compared with their younger counterparts. Thus, lymphoid organ enlargement, decrease in apoptotic indices, elevated serum anti-ssDNA and anti-dsDNA antibody levels, and impaired renal function coincided with the onset and severity of lupus disease in lpr mice. It seems likely that there is a causal relationship between defective deletion of autoreactive lymphoid cells, imperfect Fas-mediated apoptosis and development of murine SLE.

Pluripotential liver stem cells: facultative stem cells located in the biliary tree

The ability of the liver to regenerate after parenchymal damage is usually accomplished by the ephemeral entry of normally proliferatively quiescent (G0) hepatocytes into the cell cycle. However, when hepatocyte regeneration is defective, arborizing ductules which are continuous with the biliary tree, proliferate and migrate into the surrounding parenchyma. In man these biliary cells have variously been referred to as ductular structures, neoductules and neocholangioles, and have been observed in many forms of chronic liver disease, including cancer. In experimental animals similar ductal cells are usually called oval cells, and their association with defective regeneration has led to the belief that these cells represent a progenitor cell population. Oval cells are thought to take over the burden of regenerative growth after substantial hepatocyte loss, suggesting that they are the progeny of facultative stem cells. The liver is not, however, generally considered as a stem cell-fed hierarchy, although this is disputed by others. Despite this, the subject of oval cells has aroused intense interest as these cells may represent a target population for hepatic carcinogens, and they may be useful vehicles for ex vivo gene therapy. This review proposes that the liver does harbour stem cells which are located throughout the biliary epithelium, and that oval cells represent the progeny of these stem cells and function as an amplification compartment for the generation of 'new' hepatocytes. This is a conditional process which only occurs when the regenerative capacity of hepatocytes is overwhelmed and thus, unlike the intestinal epithelium, the liver is not behaving as a classical continually renewing stem cell-fed lineage. We focus on the biliary network, not merely as a conduit for bile, but also as a cell compartment with the potential to proliferate under appropriate conditions and give rise to fully differentiated hepatocytes and other cell types.

Liver damage in the rat induces hepatocyte stem cells from biliary epithelial cells

BACKGROUND & AIMS

When rat hepatocyte regeneration after partial hepatectomy is blocked by 2-acetylaminofluorene, a proliferation of biliary epithelia sends out ductules into the parenchyma. The ability of these neoductules to act as a significant progenitor compartment for hepatocytes is in dispute. This study aims to resolve this question by varying the amount of 2-acetylaminofluorene administered.

METHODS

Rats were fed 2-acetylaminofluorene fr 6 days before and up to 7 days after partial hepatectomy was performed at a dose of either 2.5 (low) or 5 (high) mg/kg(-1)/day(-1). The response was monitored by the immunohistochemical expression of intermediate filaments and cytochrome P450 enzymes.

RESULTS

No regeneration by mature hepatocytes occurred with either dose, and new ductules expressed the biliary cytokeratins 7, 8, 18, and 19 and, in addition, vimentin. At the high dose, hepatocytic differentiation was infrequent, whereas apoptosis and intestinal differentiation were common. At the low dose, almost all ductules differentiated into hepatocytes within 14 days of hepatectomy.

CONCLUSIONS

Biliary epithelium is an effective and substantiative hepatocyte progenitor compartment under appropriate conditions.

Pericardial trauma and adhesions in relation to reoperative cardiac surgery

In an attempt to appreciate the changes that favour adhesion formation we compared the morphological and fibrinolytic changes that occur in human primary and reoperative pericardium. Ten patients undergoing primary elective open heart surgery and ten undergoing first time reoperative open heart surgery were studied. Pericardial samples were taken at four time points. At 0 (time A) and 30 (time B) minutes from the time of pericardiotomy (before the commencement of CPB), 30-50 minutes (time C) after the commencement of CPB, and then finally 10 minutes (time D) after the patient had been rewarmed. The fibrinolytic activity, as measured by the plasminogen activating activity (PAA), in the pericardial samples of the ten primary cases was compared with that in 5 of the reoperative cases. For the primary group, the PAA after 30 minutes of exposure (median 6.65 IU/cm2, range 3.85-11.89 IU/cm2, p = 0.14, n = 10) was not significantly reduced when compared to the initial activity (median 8.74 IU/cm2, range 2.22-17.68 IU/cm2, n = 10). After 30-50 minutes CPB the PAA was significantly reduced (median 3.93 IU/cm2, range 1.5-13.24 IU/cm2, p = 0.028, n = 10) and still reduced after rewarming for 10 minutes (median 3.12 IU/cm2, range 0.88-19.93 IU/cm2, p = 0.047, n = 10). The simultaneous plasma tissue-type plasminogen activator activity showed a significant (p < 0.05) increase after 30-50 minutes bypass with a later decline. The changes in the reoperative pericardial PAA were similar. In addition, the degree of PAA in reoperative pericardium was consistently lower than that observed in primary tissue. The extent of primary pericardial mesothelial damage at times B, C, and D compared with that at time A showed a significant (p < 0.01 for times B, C, and D) increase. Similarly there was a significant worsening of the degree of inflammation. Compared with primary pericardium, the reoperative samples showed a significant (p < 0.01 for times A, B, and C) preponderance of damaged mesothelium at the earlier stages of the operation. It appears that, following the initial bypass surgery, the processes that cause pericardial and mesothelial healing with recovery of PAA compete with those leading to pericardial adhesions and fibrosis. The histological and biochemical outcome seen in reoperative pericardium is the result of these competitive actions.

Oval cell differentiation into hepatocytes in the acetylaminofluorene-treated regenerating rat liver

When hepatocyte regeneration after a two-thirds partial hepatectomy (PH) in rats is blocked by oral gavage of acetylaminofluorene, a proliferation of ductular cells ensues that results in a profusion of neoductules radiating from each portal tract. To examine the possibility that this population of newly emerging cells harbors cells capable of differentiating into hepatocytes, we have looked in these cells for expression of functional markers of hepatocyte commitment at both the RNA and protein levels. Expression of albumin and alpha-fetoprotein (alpha-FP) messenger RNA (mRNA) transcripts were sought in situ using antisense riboprobes, and the expression of a number of cytochrome P450 enzymes was examined immunohistochemically. Before any signs of differentiation the ductular cells strongly expressed cytokeratins 7, 8, 18, and 19 in the same manner as authentic bile ducts, but unlike the latter also expressed vimentin. In situ hybridization studies showed that small bile ducts close to the limiting plate, as well as the newly formed ducts, expressed albumin and alpha-fetoprotein messenger RNAs, and immunocytochemistry showed that the distribution of the respective proteins was similar. Beginning at 1 week after partial hepatectomy, areas of differentiation could be found in the new ducts, with cells resembling either columnar intestinal-type epithelia or hepatocytes. Intestinal-like cells expressed neither albumin, alpha-FP, nor cytochrome P450 enzymes, whereas ductular cells appearing like hepatocytes with the typical membranous distribution of cytokeratin 8 strongly expressed a variety of cytochrome P450 enzymes normally associated with functional hepatocytes. These observations further support the belief that reactive ductules, sprouted from small ducts, can represent an adaptive response of the liver to replenish lost hepatocytes, although some of the newborn cells appear to differentiate along intestinal lines.

Subcellular distribution of peptides associated with gastric mucosal healing and neoplasia

The trefoil peptides pS2 and human spasmolytic peptide are putative growth factors, particularly associated with mucus-producing cells of the gastrointestinal tract including those of the stomach. The receptor for transforming growth factor alpha (TGF alpha) takes its name from one of its alternative ligands, epidermal growth factor, and is called the epidermal growth factor receptor. Although there is immunoreactive epidermal growth factor in the stomach, it is TGF alpha and the epidermal growth factor receptor that are abundant. Immunolabelling at electron microscope level allows for subcellular localisation of antigens; pS2 and human spasmolytic peptide co-localise to cytomembranes, including the Golgi apparatus, and thecae of surface/pit mucous cells. TGF alpha is abundant on the membranes of tubulovesicles of parietal cells and is also present in chief cells: in mucous producing cells it can be detected but not in association with mucous. The distribution of the epidermal growth factor receptor mimics that of TGF alpha but with preferential clustering on the basolateral membranes of gastric cells. The trefoil peptides are associated with healing and probably act, together with mucus, to protect the gastric mucosa and maintain a viable environment. TGF alpha, transduced via the epidermal growth factor receptor, inhibits gastric acid secretion, thus aids the trefoils in the maintenance of a gastric microenvironment conducive to healing after damage. TGF alpha, however, is also a potent mitogen; while this property plays a vital part in repairing mucosal defects, if this peptide or indeed its receptor are overexpressed, the result can be neoplasia.

The resistant hepatocyte model of carcinogenesis in the rat: the apparent independent development of oval cell proliferation and early nodules

The early cellular changes in the Solt-Farber resistant hepatocyte model of carcinogenesis have been studied to clarify the relationship of oval cell proliferation to the development of early hepatocyte nodules. Cellular proliferation, intermediate filament profiles and the expression of specific cytochrome P450 enzymes were examined. At 24 h after partial hepatectomy (PH) many of the bile ductular cells were in S phase, but over the next few days DNA synthesis progressively decreased in the portal bile ducts and was more common in arborizing ductules (oval cells) radiating from the portal areas. These cells strongly expressed cytokeratins 8 and 19 and vimentin, and from 1 week after PH they frequently underwent differentiation either into hepatocytes, expressing cytochrome P450 enzymes, or into intestinal-type cells. Five days after PH, numerous basophilic foci were discernible, and these expanded rapidly. The ductular cells swirled around the foci, but their antigenic profile clearly indicated that these cells were not involved in the development of these early nodules. In normal hepatocytes, cytokeratin 8 immunoreactivity was distinctly membranous in location, and could only be readily detected in periportal hepatocytes. In the basophilic hepatocyte foci, overexpression of cytokeratin 8 was consistently associated with cells organizing into acini, with expression reminiscent of authentic bile ducts, possibly indicating a structure-function relationship. In conclusion, early foci and nodules in this model are derived from resistant hepatocytes and not ductular oval cells, the latter being a facultative multipotential stem cell compartment.

Apoptosis: regulation and relevance to toxicology

1. Apoptosis is a remarkably stereotyped morphological event across all tissues in response to a vast array of damaging agents. 2. Our very existence depends upon a willing exchange of old life for new: apoptotic cell death is our guardian and saviour from genetic damage. 3. There is a close link between cell proliferation and apoptosis: When a cell picks up the machinery to proliferate it also acquires an abort pathway--'better dead than wrong'. 4. A wide variety of highly conserved genes have been implicated in triggering apoptosis. 5. The release of DNA loops from the nuclear scaffold is a more crucial intracellular event than DNA 'laddering' in apoptotic cells. 6. The manipulation of apoptotic rates in many of the common diseases in man will be a major therapeutic strategy in the future.

Proliferating cell nuclear antigen (PCNA) immunostaining--a prognostic factor in ovarian cancer?

The measurement of tumour cell proliferation is becoming increasingly recognised in defining prognostic groups. Proliferating cell nuclear antigen (PCNA) immunolocalisation can be used as an index of cell proliferation and may define the extent of departure from normal growth control. The monoclonal antibody PC10 stains PCNA in archival paraffin-embedded tissue. This study investigates its potential as a prognostic marker in early and advanced ovarian cancer. A three-stage immunoperoxidase technique was developed to detect the monoclonal antibody PC10. Archival paraffin-embedded tissue from 19 stage I ovarian tumours (13 malignant and six borderline) and 79 advanced (stage IIb-IV) ovarian tumours (patients entered into the Third North-West Thames Ovarian Cancer Trial) was immunostained with PC10. PC10 immunostaining was performed successfully in 91.8% of cases. The PC10 labelling index (PC10 LI) ranged from 1.5% to 88% with a mean value of 47.4%. Stage I borderline tumours had significantly lower PCNA labelling indexes than stage I malignant tumours (P < 0.048). In advanced disease there was an inverse correlation between PC10 and overall survival, and in those patients who underwent good debulking surgery (37 patients with disease < 2 cm diameter) a low PC10 value (< 36.5%) correlated with improved survival (log-rank trend test for survival, chi 2 = 5.75, P = 0.017). PCNA immunostaining defines a good prognostic subgroup in adequately debulked patients with ovarian cancer.

Perioperative histologic and ultrastructural changes in the pericardium and adhesions

The presence of pericardial adhesions prolongs the operation time and increases the risk of serious damage to the heart and other major vascular structures during resternotomy. The reported incidence of such damage is 2% to 6%. Pericardial mesothelial cells exhibit fibrinolytic activity, and therefore have an actual or potential role in the breakdown of the fibrinous adhesions that serve as the initial scaffolding for the firm collagenous adhesions seen at reoperation. Ten patients undergoing primary cardiac procedures were studied to assess the morphologic changes that take place within the pericardium and to relate these to accompanying changes in the pericardial plasminogen activating activity. Samples were taken at 0, 75, and 135 minutes after pericardiotomy. Compared with samples obtained at the time of pericardiotomy, those taken at 75 and 135 minutes demonstrated a significant progression in the mesothelial cell damage (p < 0.01), together with increasing evidence of pericardial inflammation (p < 0.01). The findings from electron microscope studies confirmed and supplemented these findings. Furthermore, compared with its initial levels (median, 2.06 IU/cm2; range, 1.28 to 6.48 IU/cm2), the plasminogen activating activity of pericardial biopsy specimens was significantly reduced at 75 minutes (median, 0.64 IU/cm2; range, 0.12 to 2.44 IU/cm2; P < 0.05), with some recovery at 135 minutes (median, 1.45 IU/cm2; range, 0.12 to 4.39 IU/cm2; p = 0.059). This study has revealed that, during cardiac procedures, the pericardium undergoes inflammatory changes with concomitant damage to its mesothelium, together with a reduction in the pericardial mesothelial fibrinolytic potential.(ABSTRACT TRUNCATED AT 250 WORDS)

Liver cell death: patterns and mechanisms

Images

Squamous cell carcinoma of the feline thymus with widespread apoptosis

The present report documents a case of squamous cell carcinoma originating from the thymus of a 12-year-old short-haired male cat. The tumour had metastasised to the sternal lymphoid tissue and to the left lung. To the authors' knowledge, thymic carcinoma has not been reported in domestic animals. Furthermore, extensive apoptosis was observed in the tumour mass.

The role of growth factors in gastrointestinal cell proliferation

The epithelial lining of the gastrointestinal (GI) tract is in a state of continuous cell renewal, and the proliferating and differentiating/differentiated cell populations are spatially clearly demarcated. Members of the epidermal growth factor (EGF) family of peptides, the trefoil peptides and enteroglucagon appear to be the most important enterotrophic molecules for both normal cell renewal and healing after cell damage. Transforming growth factor-a (TGF-a) appears to be the primary physiological ligand for the EGF receptor (EGFR), promoting normal cell renewal, and TGF-a/EGFR are part of an autocrine loop in many intestinal cancers. In response to damage, a differentiating cell lineage arises from adjacent epithelium secreting EGF, TGF-a and trefoil peptides; this may be viewed as part of a 'repair kit' in damaged endodermally-derived tissue.

Expression of hepatocyte growth factor mRNA during oval cell activation in the rat liver

The customary wave of hepatocyte regeneration which occurs in the rat liver after two-thirds partial hepatectomy can be abolished by oral administration of the carcinogen 2-acetylaminofluorene. Instead, regeneration is achieved through the proliferation and differentiation of potential stem cells (oval cells) which appear to emanate from the portal space. Ultrastructural studies have illustrated the undifferentiated nature of these cells in the first 3 days after resection, but very rapidly they acquire features of small hepatocytes or biliary epithelia. Oval cell progeny can form either cohesive columns of cells within sinusoids which may later differentiate into new hepatic plates, or single cells that can insinuate within existing plates. Using a 35S antisense riboprobe to hepatocyte growth factor (HGF) mRNA, the synthesis of HGF mRNA was observed in sinusoid-lining cells. There were few HGF mRNA-expressing cells in the liver removed at resection, but numbers steadily increased in the remnant over the next 7 days. In particular, an almost nine-fold increase in the density of HGF mRNA-producing cells occurred in the periportal areas, resulting in approximately double the density present within the centrilobular parenchyma. The superabundance of HGF-producing cells in the immediate vicinity of oval cell proliferation and differentiation strongly suggests that this growth factor is involved in all aspects of stem cell behaviour--proliferation, migration, and differentiation, through a paracrine mechanism.

Bromodeoxyuridine-labelled apoptosis after treatment with antimetabolites in two murine tumours and in small intestinal crypts

Antimetabolites are S-phase specific anticancer drugs. Administration of bromodeoxyuridine (BrdUrd) to tumour bearing mice was followed by treatment with cytosine arabinoside or hydroxyurea. Anti-BrdUrd immunocytochemistry visualised susceptible tumour and intestinal crypt cells at electron microscope level, showing unequivocally that cells that were in S-phase at the time of administration of the drugs subsequently died by apoptosis.

Transforming growth factor-alpha immunoreactivity in a variety of epithelial tissues

The immunohistochemical expression of transforming growth factor-alpha (TGF alpha) has been examined in a range of normal adult epithelial tissues from both man and rat using an anti-hTGF alpha monoclonal antibody (GF10). No differences in distribution were apparent between man and rat. In the continually renewing epithelium of the gastrointestinal tract, no staining was seen within the proliferative compartments, but strong immunoexpression was noted in various differentiated populations. In the testis, the spermatogonia were unstained, but the more luminally orientated germ cells were strongly positive. In the gastrointestinal tract, at least, any mitogenic action of TGF alpha must be mediated through a relatively long paracrine loop. In contrast, the differentiated parenchyma of kidney, salivary gland and liver remained unstained apart from collecting ducts in the kidney, striated ducts in salivary glands and bile ducts in the liver. The association of TGF alpha with tubule formation was reinforced by the very strong staining of newly forming bile ducts in a model of liver oval cell proliferation. Thus, in all the epithelia studied there was a distinct spatial pattern of TGF alpha immunoreactivity.

Severe subcellular damage in the rat exocrine pancreas after pancreatobiliary diversion

Cholecystokinin (CCK) may contribute to the genesis of both pancreatic carcinoma and acute pancreatitis. By transposing a long segment of jejunum to lie between the pylorus and the duodenal papilla, pancreatobiliary diversion (PBD) causes a persistent increase in circulating CCK levels, as the normal feedback inhibition of jejunal CCK release by pancreatic juice is evaded. A number of morphological, physiological, and acinar cytokinetic changes ensue. This investigation has examined the ultrastructural changes in pancreatic acinar cells after PBD in the presence and absence of CR1409 (lorglumide), a CCK receptor antagonist. After 14 days there was degranulation and vacuolation of acinar cells with involvement of the enzyme acid phosphatase. The presence of morphologically distinct extracisternal acid phosphatase indisputably predisposed acinar cells to severe damage. Treatment with CR1409 largely prevented degranulation after PBD, but vacuolation of acinar cells still occurred, indicating a possible toxic effect of the receptor antagonist. This is the first report of CCK itself, rather than one of its analogues, causing in vivo pancreatic damage that is generally considered as a forerunner to acute pancreatitis. This is of fundamental importance to the understanding of the earliest stages of the disease.

Evaluation of a proposed technique to assess unscheduled DNA synthesis and genotoxicity

Results from a recent, new assay suggest that omeprazole, a potent inhibitor of gastric acid secretion, is genotoxic. The principle of this assay is that the non-proliferating zone of surface gastric epithelial cells can be selectively removed by controlled digestion so that any incorporation of tritiated thymidine into these cells represents unscheduled DNA synthesis. Parietal cells (which are located below the uppermost proliferating cells) and proliferating cells in semiconservative, regular DNA synthesis could always be shown in the digested fraction, and as regular DNA synthesis takes up a thousand fold more thymidine than unscheduled DNA synthesis, any signal from unscheduled synthesis would therefore be swamped. The digestion process was also uneven, as histological analysis showed denuded patches of mucosa, and gland like structures were seen in the digest. Quantification of the number of silver grains over the nuclei showed no increase in low level labelling after omeprazole administration, indicating that there was no unscheduled DNA synthesis. The labelling index of undigested gastric tissue from omeprazole treated rats was not significantly different from that of the control group, despite an increase in the plasma gastrin value.

Role of cholecystokinin in pancreatic adaptation to massive enterectomy

Since pancreatic adaptation to massive proximal small bowel resection (PSBR) may be modulated through cholecystokinin (CCK) secretion, we tested the effect of the CCK antagonist CR-1409 on this response. Male Wistar rats (n = 72) weighing 220-225 g were randomised to receive either PSBR or transection/resuture followed by saline or CR-1409 (12 mg/kg daily subcutaneously). Rats were killed one, two, and three weeks post-operatively, at which time blood was obtained for CCK assay and the pancreas was assessed for proliferative activity by three parameters: nucleic acid and protein content, bromode-oxyuridine (BrdUrd) labelling index, and proliferating cell nuclear antigen (PCNA) expression. PSBR increased plasma CCK concentration by 83-102% at 1-3 weeks, irrespective of CR-1409 administration. Total pancreatic DNA content per 100 g body weight increased by 34% at two weeks (p less than 0.05) and by 82% at three weeks (p less than 0.05), while RNA content increased by 60% and 178% (p less than 0.001) and protein content by 20% and 57% (p less than 0.05). PSBR increased the BrdUrd labelling index and the percentage of PCNA immunoreactive cells. CR-1409 completely abolished this proliferative response and also prevented the rise in nucleic acid and protein contents. Apart from growth stimulation, PSBR also enhanced pancreatic exocrine function, as shown by ultrastructural evidence of an appreciable decrease in zymogen granules; CR-1409 also inhibited this functional effect of hypercholecystokininaemia. The results confirm the tropic role of CCK after PSBR, and CR-1409 prevents this pancreatic adaptation.

The biology and pathology of programmed cell death (apoptosis)

Apoptosis is a process by which cells die in a controlled and programmed manner in response to specific stimuli, often following extrinsic and intrinsic signals which ultimately cause the "switching on" of cell death regulatory genes. Condensation of chromatin and cytoplasm, fragmentation of the cell and formation of membrane-bound bodies containing intact organelles (apoptotic bodies), and phagocytosis of these bodies by resident cells are the major structural changes associated with apoptosis. Biochemically, activation of a nonlysosomal endonuclease is a cardinal feature of this mode of cell death. Several genes have been implicated in the execution of apoptosis. A signal transduction mechanism is suspected to regulate the phenomenon. Although apoptosis is widely considered as an adaptive response to physiological or near physiological stimuli, several noxious agents can initiate the reaction and thus it is often a toxicological response.

The nature of cytotoxic drug-induced cell death in murine intestinal crypts

The nature of cell death in murine small intestinal crypts caused by potentially lethal doses of four classes of cancer chemotherapeutic agents was studied. The drugs used were cytosine arabinoside, vincristine, adriamycin and nitrogen mustard. The compounds readily induced massive cell death in the proliferating compartment of the crypt. In each case, cell death was apparent within an hour, and the incidence of dead cells peaked during the following 4-8 h. By 24 h, little damage was discernible in the crypt systems. Remarkably, dead cells or dead cell fragments were phagocytosed rapidly (within about 1 h) by neighbouring healthy enterocytes. When examined by light microscopy, transmission electron microscopy and scanning electron microscopy, the dead cells showed the characteristic features of having succumbed to an apoptotic mode of cell death without any trace of cell and organelle oedema characteristic of necrosis. The study suggests that cell death by apoptosis operates even when the cells are exposed to severe pathological perturbation and that the phenomenon is not solely a process which operates in response to either physiological stimuli or to mild physical or chemical trauma.

Apoptosis: a gene-directed programme of cell death

Apoptosis is a particular type of programmed cell death which commonly occurs in the developing embryo, in normal healthy adult tissues and in many pathological settings. In contrast to necrosis, apoptosis is not a passive phenomenon but is gene-directed, usually requiring ongoing protein synthesis. The dying cell is characterised by having a raised level of cytosolic Ca2+; this activates a non-lysosomal Ca(2+)- and Mg(2+)-dependent endonuclease which digests the chromatin into oligonucleosome length fragments. The dying cell may or may not fragment into a number of apoptotic bodies, but in all cases the cell contents are bounded by a membrane which prevents the spillage of harmful substances such as DNA. Apoptotic cells are eliminated through phagocytosis by neighbouring cells and macrophages, and cell surface changes on apoptotic cells aid their recognition and engulfment by the phagocytosing cells. Extrinsic signals can both stimulate and inhibit apoptosis, and even direct damage to the cell can activate the process. Apoptosis is widely involved in organ formation in the embryo, and its occurrence in response to noxious stimuli such as cytotoxic drugs, irradiation and hyperthermia may be viewed as an altruistic suicide. Apoptosis provides a safe disposal mechanism for neutrophils at inflamed sites, and within the immune system it is considered responsible for eliminating self-reactive T-cell clones and for the affinity maturation of antibody producing cells. A failure to undergo apoptosis has been invoked in the pathogenesis of low-grade follicular lymphoma, and the triggering of apoptosis with monoclonal antibodies specifically in tumour cells has been achieved in one or two cases.

Interleukin-1 beta stimulates human mesangial cells to synthesize and release interleukins-6 and -8

Interleukin-1 beta (IL-1 beta) and tumor necrosis factor (TNF) have been reported to stimulate human mesangial cells (HMC) to proliferate and synthesize eicosanoids. We have examined whether they also induce HMC to release cytokines. In this study we show that both IL-1 and TNF stimulate HMC to release IL-6 and IL-8. Cycling and quiescent HMC were stimulated with various concentrations of either recombinant IL-1 beta or TNF for 1 to 24 hours. IL-1 beta at doses as low as 6 pg/ml stimulated mesangial cells to synthesize mRNA for both IL-6 and IL-8 as assessed by Northern analysis; mRNA for tubulin remained constant, which demonstrated a specific increase in mRNA. Secretion of IL-6 and IL-8 into the culture medium increased (4.5 to 18 ng/ml and 4 to 40 ng/ml, respectively) measured by ELISAs. TNF had similar effects but only in high concentrations (greater than 100 ng/ml). IL-1 beta did not stimulate cells to proliferate, as measured by 3H thymidine incorporation. TNF caused proliferation but only in concentrations over 100 ng/ml. We conclude that IL-1 beta is a potent stimulator of human mesangial cell production of IL-6 and IL-8, both of which may influence injury in nephritis. TNF also stimulates mesangial cells but only in pharmacological doses.

Inhibitory effect of a cholecystokinin antagonist on the proliferative response of the pancreas to pancreatobiliary diversion

Since pancreatobiliary diversion probably stimulates pancreatic growth by increasing cholecystokinin secretion, the effect of the cholecystokinin antagonist CR-1409 on this adaptive response was tested. Male Wistar rats (n = 108) weighing 220-250g were randomised to receive either pancreatobiliary diversion (n = 60) or sham diversion (n = 48) and thereafter to receive either saline injections or CR-1409 (10 mg/kg/day, subcutaneously). Rats were killed at four, seven, and 14 days postoperatively, when blood was obtained for cholecystokinin assay and the pancreas was assessed for proliferative activity by three techniques: nucleic acid and protein assay, bromodeoxyuridine labelling, and metaphase arrest after vincristine administration (1 mg/kg, intraperitoneally). Pancreatobiliary diversion increased plasma cholecystokinin concentrations by 91% at seven days and 137% at 14 days, irrespective of CR-1409 treatment. Total pancreatic RNA content was doubled by pancreatobiliary diversion at four days (2.15 v 1.07 mg/100 g body weight: p less than 0.001) and at seven days (3.43 v 1.76 mg/100 g: p less than 0.001), and trebled at 14 days (4.27 v 1.32 mg/100 g: p less than 0.001). Pancreatobiliary diversion increased bromodeoxyuridine labelling index from 1.1 to 3.7% at seven days and the cell birth rate from 0.09 to 0.06%. CR-1409 completely abolished this proliferative response and partly prevented the rise in RNA. The results confirm pancreatic hypertrophy and increased acinar cell proliferation after pancreatobiliary diversion. CR-1409 prevents this adaptive growth, probably by blocking cholecystokinin receptors. Bromodeoxyuridine labelling and the metaphase arrest technique may be used to assess pancreatic cell kinetics.

Variations in the occurrence of silver-staining nucleolar organizer regions (AgNORs) in non-proliferating and proliferating tissues

Previous studies on the subject of silver-staining nucleolar organizer regions (AgNORs) as indicators of precise proliferative status of tissues have sometimes resulted in ambiguity. The studies, however, have most frequently addressed themselves to the prognosis of neoplasias, with the aim of using AgNORs principally to distinguish between benign and malignant tumours. This investigation was to determine a base-line relationship of AgNOR clusters to proliferation and thus concentrated on normally proliferative tissues and conditionally renewing tissues after appropriate stimulation. Two murine transplantable tumours were also examined as examples of frank malignancy. As an example of the former, variations in AgNOR clusters were noted in the small intestine of man, mouse, and rat. The conditionally renewing systems of liver, prostate, and salivary glands were stimulated into proliferation by two-thirds partial hepatectomy, castration followed by treatment with testosterone, and isoproterenol treatment, respectively, in rat models; the murine sarcoma SaF and carcinoma CaNT provided relatively simple malignant tumours for AgNOR investigation. Proliferation was monitored by noting labelling indices after injection with bromodeoxyuridine (BrdUrd) in vivo followed by immunocytochemical visualization of S-phase cells. In all tissues, an increase in the size of AgNOR clusters rather than their number correlated positively with elevated labelling, particularly with the emergence of silver-staining regions of 2-3 microns visible diameter. Thus, increased AgNOR cluster size (diameter) as representative of AgNOR cluster/nucleolus volume was found to be dependent on proliferative activity in a range of normal and neoplastic tissues.

Effect of alpha-difluoromethylornithine on the polyamine levels and proliferation in two transplantable tumours

The effect of inhibition of polyamine biosynthesis by alpha-difluoromethylornithine (DFMO) on the growth of two murine transplantable tumours was studied. Female CBA mice were implanted with either the sarcoma F (SaF) or an anaplastic mammary carcinoma (CaNT), and 3% DFMO in the drinking water was provided once the tumours were established. Over a 10-day period control SaF tumours increased exponentially from 20 mm3 to over 800 mm3, whereas DFMO-treated SaF reached only 300 mm3. CaNT grew more slowly, requiring 22 days to achieve a similar volume increase, and DFMO was as effective in retarding growth as it had been in SaF. DFMO depleted tumour tissues of putrescine and spermidine, but did not reduce spermine levels. Metaphase arrest experiments with vincristine demonstrated that DFMO could substantially reduce the rates of tumour cell production, but there was no indication the DFMO accelerated the rate of cell loss from the tumours. Despite reduced rates of cell production, labelling studies with bromodeoxyuridine failed to detect differences between control and treated tumours: an increase in transit time through the S-phase was suspected. The number of nuclear organizer regions, detected by the argyrophilia of their associated proteins, was less in DFMO-treated tumours, and within a tumour the degree of silver deposition unequivocally reflected the proliferative heterogeneity. Ultrastructural studies revealed no differences between DFMO-treated and untreated tumours.

Proliferating cell nuclear antigen immunolocalization in gastro-intestinal epithelia

Proliferating cell nuclear antigen (PCNA), also called DNA polymerase delta-associated protein, is found in the cells of the proliferative compartment of normal tissues and is essential for DNA replication. It can be recognized by many monoclonal antibodies to various epitopes on the molecule. In this investigation one of these, PC10, has been used on formalin-fixed, paraffin-embedded, human and rodent gastro-intestinal epithelial tissues to assess numerically the labelling index of PC10 and to compare it, in the rat liver and gastrointestinal tract, with the S-phase fraction as determined by bromodeoxyuridine (BrdUrd) labelling. The distribution of PC10-labelled cells was recorded with respect to cell position in the intestinal crypts of man. In tissues where both modes of assessment were used, PC10 staining in the well-established proliferative compartments was found to be more extensive than that of BrdUrd. The higher labelling index with PC10 can be explained by its identification of PCNA outside the S phase of the cell cycle and also by the long half-life of PCNA protein in post-proliferative intestinal epithelial cells as they migrate towards the villus. Nevertheless the data suggest PC10 immunostaining in gastro-intestinal epithelia is an operational marker of cell proliferation which is reproducible, quantifiable and can be performed on routinely processed tissues.

Proportions of mitotic and apoptotic cells in a range of untreated experimental tumours

The quantitative aspect of apoptosis in experimental tumours is often neglected. In this study, the apoptotic and mitotic indices for a range of tumours have been determined at light microscope level. It has been found that the apoptotic levels fall into a consistent range for all tumour types and agree well with those described by previous workers. It is suggested that these might be basic parameters of tumour expansion, as relevant to growth kinetics as mitotic levels.

Kinetic studies on a murine sarcoma and an analysis of apoptosis

A stathmokinetic method has been used to determine the cell cycle parameters, particularly the potential tumour doubling time, of a murine fat pad sarcoma. Additional information has been obtained by determining the percentage of labelled mitoses (PLM). A technique which simultaneously demonstrates autoradiographically labelled S phase nuclei and histochemically localized acid phosphatase activity has also been used at light microscope level to compare these parameters: acid phosphatase activity was demonstrated in tumour cells and macrophages. Single cell deletion by apoptosis has been investigated as distinct from necrosis. Condensed, dying apoptotic cells, have been found in proliferative areas of tumour that are not under physiological stress. The analysis of apoptosis indicated a previously unsuspected variation in apoptotic activity with tumour weight. Cell death by apoptosis initially rose as the tumour grew, but after the tumour reached a threshold weight it declined dramatically, and finally remained stable. This may reflect an initial attempt at autoregulation of tumour size which ultimately fails. Apoptosis was estimated to account for an average of 7% of the total cell loss rate in this tumour.