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The phenomenon of malignant invasion was studied with the aid of phasecontrast time-lapse microcinematography on associated cultures of normal and malignant tissues. The mixed cultures were established on a very thin plasma coat covering standard size coverslips adapted to special perfusion culture chambers. The normal tissue was represented by the outgrowth from fragments of C3H mouse embryo heart tissue. Fibroblast-like cells as well as cells of a more polygonal shape forming pavement-like sheets, probably of endothelial origin, were the main cell components of this outgrowth. The malignant cells explanted as compact colonies, close to the normal tissue fragments, were represented either by the highly malignant N1strain cells of typically fibroblast-like appearance or by the “hybrid” clone M6, of a different morphology but similarly malignant. The results of the malignant versus normal tissue confrontation were examined by frame after frame analysis of microcinematographic recordings taken on mixed cultures of various ages up to 22 days. Normal cells characterized by moderate activity of ectoplasmic membranes had a good mutual coherence and single cells detached from the colony were observed only exceptionally. The junction of two normal cell outgrowths coming into contact leads to a perfectly coherent junction without overlapping. Quite differently, the malignant N1 and M6 cells showed a very strong and abrupt activity of ectoplasmic membranes and pseudopodia. Their outgrowth in dense cultures formed a disordered non-coherent and intermingled pattern. Single “sentinel” cells frequently detached from the whole colony, especially in the case of the N1 cells. They actively and rapidly migrated at a relatively long range. When the normal and the malignant outgrowth met each other in the mixed cultures, the malignant cells had a tendency to infiltrate the normal cell colony. This penetration was essentially due to progression of the single “sentinel” cells, and occurred particularly in areas where the normal outgrowth showed gaps or lacunar arrangements. On the contrary, the progression of the malignant cells was restrained and even stopped when opposed by coherent and compact sheets of normal cells, especially the pavementlike endothelial structures. Malignant cells, when arrested in their progression, often shifted along the demarcation line changing their previous orientation. At this point, our conclusions do not entirely fit with Abercrombie's conception of complete lack of “contact inhibition” regarding malignant cells. They are in agreement, however, with other observations especially those of Leighton concerning the intracapillary blocage of malignant cell emboli observed in sponge matrix cultures of mouse embryo heart tissue.

It has been established that although an inhibitory interaction occurs when a culture of human diploid cells become crowded together (contact inhibition of growth), multiple-layered cell sheets are obtained by using a continuous medium perfusion culture. A similar effect is obtained when the culture medium is changed at frequent intervals, and this paper reports the effects of a medium change on cell growth and metabolism. A direct relationship was found between cell yield and the number of medium changes given to a culture. This was an unexpected result because normally when a culture is prolonged by additional feeding the cell yield shows a diminishing return. The amino acid and glucose uptakes and growth yields (the ratio of the amount of cell dry weight produced to substrate used) were determined and they also showed that a unit amount of growth occurred per medium change, and that cessation of growth was accompanied by cessation of nutrient uptake and metabolism. Medium changes had a profound affect on cellular metabolism, especially on DNA and protein synthesis. As a culture approached confluency, DNA, RNA and protein synthesis were sequentially inhibited. After a medium change there was a sequential stimulation of DNA, RNA and protein synthesis in the same order as they were inhibited. The inhibitory mechanism that is affected by cell crowding is obviously reversed by a medium change. The results presented in this paper suggest that contact inhibition of growth primarily affects DNA synthesis and that if the cell is able to take up a sufficient supply of nutrients in a crowded culture then this inhibition can be overcome.

Within the entire spectrum of cell and tissue work to date, perfusion culture methods have received relatively little attention. Most of the perfusion systems have been designed for single monolayer or explant type cultures, and used principally for photomicrographic studies (e.g. 1-5), beginning with the unit described by Burrows in 1912 (6). Some systems have incorporated sophisticated means for metering liquid, gas, pH, and oxidation-reduction potentials and for monitoring nutrient and metabolic product concentrations (e.g. 7-12). A summary of all perfusion type systems which had been reported up to 1965 has been made by Rose (13), who has developed recently a highly automated medium circulation system. Since 1965, several other perfusion type systems have been reported. For example, Briand (14) described an apparatus for cultivation of monolayer cultures with continuous perfusion of medium and gas, with growth of ELD cells up to 3 times as rapid as in nonperfused cultures. Castor (15) reported an ingenious system this year which employs a combination of syringe drives and a vacuum pump to obtain perfusion of small cultures mounted on microscope stages, in which long term cultivation of cells under constant conditions is accomplished. A multisurface apparatus for mass scale growth of cell monolayers has been described by Weiss and Schleicher (16), which they indicate could be adapted for continuous or semicontinuous flow of nutrients, and a spin filter device for perfusion of suspension cultures was reported by Himmelfarb et al. (17), in which concentrations of cells in suspension as high as 10" per ml were obtained. An obvious reason why perfusion type systems have not been used much in cell and tissue culture studies is that they have not been readily available commercially. However, it appears that one or more such devices will soon be produced commercially.

Inhibition of the growth of dense cell cultures was studied in perfusion culture using cinemicrographic observations of rates of cell division and movement. For cell lines from normal tissues, cell division is first inhibited when a culture becomes confluent. Since depletion of the culture medium is minimized by perfusion, it is concluded that cell-to-cell contact is the primary stimulus to the inhibition. After a culture becomes confluent the rate of division varies either with cell motility or with the average area of the culture surface occupied by a cell, suggesting that cell-to-cell contact affects the rate of division by restraining the area and movements of the cell surface. Thus effective inhibition of division would require adequate contact inhibition of movement (which describes the tendency of cells to avoid moving across one another) and also adequate adhesions between cell borders. It is not yet clear how the inhibitory stimulus is transmitted to the replicative mechanism of the cell. An hypothesis consistent with the data is that serum macromolecules, the uptake of which is limited by the cell surface, stimulate cell division.

IN a communication published some years ago' I described a number of introductory experiments which had-the object'of showing that the perfusion culture method of de Haan'4 was excellently service-' able for keeping alive rabbits' lenses in vitro for many days.. This was the first time that it seemed possible to preserve that extraordinarily vulnerable tissue in such a good condition outside the body. All attempts made in that direction had always miscarried until then11. These preliminary 'experiments convincingly demonstrated the usefulness of this method. Of course good results can only be obtained when the lens is not injured during the operation. One of the most striking advantages of, the perfusion culture method is the continual renewal of the nutrient medium. This liquid, obtained from the abdominal cavity of rabbits and in its composition resembling -aqueous humour very m-uch, passes the explants in an uninterrupted current. Probably it is for that very reason that I succeeded in obtaining such good results. The 'more important facts of my research work, showing evidence of the high vitality of the explanted lenses, may be summarized as follows: 1. Unaltered metaboliclm even in explants several days old. The lenses maintain their normal transparency for many weeks. 2. Lenses of 'young'animals show symptoms of growth, apparent from. mitotic cell divisions in the equator region. 3. Ascorbic acid can freely diffuse through the capsule in both' 'directions. When the nutrient liquid contains an adequate quantity. of this vitamin its concentiation in the lens has normal values too2. This indicates that the' permeability of the capsule of the explanted lenses does not change. 4. Regeneration of experimentally wounded capsules starts and goes on in vitro in exactly the' same way as in vivo. The final result is in both circumstances a newly formed capsule3. In subsequent experiments not only the lens but also otherocular tissues, suich as the iris, the choroid, the retina and the ciliary body were explanted with equally good results4. This enlargement of the