Time Lapse Filming of Cellular Interactions in Organ Culture

Invasion in vitro. Methods of analysis

To understand the mechanisms of tumour invasion a number of in vitro assays have been developed, in which test cells (invasive or non-invasive) have been confronted with normal cells or tissues (the host) and cultured in various ways. We will discuss here the methods by which invasion can be judged in such assays. Histology is the method that is used successfully by pathologists for the evaluation of tumour invasiveness in vivo. The major difficulty is that the relationship between the non-invasive counterpart and the host tissue is not known in vitro or in vivo. Some authors have attempted to quantify invasion on histologic sections, but a generally accepted method is not yet available. The potential contribution of time lapse cinematography to the study of the kinetic aspects of invasion is obvious. Optical systems which overcome the relative lack of transparency of three-dimensional tissues, however, do not exist. Electron microscopy has contributed by providing details about the interactions between invasive cells on the one hand and normal cells as well as intercellular matrices on the other hand. Subpopulations of highly invasive and metastatic cells were collected after repeated migration through normal tissues in two-compartment assays. Attempts to evaluate the cytotoxic effect of invasive cells confronting radiolabelled cells or tissues have produced equivocal results. In contrast, measurements of radioactivity in the medium of labelled substrata have demonstrated the lytic effect of invasive cells on the extracellular matrix. It is concluded that further search for better methods to analyze and quantitate invasion is needed. So far, corroborative analysis by various methods seems to be the best policy.

Tumor invasion and host extracellular matrix

In this review some of the major mechanistic pathways by which tumor cells are thought to invade host tissues are discussed. Tumor invasion has been conceived to be the result of pathological, close-range interactions between malignant cells and host stroma. The sequence of events that characterize invasion can be summarized as follows: (a) Tumor cell clusters break from the confinement of the primary tumor. Loss of intercellular junctions (desmosomes), alterations in the chemical composition and physical properties of the cell surface coat (loss of fibronectin and heparan sulfate; excessive amounts of hyaluronate), and loosening of cell-substrate interactions (loss of hemidesmosomes, fibronectin, and heparan sulfate), are among the most frequently listed causes of tumor cell shedding. (b) Increased proteolytic activities at the invasion front cause focal alterations in the surrounding extracellular matrix, thereby changing its physical properties. Collagenases and cathepsins, as well as elastase and other neutral proteinases are the enzymes most frequently associated with matrix destruction and invasion. In some tissues this process is effectively regulated by inhibitors of matrix-degrading, proteolytic enzymes. (c) Tumor cells migrate into the altered matrix, possibly moving as aggregates along guidance tracks provided by host structures (blood vessels, lymphatics, nerves) or matrix macromolecules (collagen and fibronectin tracks). Migration seems to be preceded by increased swelling of glycosaminoglycan (i.e., hyaluronate) in the matrix, ahead of the migrating cell population. Various host cell types (mast cells, fibroblasts, endothelial cells, macrophages, etc.) may participate in these events.

The spread of cancer in the organism. Facts and problems

In this review, cancer is conceived as an alteration of the suface-monitored social behavior of cells. Apart from impaired growth controls, loss of residency (tissue affiliation) is the most important consequence of this homeostatic disorder. It results in local spread (penetration) which is initiated by locomotive and/or desctructive activities of the neoplastic cells. Access of cancer elements to the circulation possibly leads to distant spread (metastasis). Penetration and metastasis largely depend upon reactions of the organism, which are of an ill-understood, ambiguous nature favoring both the tumor and the host.

Time-lapse filming of cellular interactions within living tissues. III. The role of cell shape

By a further development of the technique for time-lapse filming of organ cultures, it has become possible to observe the internal structure of the living tissues and to study the dynamics of intercellular contacts within the tissue. This method has been applied to a study of the interaction between the tissues of the chick chorioallantoic membrane and non-malignant fibroblasts, polyoma virus-transformed fibroblasts, and Harding-Passey melanoma cells. The behaviour of these cells in the absence and in the presence of colcemid has been investigated. From these studies it is concluded that disturbances in cell shape brought about by colcemid do not affect the invasive properties whereas the surface polypodial activity shown by the malignant cells is a major factor in tumour invasion.