Shedding of mitotic cells from the surface of multicell spheroids during growth

Abstract

During the growth of EMT6/Ro mammary tumor multicell spheroids, a large number of cells are shed into the suspension medium. The rate of cell shedding was 218 cells per square millimeter of spheroid surface per hour, or up to 1.5% of the total spheroid cell content per hour. Shed cells had a clonogenic capacity equal to that of exponential monolayer cultures and were further characterized by volume distribution, mitotic index, flow cytofluorometry, and autoradiography. The results indicated that cells are released from the spheroid surface at mitosis, presumably due to a loosening of the cell-to-cell attachment during this cycle phase. These mitotic cells, when placed in monolayer culture, attached and grew synchronously with a cell cycle time of about 13 hours. Shed cells kept in suspension culture had a similar cell cycle time, but these cells reaggregated immediately after mitosis. The results indicated that cell shedding and reaggregation both occur near the time of mitosis and are intrinsic factors regulating the initiation and subsequent growth of multicell spheroids. Although these studies were done with spheroids cultured in vitro, shedding of mitotic cells may play an important role in the in vivo process of metastasis.

Growth and Cellular Characteristics of Multicell Spheroids

“…Cells which are shed from those spheroids investigated so far seem to be viable for the most part, and result from active biological processes not affected by the spinning rate, geometry, etc. of the culture conditions (Landry et al 1981). Since mitotic cells are most commonly found, this provides a method of obtaining synchronized cells from certain types of spheroids.…”

Mathematical modelling of avascular-tumour growth

“…Mitotic inhibitors have been extracted from the necrotic core (see, for example, Levine et ai, 1984), and these prc-sumably diffuse through the spheroid affecting the mjtotic behaviour of some of the cells; however, this mechanism alone cannot be responsible for saturation since it requires that all the cells be inhibited if a continued increase in volume due to cell reproduction is to be prevented; this contradicts experimental observations. During mitosis, the reduced strength in binding between cells may cause individual cells to be shed into the surrounding matrix (Weiss, 1978;Landry et al, 1981) however, the cells that remain will still be reproducing so this mechanism cannot explain growth saturation either. In the models of Greenspan (1972) and subsequent similar studies, the crucial parameter for saturation arises in an expression stating that the contraction rate of the necrotic core is proportional to its volume; it is suggested that this is due to the process of disintegration of necrotic cellular material into simpler permeable compounds with a subsequent loss in volume.…”

Three-dimensional modeling of transport of nutrients for multicellular tumor spheroid culture in a microchannel

“…Equation (7) neglects the effects of the mitotic cell loss due to shedding on spheroid growth, which is valid if the spheroid diameter is small (Landry et al, 1981). Equation (7) can also be rewritten in term of the spheroid radius r…”