ABSTRACT
A method is described by which hanging drop cultures of chick heart fibroblasts can be photographed at stated intervals for long periods of time.
The growth of cultures in embryo extract has been analysed and the actual part played by outward migration of cells into the medium and by cell division has been evaluated.
A formula is given for expressing the growth rate of a culture in terms of the percentage of cells dividing per hour. This is termed the growth index.
Growth has been shown to occur uniformly, but at a decreasing rate for the first 36 hours and then to decline quickly and to become somewhat irregular.
The actual number of cell divisions on the field in a culture in embryo extract reaches a maximum at about the 30th hour of cultivation, but is fairly constant from about the 20th to 36th hour.
Cells divide most frequently near the periphery, where there is an optimum cell population density.
The period of mitosis remains fairly constant throughout the growth of the culture, tending to increase when the culture is growing less rapidly at the end, when the cells may stay for a considerable time in metaphase.
The intermitotic period is variable and increases greatly as the culture grows older.
The effects of certain alterations in the composition of the medium are described. Dilution of the extract with Tyrode solution caused a considerable fall in the rate of cell division. The dilution of extract with serum instead of Tyrode solution produced a similar decrease in mitotic activity, but tended to increase the number of cells on the field.
The independence of cell activity and cell division is stressed.
