Contact inhibition of replication of the established mammalian cell line 3T3 was examined during growth of the colony and compared with that of the Chinese-hamster cell line CHL-1. The growth curves of cells in the colonies conformed to the predicted exponential and linear rates for CHL-1 and 3T3 respectively. Autoradiographs of colonies in which DNA was labeled with tritiated thymidine showed that in 3T3 colonies, only peripheral cells were labeled, while CHL-1 colonies were labeled throughout.

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In previous papers (1-3) we have described methods for plating single human cells so that each multiplies in isolation to form a well defined colony which constitutes a clonal population. The procedure is simple and quantitative, and completely comparable to the standard plating procedure used by microbiologists for growth of colonies from single bacterial cells. In the work published thus far we have described application of this method only to cells of epithelioid morphology. The present communication demonstrates that fibroblasfic cells, although requiring more stringent care, can also be plated by this technique. Experiments comparing growth and genetic properties of these two cell types obtained from a variety of human organs are presented and some implications of these results for the problem of differentiation are discussed.Definitions.--The terms, done, plating e fidency, and parental strain are used as previously defined (2, 4, 5). Fibroblastic morphology: Elongated form of a glass-attached cell, which is spindle-shaped or possesses multiple, needle-like extensions of the protoplasm, as shown in Fig. 1 a. Colonies of such cells display rough edges. Epithelioid morphology: Cell form consisting of polygonal bodies whose angles are roughly equal in size, as shown in Fig. 1 b. These cells usually pack together fairly tightly, display less surface area and are less migratory than fibroblastic cells (2). In this paper these two terms refer only to the morphologic appearance of the glass-attached cells, without implications concerning their origin in vivo. Genetic or kereditary character: A character, the potentiality for which is transmitted to all the asexual progeny of the original individual even after many passages in an environment which does not permit development or expression of the character. Not all genetic potentialities are genic; i.e. under exclusive control of specific nuclear genes.

Microvilli of HeLa cells cultured in vitro were preserved for electron microscopic examination at different stages of routine cultivation procedures. By a double-embedding technique, vertical sectioning for electron microscopy was possible. It revealed that, although the microvilli were present on all sides of the cell in the dispersed stage and in the attached stage, they were not present on the bottom of the cell when it was stretched on the surface of the dish. When the cells were grown in dense colonies, they were found on top of each other, and microvilli were present on all sides, except on the bottom surface of those cells in contact with the dish. We achieved a more dramatic demonstration of the microvilli by developing a surface-replica technique which retains their spatial arrangement and permits characterization of the distribution of their number, length, and diameter.

A sample of muscle fiber, suspended in air as we have described, can be observed for many hours. If the sample is allowed to dry slowly, it becomes less and less diamagnetic as the drying proceeds. Eventually, many of our samples became paramagnetic. During the drying, the magnetic anisotropy remains; this means that at a certain stage in the process the muscle fiber is paramagnetic in the direction of the fiber axis and diamagnetic normal to that axis. Glycerated5 muscle preparations, as well as air-dried and freeze-dried muscle fibers, showed strong magnetic anisotropy. Both nerve and tendon taken from the rabbit showed asymmetric magnetic properties. In a sample of fresh beef liver no magnetic anisotropy could be detected with certainty.