Forty hairless mice were given injections of tritiated thymidine every 4th hour during 10 days. At 24 hr intervals groups of four mice were killed. The numbers of labelled basal and differentiating cells were determined by autoradiography with a stripping film technique. To determine the background activity skin sections from uninjected control mice were subjected to the same stripping film procedure. Another group of hairless mice was given one single pulse labelling with tritiated thymidine. The number of labelled mitoses was scored for 12 hr after the injection. At 10, 12 and 15 hr after the injection, the numbers of labelled basal and differentiating cells were also determined. A mathematical model of cell population kinetics in the epidermis has been suggested. The results of different simulations on this model were compared with the observed results. The curve of mean grain counts under continuous labelling increased from day to day with two well‐defined plateaux. The percentage of all labelled cells increased rapidly up to the 3rd day, and thereafter the curves gradually flattened off. When basal cells and differentiated cells were considered separately the labelling index of the basal cells increased rapidly for the first 3 days and then flattened off at the 100% level on the 5th day. The labelling index of the differentiating cells was low during the first 3–4 days. Then a steep increase in the percentage of labelled differentiating cells was seen, but the curve flattened off again close to the 100 % level after the 7th day. The labelled mitosis curve had its maximum 5 hr after the thymidine injection. The curve fell again to almost zero at 12 hr. Ten, 12 and 15 hr after the injection, 6, 7 and 7% respectively of the labelled cells were found in the spinous layer. It was concluded that three grains over each nucleus could be used as lower limit for considering a cell as labelled. On this basis, tritiated thymidine injections every 4th hour can be considered as continuous labelling.
After removing a circular area of the central corneal epithelium of the rat eye, the labelling indices and the mitotic rates were measured at various times after wounding, both in the cornea and in the adjacent conjunctival epithelium. The proliferative response was most marked in the corneal epithelium adjacent to the wound, but there was also a definite response in the epithelium covering the denuded areas, and in the conjunctival epithelium. The study demonstrated that the conjunctival epithelium. The study demonstrated that the conjunctiva itself plays a role in the healing of a central corneal epithelial wound. The similarities in the cellular response may indicate that both epithelia are under the influence of the same growth-suppressing factors (chalones), and must be looked upon as a unit. However, no support was found for the theory that the limbal area serves as a generative organ for the corneal epithelium.
The right eyes of 40 rats were exposed to a single erythemogenic dose of ultraviolet B irradiation (UVB) at 297 nm. The irradiation was directed perpendicular to the center of the cornea. The left eyes served as controls. The animals were randomly assigned into 10 groups. The labeling index (LI) after pulse labeling with tritiated thymidine and the mitotic rate (MR) after Colcemid administration were registered in the corneal epithelium at predetermined intervals up to 96 h after the irradiation. A mathematical method was used to correlate corresponding corneal areas from the different animals. In the central cornea the LI was considerably reduced up to 36 h after the irradiation. The LI increased toward the peripheral cornea and reached normal values at the limbal area. The MR was also reduced up to 36 h. However, this reduction was over the entire epithelium. The block in cell proliferation was followed by increased proliferation.
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