A link between mitotic entry and membrane growth suggests a novel model for cell size control.

Addition of new membrane to the cell surface by membrane trafficking is necessary for cell growth. In this paper, we report that blocking membrane traffic causes a mitotic checkpoint arrest via Wee1-dependent inhibitory phosphorylation of Cdk1. Checkpoint signals are relayed by the Rho1 GTPase, protein kinase C (Pkc1), and a specific form of protein phosphatase 2A (PP2A(Cdc55)). Signaling via this pathway is dependent on membrane traffic and appears to increase gradually during polar bud growth. We hypothesize that delivery of vesicles to the site of bud growth generates a signal that is proportional to the extent of polarized membrane growth and that the strength of the signal is read by downstream components to determine when sufficient growth has occurred for initiation of mitosis. Growth-dependent signaling could explain how membrane growth is integrated with cell cycle progression. It could also control both cell size and morphogenesis, thereby reconciling divergent models for mitotic checkpoint function.

Catch-up reproductive maturation in rural Tonga girls, Zambia?

To compare the timing of reproductive maturation among urban and rural Tonga girls in Zambia, anthropometric measures and Tanner stages of breast development were obtained. Subjects were 774 (282 rural, 492 urban) girls ages 6-18. Results indicate that rural girls are shorter and have smaller triceps and subscapular skinfolds than their urban counterparts. Median age at menarche for the entire sample, as estimated by probit analysis, was 14.8 years (95% CL = 14.34-15.40). Onset of breast development among urban girls was significantly younger than for the rural girls: 11.47 (95% CL 11.22-11.71) years vs. 13.15 (95% CL 12.40-14.15) years. In contrast, the two groups did not differ in timing of pubertal completion as assessed by median age for Tanner Breast Stage 5: 17.01 (95% CL 16.30-18.33) vs. 16.96 (95% CL 16.37-17.56) years. Predictors of pubertal onset, based on multivariate logistic regression, included dental maturation, height, and triceps skinfold. Triceps skinfold was the only significant predictor of pubertal completion. These results suggest that rural girls progress through puberty more rapidly than the urban girls despite their later start. This finding of maturational catch-up contrasts with earlier urban/rural comparisons of girls as well as previous results among Gwembe boys, for which later pubertal onset is associated with longer duration of pubertal maturation. While the mechanism remains unclear, biocultural explanations suggest preferential feeding during adolescence as a source for rural girl's maturational catch-up.