Figure 3.

Model for induction of a Rappaport furrow indirectly as a result of local depletion of contractile elements near the primary furrows, followed by White/Borisy focusing. (a) In metaphase the contractile elements (circles) are uniformly distributed in the cortex. These elements presumably include myosin-II filaments and actin filaments. (b) At anaphase, the spindle midzones signal as in Fig. 1 (arrows), leading to local enrichment of contractile elements and primary furrow assembly. As a consequence, regions of cortex depleted in contractile elements are generated next to the furrows (denoted by brackets). (c) The gradient of contractile elements in the mid-region of the cell self-focuses by the White/Borisy mechanism (arrows), leading to assembly of a Rappaport furrow. This model predicts the absence of overlap microtubules, and of proteins normally associated with midzone signaling, in the cell center in the Rieder et al. experiment (3). Myosin and actin are predicted to be enriched in the Rappaport furrow, along with other proteins that have a role in furrow function.