On the biomechanics of cytokinesis in animal cells

The material properties of the cell membrane are discussed. Various theories concerning the mechanism of cytokinesis in animal cells are presented. The currently accepted mechanism is that of active muscle-like contraction of the furrow base itself. A mathematical model is developed based on this theory. The cell membrane is modelled as a spherical membrane of nonlinear, elastic material. The membrane undergoes large deformations under the action of a contractile ring force in its equatorial plane. The numerical procedure employed in the solution of the governing equations is explained. The numerical results are compared with the experimental observations available in the literature. It is concluded that the cell membrane stiffness increases during the early stages of cleavage and it, later, decreases. The cell membrane division is a biomechanical instability problem. The factors that may facilitate or block cleavage are discussed. The experimental evidences that support the conjectures of the model are pointed out.     

Effects of caffeine,cyclic 3′, 5′ adenosine monophosphate and cyclic 3′, 5′ guanosine monophosphate in the development of the mycelial form of Sporothrix schenckii

The kinetics of the development of the mycelial form of Sporothrix schenckii from yeast cells and conidia in a minimal basal medium with glucose at pH 4.0 and 25 °C were established. Germ tube formation was used as the index of germination for both yeast cells and conidia. Yeast cells were first observed to develop germ tubes after 3 h of incubation, reaching 92±5%, after 12 h of incubation. Germ tubes were first detected in conidia after 9 h of incubation, and 12 h after inoculation 92±6% of the conidia had germ tubes. After 24 h of incubation, fully developed, sporulating mycelia were observed from both yeast cells and conidia. A delay in germ tube formation from yeast cells was observed when But₂cAMP(10 mM) and But₂cGMP (10 mM) were added to the medium. Also the addition of caffeine, a cyclic nucleotide phosphodiesterase inhibitor, inhibited the yeast to mycelial transition. Conidial germination into the mycelial form was also inhibited when cAMP, But₂cAMP and caffeine were added to the medium. These results suggest the possible involvement of cyclic nucleotides in the control of dimorphism in S. schenckii.     

Effects of zinc on macromolecular synthesis and nuclear division during the yeast to mycelium transition in Sporothrix schenckii

Zinc ions (10 mM) have been reported previously to inhibit the yeast to mycelium transition in Sporothrix schenckii. Yeast cells of this fungus were harvested, selected by filtration and allowed to form germ tubes in a basal medium with glucose in the presence of 10 mM zinc and the effects of this ion on protein, RNA and DNA synthesis and nuclear division recorded. All of these processes were affected by the addition of 10 mM zinc to the medium. Nevertheless, the inhibition of protein synthesis was observed earlier than that of RNA or DNA synthesis and was of a greater magnitude than that observed for both of these processes. Protein synthesis was inhibited within the first hour after inoculation, at which time this process begins in the control cells. RNA synthesis was inhibited during the 3 to 6 h interval after inoculation, that is, 3 h after the start of this process in the control cells. After 9 h of incubation, the inhibition of protein synthesis had reached its maximum at 70%, while that of RNA synthesis was only 52%. DNA synthesis was slightly inhibited, with maximum inhibition being observed 9 h after inoculation. Nuclear division in cells forming germ tubes in the presence of 10 mM zinc took place with a 3 h delay in relation to the control cells. These observations suggest that the inhibition of protein synthesis might be the most important mechanism by which zinc inhibits the yeast to mycelium transition in S. schenckii.