SYNCHRONIZATION OF CELL DIVISION

Most living organisms display many types of biological rhythms. We describe how a growing population of cells may be distributed between age classes or cell types, and define conditions necessary to produce synchronous population development. A probabilistic model describing the changes in cell numbers during proliferation is presented. The model predicts that during cell reproduction with constant parameters any cell population approaches a stationary behavior. According to this model, synchronization of cell growth is possible if there is a uniform parameter set for cell division. This point is illustrated by a set of graphs showing snapshots of model simulations with different parameter sets for transient and stationary behaviors.

Involvement of plasma membrane complexes in cellulose deposition in desmids

Deposition of cellulose microfibrils in oriented patterns in plant cell walls is believed to be one of the mechanisms by which cell shape is controlled. In some desmids, such as Closterium. circumferentially oriented microfibrils appear to help generate an elongated spindle shaped cell. In contrast, the primary cell wall of Micrasterias contains an apparently random meshwork of microfibrils, yet an elaborate multi-lobed semicell is formed. Freeze-etch electron microscopy of unfixed, ultrarapidly frozen desmids was undertaken to identify plasma membrane and cytoskeletal elements that may participate in the deposition and orientation of cellulose microfibrils.Micrasterias denticulata and aClosterium sp. were grown under a light/dark cycle to induce partial synchronization of cell division. Groups of several cells at the desired stage of semicell regeneration were loaded between two modified Balzers gold planchets coated with a slurry of yeast. A slot grid spacer prevented the cells from being crushed. Specimens were frozen in a propane jet freezing device designed by Gilkey and Staehelin. Replicas were produced in a Balzers BA 360 freeze-etch device and examined in a JEOL 100 CX electron microscope.