Endoreduplication induced by hydroxylamine sulfate in Zea mays root tip nuclei

Significance of polyploidy in megakaryocytes and other cells in health and tumor disease

Polyploidy--the doubling of chromosome sets of cells caused by a stop of mitosis at different levels of the mitotic cycle--is a phenomenon widely observed in plants, protozoa, metazoa, and animals. In man obligate polyploid tissues are found in liver parenchyma, heart muscle cells, and bone marrow megakaryocytes. Polyploidy occurs mostly in stable and highly differentiated cells and tissues. Besides age, stimulation of proliferation and increased metabolic function lead to polyploidization in these organs. Aneuploidy, however, is exclusively found in tumor cells. Megakaryocyte differentiation and polyploidy are controlled by thrombopoietin-like activities, of which the loci of production are still unknown. Megakaryocytes are unique among polyploid mammal cells. On the precursor level they maintain their proliferative activity independently of the mammal's age. Once having entered the incomplete mitotic cycle they stop cytokinesis and develop into highly polyploid cells. Polyploidization of megakaryocytes is the basic requirement for establishing highly effective hemostasis in mammals, which exhibit blood circulation based on high blood pressures. Every polyploidization results in increased production of membrane materials with which the platelet becomes endowed. By shedding cytoplasmic fragments approximately 3000 platelets are set free from a 32c megakaryocyte, compared with only 16 nucleated thrombocytes by mitotic division. There is further evidence that the heterogeneity of platelets mostly depends on the different polyploidy classes of the megakaryocytes from which they are derived. Changes in the polyploidy pattern of megakaryocytes could therefore have consequences for hemostatic disorders in several human diseases, particularly in malignancy.

CYCLOHEXIMIDE-INDUCED NUCLEAR ALTERATIONS IN MAIZE ROOT TIPS

Cycloheximide appears to act in G2 and also at the end of metaphase in disrupting the normal progression of chromosomes through the nuclear cycle in Zea mays L. A dose of 7.5 µg/ml resulted in the accumulation of metaphase figures at 60 min. Regardless of the length of treatment (60 or 120 min), a drop in mitotic index occurred after 60 min due to a decrease in the frequency of prophases. This reduction is attributed to a cycloheximide-sensitive transition point in G2 approximately 1 to 1.5 h before the start of mitosis. The incorporation of 14C-leucine into proteins was found to be inhibited by cycloheximide at concentrations ranging from 0.75 µg/ml (very slight inhibition) to 750 µg/ml (almost complete inhibition). Label incorporation into those proteins found in the nuclear fraction was inhibited sooner and to a lesser extent than into total proteins. Cycloheximide at 75.0 µg/ml induces a variety of cytogenetic variants of which nine classes were observed. These included alterations in both chromosome morphology and nuclear organization and occurred at frequencies significantly higher than the controls. The results suggest that cycloheximide can act at several levels of organization which together are essential for establishing and maintaining the events of the nuclear cycle.

Effects of actinomycin D and puromycin on colchicine-induced endoreduplication in Chinese hamster cells in vitro

Experiments were carried out to study the events through which the process of endoreduplication occurs after treatment with colchicine in Chinese hamster cells cultivated in vitro, and to analyse the differences between the processes leading to normal mitosis and those leading to endoreduplication. Chinese hamster cells (line C-125) were treated with colchicine (10(-4) M) for 1 h, in order to induce endoreduplication. Before and after colchicine treatment the cultures were treated at various intervals with actinomycin D and puromycin. For preparation of mitosis specific RNA synthesis is required, and also synthesis of a specific protein, which is not required for endoreduplication. Moreover this suggests not only that the cells that endoreduplicate do not need new RNA and protein synthesis, but that inhibition of their synthesis apparently favours the processes of endoreduplication.