EARLY STAGES OF SPINDLE FORMATION AND INDEPENDENCE OF CHROMOSOME AND MICROTUBULE CYCLES IN HAEMANTHUS ENDOSPERM

We analyzed transformation of the interphase microtubular cytoskeleton into the prophase spindle and followed the pattern of spindle axis de termination. Microtubules in endosperm of the higher plant Haemanthus (Scadoxus) were stained by the immunogold and immunogold silver-enhanc ed methods. Basic structural units involved in spindle morphogenesis w ere ''microtubule converging centers.'' We emphasized the importance o f relative independence of chromosomal and microtubular cycles, and th e influence of these cycles on the progress of mitosis. Cells with mod erately desynchronized cycles were functional, but extreme desynchroni zation led to aberrant mitosis. There were three distinct phases of sp indle development, The first one comprised interphase and early to mid -prophase. During this phase, the interphase microtubule meshwork radi ating from the nuclear surface into the cytoplasm rearranged and forme d a dense microtubule cage around the nucleus. The second phase compri sed mid to late prophase, and resulted in the formation of normal (bip olar) or transitory aberrant (apolar or multipolar) prophase spindles. The third phase comprised late prophase with prometaphase. The onset of prometaphase was accompanied by a rapid association of microtubule converging centers with kinetochores. In this stage aberrant spindles transformed invariably into bipolar ones. Lateral association of a few bipolar kinetochore fibers at early prometaphase established the core of the bipolar spindle and its alignment. We concluded that (1) spind le formation is a largely independent microtubular process modified by the chromosomal/kinetochore cycle; and (2) the initial polarity of th e spindle is established by microtubule converging centers, which an a functional substitute of the centrosome/MTOC. We believe that the dyn amics of microtubule converging centers is an expression of microtubul e self-organization driven by motor proteins as proposed by Mitchison [1992: Philos. Trans. R. Soc. Lond. B. 336:99]. (C) 1998 Wiley-Liss, I nc.