The metaphase micronucleus is spheroidal in shape (about 4.7 pm in length and about 4.1 pm in equatorial diam eter)(Fig. 135). There are many intra nu cle ar microtubules (19 - 24 nm in diameter). As in other ci liâ te s they form a f a i r l y ty p ic a l m ito tic spindle at th is stage. Most microtubules run p a ra lle l to the lon g itu d in a l axis o f the nucleus although some others are oriented at r ig h t angles to th is axis. Figure 135 shows the d is tr ib u tio n o f chromatin and microtubules at metaphase. Some o f the tubules are probably attached to kinetochores; they are associated w ith chromatin (Fig. 136). Other microtubules terminate at the poles amongst fin e ly fibrous osm iophilic material or a t the nuclear envelope (Figs. 137 & 138),
(b) Early separation spindle
A fte r metaphase the micronucleus elongates to form a dumbell-shaped stru ctu re . Chromatin is concentrated in the two 'term inal knobs' which are joined by a more slender portion o f the nucleus th a t contains the separation spindle.
When the post-metaphase elongation begins a w ell-defined separation spindle has formed. The micronuclear length has increased to about 15 pm. Concommitantly the diameter o f the micronucleus has decreased to about 2.3 pm in the mid-region (Fig. 139). As the terminal knobs are approached, the micronucleus tapers o f f (Fig. 140). Micronuclear diameter is smallest at the bases o f the terminal knobs (0.4 pm)(Fig. 142). Microtubules (about 24 nm in diameter) and m ic ro fila m e n t-lik e f i b r i l s
(about 6 nm in diameter) are conspicuous w ith in the nucleoplasm o f terminal knobs (Fig. 141). Cross-sections o f micronuclei at th is stage
reveal the d is tr ib u tio n o f microtubules forming separation spindles (Fig. 139). There is a change in microtubule arrangement and size compared w ith th a t present during metaphase. One row o f microtubules forms an almost continuous 'sheath' ju s t inside the nuclear envelope.
Some o f these sheath microtubules have a greater diameter (about 26 - 28 nm) than the more c e n tra lly positioned microtubules (19 - 24 nm). Short
bridges appear to attach the microtubules o f the peripheral sheath d ir e c tly to the nuclear envelope and/or to adjacent sheath microtubules
(Fig. 139). More c e n tra lly positioned microtubules are arranged in to large clusters o f varying sizes. These microtubules are from 19 - 24 nm in diameter lik e those in metaphase m icronuclei. Short bridges are sometimes apparent between these tubules.
(c) Late separation spindle
In micronuclei th a t have reached lengths o f 80 - 100 pm, the mid region o f the spindle is about 0.8 pm in diameter whereas at the ends, the spindle diameter is about 0,4 pm. Correlated w ith th is is a
v a ria tio n in microtubule number. The maximum microtubule number/spindle cross-section occurs in the mid-region (about 180 tubules). The number o f tubules f a l ls o f f s te a d ily to about 50 tubules towards the terminal knobs (Figs. 144 & 147). The ordered p a ra lle l array o f microtubules in the separation spindles permits exact assessments o f microtubule number in accurately oriented spindle cross-sections. Attempts were made to obtain accurately oriented spindle cross-sections. Thin cross-
sections at selected in te rv a ls along the lengths o f two micronuclei in the same organism were obtained. Their microtubule d is tr ib u tio n p ro file s were assessed. These show th a t the mid-region o f the
spindle contains considerably more tubules than the regions on e ith e r side, and also th a t there is a gradual decrease in microtubule number
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from the spindle mid-region to the ends (near the terminal knobs) (Fig. 143). The to ta l number o f microtubules at equivalent points along the two d iffe r e n t spindles was very s im ila r (Fig. 143).
Not only is there a v a r ia b ilit y in spindle cross-sectional
diameter and microtubule number/cross-section along the length o f the micronucleus, but the diameters o f the spindle microtubules also vary. In the spindle m id-region, microtubules are predominantly 28 - 31 nm in diameter (Fig. 145). Microtubules o f th is magnitude are not present in e ith e r metaphase or early separation spindles. Interspersed amongst these very large microtubules are microtubules o f smaller diameter
(mostly about 24 nm^^. Areas o f osm iophilic fin e ly
f i b r i l l a r m aterial are prevalent between groups o f microtubules (Fig. 145) Nearer the ends o f the spindle, the microtubules are more closely packed as the micronuclear diameter narrows (Fig. 146), Microtubules o f about 28 - 31 nm diameter predominate and the areas o f f i b r i l l a r in te rtu b u le m atrix are reduced. At the very ends o f spindles, where they meet the terminal knobs, microtubules are predominantly of the 24 nm v a rie ty
(Fig, 147). Some microtubules extend in to the terminal knobs. These microtubules are a ll about 24 nm in diameter. Dispersed amongst the chromatin and microtubules are microfilamentous strands o f about 6 nm diameter (Fig. 148),
2 PARAMECIUM PRIMAURELIA