The first cleavage furrow in eggs of Arbacia (sea urchin) is accompanied by a uniform ring of aligned microfilaments, called the contractile ring . Individual contractile ring filaments measure 35-60 A and occasionally appear "hollow ." The contractile ring exists from about 20 sec after anaphase to the end of furrowing activity, i .e ., 6-7 min at 20°C . It is closely associated with the plasma membrane at all times, and is probably assembled there . It is about 8 p wide and 0.2 p thick throughout cleavage . Its volume decreases, however, suggesting a contraction-related disassembly of contractile ring filaments, rather than a sliding-filament mechanism in the strict sense . Cytochalasin B ( >I 0-s M) arrests cleavage within 60 sec, by which time contractile ring filaments are no longer visible ultrastructurally . The furrow may be seen to recede within this time . Karyokinesis is unaffected . Simultaneous disruption of furrowing activity and of the contractile ring largely confirms the vital role of the contractile ring as the organelle of cell cleavage .

Many microfilaments and microtubules are well preserved after glycerol-extraction of HeLa cells at room temperature (220). Incubation in heavy meromyosin from rabbit skeletal muscle results in conspicuous and characteristic "decoration" of microfilaments of the contractile ring. Decoration is completely prevented by 10 mM ATP or 2 mM pyrophosphate, and fails to occur if heavy meromyosin is either omitted or replaced by egg albumin, a nonspecific protein. Decorated microfilaments have a substructure consisting of polarized, repeating arrowheads 27-35 nm apart. The specificity of these results strongly suggests that microfilaments of the contractile ring in HeLa cells are closely related to muscle actin. Very thin undecorated strands among the microfilaments of the contractile ring possibly represent a myosin component. These findings are discussed in terms of: the actomyosin-like properties of the contractile ring as a mechanochemical organelle that causes cell cleavage; the probable universal occurrence of actin-like protein in all dividing animal cells; and the contractile ring's combined sensitivity to cytochalasin B and its affinity for heavy meromyosin, a combination unique among microfilamentous organelles.

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We report on the internal ultrastructure of long, finger-like microvilli which cover the surface of the fertilized sea urchin egg. Eggs were attached to polylysinecoated surfaces; their upper portions were sheared away with a stream of buffer which left behind only their plasma membranes and adjacent cytoplasmic structures. Scanning electron microscopy (EM) of such fragments revealed intact thin, protoplasmic projections radiating away from the body of the cortex. By transmission EM of cortices similarly prepared on grids, small bundles of microfilaments appear as cores within the thin cytoplasmic projections. These microfilaments are shown to be composed of actin by their ability to interact with muscle heavy meromyosin (HMM). HMM-decora~ed microfilaments possess repeating arrowheads which uniformly point toward the cell interior. Actin bundles in the microvilli of sea urchin eggs may mediate microvillus support and elongation.

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