To enable our research it was essential to develop an oocyte model system exquisitely suited for quantitative live imaging assays. We established starfish oocytes as a model system ideally suited for this purpose that has proven to be a rich source to discover mechanisms of adaptation to meiosis-specific functions:


A contractile actin network collects chromosomes scattered in the large oocyte nucleus

Chromosomes scattered in the exceptionally large nucleus of starfish oocytes are located simply too far from the spindle to be efficiently captured by microtubules, as it occurs in somatic cells. Instead, we showed that an actin-dependent mechanism is required to transport chromosomes to within the ‘reaching distance’ of spindle microtubules (Lenart et al., 2005).

We recently revealed key mechanistic details of this novel actin-driven mode of chromosome transport. We showed that transport is based on ‘sieving’, i.e. chromosomes are trapped and carried along in an actin network that has a mesh size smaller than chromosomes. The direction of the contraction is set by anchoring the network to the cell cortex, transporting chromosomes to the cortical microtubule asters (Mori et al., 2011).

A burst of actin polymerization facilitates nuclear envelope rupture

Nuclear envelope breakdown (NEBD) proceeds in two steps: first peripheral nuclear pore components are released, resulting in a partial permeabilization of the nuclear envelope that is followed by a rapid and complete disruption of the boundary between cytoplasm and nucleus (Lenart et al., 2003).

In recent work, we showed that the rapid rupture of the nuclear envelope in the second step of NEBD is mediated by a transient burst of actin polymerization by the Arp2/3 nucleation complex (Mori et al., 2014).

The mechanism of centriole elimination in oocyte meiosis

If sperm and egg each contributed a somatic set of centrioles, the zygote would contain a double set leading to multipolar spindles and failed embryonic development. Therefore, common to animal species, centrioles are eliminated from the egg. By imaging the entire process live in starfish oocytes, we recently proposed the first comprehensive model for elimination of centrioles. We show that the somatic centriole cycle is modified in a way that mature mother centrioles are extruded into polar bodies in order to be eliminated. Unlike mothers, we show that disengaged daughter centrioles are intrinsically unstable and are eliminated in the egg cytoplasm (Borrego-Pinto et al., 2016).

Microtubule independent patterning of cytokinesis in large cells

The extremely asymmetric polar body cytokinesis (a.k.a. polar body extrusion) is accompanied by a dramatic wave of cortical contraction in most animal oocytes, with so far unknown mechanism and function. In recently published work we show that these surface contraction waves use the conserved RhoA-Rho-kinase-myosinII molecular module also involved in cytokinetic ring assembly. However, while the contractile ring is positioned by spindle microtubules in somatic cells, we show that surface contraction waves are patterned by a spatiotemporal gradient of the cell cycle kinase cdk1-cyclinB. Our results thus show that while in somatic cells microtubules dominate positioning of the contractile ring, spatial patterning is an inherent property of the conserved biochemical pathway, and is the dominant mechanism in large cells (Bischof et al., 2017).




We are using the oocytes of bat stars (Patiria miniata) as experimental model system. This species is common along the West Coast of the United States, although the population greatly suffered in the past years from the starfish wasting disease. We obtain the animals from providers in California, South Coast Bio-Marine, Marinus Scientific or Monterey Abalone. We maintain animals at EMBL's Marine Facility in temperature, light, pH and salinity controlled aquariums supplied with natural sea water from the North Sea. The Facility provides us with experimental material year around.