Oocytes are the largest mononucleate cells that most animals make*, because these single cells have to stockpile everything needed to make it through embryogenesis.  (The only unique thing the sperm brings, at fertilization, is the centriole.)  In order to sustain their enormous biosynthetic activity, the oocyte nucleus swells to correspondingly enormous proportions, with a nucleolus – the part engaged in ribosome assembly – that exceeds the size of many tissue cells.  The giant nucleus of the oocyte is called the “germinal vesicle”. 

When the oocyte undergoes maturation (i.e., upon spawning in the case of starfish, or in this case after treatment with the hormone 1-methyladenine) the first overtly-visible consequence is the sudden breakdown of the germinal vesicle.  Thereafter, centrosomes and chromosomes have to find each other in the sea of dispersed nucleoplasm and form the meiotic spindle.

In most animal cells, nuclear envelope breakdown (total or partial) admits microtubules growing from the centrosomes, which, by growing and shrinking in all directions, search for and capture the kinetochores on each condensed chromosome.  But the collapse of the germinal vesicle leaves chromosomes scattered over a larger volume than even the mature mitotic spindle would span.  It seems difficult to imagine that random microtubule growth could encounter and capture them all in a reasonable amount of time.

Indeed, the starfish oocyte gets its chromosomes together in a very different way, as first demonstrated by Lénárt and colleagues.  They showed that treatment with microtubule poisons has little effect on the collection of chromosomes near the centrosome after germinal vesicle breakdown (although of course without microtubules the spindle can’t develop).  In contrast, treatment with actin poisons strongly inhibits chromosome collection.  Lenart and colleagues showed that germinal vesicle breakdown is accompanied by rapid assembly of an extensive actin network, which contracts; like a fisherman’s purse-seine, the actin network traps the condensed chromosomes and brings them within striking distance of the microtubules growing from the centrosomes.

— text by George von Dassow

*Insects are notable exceptions, although their oocytes are also very large; but the pervasive endoreplication of insect tissues means that some insect cells, such as the massively polytene cells of the salivary gland, are on the same scale as oocytes.  Other exceptions include the giant neurons of many molluscs.