Absence of centrioles in the first and second meiotic spindles of mouse oocytes.

Centrioles are present in oogonia and oocytes of hamsters, mice, rats, gerbils and man up to the pachytene stage. In subsequent meiotic stages, centrioles are absent. At the time of germinal vesicle breakdown in the mouse several foci composed of electron-dense fibrillar material from which microtubules radiate appear near the nucleus. These fibrillar aggregates, referred to as microtubule foci, form small asters and collectively perform the role of the spindle pole during the two meiotic divisions. There are multiple centres at each pole and their number may vary. Thus, centrioles are not obligatory organelles of the spindle apparatus.