Microtubule organization, acetylation, and nucleation in Xenopus laevis oocytes: II. A developmental transition in microtubule organization during early diplotene.

Confocal immunofluorescence microscopy of ovaries from juvenile frogs revealed changes in the organization, acetylation, and nucleation, of microtubules (MTs), and redistribution of gamma-tubulin (gamma-TB), during early oogenesis in Xenopus laevis. Interphase oogonia contained sparse, radially organized, MT arrays and prominent centrosomes, Acetylated MTs were not commonly found in oogonia. In contrast, small (approximately 12-25 microns), postmitotic (stage 0) oocytes contained dense, highly polarized, MT networks that exhibited little or no evidence of radical organization. Examination of stage 0 oocytes stained with antibodies to gamma-TB, in conjunction with assays of MT nucleation activity, revealed that stage 0 oocytes do contain active centrosomes. In addition, stage 0 oocytes contained numerous acetylated MTs, suggesting that arrest in meiotic prophase is accompanied by MT stabilization. Early stage I oocytes (diameters from approximately 35-50 microns) exhibited a rounded morphology and contained a dispersed, apparently disordered, MT array with a substantial population of acetylated MTs. Examination of stage I oocytes stained with gamma-TB antibodies revealed that this centrosomal protein was present in multiple cytoplasmic foci which did not function as MTOCs following cold-induced MT disassembly. The results presented indicate that the maternal centrosome is inactivated during early stage I, roughly coincident with the onset of the diplotene stage of meiotic prophase and prior to assembly of the mitochondrial mass. Our observations place constraints on the role of MTs and the maternal centrosome during specification of the animal-vegetal axis of Xenopus oocytes and raise questions regarding the mechanisms by which MT assembly and organization are regulated during oocyte differentiation.