Microtubules in Xenopus oocytes are oriented with their minus-ends towardsthe cortex

Despite lacking centrosomes, stage VI Xenopus oocytes contain extensive net works of cytoplasmic microtubules (MTs). To gain additional insight into th e factors regulating MT organization during oogenesis, we have used electro n microscopy and "hook decoration" to examine the distribution and orientat ion of MTs in Xenopus oocytes. A limited survey of two "undecorated" stage VI oocytes revealed 218 MTs in images covering similar to 2,500 mu m(2), an d indicated that the MT number density of the animal cytoplasm was greater than that of the vegetal cytoplasm. Examination of five "decorated" stage V I oocytes (three animal and five vegetal hemispheres) revealed 653 MTs. Of these, 76% could be scored as having exclusively counterclockwise (CCW) or clockwise (CW) hooks. In the animal hemispheres, 93% of the scored MTs exhi bited CCW hooks when viewed from the direction of the cortex, indicating th at most MTs were oriented with their minus-ends out. MT orientation appeare d relatively uniform throughout the animal cytoplasm: more than 90% of the scored MTs in the cortical (90%), subcortical (96%), or perinuclear (98%) c ytoplasm were oriented with their minus-ends out. In the vegetal hemisphere s, similar to 80% of the scored MTs exhibited CCW hooks, and thus were orie nted with their minus-ends out; 96% of the scored MTs in stage III oocytes were oriented minus-end out. These observations support a model in which th e cortex plays a significant role in MT nucleation and organization in Xeno pus oocytes, and have significant implications fur the MT-dependent transpo rt and localization of cytoplasmic organelles and RNAs during oogenesis. (C ) 1999 Wiley-Liss, Inc.