CORTICAL CYTOPLASM, WHICH INDUCES DORSAL AXIS FORMATION IN XENOPUS, IS INACTIVATED BY UV IRRADIATION OF THE OOCYTE

Localized maternal determinants control the formation of dorsal axial structures in Xenopus embryos. To examine the spatial distribution of dorsal determinants, we injected cytoplasm from various regions of the egg and 16-cell embryo into the ventral vegetal cells of a 16-cell re cipient embryo. Cortical cytoplasm from the egg vegetal surface induce d the formation of a secondary dorsal axis in 53% of recipients. In co ntrast, animal cortical, equatorial cortical and vegetal deep cytoplas m never induced secondary axis formation. We also compared the axis-in ducing ability of animal versus vegetal dorsal cortical cytoplasm from 16-cell embryos. Significantly more dorsalizing activity was found in vegetal dorsal cytoplasm compared to animal dorsal cytoplasm at this stage. Previous work has shown that UV irradiation of the vegetal surf ace of either prophase I oocytes, or fertilized eggs, leads to the dev elopment of embryos that lack dorsal structures. Egg vegetal cortical cytoplasm was capable of restoring the dorsal axis of 16-cell recipien t embryos derived from UV-irradiated oocytes or fertilized eggs. We al so tested the axis inducing ability of cytoplasm obtained when UV-irra diated oocytes and eggs were treated as donors of cytoplasm. While veg etal cortical cytoplasm from UV-irradiated fertilized eggs retains its dorsalizing activity, cytoplasm obtained from eggs, UV irradiated as oocytes, does not. The egg vegetal cortex provides a suitable source f or the isolation of maternal dorsal determinants. In addition, since U V irradiation of the oocyte vegetal surface destroys the dorsalizing a ctivity of transferred cytoplasm, UV can be used to further restrict p ossible candidates for such determinants.