Figure 4. Hypothesis: the dorsal determinants of the Xenopus laevis egg may be endosomal components.

The vegetal pole of the frog egg contains ‘maternal determinants’ of unknown composition. We propose here that they may correspond to wnt-containing early endosomes that become incorporated into multivesicular bodies (MVBs) on the dorsal side of the zygote. This would sequester glycogen synthase kinase 3 (gsk3) and axis inhibition protein (axin) inside MVBs, triggering the earliest wnt signal in vertebrate embryonic differentiation, which induces the Nieuwkoop signalling centre. a | In the unfertilized egg, membrane vesicles are observed in the vegetal pole⁶¹. b | The sperm brings with it the centriole, giving rise to a centrosome that organizes a cortical network of microtubules. Early endosomes from the vegetal pole are transported along microtubules to the dorsal side, which forms opposite the sperm entry point. The new embryonic axis forms where vegetal material and animal cytoplasm mix⁵⁵. Cortical microtubules cause a rotation of the egg cortex towards the sperm entry point, displacing the superficial pigment of the egg⁵³. This forms a lighter ‘dorsal crescent’ that marks the dorsal side. One possibility is that maternal wnt is secreted by the oocyte and internalized into early endosomes at the vegetal pole. Upon fertilization, the formation of MVBs at the dorsal side might promote gsk3 sequestration and thereby allow persistent wnt signalling to promote axis formation. dvl, dishevelled; lrp6, low-density lipoprotein receptor-related 6.