Dorsal Axis Formation Research Articles

Nuclear transplantation in goats

Xenopus embryos develop dorsal/ventral and anterior/posterior axes as a result of the activity of a maternal Xwnt pathway, in which β-catenin is an essential component, acting as a transactivator of transcription of zygotic genes. However, the questions of where and when β-catenin is required in early embryogenesis have not been addressed directly, because no loss-of-function method has been available. Here we report the use of a novel antisense approach that allows us to target depletion of protein to individual blastomeres. When a "morpholino" oligo complementary to β-catenin mRNA is injected into early embryos, it depletes β-catenin protein effectively through the neurula stage. By targeting the oligo to different cleavage blastomeres, we block β-catenin activity in different areas and at different times. Dorsal vegetal injection at the 2- and 4-cell stages blocks dorsal axis formation and at the 8-cell stage blocks head formation, while A-tier injection at the 32-cell stage causes abnormal cement gland formation. This approach shows the complex involvement of Xwnt pathways in embryonic patterning and offers a rapid method for the functional analysis of both maternal and early zygotic gene products in Xenopus.

A cytoplasmic determinant for dorsal axis formation in an early embryo of Xenopus laevis

In Xenopus laevis, dorsal cells that arise at the future dorsal side of an early cleaving embryo have already acquired the ability to cause axis formation. Since the distribution of cytoplasmic components is markedly heterogeneous in an egg and embryo, it has been supposed that the dorsal cells are endowed with the activity to form axial structures by inheriting a unique cytoplasmic component or components localized in the dorsal region of an egg or embryo. However, there has been no direct evidence for this. To examine the activity of the cytoplasm of dorsal cells, we injected cytoplasm (dorsal cytoplasm) from dorsal vegetal cells of a Xenopus 16-cell embryo into ventral vegetal cells of a simultaneous recipient. The cytoplasm caused secondary axis formation in 42% of recipients. Histological examination revealed that well-developed secondary axes included notochord, as well as a neural tube and somites. However, injection of cytoplasm of ventral vegetal cells never caused secondary axis and most recipients became normal tailbud embryos. Furthermore, about two-thirds of ventral isolated halves injected with dorsal cytoplasm formed axial structures. These results show that dorsal, but not ventral, cytoplasm contains the component or components responsible for axis formation. This can be the first step towards identifying the molecular basis of dorsal axis formation.

A cytoplasmic determinant for dorsal axis formation in an early embryo of Xenopus laevis.

In Xenopus laevis, dorsal cells that arise at the future dorsal side of an early cleaving embryo have already acquired the ability to cause axis formation. Since the distribution of cytoplasmic components is markedly heterogeneous in an egg and embryo, it has been supposed that the dorsal cells are endowed with the activity to form axial structures by inheriting a unique cytoplasmic component or components localized in the dorsal region of an egg or embryo. However, there has been no direct evidence for this. To examine the activity of the cytoplasm of dorsal cells, we injected cytoplasm (dorsal cytoplasm) from dorsal vegetal cells of a Xenopus 16-cell embryo into ventral vegetal cells of a simultaneous recipient. The cytoplasm caused secondary axis formation in 42% of recipients. Histological examination revealed that well-developed secondary axes included notochord, as well as a neural tube and somites. However, injection of cytoplasm of ventral vegetal cells never caused secondary axis and most recipients became normal tailbud embryos. Furthermore, about two-thirds of ventral isolated halves injected with dorsal cytoplasm formed axial structures. These results show that dorsal, but not ventral, cytoplasm contains the component or components responsible for axis formation. This can be the first step towards identifying the molecular basis of dorsal axis formation.

Axis and germ line deficiencies caused by u.v. irradiation of Xenopus oocytes cultured in vitro

ABSTRACT An intriguing aspect of developmental biology is the extent to which early development is controlled by the spatial arrangement of molecules in the oocyte. Ultraviolet (u.v.) irradiation of the vegetal pole of the fertilized egg of Xenopus laevis affects both the development of the embryonic dorsal axis and also the formation of primordial germ cells (PGCs). However, the importance of cytoplasmic localization in the oocyte has been difficult to assess because, until recently, it has proved impossible to mature and fertilize cultured oocytes routinely. In this report, we describe a method for routinely maturing and fertilizing cultured oocytes of Xenopus. We find that the u.v.-sensitive period for PGC and dorsal axis formation extends back into stage-VI oocytes, thus demonstrating a true oocyte contribution to these processes. This method also allows greater time for experimental intervention and should facilitate the eventual isolation of the molecules concerned.