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N K Björklund

Publications and source records attributed to N K Björklund.

6 recordsLinked to original sources
PubMed1996

Surface contraction and expansion waves correlated with differentiation in axolotl embryos. II. In contrast to urodeles, the anuran Xenopus laevis does not show furrowing surface contraction waves.

We have observed a number of contraction waves traversing the axolotl (Ambystoma mexicanum) embryo (a urodelan amphibian) from the midblastula transition up to at least neural tube closure, and wished to learn if similar "differentiation waves" appear on the popular laboratory anuran amphibian, the South African clawed toad, Xenopus laevis. Time lapse video microscopy showed that no contraction waves are visible on the surface of Xenopus from gastrulation through neurulation. It is possible that cell intercalations in the double-layered ectoderm of the Xenopus embryo are homologous to the surface waves in the single layered ectoderm of the axolotl embryo. In any case, a simple, universal correspondence between surface waves and induction phenomena and differentiation does not exist.

Ambystoma↗
PubMed1994

Furrowing surface contraction wave coincident with primary neural induction in amphibian embryos.

We predicted, and have now observed, a surface contraction wave in axolotl (Ambystoma mexicanum) embryos that appears to coincide temporally and spatially with primary neural induction and homoiogenetic induction, and with involution of the chordomesoderm. The wave starts from a focus anterior to the dorsal lip of the blastopore and spreads as an ellipse, until part of it encounters the rim of the blastopore and vanishes there. The remaining arc then continues over the dorsal hemisphere until it reforms an ellipse that decreases in size. About 9 to 12 hours after it begins, the wave vanishes at a focus diametrically opposite its point of origin. The wave involves both local contraction and furrowing in the monolayer ectoderm. To a good approximation, the hemispherical portion of the ectoderm traversed by the wave becomes neuroepithelium, while the ectoderm not traversed by the wave becomes epidermis. The wave might provide a mechanism to determine the time and location at which neuroepithelial differentiation occurs.

Ambystoma mexicanum↗
PubMed1994

Surface contraction and expansion waves correlated with differentiation in axolotl embryos--I. Prolegomenon and differentiation during invagination through the blastopore, as shown by the fate map.

We have discovered a series of expansion and contraction, solitary waves that correlate with discrete steps of differentiation in the urodele amphibian axolotl embryo (Ambystoma mexicanum). Here we examine in detail the proposition that the blastopore is a set of differentiation waves. We superimposed the image of the axolotl fate map onto our digitized video images of normal gastrulation and matched the fate map to pigmentation irregularities on the embryo. We were then able to track the invagination of the fate map by tracking the variegated pigmentation on several embryos as gastrulation proceeded. We show a particular expansion and contraction wave sequence for every tissue in the blastula stage fate map and can now explain precisely why the fate map has the shape it does and its relationship to the embryo at subsequent stages. Each tissue can be assigned a differentiation code and placed on a hierarchical, binary differentiation tree.

Ambystoma mexicanum↗