DORSAL LIP OF THE BLASTOPORE HOW TO

In (C) you can see why the dorsal lip tissue was called the organizer. Notice that the secondary invagination includes many unlabeled host cells (B) and later the induced secondary axis includes many structures formed by unlabeled host cells (somites, neural tube, etc.). In the figures at right the transplanted dorsal lip cells are labeled in red. Spemann and Mangold actually performed their transplantation studies using two different species of newt embryos that were differently pigmented so the cells of each species could be unambigously identified. Proof that the blastomeres of the dorsal lip were inducing host cells to take on new fates came from transplants using 'marked' cells. The dorsal lip of the blastopore was named the 'organizer' because of its unique inductive abilities. The fate changes induced by the dorsal lip transplant were dramatic an entirely new embryonic axis was induced. The dorsal lip of the blastopore (the region fated to give rise to dorsal mesoderm) not only did not change fate when transplanted, but induced surrounding host cells to change their fate. They found that there was one region of the late blastula and early gastrula that behaved in an extraordinary way when transplanted to a ectopic host position. The key experiments were done by Hans Spemann and his graduate student Hilde Mangold in 1924. The question is exactly when and how does this determination occur? What is the source of the patterning and cell determination signals. You should not be surprised by these results as we have discussed the progressive cell determination that occurs during development. Now the transplanted tissue exhibited 'independent development' and developed 'autonomously' based on its identity in the donor. However, the same heterotopic transplants performed at the late gastrula stage gave very different results.

DORSAL LIP OF THE BLASTOPORE HOW TO

The transplanted tissue 'depended' on its new host neighbors to tell it how to develop (Dependent 'conditional' development). Heterotopic transplantations of early gastrula tissue resulted in a normal embryo. The transplantation experiments shown at right illustrate this. The animal-vegetal axis is obvious due to the pigmentation and yolk differences that are determined during oogenesis.Įxcept for the isolation experiment described above, most experiments on frog blastula and early gastrula stage embryos suggested that cell fate was not determined until the mid gastrula stage. Where do the first asymmetries arise in vertebrate development? How are the anterior-posterior and dorsal-ventral axis established? How does embryonic patterning arise? When you look at a frog egg you can clearly see one maternally derived asymmetry. We are interested in the same questions that we addressed in Fly development. This suggests that some important cytoplasmic determinants are localized in the early vertebrate embryo. The one receiving the gray crescent material develops into a normal embryo, but the other forms a disorganized mass of tissue called a 'belly piece' because it contains mainly ventral tissues. However, if you experimentally alter the first cleavage so that one blastomere gets all the gray crescent cytoplasm and the other gets none, then the isolated blastomeres behave very differently. Isolated blastomeres give rise to perfectly normal duplicated embryos. Normally, the first cleavage bisects the 'gray crescent'. In the figure at right showing the first cleavage of a frog embryo you see the results of isolating the blastomeres. All animals use a spectrum of molecular mechanisms to regulate different stages of cell specification and differentiation. However, as we begin to understand the molecular mechanisms underlying a cells potency and progressive cell fate restrictions we see that there is no fundamental difference between mosaic and regulative development. The remaining cells can alter their fate based on their new neighbor relationships (position within the embryo) and compensate for the missing cell's progeny. In contrast, if you ablate a cell in the early embryo of an animal like the frog you often see no effect on development. If you ablate a C.elegans founder cell then all cells that normally arise from that lineage are missing from the embryo. Historically, animal development has been thought to be either mainly mosaic and lineage dependent or regulative and dependent on position.

Dorsal lip of the blastopore