During gastrulation in amphibians, cells at the dorsal lip of the blastopore roll inward (involute) to become mesoderm. If the dorsal lip is transplanted to a different location on a host embryo, what happens?
ANothing — the transplanted cells die in the new location
BA second, complete body axis forms at the transplant site, because the dorsal lip (Spemann's organizer) secretes signals that induce and organize surrounding host cells into a new axis
CThe transplanted cells form a disorganized mass of mesodermal tissue
DThe host embryo rejects the transplanted tissue through an immune response
This is Spemann and Mangold's Nobel Prize-winning experiment (1924). The dorsal lip of the blastopore — the 'Spemann organizer' — is the most potent signaling center in the early embryo. It secretes inhibitors of BMP signaling (Chordin, Noggin, Follistatin) that create a permissive environment for neural and dorsal mesodermal development. When transplanted to the ventral side, it induces surrounding host cells to form neural tissue and somites, producing a twinned body axis. The organizer both self-differentiates into notochord and induces neighboring cells to adopt fates they would not otherwise assume.
Question 2 True / False
Gastrulation converts a two-dimensional sheet of cells into a three-dimensional body plan by moving cells from the surface to the interior.
TTrue
FFalse
Answer: True
Before gastrulation, the embryo is essentially a surface — a hollow ball (blastula) or flat disc of cells. Gastrulation moves cells from this surface to the interior through coordinated cell movements: invagination (infolding of a cell sheet), involution (rolling of cells over a lip), ingression (individual cell migration inward), and epiboly (spreading of cells over the surface). The result is an embryo with three tissue layers — ectoderm remaining on the surface, endoderm lining the newly formed gut, and mesoderm between them. These spatial relationships, established during gastrulation, determine all subsequent organ formation.
Question 3 Short Answer
What role does convergent extension play during gastrulation, and how does it differ from invagination?
Think about your answer, then reveal below.
Model answer: Convergent extension is a cell rearrangement process where cells intercalate (insert between each other) along one axis, causing the tissue to narrow (converge) in that direction and lengthen (extend) perpendicular to it. Unlike invagination (which moves a cell sheet from outside to inside), convergent extension reshapes a tissue by rearranging cells within the plane. During gastrulation, convergent extension elongates the forming body axis — it is responsible for the dramatic anterior-posterior lengthening seen in amphibian and fish embryos. Convergent extension is driven by planar cell polarity signaling, which aligns cell movements so that intercalation is directional rather than random.
Convergent extension was first characterized by Ray Keller in Xenopus gastrulation. It is a remarkably powerful morphogenetic engine: the tissue can narrow to half its width while doubling in length, driven entirely by cells crawling between their neighbors. Disrupting planar cell polarity (e.g., mutating Wnt/PCP pathway components) blocks convergent extension and produces short, wide embryos with severe axis defects.