The cell movements of gastrulation convert the blastula into an embryo with three tissue layers. New contacts between cells set up inductive signaling interactions that determine cell fates. Dorsal lip tissue is the source of organizer cells that induce development of preliminary head, trunk, and tail structures.
learning outcomes
You should be able to:
Compare and contrast gastrulation in frogs and sea urchins.
Explain the experiment that revealed the role of the dorsal lip of the amphibian blastopore in embryonic development.
Describe the cell movements that occur during gastrulation in sea urchins and amphibians.
Summarize how organizer cells achieve a pattern by appropriately expressing growth factor antagonists.
Explain the meaning of necessity and sufficiency in investigating potential inductive signals.
Compare gastrulation in reptiles and amphibians.
The sea urchin blastula develops into a radially symmetrical larva, but the frog blastula develops into a bilateral embryo. How does this difference relate to processes of gastrulation in these two types of animals?
Gastrulation in both the sea urchin and the frog proceeds with an involution forming a blastopore. In the sea urchin, the cells involuting are similarly determined, but in the frog, tissue interactions differ according to location around the blastopore, and the involuting cells become differentially determined.
What is meant by satisfying the criteria of necessity and sufficiency, and how did the Spemann–
The criterion of necessity means that a component of a process must be present for that process to occur. Spemann’s experiment, constricting the fertilized egg so that the gray crescent was only in one daughter cell or in both daughter cells, showed that the gray crescent was necessary for embryo development. The criterion of sufficiency means that a component of a process can alone cause the process to occur. In the Spemann–Mangold experiments, sufficiency was demonstrated by showing that when the dorsal lip of the blastopore was transplanted to another location on the blastula, it stimulated another axis of development.
How do the cells of the primary organizer induce different structures on the anterior−posterior axis as they migrate from the dorsal lip to the head region of the embryo?
The first involuting cells of the primary organizer express the transcription factor Goosecoid, and as those cells move anteriorly, Goosecoid suppresses the expression of certain transcription factors in neighboring tissues that have to be suppressed so that organs appropriate for the head region can develop. Cells that leave the dorsal lip of the blastopore later express different transcription factors that also have suppressive action on expression of other transcription factors in neighboring tissues, enabling the induction of region-
How does gastrulation in reptiles compare with that in amphibians?
Gastrulation in reptiles and amphibians differs because the blastula of amphibians is a sphere, but the equivalent stage of reptilian development is a flat sheet of cells, the blastodisc. In amphibians, the cell movements of gastrulation take place through a hole in the blastula (blastopore), while in reptilian blastodiscs, a longitudinal slit (primitive streak) forms and the cell movements of gastrulation take place through that slit. In amphibians, the ingress of cells through the blastopore creates the archenteron (primitive gut). No archenteron forms in the reptilian embryo, but the endodermal and mesodermal cells that pass through the primitive streak migrate forward to become gut and other structures.
We have described how the fertilized egg develops into an embryo with three germ layers and how cellular signals trigger different patterns of differentiation. In the next section we will describe how organs and organ systems develop.