key concept 43.3 Gastrulation Generates Multiple Tissue Layers

The blastula is typically a fluid-filled ball of cells. How does this simple ball of cells become an embryo made up of multiple tissue layers with head and tail ends and dorsal and ventral sides? Gastrulation is the process whereby the blastula is transformed by massive movements of cells into an embryo with multiple tissue layers and distinct body axes. The resulting spatial relationships between tissues make possible the inductive interactions between cells that trigger differentiation and organ formation.

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  • Cell movements during gastrulation result in three germ layers and new tissue interactions.

  • Gastrulation in frogs is more complex than in sea urchins, owing to frog blastulas having more yolk and more cell layers.

  • The dorsal lip of the blastopore is the primary organizer in the amphibian embryo.

  • Showing that a protein is an inductive signal requires that it be both necessary and sufficient for the proposed effect.

  • In amphibians, organizer cells moving anteriorly from the dorsal lip of the blastopore initiate the formation of different organs along the anterior–posterior axis.

  • Inductive tissue interactions can suppress as well as activate.

  • Because bird and reptile eggs have a large yolk mass, the embryos have a flattened blastodisc, and display a pattern of gastrulation very different from that of amphibians.

In the triploblastic animals (see Key Concept 30.1), three germ layers (also called cell layers or tissue layers, and not to be confused with germ cells) form during gastrulation:

  1. The endoderm is the innermost germ layer, created as some blastomeres move to the inside of the embryo. The endoderm gives rise to the epithelial lining of the digestive tract, respiratory tract, pancreas, thyroid, and liver.

  2. The ectoderm is the outer germ layer, formed from those cells remaining on the outside of the embryo. The ectoderm contributes to the nervous system, including the eyes and ears; and to the epidermal layer of the skin and structures derived from skin, such as hair, feathers, nails or claws, sweat glands, oil glands, and even teeth and other tissues of the mouth.

  3. The mesoderm is the middle layer and is made up of cells that migrate between the endoderm and the ectoderm. The mesoderm contributes tissues to many organs, including the heart, blood vessels, muscles, and bones.

The three germ layers are illustrated for a very early embryo in the fate map shown in Figure 43.6.

Some of the most interesting and important challenges in animal development have dealt with two related questions: what directs the cell movements of gastrulation, and what is responsible for the resulting patterns of cell differentiation and organ formation? Scientists have made significant progress in answering both these questions at the molecular level. In the following discussion we will begin with sea urchin gastrulation because it is the simplest to conceptualize in spatial terms. We will then describe the more complex pattern of gastrulation in frogs, and then the still more complex patterns in reptiles (including birds), and mammals.

Animation 43.1 Gastrulation

www.life11e.com/a43.1