Figure 21–28 Blastula fate map in a frog embryo. the endoderm derives  from the most vegetal blastomeres (yellow), the ectoderm from the most animal (blue), and the mesoderm from a middle set (green) that contributes also to endoderm and ectoderm. Different cell types derive from different positions along the dorsoventral axis.
Figure 21–29 Induction of a secondary axis by the Organizer. an amphibian embryo receives a graft of a small cluster of cells taken from a specific site, called the Organizer region, on the dorsal side of another embryo at the same stage. Signals from the graft organize the behavior of neighboring cells of the host embryo, causing development of a pair of conjoined (Siamese) twins. See Movie 21.4. [after
J. holtfreter and V. hamburger, in analysis of Development (B.h. Willier, p.a. Weiss and V. hamburger, eds), pp. 230–296. philadelphia: Saunders, 1955.] 1168 Chapter 21: Development of Multicellular Organisms that diffuse through the embryo from cell to cell, not by transcription regulators that move through the cytoplasm of a syncytium. Not surprisingly, the Organizer is now known to be a major source of secreted protein signals.
a competition Between Secreted Signaling proteins patterns the Vertebrate embryo
The signal molecules that pattern the frog embryo along the animal-vegetal (A-V) axis belong to the TGFβ family: they are secreted by a signaling center at the vegetal pole and form concentration gradients along the A-V axis. The Nodal protein acts over a relatively short range: cells near the vegetal pole are exposed to high levels of it and respond by switching on genes that promote the development of endoderm; cells further away are exposed to lower levels and activate genes that promote the formation of mesoderm. The cells at the vegetal pole that produce Nodal also produce a more rapidly diffusing TGFβ-like protein called Lefty, which antagonizes Nodal. The result is a high ratio of Lefty to Nodal at the animal pole, where Lefty predominates and Nodal signaling is blocked; this causes the cells there to develop as ectoderm (Figure 21–30A). Thus, a mid-range activation by Nodal, combined with a long-range inhibition by Lefty, sets up the pattern of progenitors along the A-V axis for the three germ layers—endoderm, mesoderm, and ectoderm.
The frog’s dorsal signaling system uses a different set of secreted signals from that of the vegetal signaling system to subdivide the germ-layer territories according to location along the D-V axis of the embryo. It exerts its influence by secreting two inhibitory signal proteins, called Chordin and Noggin. These antagonize the action of bone morphogenetic proteins (BMPs; members of yet another subclass of the TGFβ family), which themselves are secreted throughout the embryo. In this way, Chordin and Noggin form a dorsal-to-ventral gradient that blocks BMP signaling on the dorsal side but allows it to remain high on the ventral side (Figure 21–30B). Ectodermal cells that experience high levels of BMP signaling are driven to epidermal fates, whereas cells that experience little or no BMP signaling remain neural.
Knowing the signals that specify the three germ layers and various tissue types of the vertebrate body, one can reproduce this specification in a culture dish. Frog cells taken from the animal-pole region of the embryo, for example,
Figure 21–30 How Nodal and bone morphogenic protein (BMP) signaling pattern the embryonic axes. Nodal and its antagonist Lefty pattern the animal-vegetal axis, while BMp and its antagonists chordin and Noggin pattern the dorsoventral axis.