Interactions With Neighboring Tissues In Making Neural Tissue

The three basic layers of the embryo—the endo-derm, mesoderm, and ectoderm—arise through the complex movements of gastrulation. These movements also create new tissue relations. For example, after gastrulation in the frog, presumptive mesoderm

FIGURE 1.11 Development of the chick embryo. The blastoderm (area opaca) sits on top of the large yolk and is the result of a large number of cleavage divisions. At the start of gastrulation, cells move posteriorly (arrows) and migrate under the area opaca. The embryo begins to elongate in the anterior-posterior axis, and the region where the cells migrate underneath the area opaca is now called the primitive groove and then primitive streak. Details of the cellular movements are shown in the cutaway view. The cells migrate into the blastocoel to form the mesoderm. At the anterior end of the primitive streak an enlargement of the streak is called Hensens node.

Area opaca Area pellucid

Thickening area of blastoderm

Area opaca

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Koller's sickle

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Koller's sickle

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Thickening area of blastoderm

Anterior

Thickening area of blastoderm

Hensen's node

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Area opaca

Primitive groove

Hensen's node

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Prosencephalon Mesencephalon

S1 C

S1 C

now underlies the dorsal ectoderm. A large number of experimental studies in the early part of the twentieth century revealed that these new tissue arrangements were of critical importance to the development of a normal animal. By culturing small pieces of embryos in isolation, it was possible to determine the time at which each part of the embryo acquired its character or fate (Figure 1.13). When the dorsal ectoderm was cultured in isolation prior to gastrulation, the cells differentiated into epidermis, while when roughly the same piece of tissue was isolated from gastrulating embryos, the piece of ectoderm now differentiated into neural tissue, including recognizable parts of the brain, spinal cord, and even eyes. These results led Hans Spemann, a leading embryologist of the time, to speculate that the ectoderm became fated to generate neural tissue as a result of the tissue rearrangements that occur at gastrulation (Hamburger et al., 1969). One possible source of this "induction" of the neural tissue was the involuting mesoderm, known at the time as the archenteron roof. As noted above, the involuting tissue is led into the interior of the embryo by the dorsal lip of the blastopore. To test the idea that the involuting mesoderm induces the overlying ectoderm to become neural tissue, Spemann and Hilde Mangold carried out the following experiment (Figure 1.14). The dorsal lip of the blastopore was dissected from one embryo and transplanted to the interior of another embryo, and the latter embryo was allowed to develop into a tadpole. Spemann and Mangold found that an entire second body axis, including a brain, spinal cord, and eyes developed from the ventral side of the

2 cell stage

4 cell stage

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Blastula Chick

Inner cell mass

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Primitive streak

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FIGURE 1.12 Development of the human embryo. The initial cleavage divisions are symmetric and produce apparently identical blastomeres. There is not much yolk in the mammalian embryo, since most nutrients are derived from the placenta. After multiple cleavage divisions, the embryo is called a blastocyst and develops a distinct inner cell mass and an outer layer of cells. The inner cell mass will develop into the embryo, while the outer cells will contribute to the placenta. After implantation, the embryo begins to elongate, and a section through the amniotic cavity shows that the human embryo develops a primitive streak much like that present in the chick embryo. The primitive streak is a line of cells migrating into the blasto-coel that will form the mesoderm, and the neural tube will form from the ectoderm overlying the involuting mesoderm. The tube rolls up and forms the brain and spinal cord in a process much like that described for the other vertebrates.

embryo, where neural tissue does not normally arise. To determine whether the new neural tissue that developed in these twinned embryos came from the dorsal lip tissue, they transplanted the dorsal blasto-pore lip from the embryo of a normal pigmented frog into the embryo of a nonpigmented strain of frogs. They found that the second body axis that resulted was made of mostly nonpigmented cells, indicating that it came largely from the host blastula, not the transplanted dorsal lip. Thus, the grafted blastopore cells have the capacity to induce neural tissues from a region of the ectoderm that would normally not give rise to a nervous system. In addition to the neural tissue in these embryos, they found that mesodermally derived structures also contributed to the twinned embryo. They concluded that the dorsal lip acts not only as a neural inducer but also as an "organizer" of the entire body axis. As a result of these experiments, this region of the embryo is known as the Spemann organizer.

In the years following these initial studies of Spemann and Mangold, several embryologists tried to further characterize the induction process, as well as to identify the inducing principle or factor. One of the

Sanes Figure

FIGURE 1.13 Isolation of fragments of embryos at different stages of development demonstrates when tissue becomes committed to the neural lineage. If the animal cap is isolated from the rest of the embryo (left), the cells develop as epidermis, or skin. If the same region of the embryo is isolated a few hours later, during gas-trulation (right), it will develop into neural tissue (shown in the figure as red neurons). Experiments like these led to the idea that the neural lineage arises during gastrulation.

FIGURE 1.13 Isolation of fragments of embryos at different stages of development demonstrates when tissue becomes committed to the neural lineage. If the animal cap is isolated from the rest of the embryo (left), the cells develop as epidermis, or skin. If the same region of the embryo is isolated a few hours later, during gas-trulation (right), it will develop into neural tissue (shown in the figure as red neurons). Experiments like these led to the idea that the neural lineage arises during gastrulation.

first realizations that came from these additional studies was that "the organizer" has subdivisions, each capable of inducing specific types of differentiation. Holtfreter subdivided the organizer region into pieces, and, using the same transplantation strategy, he found that when more lateral aspects of the dorsal lip were used, tails were induced, whereas when more medial regions of the organizer region were transplanted, heads were induced. In an attempt to more precisely define the heterogeneity of the region, Holtfreter also cultured small bits of the dorsal lip and found that that these develop into more or less well-defined structures, such as single eyes or ears! As Holtfreter succinctly summarizes: "even at the gas-trula stage the head organizer is not actually an equipotential entity, but is subdivided into specialized inductors although distinct boundaries between them do not seem to exist."

Neural induction does not appear to act solely through a vertical signal passed from the involuting mesoderm; there is also evidence that a neural inducing signal can be passed through the plane of the ectoderm. As noted above, when blastulas are placed in hypotonic solutions just prior to gastrulation, the IMZ cells fail to involute, and instead evaginate to produce an exogastrula. Under these conditions, the process of signaling between the mesoderm and the ectoderm should be blocked, since involuting mesoderm is no

FIGURE 1.14 Spemann and Mangold transplanted the dorsal lip of the blastopore from a pigmented embryo (shown as red) to a non-pigmented host embryo. A second axis, including the neural tube, was induced by the transplanted tissue. The transplanted dorsal blastopore lip cells gave rise to some of the tissue in the secondary axis, but some of the host cells also contributed to the new body axis. They concluded that the dorsal lip cells could "organize" the host cells to form a new body axis, and they named this special region of the embryo the organizer.

longer underneath the ectoderm. Surprisingly, however, some neural induction does appear to take place. Although this was difficult to determine in Holtfreter's time, the use of antibodies and probes for neural-specific proteins and gene expression clearly shows that organized neural tissue forms from such exogas-trulated ectoderm (Holtfreter, 1939; Ruiz i Altaba, 1992). This so-called planar induction can also be demonstrated in a unique tissue combination invented by Ray Keller that bears his name, the Keller sandwich. In this preparation, the presumptive neural ectoderm and the dorsal lip are dissected from two embryos and sandwiched together. In these, the mesoderm does not move inside of the sandwich, but rather extends away from the neurectoderm like an exogastrula (Keller et al., 1992; Figure 1.15). Only a thin bridge of tissue connects the mesoderm with the neural ectoderm, but nevertheless, extensive and patterned neural development occurs in these cultures (Figure 1.15).

Keller Sandwich And Neural Induction

FIGURE 1.15 Planar neural induction can be contrasted from vertical neural induction by Keller sandwiches. The organizer region, including the IMZ cells, along with some of the surrounding ectodermal tissue, can be cultured in isolation and will not involute when two IMZ regions are placed back-to-back. The tissue undergoes morphological changes similar to those that occur during gastrulation, except the tissue extends rather than involutes. Nevertheless, neural tissue (red) is induced in the attached ectoderm, indicating that the signals for neural induction can be passed through the small region that connects the mesodermal cells and the ecto-dermal cells.

FIGURE 1.15 Planar neural induction can be contrasted from vertical neural induction by Keller sandwiches. The organizer region, including the IMZ cells, along with some of the surrounding ectodermal tissue, can be cultured in isolation and will not involute when two IMZ regions are placed back-to-back. The tissue undergoes morphological changes similar to those that occur during gastrulation, except the tissue extends rather than involutes. Nevertheless, neural tissue (red) is induced in the attached ectoderm, indicating that the signals for neural induction can be passed through the small region that connects the mesodermal cells and the ecto-dermal cells.

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