The Pita Sandwich

The giant explant preserves the normal dorsoventral planar tissue relationships in the gastrula, except at the ventral midline, where it has a free edge. Moreover, the corresponding inner surfaces of the explant are apposed, and the rolling of the neural tubes occurs opposite one another, both abnormal configurations that might affect morphogenesis. Another type of explant, the "pita" sandwich, both retains the normal planar tissue relationships of the gastrula by keeping the midventral line intact and avoids the problem of basal surface

Fig. 12. The "pita" sandwich is similar to the giant sandwich, but consists of tissues taken from only one embryo, sandwiched together in the dorsoventral direction. This explant is made by cutting around the VE (dashed lines, A), and then sticking the eyebrow hair animally, along the interface between postinvolution and preinvolution tissues. The hairloop is used to hold the VE and postinvolution material, and the eyebrow hair is moved outward, pulling the gastrula wall away from the postinvolution tissues (B,C). The hairloop is then used to extract all the postinvolution material and VE out through this artificial "blastopore," leaving a pita-like affair, which is compressed in the dorsal-ventral direction (D). This explant develops much like the giant sandwich, but has a neural groove only on one side (E). Prospective areas or the corresponding differentiated tissues are shown, including notochord (N), somitic mesoderm (S), epidermis (Ep), forebrain/midbrain (F), rhombencephalon (RH), spinal cord (SC), and neural tissue (Ne). The mesodermal prospective fates are shown as they would appear if the overlying epithelial endoderm were removed.

Fig. 12. The "pita" sandwich is similar to the giant sandwich, but consists of tissues taken from only one embryo, sandwiched together in the dorsoventral direction. This explant is made by cutting around the VE (dashed lines, A), and then sticking the eyebrow hair animally, along the interface between postinvolution and preinvolution tissues. The hairloop is used to hold the VE and postinvolution material, and the eyebrow hair is moved outward, pulling the gastrula wall away from the postinvolution tissues (B,C). The hairloop is then used to extract all the postinvolution material and VE out through this artificial "blastopore," leaving a pita-like affair, which is compressed in the dorsal-ventral direction (D). This explant develops much like the giant sandwich, but has a neural groove only on one side (E). Prospective areas or the corresponding differentiated tissues are shown, including notochord (N), somitic mesoderm (S), epidermis (Ep), forebrain/midbrain (F), rhombencephalon (RH), spinal cord (SC), and neural tissue (Ne). The mesodermal prospective fates are shown as they would appear if the overlying epithelial endoderm were removed.

apposition. The pita sandwich is made by cutting through the IMZ, along the blastoporal lip, as described above (Fig. 4B). The IMZ is teased outward and an eyebrow hair inserted into the blastocoel, toward the animal pole (Fig. 12A). The IMZ-NIMZ-AC is then pulled away from the involuted tissue (Fig. 12B,C). By gently stretching the tissue, the entire IMZ-NIMZ-AC can be removed from the involuted mesodermal tissues at any stage from 10 to 11.5, and flattened or cultured as a vesicle (Fig. 12D). The pita sandwich converges, extends, and differentiates dorsal neural and mesodermal tissues as well as epidermis and other tissues (Fig. 12E). This explant, though difficult to make, preserves some relationships of the normal embryo not maintained in the giant or standard explant.

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