XB-IMG-102252
Xenbase Image ID: 102252
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Fig. 1. Diagrams illustrating the model of embryonic lens induction and development in Xenopus laevis. The various graphs represent dif- ferent tissue properties revealed by tissue transplantation and explant culture experiments (Henry and Grainger, 1987, 1989; Servetnick and Grainger, 1991). The X axes indicate developmental stages (according to Nieuwkoop and Faber, 1956). The Y axis represents the relative intensity of that property. A: Embryonic lens formation appears to be directed by two principal phases of induction, which include essential “early,” as well as “late” tissue interactions. Some slight overlap of these phases of induction occurs around stage 19 when the neural folds undergo fusion, which eventually separates the neural plate from the adjacent placodal ectoderm (terminating planar signals), and the optic vesicles begin to protrude from the developing forebrain and contact the presumptive lens ectoderm. B: Embryonic ectoderm gains an autonomous window of “competence” (A.C. in Fig. 4) to respond to lens inductive interactions, which is tied to its ability to respond to these interactions in the appro-
priate context. This property reaches a maximum at approximately stage 11.5, as indicated. Presumably competence to respond to these signals is sustained while inductive interactions continue to take place during later stages. C: When competent tissues receive these inductive signals, they gain an increasing lens-forming “bias” or propensity to form lens cells. After an initial period of “specification,” sufficient inductive signals reach the responding tissue such that when it can then be cultured in isolation to produce a lens, the tissue is then considered to be “specified” (stage 19). Ultimately, the tissue becomes “committed” to the lens cell fate, when it can no longer respond to other signals (definitions according to Slack, 1991; Grainger, 1992, 1996). Presumably this occurs by stage 26, when the lens undergoes cellular differentiation, after a phase of “commitment.” D: Hypothetical (temporal) patterns of gene expression that might be correlated with and establish each of the different properties described above. See text for further details. Image published in: Henry JJ et al. (2002) Copyright © 2002. Image reproduced with permission of the Publisher, John Wiley & Sons. Larger Image Printer Friendly View |