Servetnick M and Grainger RM (1991), Changes in neural and lens competence in Xenopu...

XB-ART-24876

Development 1991 May 01;1121:177-88.

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Changes in neural and lens competence in Xenopus ectoderm: evidence for an autonomous developmental timer.

Servetnick M , Grainger RM .

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The ability of a tissue to respond to induction, termed its competence, is often critical in determining both the timing of inductive interactions and the extent of induced tissue. We have examined the lens-forming competence of Xenopus embryonic ectoderm by transplanting it into the presumptive lens region of open neural plate stage embryos. We find that early gastrula ectoderm has little lens-forming competence, but instead forms neural tissue, despite its location outside the neural plate; we believe that the transplants are being neuralized by a signal originating in the host neural plate. This neural competence is not localized to a particular region within the ectoderm since both dorsal and ventral portions of early gastrula ectoderm show the same response. As ectoderm is taken from gastrulae of increasing age, its neural competence is gradually lost, while lens competence appears and then rapidly disappears during later gastrula stages. To determine whether these developmental changes in competence result from tissue interactions during gastrulation, or are due to autonomous changes within the ectoderm itself, ectoderm was removed from early gastrulae and cultured for various periods of time before transplantation. The loss of neural competence, and the gain and loss of lens competence, all occur in ectoderm cultured in vitro with approximately the same time course as seen in ectoderm in vitro. Thus, at least from the beginning of gastrulation onwards, changes in competence occur autonomously within ectoderm. We propose that there is a developmental timing mechanism in embryonic ectoderm that specifies a sequence of competences solely on the basis of the age of the ectoderm.

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Species referenced: Xenopus laevis
Genes referenced: dnai1 eno2 ncam1 tbx2

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	Fig. 1. Transplant and culture experiments to assay lens competence (A) Ectoderm was removed from FLDxlabelled early gastrula (stage 10) embryos, and the dorsal, middle or ventral portion of the animal cap was transplanted to the presumptive lens region (PLR, shown in gray) of a neural plate stage (stage 14) embryo from which the presumptive lens ectoderm had been removed. (B) Presumptive ventral ectoderm was removed from FLDx-labelled embryos at different stages of gastrulation and transplanted to the stage 14 PLR (shown in gray). (C) Ectoderm was removed from FLDx-labelled early gastrula (stage 10) embryos and placed in culture for various periods of time, after which it was transplanted to the PLR of a stage 14 embryo. The developmental age of the ectoderm was monitored using unoperated sibling embryos In all experiments, host embryos were cultured to swimming tadpole stage, fixed, sectioned and stained by lmmunofluorescence with antibodies that recognize either lens proteins or N-CAM.
	Fig. 2. Competence of early gastrula ectoderm. Ectoderm was removed from FLDx-labelled early gastrula embryos, and either the dorsal, middle or ventral portion of the animal cap was transplanted to neural plate stage embryos, as shown in Fig 1A. Induced lenses were scored on the basis of immunofluorescence, neural tissue was scored morphologically. Bars show the proportion of transplants which formed lens or neural tissue. n=number of cases scored.
	Fig. 3. Structures induced from transplanted early gastrula ectoderm Each row shows a case of a transplant of early gastrula ectoderm to the neural plate stage presumptive lens region (as shown in Fig 1A). In each row, left column shows section viewed with differential interference contrast (DIC); middle column shows FLDx fluorescence, and right column shows immunofluorescence (E stained with anti-N-CAM antibody, H stained with anti-lens protein antibody). Abbreviations: he, host eye; ie, induced eye; in, induced neural tissue, ll, induced lens; pr, pigmented retina. (A,B) Section through host eye (he; note pigmented retina [pr] at far right). Transplanted tissue (labelled tissue in B) has formed an induced neural tube-like structure (labelled in, however, the neural nature of this tissue has not been confirmed by antibody staining in this case) and eye tissue (labelled ie), as judged by the presence of pigmented retina, which has fused to the host eye. (C-E) Induced neural tissue The transplant (labelled tissue in D) has made large structures (labelled in), which stain brightly with anti-N-CAM antibody (E), showing that they are neural. (The host eye tissue also stains with N-CAM ) Part of the transplant has also made induced eye tissue (ie), as judged by the appearance of pigmented retina in this portion of the transplant, which is fused to the host eye (he). (F-H) Induced eye and lens. The transplant (labelled tissue in G) has made what is apparently induced eye tissue (ie), as judged by the appearance of pigmented retina, and a small induced lens (ll), as shown by anti-lens antibody staining (H). Magnifications are xllO (A,B), x95(C-E), x!20(F-H).
	Fig. 4. Competence of gastrula ectoderm Ectoderm was removed from FLDx-labelled embryos at various stages of gastrulation and transplanted to neural plate stage embryos, as shown in Fig. IB. Tissues were scored as described in Fig. 2. Bars show the proportion of transplants that formed lens or neural tissue at each of the stages tested. Data for stage 10 are from Fig. 2. n=number of cases scored.
	Fig. 5. Structures induced from transplanted mid-gastrula ectoderm. Each row shows a case of a transplant of mid-gastrula ectoderm to the neural plate stage presumptive lens region (as shown in Fig. IB) In each row, figure at left shows section viewed with DIC; middle figure shows FLDx fluorescence; right figure shows lmmunofluorescence (C,I stained with anti-NCAM antibody; F stained with anti-lens protein antibody). Abbreviations, e, eye; in, induced neural tissue; il, induced lens. (A-C) Induced neural tissue. Transplant (labelled tissue in B) has formed a small mass of induced neural tissue (in) near the host eye (e) The induced tissue has not detached from the ectoderm, and anti-N-CAM staining shows that the transplant stains in a patchy manner (arrow in C; cf Fig. 3E), indicating poor neural development (D-F) Induced lens. Transplant (labelled tissue in E) has formed a large, morphologically well-developed lens (il), as shown by immunostaining (F). (G-I) Induced cranial nerve. Transplant (labelled tissue, marked by arrows in H), has the appearance of a nerve, which stains brightly for N-CAM (arrows in I). Magnifications are x!60 (A-C), X140 (D-F), x!20 (G-I).
	Fig. 6. Competence of ectoderm cultured in vitro. Ectoderm was removed from FLDx-labelled embryos at the early gastrula stage (stage 10), cultured in vitro for various periods of time, and was then transplanted to neural plate stage embryos, as shown in Fig. 1C. The age of the ectoderm was monitored with unoperated sibling embryos. Tissues were scored as described in Fig. 2. Bars show the proportion of transplants that formed lens or neural tissue at each of the stages tested. Data for stage 10 are from Fig 2. n=number of cases scored
	Fig. 7. Structures induced from early gastrula ectoderm, which was cultured in vitro, then transplanted to the presumptive lens region of a neural plate stage embryo (as shown in Fig. 1C). A,D are viewed with DIC; B,E show FLDx fluorescence; C shows immunofiuorescence. Abbreviations: e, eye; il, induced lens; 10, induced otic vesicle; nt, neural tube (A-C) Lens induced from ectoderm cultured to midgastrula (stage 11). Transplant (labelled tissue in B) has formed an induced lens (il), as judged by anti-lens antibody staining (C), showing that ectoderm has acquired lens competence during the period of culture. (D,E) Ear vesicle induced from ectoderm cultured to the equivalent of early neurula (stage 13). Transplanted ectoderm has given nse to an otic vesicle (io), indicating that even after long periods of culture, ectoderm still has ear competence. Magnifications are X125 (A-C), X145 (D,E).
	Fig. 8. Structures induced from early gastrula ectoderm, which was divided into dorsal and ventral halves, cultured in vitro until stage 11 (mid-gastrula), then transplanted to the presumptive lens region of a neural plate stage embryo. Tissues were scored as described in Fig. 2 Bars show the proportion of transplants that formed lens or neural tissue n=number of cases scored.
	Fig. 9. Successive changes in ectodermal competence during development Periods of competence which are well-established are shown as solid lines, those that are not unambiguously known are shown as dashed lines. The timing of gain and loss of mesoderm competence and loss of neural competence are well-documented (see text for references); gain and loss of lens competence are from data presented here The gain of neural competence has not been extensively studied using natural inductor tissues (see Nieuwkoop et al. 1985). Ear competence is known to be present during neurula stages (Gallagher et al. in preparation) and has been lost from cultured ectoderm by stage 18 (M. Servetnick, unpublished observations), but gain and loss of ear competence have not been studied in detail. The period of competence for cranial neural placodes is based on observations made here, and by Nieuwkoop (1958). In general, the end of competence can be readily determined; gain of competence is more difficult to define experimentally, especially if the period during which the inductive signal is present is not known precisely, since the transplant may acquire competence some time after transplantation and still be exposed to inductive influences, which often persist for a greater penod of time than the window of competence (see text for examples; see also Waddington, 1936). Although individual periods of competence are shown to overlap, it has not been clearly established whether individual cells have more than one competence at a given time. [Some of the features of this model were originally proposed by Nieuwkoop (1973).]