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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 gastrulaectoderm 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 gastrulaectoderm 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.
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 lensectoderm had been removed.
(B) Presumptive ventralectoderm 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 14embryo. 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).]
