Gont LK et al. (1993), Tail formation as a continuation of gastrulatio...

XB-ART-21967

Development 1993 Dec 01;1194:991-1004.

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Tail formation as a continuation of gastrulation: the multiple cell populations of the Xenopus tailbud derive from the late blastopore lip.

Gont LK , Steinbeisser H , Blumberg B , de Robertis EM .

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Three lines of evidence suggest that tail formation in Xenopus is a direct continuation of events initiated during gastrulation. First, the expression of two gene markers, Xbra and Xnot2, can be followed from the blastopore lip into distinct cell populations of the developing tailbud. Second, the tip of the tail retains Spemann's tail organizer activity until late stages of development. Third, lineage studies with the tracer DiI indicate that the cells of the late blastopore are fated to form specific tissues of the tailbud, and that intercalation of dorsal cells continues during tail elongation. In particular, the fate map shows that the tip of the tail is a direct descendant of the late dorsal blastopore lip. Thus, the tailbud is not an undifferentiated blastema as previously thought, but rather consists of distinct cell populations which arise during gastrulation.

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Species referenced: Xenopus
Genes referenced: epha4 fubp1 not tbxt

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	Fig. 1. Xnot2 marks the tip of the tail and its expression can be followed back through development to the dorsal lip. Embryos of various stages were hybridized with antisense Xnot2 RNA probes. Whole-mount hybridization was visualized with an alkaline phosphatase reaction. (A) Stage 35 (48 h) embryo, staining at the tip of the tail. (B) Stage 22 (24 h) embryo, staining at the tailbud. (C) Stage 11 (midgastrula,12 h), staining in prospective notochord cells. (D) Stage 10 (early gastrula, 10 h), staining on the dorsal marginal zone prior to convergence and extension movements. Arrow indicates the dorsal lip.
	Fig. 2. Expression of Xnot2 and Xbra in the blastopore lip and tailbud. Top row: Xnot2 whole-mount in situ hybridizations. Bottom row: Xbra hybrididizations. (a,A') Expression of Xnot2 is restricted to the prospective notochord and dorsal lip (A,B), whereas Xbra is expressed in the propective notochord and the entire circumblastoporal region.
	(CURATORS NOTE:line drawings not shown due to poor reproduction quality- see original article for details) Fig. 3. Xnot2 and Xbra are expressed in distinct regions of the tailbud. Embryos were hybridized with Xnot 2 (A,B,C) or Xbra (ABC antisense RNA, developed with alkaline phosphatase, embedded in paraffin wax and sectioned. (A,A Stage 23, sagittal section; (B,B stage 28, sagittal section; (C, C stage 31, frontal section. (A,B,C) Schematic drawings indicating the anatomy of the region based on analysis of serial sections. Note that Xnot2 is expressed in the chordoneural hinge (anterior wall of the neurenteric canal) and ventral spinal cord. Xbra, in addition to the chordoneural hinge and notochord, is expressed in the posterior wall of the neurenteric canal and in cells of the roof of the spinal cord (arrowheads). Note that the neurenteric canal is visible in the frontal sections (C).
	(CURATORS NOTE: schematic drawings not shown due to poor reproduction quality- see original article for details) Fig. 4. Xnot2 marks the ventral spinal cord, notochord and chordoneural hinge. Sections of stage 23 (1-day, early tailbud) embryos hybridized with Xnot2. (A,B) transverse section showing Xnot2 expression in the notochord and ventral half of the neural tube. (C,D) Sagittal section showing Xnot2 expression in the chordoneural hinge; the neurenteric canal can be seen. (B,D) Schematic drawings indicating anatomical structures.
	(CURATORS NOTE: schematic drawings (A) not shown due to poor reproduction quality- see original article for details) Fig. 5. Formation of the neurenteric canal in Xenopus. (A) Diagram of an early neurula. (B) Photograph of a stage 13 neurula (15 h).(C) Stage 15 embryo with a fully formed neural plate. Note that the lateral lips of the slit blastopore rise and fuse, forming the neurenteric canal which connects the neurenteric opening and the anus. Arrow indicates the dorsal neurenteric opening, arrowhead indicates the anus.

	Fig. 7. Cell intercalation continues during tail formation. Small groups of cells of stage 13 dorsal (A) or lateral (B) lips were injected with DiI and the tail of the embryos analyzed at stage 40 by epifluorescence microscopy. (AB Same tails shown in bright field. Note that, in the notochord, labelled and unlabelled cells are interspersed (arrows in A), which is indicative of cell intercalation. No (notochord); So (somite).
	Fig. 8. Tail organizer activity persists in the chordoneural hinge. All embryos shown here result from the transplantation of tissue fragments into the blastocoele of early gastrulae by the Einsteck procedure. (A) Chordoneural hinge (bottom embryo) and prechordal plate (middle embryo) grafts from early tailbud (stage 25) embryos and a sham-operated embryo (top embryo) are shown. (B) contribution of FDA-labelled chordoneural hinge to the =induced secondary tail. The graft formed only a a small part of the induced tail; by histological analysis the labelled cells formed chordoneural hinge, notochord and some neural tissue ( not shown). (C) Low power view of the same embryo shown in B, showing the orientation of the secondary tail with respect to the host embryo. Note the presence of well-formed dorsal and ventral fins in the secondary tail. (D) Section of a secondary tail resulting from implantation of the tip of the tail of a 2-day tadpole ( stage 35); note the presence of notochord ( No) muscle blocks ( Mu) and neural tube (NT). (E) Graft of the tip of the tail of an unlabelled donor into a host embryo whose cytoplasm was labelled with rhodamine dextran; the majority of the induced tail structures , in particular the notochord and muscle, are red and therefore derived from the host. The tip pf the secondary tail ( outline indicated with arrowheads), and the neural tissue are unlabelled and presumably derived from the graft. The histology of the region indictated by brackets of the same embryo is shown in D. (F) Graft fragment of a rhodamine dextran marked neural tissue obtained from the hindbrain of a stage 30 embryo; (F') Histological section through a similar graft. Hindbrain grafts fail to induce structures. (G) Implanted rhodamine dextran-labelled notochord (stage 30, from the middle third of the embryo and of a similar size to the implant in B); (G') histological section through a similar graft. Notochord does not induce significant structures in the Einstick assay. Cg, cement gland; CNH, chordoneural hinge; DF dorsal fin; VF ventral fin; 1', primary tail, 2', secondary tail; Ne neural tissues; No, notochord.