Gene Expression Patterns 3 (2003) 509–519
www.elsevier.com/locate/modgep
Novel gene expression domains reveal early patterning
of the Xenopus endoderm
Ricardo M.B. Costaa, Julia Masona, Monica Leeb, Enrique Amayaa, Aaron M. Zorna,b,*
b
a
Wellcome Trust/Cancer Research UK Institute of Cancer and Developmental Biology, Tennis Court Road, Cambridge, CB2 1QR, UK
Cincinnati Children’s Hospital Medical Center Research Foundation. Division of Developmental Biology, 3333 Burnet Avenue, Cincinnati, OH 45229, USA
Received 1 January 2003; received in revised form 2 February 2003; accepted 3 March 2003
Abstract
The endoderm gives rise the respiratory and digestive tract epithelia as well as associated organs such as the liver, lungs and pancreas.
Investigations examining the molecular basis of embryonic endodermal patterning and organogenesis have been hampered by the lack of
regionally expressed molecular markers in the early endoderm. By differentially screening an arrayed cDNA library, combined with an in situ
hybridization screen we identified 13 new genes regionally expressed in the early tailbud endoderm of the Xenopus embryo. The putative
proteins encoded by these cDNAs include a cell surface transporter, secreted proteins, a protease, a protease inhibitor, an RNA-binding protein,
a phosphatase inhibitor and several enzymes. We find that the expression of these genes falls into one of three re-occurring domains in the
tailbud embryo; (1) a ventral midgut, (2) posterior to the midgut and (3) in the dorsal endoderm beneath the notochord. Several of these genes are
also regionally expressed at gastrula and neurula stages and appear to mark territories that were previously only predicted by the endoderm fate
map. This indicates that there is significant positional identity in the early endoderm long before stages 28 – 32 when regional specification of the
endoderm is thought to occur. These new genes provide valuable tools for studying endodermal patterning and organogenesis in Xenopus.
q 2003 Elsevier Science B.V. All rights reserved.
Keywords: Xenopus; Endoderm; Patterning; Screen; Gut; Liver; Pancreas; Organogenesis; Hex; Vito; Endocut; Chito; Figo; Fetuinish; Vent-2; Gene
expression
1. Results
The endodermal germ layer gives rise to the epithelial
cells of the gastrointestinal and respiratory systems and
contributes to organs such as the lungs, liver, pancreas,
thyroid and thymus (Wells and Melton, 1999). Recent
advances in Xenopus and zebrafish have elucidated a
molecular pathway controlling initial endoderm formation
(reviewed in; Shivdasani, 2002; Stainier, 2002; Xanthos
et al., 2001) and by twelve hours of development, after
gastrulation the vegetal cells are committed to the
endodermal lineage (Wylie et al., 1987). How the endoderm
is subsequently patterned and generates this vast array of
highly specialized cell types of the gastrointestinal and
respiratory system is poorly understood. By three days of
Xenopus development (, stage 40), regional specification of
the endoderm is manifested by the formation of distinct
organ buds at their appropriate anterior-posterior (A– P) and
dorsal-ventral (D – V) position along the gut tube. Progress
* Corresponding author. Tel.: þ 1-513-636-3770; fax: þ1-513-636-4317.
E-mail address: aaron.zorn@chmcc.org (A.M. Zorn).
in understanding the molecular basis of endodermal
patterning has been hampered by the lack of regional
expressed molecular markers in the early endoderm.
The utility of good molecular markers and the use of
Xenopus embryos for studying early embryonic pattering is
evident by the numerous studies over the past 10 years,
resulting in a detailed understanding of the molecular
mechanisms that pattern the vertebrate mesoderm and
ectoderm. With the aim of applying the same approaches
to endoderm patterning we set out to identify new genes
expressed in the early Xenopus endoderm. We focused on
the poorly understood period of time between gastrula,
when Hex and Cerberus are expressed, and just prior to
organ bud formation, when Hex, Pdx, Cdx and Nkx2.1 mark
differentiating tissues (Fig. 1A).
1.1. Differential hybridization screen
Our goal was to identify genes expressed in the Xenopus
tailbud endoderm in order to provide tools for studying
endodermal patterning. We constructed an arrayed cDNA
1567-133X/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1567-133X(03)00086-3