XB-ART-18464Dev Dyn 1996 Mar 01;2053:265-80. doi: 10.1002/(SICI)1097-0177(199603)205:3<265::AID-AJA7>3.0.CO;2-G.
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A sticky problem: the Xenopus cement gland as a paradigm for anteroposterior patterning.
The cement gland is a mucus-secreting organ found at the extreme anterior of frog embryos. It attaches the embryo to a solid support before swimming and feeding begin, and also serves a related sensory function that stops the embryo from moving once it is attached. Cement gland is an extremely useful anterior marker, whose study continues to yield fundamental information concerning vertebrate axial patterning. Cement gland arises from the outer layer of the embryonic ectoderm and, in Xenopus, forms a cone of columnar epithelium. It is the first ectodermal organ to differentiate, beginning to do so by late gastrula. A battery of genes expressed in the developing and mature cement gland serve as useful markers. Cement gland development can be influenced by both stimulatory and inhibitory cell interactions. Stimulatory signals arise from the anterior neural plate, head endoderm, and the dorsal mesoderm. Inhibitory signals are present in the posterior dorsal mesoderm and in ventral ectoderm and mesoderm. Further, signalling between the ectodermal layers may restrict cement gland differentiation to the outer ectodermal cells. Several secreted molecules are able to induce or repress cement gland formation: these include noggin, follistatin, hedgehog, chordin, retinoic acid, embryonic fibroblast growth factor (eFGF), Bone Morphogenetic Protein-4 (BMP-4), and Xwnt-8. Several of these factors alter expression of the homeodomain gene Xotx2, which may be a transcriptional activator of cement gland differentiation genes. The significance of the cell interactions and factors described in positioning cement gland at the front of the embryo is explored.
PubMed ID: 8850563
Article link: Dev Dyn
Species referenced: Xenopus
Genes referenced: ag1 bmp4 chrd.1 fgf4 fst muc2 muc2l nog otx2 shh tbx2 wnt8a xa-1
Article Images: [+] show captions
|Fig. 1. Morphology, histology, and morphogenesis of the anuran cement gland. A: The Xenopus laevis cement gland. Xenopus embryos have a single conical cement gland. A whole tailbud stage embryo is shown on the left (redrawn from Nieuwkoop and Faber, 1967). Arrowhead: cement gland. A longitudinal section through half of the cement gland is shown on the right. Plane of section is shown on whole embryo. Inner ectoderm: the inner layer of the ectoderm beneath the cement gland. Redrawn from Lyerla and Pelizzari (1976). CG: the length of cement gland cells. Storage vesicle: contains lipids. Epidermis: the outer layer of the ectoderm covers the cement gland cells leaving only the secretory tips uncovered. 6: The cement glands of Rana plafyrrhinus and R. esculenta are bifurcated. A whole R. plafyrrhinus embryo at hatching stage is shown on the left, and on the right a longitudinal section through half of one of the cement glands of its close relative R. esculenta. Plane of section is shown on whole embryo. Redrawn from Lieberkind, 1937. Arrowhead: Cement gland. C: Development of cement glands in R. esculenfa, on which Johannes Holtfreter did much of his work. The cement gland primordium begins as a pigmented patch of cells which invaginate to form a V-shaped groove through which the elongated cement gland cells project. By hatching stage, the cement gland has completely separated into a paired organ. Arrowheads indicate the position of the cement gland. Redrawn from Lieberkind (1937).|
|Fig. 2. Developmental time course of three cement gland markers in Xenopus. In situ hybridization analysis using digoxygenin labeled antisense probes hybridized to a developmental series of whole Xenopus embryos. XCG (A-D) and XAG (E-H) (Sive et al., 1989) transcripts are first detected at late gastrula, in dispersed cells within the outer layer of the dorsal ectoderm, just anterior to the presumptive neural plate (A, E). XCG is detected in presumptive cement gland cells only, whereas XAG is detected in cement gland cells (arrowheads) and more weakly in the adjacent, posterior hatching gland cells (arrows). XA transcripts (CL) (Sive et al., 1989) are initially detected at late gastrula stage in the hatching gland primordium (I), and then at early neurula stage, in the cells of the posterior cement gland (J). Refer to Cement Gland Anatomy: Morphology and Molecules in the text for discussion of these data. In situ hybridization method adapted from Harland (1991).|
|Fig. 3. Timing of Xenopus cement gland specification. Using specification assays, where ectoderm is explanted from the embryo at various stages and allowed to differentiate in culture, the following conclusions can be reached regarding timing of cement gland induction. See Timing of Cement Gland Induction in the text for discussion. A: At the onset of gastrulation the inner layer of the ectoderm is specified as pre-cement gland, both dorsally and ventrally. V: ventral. D: dorsal. B: By early gastrula, in addition to continued cement gland specification in the inner layer, the outer dorsal ectoderm is weakly specified as cement gland. A: anterior. P: posterior. C: At mid to late gastrula the cement gland primordium is specified in the outer layer of the anterior dorsal ectoderm, while the inner layer of the dorsal ectoderm has lost this specification and presumably been reprogrammed towards a neural fate|
|Fig. 4. Cell interactions that can modulate cement gland (CG) induction. A: Schematic of a mid-gastrula stage Xenopus embryo. B: Expansion of boxed area in A, showing potential cellular interactions revealed by explant, transplant and einsteck assays. See Mutipie Signalling Tissues Influence Cement Gland Formation in the text and Table 2 for details. Positive inducing signals (arrows) are found in the yolky endoderm (1) at mid-gastrula stage and in the pre-chordal plate (2) as well as more posterior dorsal ectoderm (3) (including presumptive neural plate). The inductive signals derived from the mesendoderm (1 and 2) appear to be attenuated by the inner ectoderm. Inhibitory signals (T-bars) are found in the outer dorsal ectodermal layer (4) of early-mid-gastrula embryos and in the presumptive notochord (5) as well as in the ventral ectoderm (6) in late gastrula embryos. As discussed in the text, presumptive notochord can also be a strong inducer of cement gland, depending on the ectodermal substrate. The cement gland primordiurn lies approximately at the boundary between the dorsal and ventral regions. A, anterior; P, posterior; D, dorsal; V, ventral; ARC., archenteron.|
|xa-1 (anterior and ectodermic-specific protein) gene expression in Xenopus laevis embryos, NF stage 28, as assayed by in situ hybridization, anterior view, dorsal up.|