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 anter ior of frog embryos, It attaches the embryo to a solid support before swimming and feeding bean, 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 f undamental information concerning vertebrate axial patterning. Cement gland arises from the outer layer of the embryonic ectoderm and, in Xe nopus, forms a cone of columnar epithelium. It is the first ectodermal organ to differentiate, beginning to do so by late gastrula. A batter y of genes expressed in the developing and mature cement gland serve a s useful markers, Cement gland development can be influenced by both s timulatory and inhibitory cell interactions, Stimulatory signals arise from the anterior neural plate, head endoderm, and the dorsal mesoder m. Inhibitory signals are present in the posterior dorsal mesoderm and in ventral ectoderm and mesoderm. Further, signalling between the ect odermal layers may restrict cement gland differentiation to the outer ectodermal cells. Several secreted molecules are able to induce or rep ress cement gland formation: these include noggin, follistatin, hedgeh og, chordin, retinoic acid, embryonic fibroblast growth factor (eFGF), Bone Morphogenetic Protein-4 (BMP-4), and Xwnt-8. Several of these fa ctors alter expression of the homeodomain gene Xotx2, which may be a t ranscriptional activator of cement gland differentiation genes, The si gnificance of the cell interactions and factors described in positioni ng cement gland at the front of the embryo is explored. (C) 1996 Wiley -Liss, Inc.