Amphibian cleavage is holoblastic, but unequal due to the presence of yolk in the vegetal hemisphere. Amphibian gastrulation begins with the invagination of the bottle cells, followed by the
coordinated involution of the mesoderm and the epiboly of the ectoderm. Vegetal rotation plays a
significant role in directing the involution. The driving forces for ectodermal epiboly and the convergent extension of the mesoderm arethe intercalation events in which several tissue layers merge. Fibronectin plays a critical role in enabling the mesodermal cells to migrate into the embryo.The dorsal lip of the blastopore forms the organizer tissue of the amphibian gastrula. This tissue dorsalizes the ectoderm, transforming it into neural tissue, and it transforms ventralmesoderm into lateral mesoderm.The organizer consists of pharyngeal endoderm, head mesoderm, notochord, and dorsal blastopore lip. The organizer functions by secreting proteins (Noggin, chordin, and follistatin) that block the BMP signal that would otherwise ventralize the mesoderm and activate theepidermal genes in the ectoderm. In the head region, an addition set of proteins (Cerberus, Frzb, Dickkopf) block the Wnt signal from the ventral and lateral mesoderm.The organizer is itself induced by the Nieuwkoop center, located in the dorsalmost vegetal cells. This center is formed by cortical rotation during fertilization, which translocates the Dishevelled protein to the dorsal side of the egg.The Dishevelled protein stabilizes β-catenin in the dorsal cells of the embryo. Thus, the Nieuwkoop center is formed by the accumulation of β-catenin, which can complex with Tcf3 to form a transcription factor complex that can activate the transcription of the siamois gene.The siamois product and a TGF-β signal (perhaps from Vg1) can activate the goosecoid gene in the organizer. The goosecoid gene can activate other genes that cause the organizer to function.Other posteriorizing signals (Wnt3a, retinoic acid, eFGF) can influence the anterior-posteriorspecification of the neural tube.The left-right axis appears to be initiated at fertilization through the Vg1 protein. In a still unknownfashion, this protein activates a Nodal protein solely on the left side of the body. As in other vertebrates, theNodal protein activates expression of Pitx2, which is critical in distinguishing left-sidedness from rightsidednessin the heart and gut tubes.
coordinated involution of the mesoderm and the epiboly of the ectoderm. Vegetal rotation plays a
significant role in directing the involution. The driving forces for ectodermal epiboly and the convergent extension of the mesoderm arethe intercalation events in which several tissue layers merge. Fibronectin plays a critical role in enabling the mesodermal cells to migrate into the embryo.The dorsal lip of the blastopore forms the organizer tissue of the amphibian gastrula. This tissue dorsalizes the ectoderm, transforming it into neural tissue, and it transforms ventralmesoderm into lateral mesoderm.The organizer consists of pharyngeal endoderm, head mesoderm, notochord, and dorsal blastopore lip. The organizer functions by secreting proteins (Noggin, chordin, and follistatin) that block the BMP signal that would otherwise ventralize the mesoderm and activate theepidermal genes in the ectoderm. In the head region, an addition set of proteins (Cerberus, Frzb, Dickkopf) block the Wnt signal from the ventral and lateral mesoderm.The organizer is itself induced by the Nieuwkoop center, located in the dorsalmost vegetal cells. This center is formed by cortical rotation during fertilization, which translocates the Dishevelled protein to the dorsal side of the egg.The Dishevelled protein stabilizes β-catenin in the dorsal cells of the embryo. Thus, the Nieuwkoop center is formed by the accumulation of β-catenin, which can complex with Tcf3 to form a transcription factor complex that can activate the transcription of the siamois gene.The siamois product and a TGF-β signal (perhaps from Vg1) can activate the goosecoid gene in the organizer. The goosecoid gene can activate other genes that cause the organizer to function.Other posteriorizing signals (Wnt3a, retinoic acid, eFGF) can influence the anterior-posteriorspecification of the neural tube.The left-right axis appears to be initiated at fertilization through the Vg1 protein. In a still unknownfashion, this protein activates a Nodal protein solely on the left side of the body. As in other vertebrates, theNodal protein activates expression of Pitx2, which is critical in distinguishing left-sidedness from rightsidednessin the heart and gut tubes.
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