Fertilization can occur anywhere in the animal hemisphere of the amphibian embryo, but the point of sperm entry is very imperative because it determines the orientation of the dorsal-ventral axis of the larva (tadpole). The point of sperm entry will mark the ventral side of the embryo, while the site 180 degrees opposite the point of sperm entry will mark the dorsal side.
Cleavage in most frog embryos is radially symmetrical and holoblastic, just like echinoderm cleavage. As we mentioned that amphibian cleavage is holoblastic, but it is unequal due to the presence of yolk in the vegetal hemisphere.
Gastrulation in amphibian begins with the invagination of the bottle cells, followed by the coordinated involution of the mesoderm and the epiboly of the ectoderm. The rotation of the vegetal pole plays a important role in directing the involution pattern. During gastrulation, the driving forces for ectoderm epiboly and the convergent extension of the mesoderm are the intercalation events in which several tissues layers merge. What play important role to make sure that mesodermal cell to migrate into the embryo? It is the Fibronectin.
The dorsal lip of the blastopore form the organizer tissue of the amphibian gastrula. The tissue dorsalizes the ectoderm, transforming it into neural tissues and transforms ventral mesoderm into lateral and dorsal mesoderm. The organizer comprises of pharyngeal endoderm, head mesoderm, notochord, and dorsal blastopore lip tissues. The organizer functions by secreting protein (Noggin, chord in, and follistatin) that block the BMP signal that would otherwise centralize the mesoderm and activate the epidermal gene in the ectoderm.
The organizer itself induced by the Nieuwkoop center, located in the dorsal most vegetal cells. This center is formed by the translocation of the Disheveled protein and Wnt11 to the coral side of the egg to stabilize beta-catenin in the dorsal cells of the embryo.
The Siamois and Twin protein collaborate with Smad2/4 transcription factors generated by the TGF beta pathway (Nodal, Vg1, and activin) to activate genes encoding BMP inhibitors. Noggin, chord in, and follistatin, as well as the transcription factor Goosecoid are the BMP inhibitors. When BMP inhibitors are present, the ectoderm cells form neural tissue, but the action of BMP on ectoderm cells causes them to become epidermis. For the head region, an additional set of proteins (Cerberus, Frzb, Dickkopf) block the Wnt signal from the ventral and lateral mesoderm.
Insulin-like growth factor (IGF) help transform the neural tube into anterior (forebrain) tissue.
This is just snap view of amphibian early development and you can look into other biology texts books or online developmental biology for more details and thanks for reading.
References:
www.wikipedia.com
Developemental Biology 9 edition by Scott F. Gilbert.
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