Lecture 71 of 373: Axis and Pattern Formation in Amphibia and Chick (58 mins) | CUET (Common University Entrance Test) PG Zoology (SCQP28) | Complete Video Course 373 Lectures [222 hrs : 42 mins]

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Axis and pattern formation in Amphibia – An Overview:

The dorsal lip of the blastopore forms the organizer tis sue of the amphibian gastrula. This tissue dorsalizes the ectoderm, transforming it into neural tissue, and it transforms ventral mesoderm into lateral and dorsal mesoderm.

The organizer consists of pharyngeal endoderm, head mesoderm, notochord, and dorsal blastopore lip tissues. The organizer functions by secreting proteins (Noggin, Chordin, and Follistatin) that block the BMP signal that would otherwise ventralize the mesoderm and activate the epidermal genes in the ectoderm.

Dorsal-ventral specification begins with maternal messages and proteins stored in the vegetal cytoplasm. These include Nodal-like paracrine factors, transcription factors (such as VegT), and agents that protect β-catenin from degradation.

The organizer is itself induced by the Nieuwkoop center, located in the dorsalmost vegetal cells. This center is formed by the translocation of the Disheveled protein and Wn11 to the dorsal side of the egg to stabilize β-catenin in the dorsal cells of the embryo.

The Nieuwkoop center is formed by the accumulation of β-catenin, which can complex with Tcf3 to form a tran scription factor complex that can activate the transcription of the siamois and twin genes on the dorsal side of the embryo.

The Siamois and Twin proteins collaborate with activat ed Smad2 transcription factors generated by the TGF-β pathway (Nodal, Vg1) to activate genes encoding BMP inhibitors. These inhibitors include the secreted factors Noggin, Chordin, and Follistatin, as well as the transcrip tion factor Goosecoid.

In the presence of BMP inhibitors, ectodermal cells form neural tissue. The action of BMP on ectodermal cells causes them to become epidermis.

In the head region, an additional set of proteins (Cerberus, Frzb, Dickkopf, Tiki) blocks the Wnt signal from the ventral and lateral mesoderm.

Wnt signaling causes a gradient of β-catenin along the anterior-posterior axis of the neural plate that appears to specify the regionalization of the neural tube.

Insulin-like growth factors (IGFs) help transform the neural tube into anterior (forebrain) tissue.

The left-right axis appears to be initiated by the activation of a Nodal protein solely on the left side of the embryo. In Xenopus, as in other vertebrates, Nodal protein activates expression of pitx2, which is critical in distinguishing left-sidedness from right-sidedness.

Cleavage in fish is meroblastic. The deep cells of the blastoderm form between the yolk syncytial layer and the enveloping layer. These deep cells migrate over the top of the yolk, forming the hypoblast and epiblast.

On the future dorsal side, the hypoblast and epiblast intercalate to form the embryonic shield, a structure homologous to the amphibian organizer. Transplantation of the embryonic shield into the ventral side of another embryo will cause a second embryonic axis to form.

In both amphibians and fish, neural ectoderm is per mitted to form where the BMP-mediated induction of epidermal tissue is prevented. The fish embryonic shield, like the amphibian dorsal blastopore lip, secretes the BMP antagonists. Like the amphibian organizer, the shield receives its abilities by being induced by β-catenin and by underlying endodermal cells expressing Nodal related paracrine factors.

Axes and Pattern Formation in Chick – An Overview

The prechordal plate helps induce formation of the forebrain; the chordamesoderm induces formation of the midbrain, hindbrain, and spinal cord. The first cells migrating laterally through the primitive streak become endoderm, displacing the hypoblast. The mesoderm cells then migrate through the primitive streak. Meanwhile, the surface ectoderm undergoes epiboly around the yolk.

In birds, gravity helps determine the position of the primitive streak, which points in a posterior-to-anterior direction and whose differentiation establishes the dorsal-ventral axis. The left-right axis is formed by the expression of Nodal protein on the left side of the embryo, which signals Pitx2 expression on the left side of developing organs.

The hypoblast helps determine the body axes of the embryo, and its migration determines the cell movements that accompany formation of the primitive streak and thus its orientation.