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Ectodermal appendages

What Are Ectodermal Placodes? What Sensory Organs Do They Form?

A placode is a thickened region of the ectoderm, and are induced at the anterior end of the neural plate where BMP levels are intermediate (this same region gives rise to neural crest cells). Ectoderm is epithelial, while the mesoderm contains both mesenchymal and epithelial cells.

Sensory placodes and their determining signals: nasal placodes (Pax6); lens placodes (Pax6); trigeminal nerve (Pax3/8); facial nerve (?); ear (Pax2); glosso-pharyngeal nerve (?); vagus nerve (?). Pax6 expression is inhibited by high concentrations of Shh and BMP, causing holoporsencephaly.

Describe Induction in Vertebrate Eye Development. What Is Competence? What Is Pax6?

The optic vesicle induces the lens placode; the lens vesicle induces the optic cup. Competence is the ability to respond to a signal. Only anterior ectoderm is competent to respond to the inductive signal from neural tissue. Transplanting an optic vesicle outside the anterior ectoderm will not result in ectopic eye formation.

Human Pax6 is needed for eye formation; its homologs in other species are murine Smalleye and Drosophila Eyelesss. In Drosophila, ectopic expression of human Pax6 is sufficient to induce ectopic eye formation! In mice and humans, Pax6-/- mutants never even develop an optic cup, and wind up with eyes nor a nose.

Pax6 is expressed in the optic vesicle (neural tissue) and in the lens vesicle it induces (an ectodermal placode). Researchers wondered whether Pax6 is required in both or only tissue to induce the lens. Tissue recombination experiments between Pax6+/+ and Pax-/- embryos answered this question.

Optic VesicleSurface (Lens) EctodermLens Formation?
WTWTYes
WTPax6-/-None
Pax6-/-WTYes
Pax6-/-Pax6-/-None

Conclusion: Pax6 is required in the lens ectoderm for lens induction (required for competence to respond).

Describe Regional Specificity of Induction

Epithelial-mesenchymal interactions that induce cutaneous structures are described as regionally specific. Skin is composed of two main tissues: an outer epidermis (ectoderm-derived epithelium); and an inner dermis (mesoderm-derived mesenchyme). Chick epidermis secretes Shh and TGF-β, which induces underlying dermis to condense; this condensed dermal mesenchyme in turn secretes factors that cause the epidermis to form regionally specific cutaneous structures.

Mesoderm specifies the identity of regional overlying structures. Researchers separated embryonic epithelium and mesenchyme from each other; the embryonic epithelium and mesenchyme was recombined in various ways. In every case, the a given mesenchyme imparted the same identity to any of the overlying embryonic epithelia. For example, chick leg mesenchyme transplanted onto wing embryonic epithelium, will induce leg feathers in the wing embryonic epithelium.

Describe Genetic Specificity of Induction

Epithelial-mesenchymal interactions that induce cutaneous structures are described as genetically specific. Mesenchyme instructs epithelium to activate certain genes in a regionally specific manner. However, the epithelium responds only as its genome permits. This was discovered by transplanting together tissues from different species.

Spemann and Schotté transplanted frog flank ectoderm to the oral region of a newt gastrula. Similarly, they transplanted newt flank ectoderm to the oral region of a frog gastrula. The newt gastrula developed a froglike mouth; the frog gastrula developed a newtlike mouth. In other words, the mesoderm instructed the ectoderm to make a mouth but the foreign ectoderm did so according to its own genome.

This reveals that mesenchymal tissue can induce overlying ectoderm across species barriers, but that the epithelium responds in a species-specific manner. The type of organ induced is controlled by mesenchyme, but species specificity is controlled by the responding epithelium.

What Is Ectodermal Dysplasia? What Does It Reveal About Ectodermal Appendage Development

Ectodermal Dysplasia is the abnormal development of cutaneous structures: fewer, smaller teeth, sparse hair, small fingernails and toenails, few sweat glands and epithelial placode retardation. Edar is expressed in ectodermal placodes. Eda is expressed in the surrounding epidermis. BMP-4 is a placode inhibitor.

Foxi3 is transcription factor involved in appendage formation. Foxi3 mutants have defects in hair, teeth, sweat glands. Thus, studies of ectodermal dysplasia have revealed that ectodermal appendage development requires expression of Foxi3. Without Foxi3, ectodermal appendages cannot develop properly.