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Ectoderm

The "ectoderm" is one of the three primary germ cell layers in the early embryo. The other two layers are the mesoderm (middle layer) and endoderm, with the ectoderm as the most exterior layer. It forms from the outer layer of germ cells.

Ectoderm is the external layer and forms the nervous system, including the brain, spinal cord, peripheral nerves. Ectoderm also gives rise to the skin of the integumentary system, as well as nails, sweat glands, nostrils and anus.

In vertebrates, the ectoderm has three parts: external ectoderm (also known as surface ectoderm), the neural crest, and neural tube. The latter two are known as neuroectoderm.

Specification of Ectoderm

It is a topic of great research to elucidate the factors used to determine the specification of ectoderm, but also mesoderm. One of the earliest contributing factors found was Ectodermin- a ubiquitin ligase. Others like POU91, FOXle and FAM have also been identified.

Role of Ectodermin

The protein Ectodermin, firstly identified in Xenopus embryos, promotes ectodermal fate and suppresses the mesoderm formation mediated by the signaling of Transforming Growth Factor β (TGFβ) and Bone Morphogenic Proteins (BMP), members of the TGFβ-superfamily. When the TGFβ ligands bind to TGFβ receptors, they cause the activation of the signal transducers R-Smads (Smad2, Smad3). Smad4 forms a complex with activated R-Smads and activates transcription of specific genes in response to TGFβ signal. The BMP pathway transmits its signals in a similar way but through other types of R-Smads (Smad1, Smad5 and Smad8). The transcription factor Smad4 is the only common mediator shared between both TGFβ and the BMP pathways.During ectoderm specification, the function of Smad4 is regulated by ubiquitination and deubiquitination made by Ectodermin and FAM (acronym of Fat acets in mammals) respectively. The ubiquitination state of Smad4 will determine if it is be able to respond to signals derived from TGFβ and BMP.The equilibrium of the activity, localization and timing of TGFβ and BMP transducers, Smad4, FAM and of Ectodermin should be achieved in order to be able to modulate the gene expression of genes required for germ layer formation. Derivatives of Ectoderm

Neurectoderm, Mesectoderm.