Development of placodes in the epithelium involves multiple inductive interactions between the epithelium and other tissues.
In the case of the sensory placodes, the neural tissue induces placode formation. In the case of ectodermal appendages, the mesenchyme is a source of inductive signals. In all cases, there is reciprocal signaling between the epithelium and the mesenchyme.
In addition to positive signals that induce placode formation (Shh, Wnts, FGFs, BMP antagonists), there are negative signals (such as BMPs) that are important for allowing spacing of ectodermal appendages like teeth and hair follicles.
The developing brain induces the overlying ectoderm to develop into sensory organs.
The neural crest arises at the posterior border of the neural plate and the epidermis; ectodermal placodes arise at the anterior border of the neural plate and the epidermis. Via invagination, ectodermal places then develop into ganglia (nerve bundles) and parts of the ear, eye and nose (sensory organs of the head).
Ectodermal placodes arise from cells at the border of the neural plate and the epidermis in anterior regions of the embryo (in the posterior region, this same border gives rise to the neural crest).
The brain induces development of two dorsolateral rows of ectodermal placodes in the head. The rhombencephalon induces the otic placode -- the first ectodermal placode to develop -- whose invagination forms the inner ear. The telencephalon induces the nasal placodes, whose invagination ultimately connects to the oral cavity. The diencephalon induces the lens placode, whose invagination is part of eye development.
Epithelial placodes do not properly develop in Individuals with ectodermal dysplasia, thus retarding development of ectodermal appendages.
Mice and humans with ectodermal dysplasia have little or no hair, fewer and smaller teeth, few or no sweat glands and small nails. Cloning of mutated genes in ectodermal dysplasia patients led to the discovery of a secreted factor called ectodysplasin-A (EDA) and its receptor (EDAR).
EDA is expressed throughout the epidermis, and EDAR is induced by signals from the mesenchyme (dermis). Islands of EDAR expression are induced by further signals to grow downward into the mesoderm and form placodes.