Placodes

Placodes are thickenings of the cranial surface ectoderm in vertebrate embryos that give rise to a variety of sensory structures and related neural elements. They form in the head region during early development and contribute to peripheral sensory systems as well as certain non-neural tissues. Placodes arise within the pre-placodal region and differentiate through interactions with neighboring tissues, including neural crest cells and underlying mesenchyme, under the influence of signaling pathways such as BMP, FGF, and Wnt.

Placodes are commonly divided into neurogenic and non-neurogenic groups. Neurogenic placodes generate sensory neurons and associated epithelia that contribute to cranial sensory ganglia and sense organs. Notable examples include the olfactory placode, which forms the olfactory epithelium and its sensory neurons; the otic placode, which invaginates to form the inner ear structures responsible for hearing and balance; and the epibranchial placodes, which give rise to sensory neurons of certain cranial nerves. Some classifications expand this group to include placodes that contribute to diverse cranial sensory pathways.

Non-neurogenic placodes include the lens placode, which forms the lens of the eye, and the adenohypophyseal (hypophyseal) placode, from which the anterior pituitary gland (adenohypophysis) develops. These placodes do not principally produce neurons but instead contribute essential non-neural tissues to the head.

In summary, placodes are key embryonic ectodermal thickenings that organize the formation of major sensory organs and related structures in the vertebrate head, playing a central role in the development of the cranial sensory system and pituitary axis.