Placodes

Introduction

In human development, during week 4 a series of thickened surface ectodermal patches form in pairs rostro-caudally in the head region.

Recent research suggests that all sensory placodes may arise from common panplacodal primordium origin around the neural plate, and then differentiate to eventually have different developmental fates. (More? Schlosser G.)

These sensory placodes will later contribute key components of each of our special senses (vision, hearing and smell). Other species have a number of additional placodes which form other sensory structures (fish, lateral line receptor). Note that their initial postion on the developing head is significantly different to their final position in the future sensory system (More? Sensory Notes).

Otic placode- in the stage 13/14 embryo (shown below) the otic placode has sunk from the surface ectoderm to form a hollow epithelial ball, the otocyst, which now lies beneath the surface surrounded by mesenchyme (mesoderm). The epithelia of this ball varies in thickness and has begun to distort, it will eventually form the inner ear membranous labyrinth.

Optic (Lens) placode- lies on the surface, adjacent to the outpocketing of the nervous system (which will for the retina) and will form the lens.

Nasal placode- has 2 components (medial and lateral) and will form the nose olefactory epithelium.