Early development of vestibular receptors in human embryos. An electron microscopic study.

The development of the vestibular receptors in 7 to 9-week-old human embryos was investigated by light, scanning and transmission electron microscopy. The greater part of the vestibular epithelium is undifferentiated in the 7-week-old embryo. It is composed of polystratified epithelial cells. Some afferent endings are found at the base of the epithelial cells but no synaptic specializations are detected. A few afferent endings reach the upper part of the epithelium. Thus, at this stage the fibers are present in the vestibular epithelium before the differentiation of the sensory cells. The apical pole of the epithelial cells presents basal bodies linked with striated rootlets and other typical vestibular epithelium cell structures. Short hair bundles are found in a very small part of the vestibular epithelium. At 8-9 weeks of gestation, numerous nerve endings surround the base of the sensory cells. Densifications of the pre- and postsynaptic membranes and synaptic bodies are seen. The newly afferented cells present polarized hair bundles. We suspect that the hair cells are important for the guidance of afferent terminals perhaps even before their morphological differentiation.