The anatomy of two functional types of mechanoreceptive 'free' nerve-ending in the head skin of Xenopus embryos.

The structure of the two functional types of 'free' nerve-ending in the head skin of late Xenopus embryos has been examined by horseradish peroxidase staining through their cells in the trigeminal ganglion and by electron microscopy. Type I neurites are identified as the 'movement' detectors by their purely homolateral innervation. They have many fine branches between the superficial skin cells, bearing numerous large varicosities. Type II neurites cross the midline to innervate both sides of the head as do the 'rapid transient' detectors found by physiology. They have a few fairly straight branches between the skin cell layers with few elongated varicosities.