Figure 5.

Sequential development of whisker- and skin-innervating somatosensory nerve endings. A, Coronal sections of developing mystacial pads. At E12.5 (Aa) and E14.5 (Ab), most axons were found at the developing FSC and form a plexus, which are presumptive DVN axons (arrows), whereas the nociceptive axon tips are first found at E14.5 at the surface skin between each whisker (arrowheads). At E16.5, Merkel endings at the ring sinus level are first observed (arrows). SVN axons are not yet found at this stage (Ac, arrowheads). At P0, Merkel endings and transverse lanceolate endings, but not other type of endings, are observed (Ad, arrows). B, Trigeminal axons innervating the small whiskers are detected at P0. Mechanosensory Merkel endings, transverse and longitudinal lanceolate endings (arrows), as well as nociceptive circumferential free endings (arrowheads) are observed. C, Transverse sections of developing forelimbs. By E14.5, the sensory axons reach their targets and form a plexus underneath the presumptive dermal ridges (Ca). At P0, the innervations are distributed to cover the entire skin but have not formed specialized nerve endings (Cb). Scale bars, 100 μm.