Proliferation patterns of dorsal root ganglion neurons of cutaneous, muscle and visceral nerves in the rat

In a previous study we provided evidence that dorsal root ganglion (DRG) neurons of different phenotypes have different birthdates. The present study aimed at determining if birthdates of DRG neurons are related to different types of peripheral nerves, namely cutaneous versus muscle, and somatic versus visceral. Pregnant rats were injected intraperitoneally with bromodeoxyuridine (BrdU) to label the neurons on one of the embryonic days E12-E16. When the progeny rats reached adulthood, a mixture of 1% B-fragment of cholera toxin and 1% isolectin B4 from Griffonia simplicifolia I was injected into the peripheral nerves, or a 5% Fluoro-Gold solution was applied to the transected end of the nerves. The saphenous and sural nerves were used as cutaneous nerves, the gastrocnemius nerve as a muscle nerve, the intercostal nerves T9-11 as somatic nerves and the greater splanchnic nerve as a visceral nerve. Cell size measurements were made of DRG neurons labeled from the two cutaneous nerves and the muscle nerve, as well as of neurons of the saphenous and gastrocnemius nerves labeled by BrdU at different embryonic stages. Most of the DRG neurons of the muscle and intercostal nerves were generated early, with peaks at E13, and those of the cutaneous and visceral afferent nerves later, with peaks at E14. The temporal differences were reflected in the cell size spectrum, the muscle nerve having a greater proportion of large neurons compared to the cutaneous nerves. The findings add to previous knowledge regarding the sequence of development of different DRG phenotypes.