Innervation of the thymus in normal and bone marrow reconstituted severe combined immunodeficient (SCID) mice

The innervation of the thymus was studied in SCID mice: There was a relatively more dense innervation pattern in SCID mice as compared to normal BALB/c mice (from which SCID mice are derived), including nerve fibres immunoreactive for protein gene product 9.5 (PGP 9.5), tyrosine hydroxylase (TH), neuropeptide tyrosine (NPY) and vasoactive intestinal peptide (VIP), although there was no reactivity to substance P (SP) or leucine enkephalin (ENK). Only a few acetylcholinesterase (AChE)-positive nerve fibres were observed in the SCID thymus. Ten weeks after the transfer of bone marrow from normal BALB/c mice into SCID mice no immunoreactivity to the above markers was found, nor was there any AChE reaction, although histologically the thymus appeared normal and dot-blot assays demonstrated the presence of immunoglobulin indicating a return to normal bone marrow function in SCID mice. Both innervation and morphology were restored 6 months after bone marrow transfer. In conclusion, the thymus of SCID mice lacking thymocytes has visible neurotransmitter levels in the nerves, but after thymocyte repopulation by bone marrow transplantation the transmitters are generally not demonstrable. This indicates that the innervation may be more important for the establishment of the microenvironment rather than the maintenance of thymocyte differentiation.