Histochemistry of sympathetic neurons allotransplanted from young and aged mice to the submandibular gland

Sympathetic ganglion tissue of young (3 months) and aged (24 months) NMRI mice was allotransplanted into the submandibular gland to study the influence of aging on the survival of grafted neurons. The submandibular gland (SMG) was chosen as a host tissue because of its high concentration of NGF and good blood supply. Four weeks postgrafting the viability of transplants was evaluated using the formaldehyde-induced fluorescence technique, tyrosine hydroxylase (TH) immunohistochemistry, and morphometry. The density of neurons, catecholamine fluoresence and TH immunoreactivity (THIR) appeared to be almost unchanged when the transplant was completely surrounded by the SMG tissue, whereas transplants located within the interlobular septum and capsule, or even outside the capsule, showed significantly reduced neuronal survival. The shape of most of the transplanted neurons was not different from those in the intact ganglia. The average diameter of the transplanted young neurons was significantly decreased; this was not the case with the aged neurons. The histograms of grouped diameter values showed a shift to smaller cells in ganglion transplants in both age groups. The transplants in mice treated with 6-OH-dopamine showed considerable regrowth of adrenergic nerve fibers. There seemed to be no marked difference in the survival of transplanted neurons between aged and young animals. The results indicate that the sympathetic neurons from both young adult and aged animals survive the allotransplantation procedure. The neurotrophic factors together with dense vascularization present in the mouse submandibular gland may be beneficial for the restoration of the integrity of mature and aged adrenergic neurons.