Utility of neonatally tolerized rats for cell tracking. To test whether neonatal exposure of wild-type rats to transgenic cells results in long-lasting tolerance to the marker gene, skin from hPLAP-tg donor was grafted into a neonatally tolerized, 4-month-old F344 wild-type rat. Four weeks post-transplantation, the skin graft from the transgenic donor (upper part of the section) shows strong expression of hPLAP in all cells (A). The border between transgenic donor tissue and wild-type recipient tissue is sharply delineated (arrowheads). A sebaceous gland (arrow in A), and a few capillaries (arrows in B) of transgenic donor origin are found in the surrounding wild-type skin. Keratinocytes do not cross the sharp border between wild-type and transgenic epidermis. Fig. 5B is a high power view of the frame shown in (A). When hPLAP-tg BMC were intravenously injected in neonatally tolerized F344 WT recipients, transgenic donor leukocytes are clearly present in an abdominal lymph node, 2 months post-injection (C-D). Fig. 5D is a high power view of the frame shown in (C). However, hPLAP-tg BMC injected intravenously in neonatally tolerized F344 WT recipients do not home to bone, bone marrow, or cartilage (E-F). Five-μm-thick paraffin or MMA sections of ethanol-fixed skin biopsy (A-B), abdominal lymph node (C-D), tibia (E), and knee joint (F) stained for hPLAP activity after heat pretreatment and counterstained with nuclear fast red. Bar = 100 μm (A, C) or 50 μm (B, D-F).