HAMSTER CHEEK POUCH SKIN HOMOGRAFTS IO5 



of the three hamster strain combinations employed for the present 

 study it has also been found that a significant proportion of skin 

 homografts from near term, or early postnatal donors long outlive 

 skin homografts from adult donors. An analysis of the mechanism 

 involved is in progress. Prehminary results indicate that the hosts 

 behave as if they have failed to receive an adequate immuno- 

 logical stimulus from the healthy-looking grafts. (2) Merwin and 

 Hill's (1954) observation that the survival of very small homo- 

 grafts of thyroid or Harderian gland tissue implanted sub- 

 cutaneously into mice was greatly, if not indefmitely, prolonged 

 if their initial vascularization was delayed for a few weeks. 

 Nevertheless, vascularized homografts manifesting prolongation 

 of survival remained fully susceptible to resistance elicited in the 

 hosts by subsequent grafts of similar genetic origin. 



With the exception of hamster pouch skin, the only other 

 example of an immunologically privileged tissue is cartilage. 

 The long-term survival of cartilage homografts is now well- 

 documented (see Craigmyle, 1958; Gibson, Davis and Curran, 

 1958) and seems to depend principally upon distinctive physico- 

 chemical properties of the matrix in which the chondrocytes 

 reside. Besides underlying the weak isoantigenicity of cartilage 

 homografts, tliis matrix affords the cells complete protection even 

 in specifically immunized hosts (Craigmyle, i960). The situation 

 here is comparable with that of Woodruff's (1957) orthotopic 

 split-thickness skin homografts on rats, separated from their hosts 

 by a cell-impermeable Millipore membrane; and with that of 

 homologous cells enclosed in Millipore chambers inserted into 

 peritoneal cavities (Weaver, Algire and Prehn, 1955). In none of 

 these circumstances were the grafted cells or tissues antigenically 

 effective, nor could a state of pre-existing sensitization on the part 

 of the host be brought to bear upon the potentially susceptible 

 target cells so long as the entry ojhost cells was prevented. 



The results of heterotopic grafting tests have shown that cornea 

 is not an immunologically privileged tissue, though the intact 



