THE aHAJ'TIXG OF TISSUES 



703 



toiionotically. and also those strueturos 

 ^vhiell have the greatest power to regen- 

 erate, best withstand the injuries re- 

 sulting from the process of grafting. 

 Thus, the excretory ducts are more re- 

 sistant tlian the secreting portion of the 

 glands, the small follicles of the ovary 

 are more resistant than the large fol- 

 licles, and the ordinary connective tis- 

 sue cells are more resistant than the 

 corpus hit emu. 



The transplantability of tissues,how- 

 ever,is greater than is usually assumed. 

 The spleen tissue may live for a consid- 

 erable period of time, if transplanted 

 into near relatives. After transplanta- 

 tion of liver, carried out under favor- 

 able conditions, not only bile ducts and 

 connective tissue may survive, but also 

 the liver tissue proper. The liver cells 

 even may multiply by mitosis under 

 these conditions. 



There exists a definite graded rela- 

 tion between the structure of a tissue 

 and the amount of unfavorable condi- 

 tions which such a tissue can tolerate 

 without being destroyed. Such unfa- 

 voraljle conditions are always present 

 after transplantation. Thus the ex- 

 cretory ducts can withstand a greater 

 combination of unfavorable conditions 

 than liver cells or the cells of the con- 

 voluted tubules of the kidney. 



There are other more or less acciden- 

 tal factors which play a part in deter- 

 mining the result of transplantation. 

 It is found that after transplantation 

 of skin, the epidermis is destroyed in a 

 number of cases even under otherwise 

 favorable conditions, and this in spite 

 of the simple structure of the epidermis 

 which, as such, would readily lend itself 

 to transplantation. The presence of 

 hair (combined perhaps with the hard- 

 ness of the keratin, its chief constitu- 

 ent which is constantly produced by 

 growing epidermis) leads to its de- 

 struction. Almost invariably a piece 

 of skin transplanted subcutaneously 

 forms a cyst. Around the hair the 

 transplanted epithelium is injured and 



the stimulus of the hair acting as a 

 solid foreign body causes the connective 

 tissue to grow. Along the hair this 

 connective tissue pushes into the cavity 

 of the cyst, gradually filling it and 

 causing a destruction of the epidermis. 

 Another accidental factor of impor- 

 tance occurs in grafting a lobe of the 

 thyroid gland when it sometimes hap- 

 pens that fat adhering to the thyroid 

 remains attached to the organ and is 

 carried into the host together Avith the 

 graft. The peripherally situated fat 

 tissue prevents the thyroid from devel- 

 oping and consequently there is pro- 

 duced a deficiency in acini in the 

 thyroid graft wherever fat tissue was 

 adherent to the thyroid proper. There- 

 fore great attention must be given to 

 removing carefully all extraneous tissue 

 in carrying out such transplantations. 



Again, the fate of the graft is deter- 

 mined by the relationship between the 

 transplant and host. Each individual 

 belonging to a certain species differs 

 from every other individual in the 

 structure of certain proteins which are 

 a part of all, or almost all, of the va- 

 rious kinds of cells as well as of the 

 blood of each individual. Even chil- 

 dren usually differ from their parents 

 and from their brothers and sisters, in 

 this respect. Greater than between rel- 

 atives is the chemical difference be- 

 tween unrelated individuals; greater 

 still is the chemical difference between 

 different strains belonging to the same 

 species. White mice, for instance, 

 which have been bred in different coun- 

 tries and may be otherwise indistin- 

 guishable from one another, may show 

 marked chemical differences, while be- 

 tween different species the chemical 

 differences are greater still, and the 

 differences increase the farther apart 

 the species are in the genealogical sys- 

 tem. Thus the fate of the graft de- 

 pends upon the chemical relationship 

 between graft and host. Generally it 

 can l)e staled iliat the greater the chem- 

 ical difference between orraft and host. 



