CHAPTER 2 



THE EMBRYOGENESIS OF 

 LYMPHOCYTIC TISSUE 



WILLIAM BLOOM 



DEFINITION OF TERMS 



It is impossible to define to the satisfaction of all the terms "lymphocytes" 

 and "lymphocytic tissue." It would be relatively simple if we agreed that the 

 term "lymphocyte" meant the smaller lymphocytes as represented by the non- 

 monocytic, nongranular leucocytes which gain access to the peripheral blood. 

 This definition is, however, insufficient since it does not consider the relation 

 of these "lymphocytes" to the larger free stem cells in the various hemato- 

 poietic tissues — cells which rarely, if ever, enter the blood stream under 

 normal conditions. There is less difficulty in defining lymphocytic tissue if by 

 this is meant the cellular stroma and the included lymphocytes and free 

 phagocytes which comprise the lymph nodes, white pulp of the spleen, and 

 the solitary and aggregated nodules (and tonsils) of the lamina propria of 

 gastrointestinal, respiratory, and, occasionally, generative tracts of postnatal 

 mammals. But we face further problems in attempting to delimit the thymus, 

 the splenic red pulp, and the accumulations of lymphatic tissue in normal 

 and abnormal bone marrow. 



Nevertheless, in any discussion of the embryogenesis of lymphocytes and 

 lymphocytic tissue, one cannot avoid the obligation of at least trying to define 

 these terms. To this end, it is necessary to consider briefly the hematopoietic 

 tissues of the vertebrates in which there are some striking and illuminating 

 parallels in the phylogenetic and ontogenetic development of blood cells 

 (see Jordan 5 ) . 



COMPARATIVE HEMATOPOIESIS IN VERTEBRATES 



The blood of practically all vertebrates contains erythrocytes, nucleated 

 thrombocytes (nonnucleated platelets in mammals) , lymphocytes, monocytes 



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