2 The Lymphocyte and Lymphocytic Tissue 



are relatively sparse, and thus an appreciation of which cell types divide is 

 difficult to attain. The exact nature of the dividing cell is seldom indisput- 

 able; one judges the cell type for the most part on the basis of the size of the 

 mitotic figure and the nature of the cytoplasm, both of which are altered by 

 the process of division. 



Studies of section and imprint material from human lymph nodes have 

 resulted in the conclusion that there are many transitional stages 8 of develop- 

 ment from reticular cells to lymphocytes; mitoses occur in all of the stages 

 of development, but they appear to be exceedingly rare in cells comparable to 

 the small lymphocytes of the blood. Mitoses have been said to be most nu- 

 merous in the medium-sized and large lymphocytes of lymph nodes 9 " 12 but it 

 should be realized that these cells only rarely are found in the circulating 

 blood. In infants and in laboratory animals, immature lymphocytes and retic- 

 ular lymphocytes are regarded as possible constituents of the blood. 



A previous concept of lymphocytogenesis, 1 - H with which the late Dr. Hal 

 Downey agreed, can be expressed in the following schema. 



Sections of lymphatic tissue Imprints of lymphatic tissue 



Primitive reticular cells Primitive reticular cells 



(mitoses) (mitoses) 



Giant and large lymphocytes Hematopoietic reticular cells 



(hemocytoblasts) (reticular blast forms) 



(mitoses) (mitoses) 



{ I 



Medium-sized lymphocytes Reticular lymphocytes 



(mitoses) Immature lymphocytes 



\ 



Small lymphocytes 

 (mitoses rare) 



(prolymphocyts) 

 (mitoses) 



\ 



Large, medium-sized, and small lymphocytes 



(mitoses rare) 



The cells that function as lymphoblasts of the node have been considered 

 as progeny of reticular cells in order to explain one facet of the process of 

 lymphocytogenesis, 1 7_13 but the fact that lymphocytes may enlarge and be- 

 come indistinguishable from histiocytes, macrophages, and reticular cells has 

 been demonstrated by many investigators. 14 " 1 " This reverse (?) transforma- 

 tion of lymphocytes to large basophilic cells resembling hematopoietic reti- 

 cular cells' 7 has been acknowledged, 13 and the fact of ultimate transformation 

 of these cells to histiocytes, macrophages, epithelioid cells, giant cells, and 

 fibroblasts has become part of the general morphologic understanding of in- 



