CHAPTER 4 



Blood Cells From Hematopoietic Organs 



of the Embryo 



There is general agreement that dnring the life 

 of the chicken, erythrocytes have an intravascular 

 origin and myelocytes an extravascular origin. 

 From the present study, and the work of others, 

 it appears prohajjle that the broader generaliza- 

 tion can be made that viable erythrocytes and 

 thrombocytes have an intravascular origin and 

 all leukocytes have an extravascular origin. 

 Basically, this agrees with Dantschakoff's (1908a 

 and 1909a) concept, except that she calls a prim- 

 itive blood cell a lymphocyte; whereas, in this 

 study a lymphocyte is considered to be the mature 

 stage of a distinct leukocyte line and not a stem 

 cell or primitive blood cell. 



The erythrocytes and the thrombocytes are the 

 only two cell types normally present in the circu- 

 lating blood of the chick embiyo. All others that 

 develop within hematopoietic organs are held 

 there until after hatching. Granulocytes appear 

 in great abundance in bone marrow, spleen, kid- 

 ney, pancreas, and sometimes in the liver and 

 other parenchymal organs. According to Dant- 

 schakoff (1908a and b) and Danschakoff (19]61j 

 and c), granulocytes also develop in the yolk sac 

 and in the thymus, and Nonidez (1920) found 

 developmental stages in the ovary after hatching. 

 The bird in its hematology is reminiscent of some 

 of its reptilian and amphibian ancestry where 

 blood-cell development is widely scattered over 

 the body (Dawson, 1932). Danschakoff 

 (1916c) observed that in embryo chicks there 

 was a similar wide potency of the mesenchyme to 

 form blood cells. She found that during normal 

 development this potency was restricted to certain 

 tissues ])ut if transplants of pieces of adult organs 

 were made on the allantois of the early embryo, 

 the mesenchyme cells among the striated muscle 

 fibers, and among the cells of liver, kidney, and 

 ovary, and in the walls of major vessels, took 

 on hematopoietic functions. Often the circulat- 

 ing blood simulated that of leukemia. 



Were we to judge from information on mam- 

 mals, we might expect the liver to be an active 

 embryonic hematopoietic organ; but in the 

 chicken it remains practically free of developing 

 blood cells (Bizzozero, 1889, and Dantschakoff, 

 1908a and b). The difference between birds 

 and mammals may be due to the existence of a 

 large yolk sac in birds and a reduction to a rudi- 

 mentary condition in mammals. Dantschakoff 

 observed that the liver of the normal chick em- 

 bryo was not a hematopoietic organ, but accord- 

 ing to Haff (1914) it does have such a function 

 for jjotli erythrocytes and granulocytes. The 

 endothelial cells of the liver sinusoids proved to 

 be the point of origin by way of a "large lympho- 

 cyte" for the different cell types. Wislocki 

 (194.3) observed that in a species of monkey that 

 had adapted the placenta to the function of a 

 hematopoietic organ, the liver had been relieved 

 of this function and showed no blood-cell de- 

 velopment. The pancreas of the chick at 19 days 

 of incubation is packed with heterophils. 

 Wliether this organ serves as a hematopoietic 

 center or as a storage depot for the granulocytes 

 produced by spleen and bone marrow has not 

 been determined. It was noted by Mrs. Effie 

 M. Denington, a member of the staff of this Lab- 

 oratory who made these obsei-vations, that, among 

 different embryos, there is wide variability in 

 the number of heterophils present. 



EMBRYO BONE MARROW 



Certain differences l^etween bone-marrow for- 

 mation in birds and in mammals has been pointed 

 out by Hamilton (1952), who says (p. 508): 



"There is never any independent epiphysial 

 center of ossification in long bones of birds, as 

 there is in mammals. The ends of the bones re- 

 main cartilaginous for awhile and provide for 



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