Blood Cells From Hematopoietic Organs of the Embryo
There is general agreement that during 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 probable that the broader generalization can be made that viable erythrocytes and thrombocytes have an intravascular origin and all leukocytes have an extravascular origin. Basically, this agrees with Dantschakoff ( 1908a and 1909a) concept, except that she calls a primitive 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 circulating blood of the chick embryo. All others that develop within hematopoietic organs are held there until after hatching. Granulocytes appear in great abundance in bone marrow, spleen, kidney, pancreas, and sometimes in the liver and other parenchymal organs. According to Dantschakoff ( 1908a and b) and Danschakoff ( 1916b 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 but 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 circulating blood simulated that of leukemia.
Were we to judge from information on mammals, 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 rudimentary condition in mammals. Dantschakoff observed that the liver of the normal chick embryo was not a hematopoietic organ, but according to Haff ( 1914) it does have such a function for both erythrocytes and granulocytes. The endothelial cells of the liver sinusoids proved to be the point of origin by way of a "large lymphocyte" for the different cell types. Wislocki ( 1943) 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 development. The pancreas of the chick at 19 days of incubation is packed with heterophils. Whether 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 Laboratory who made these observations, that, among different embryos, there is wide variability in the number of heteropbils present.
Certain differences between bone-marrow formation 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 remain cartilaginous for awhile and provide for