242 
MALARIA 
a reparative role under the same conditions 
as do the fibroblasts and are largely limited 
to the production of new blood vessels of 
developing connective tissue. The frequent 
reports of phagocytic endothelial cells are 
due chiefly to (1) the reversion to the wide¬ 
spread older conception that the macro¬ 
phages in categories 1, 2 and 3 originate 
embryologically and, frequently, in the 
adult organism from true endothelial cells 
and that all endothelium can be phagocytic 
either in situ or after desquamation; (2) 
confusion of adventitial cells (both undif¬ 
ferentiated perivascular cells and macro¬ 
phages of the perivascular loose connective 
tissue) with endothelial cells; and (3) con¬ 
fusion of circulating macrophages which 
have become closely applied to the vessel 
wall with endothelial cells. Although not 
well substantiated, true endothelial cells 
may be phagocytic under unusual condi¬ 
tions, such as in stasis associated with 
occlusion of the brain capillaries in P. falci¬ 
parum. 
Free Connective Tissue and Blood 
Cells : Cells of the Blood 
and Lymph 
The cells of the blood and lymph are 
generally divided into those of myeloid and 
those of lymphoid origin, although during 
ectopic myelopoiesis the myeloid cells may 
arise in organs other than the bone mar¬ 
row, and, under certain conditions, lym¬ 
phatic nodules may occur in the bone 
marrow. 
(1) Myeloid cells. The myeloid cells in¬ 
clude (1) the red blood cell series, of which 
the youngest free cell is the erythroblast 
and the mature cell is the. erythrocyte; (2) 
the blood platelets, which probably develop 
from the megakaryocytes; and (3) the 
granulocyte series, of* 1 which the youngest 
free cells are the three types of myelocyte 
and the mature cells are the heterophil or 
special (= in man the neutrophil poly¬ 
morphonuclear), eosinophil and basophil 
leucocytes. In malaria erythropoiesis is 
important as a “reparative proliferation” 
because red cells are destroyed either di¬ 
rectly by the parasite or indirectly by 
toxic action and erythrophagocytosis (Tali¬ 
aferro and Cannon 1936). Among the re¬ 
maining cells only the heterophils show 
evidence of a functional role (see below 
under hematology). They are rarely 
phagocytic under ordinary conditions, but 
may be phagocytic in heavy infections of 
P. falciparum. 
(2) Free mesenchymal cells: lymphoid 
cells. There is fair agreement among in¬ 
vestigators regarding the nature of the pre¬ 
dominantly fixed cells with mesenchymal 
potencies, but there is a wide diversity of 
opinion regarding the nature of the free 
cells. It is generally agreed that in addi¬ 
tion to the lymphocytes of lymphatic tis¬ 
sue, free lymphoid cells, more or less simi¬ 
lar to lymphocytes, occur under physiologi¬ 
cal and pathological conditions in the 
myeloid and loose connective tissues and 
that they are the stem cells of the various 
myeloid cells of the preceding paragraph,' 
but there is no unanimity of opinion as to 
whether they are all lymphocytes or differ 
in structure and in developmental poten¬ 
cies. 
The work of the present author (Tali¬ 
aferro and Mulligan 1937) has convinced 
him of the essential soundness of the Uni¬ 
tarian view of Maximow (1927a, 1927b) 
and Bloom (1938a) to the effect that all of 
the stem cells of the myeloid tissue (hemo- 
cytoblasts) are identical in morphology 
and developmental potencies with the 
lymphocytes of lymphatic tissue. This does 
not mean that the lymphocytes of lym¬ 
phatic tissue and the hemocytoblasts of 
myeloid tissues develop indiscriminately 
into myeloid and lymphoid cells under 
physiological conditions. This view holds 
rather that the lymphocytes of lymphatic 
tissue give rise only to lymphoid cells and 
the hemocytoblasts of myeloid tissue only 
to myeloid cells under ordinary conditions, 
but that both types of cells are able to 
develop into any cells of the blood and con¬ 
nective tissue under abnormal stimuli. 
Thus, lymphocytes develop into macro¬ 
phages in malaria, as is described later. 
As a corollary to this concept of the free 
mesenchymal cells, the author agrees with 
