Cytological Studies of Mammalian Embryonic Blood Cells

Abstract

Blood cells are most readily classified when seen in blood smear preparations or dry imprints (smears) of tissues stained with Romanowsky dyes. In most laboratories, however, only paraffin sections are studied when the hematologist or pathologist is interested in the hemopoietic activity of spleen, liver, lymph nodes, etc. American investigators have studied mammalian embryonic hemopoiesis from paraffin or celloidin sections only. It was deemed worthwhile to study embryonic hemopoietic organs using the dry imprint method, checking our observations with paraffin sections. The embryonic tissue (yolk-sac, liver, spleen, bone marrow) was touched gently to a chemically clean cover glass and waved vigorously until the imprinted material was dry. May-Grünwald-Giensa staining was used immediately as in the staining of blood smears. Preparations made in this way make possible a comparison between embryonic blood cells and elements seen in blood smears and organ imprints of the adult under normal or various physiologic and pathologic states. It is important to know which cells are found normally only in the embryo, and the elements of the embryonic hemopoietic organs which are identical with normal cells of the adult. The material used for this study consisted of rat embryos from 9–22 days'gestation, plus various stages of rabbit, human, mouse and pig embryos and foetuses. The following were conclusions drawn: 1. The first circulating blood cells of the rat are not lymphocytes (Maximow, Jolly, Jordan), but are immature red cells of the embryonic megaloblastic series. 2. Two generations of red cells are produced in all mammalian embryos, the megaloblastic, formed primarily in the yolk-sac, and the normoblastic which appears in the liver in great numbers. The normoblasts of the liver are cytologically identical with the nucleated red cells of the fetal spleen and fetal and adult bone marrow.