The development of the embryonic hemopoietic organs from yolk sac to liver and spleen: a cytometrical observation.

两年的研究结果表明：(1)小鼠卵黄囊内的造血细胞来源于卵黄囊，小鼠胚胎卵黄囊的造血始于妊娠第8天，在卵黄囊内形成卵黄血管后，第10天红系生成活跃。造血灶含有不同发育阶段的原始红细胞，其中存在少量大的、未成熟的造血细胞，其核直径超过8微米。这种类似晚期血管母细胞的未成熟造血细胞存在于10天胚胎的外周循环中。(2)肝造血和脾造血妊娠11天时，肝组织含有未成熟造血细胞，从核和核仁的外观来看，它们类似于晚期血管母细胞。这种造血细胞的直径大于8微米，在妊娠15天的肝脏中可以看到。(3)脾的造血细胞和淋巴细胞在妊娠16天时，首先出现未成熟的造血细胞和小淋巴细胞，然后脾以红系为主。胎脾可能是骨髓造血前B淋巴细胞群的来源。(4)巨核细胞卵黄囊卵黄血管内含有少量游离巨核细胞。卵黄囊巨核细胞的大小明显小于骨髓巨核细胞，可分为YM1、YM2和YM3三种类型。11日龄胚胎肝巨核细胞大多具有与YM1或YM2细胞相似的超微结构特征。

Our study of these two years gave the following results.(1) Yolk sac hemopoiesis and circulating erythroblasts of yolk sac originHemopoiesis in the yolk sac of the mouse embryo begins at 8 days of gestation, after the formation of vitellin vessels, erythtopoiesis appears to be active at 10 days. The hemopoietic foci contain primitive erythroblasts of various stages of development, and among them there exist small number of large, immature, hemopoietic cells with nuclei over 8 um in diameter. Such immature hemopoietic cells resembling late angioblasts are present in the peripheral circulation of a 10-day embryo.(2)Liver hemopoiesis and spleen hemopoiesisAt 11 days of gestation, hepatic tissues contain immature hemopoietic cells which resemble late angioblasts on the basis of their nuclear and nucleolar appearance. Such hemopoietic cells measure larger than 8 um nuclear diameter and can be seen in the liver unitl 15 days of gestation. Then the immature hemopoietic cells appear in the splenic cords, and the cells can be seen in the spleen even at 2 days after birth.(3)Spleen hemopoiesis and lymphocytesAt 16 days of gestation, immature hemopoietic cells and small lymphocytes first appear and then the spleen become predominantly erythroietic. The fetal spleen is the possible source of B-lymphocyte population before the bone marrow hemopiesis.(4)MegakaryopoiesisYolk sac vitelline vessels contain a few free megakaryopoietic cells. Yolk sac megakaryopoietic cells, which are significantly smaller in cell size than bone marrow megakaryopoietic cells, can be classified into three types: YM1, YM2, and YM3. Most of the hepatic megakaryopoietic cells of an 11-day embryo have ultrastructural features similar to those of YM1 or YM2 cells.