By seven days of gestation, the yolk sac of the mouse has a sheet of mesoderm adjacent to the basement membrane separating it from the endodermal epithelium. Localized proliferations of this mesoderm produces thickened cellular regions which transform into the angioblastic cords; all of these developmental cells are attached by tight junctions and desmosomes. By eight and onehalf days, lumina appear within the angioblastic cords; the peripheral cells become attenuated and form endothelial cells which will line the primitive vessels while the more central cells become the primitive erythroblasts of the blood island.The process of vasculogenesis and lumenization occurs between eight and onehalf and nine days of gestation and has been correlated with the reduction of cellular junctions between angioblasts and fixed primitive erythroblasts, a loss of the visceral basement membrane and the formation of wide intercellular channels between endodermal epithelial cells. The primitive erythroblasts comprising the blood islands have abundant polysomes, sparse rough endoplasmic reticulum and possess coated vesicles and ferritin aggregates in their cytoplasm and coated invaginations of their plasma membrane. By nine days of gestation, the primitive erythroblasts lose their attachments and become free in the vitelline vessels. Mitochondria of the primitive and free erythroblasts are slightly enlarged and have lighter matrices than angioblasts and mesodermal cells. By 10 to 11 days of gestation, as differentiation proceeds, coated vesicles and invaginations become more numerous and the developing erythroblasts gradually decrease in both cell and nuclear size. Concomitant with these changes is the decrease in the number and size of the mitochondria, a decrease in polysomal numbers and an increase in hemoglobin and cytoplasmic density.The inter-and intracellular events that occur during the formation of the primitive blood vessels and erythrocytic precursors of the yolk sac have not been thoroughly documented from an ultrastructural standpoint. The developmental stages in the process of hemopoiesis occur rapidly with the undifferentiated cells undergoing irreversible specialization along distinct pathways. Submicroscopic analysis has the dual potential of determining the earliest morphological features which signal the initial steps of differentiation and illuminating the changes in structural and functional relationships that occur during subsequent maturation and specialization. Previous studies have dealt with such features as the cellular versus syncytial nature of the primitive angioblastic cords, how ANAT. REC., 170: 199-224. vascular lumen form and the developmental pathway and structural features of erythrocyte differentiation and maturation (Bloom, '38; Danchakoff, '16a,b; Haar, '70; Jones, '64; Kovach et al., '67; Maximow, '09, '24; Sorenson, '61). It is the purpose of this correlated phase and electron microscopic study to expand and clarify the early developmental processes involved in erythropoiesis and vas...

The Krü ppel-like factors (KLFs) are a family of C2/H2 zinc finger DNA-binding proteins that are important in controlling developmental programs. Erythroid Krü ppel-like factor (EKLF or KLF1) positively regulates the ␤-globin gene in definitive erythroid cells. KLF2 (LKLF) is closely related to EKLF and is expressed in erythroid cells. KLF2 ؊/؊ mice die between embryonic day 12.5 (E12.5) and E14.5, because of severe intraembryonic hemorrhaging. They also display growth retardation and anemia. We investigated the expression of the ␤-like globin genes in KLF2 knockout mice. Our results show that KLF2 ؊/؊ mice have a significant reduction of murine embryonic Ey-and ␤h1-globin but not -globin gene expression in the E10.5 yolk sac, compared with wild-type mice. The expression of the adult ␤ maj -and ␤ min -globin genes is unaffected in the fetal livers of E12.5 embryos. In mice carrying the entire human globin locus, KLF2 also regulates the expression of the human embryonic ⑀-globin gene but not the adult ␤-globin gene, suggesting that this developmentalstage-specific role is evolutionarily conserved. KLF2 also plays a role in the maturation and/or stability of erythroid cells in the yolk sac. KLF2 ؊ IntroductionHematopoiesis represents a complex differentiation pathway involving many transcription factors and growth factors that interact in a concerted fashion during mammalian development. 1 Transcription factors often exist as multigene families of structurally and functionally related members that are involved in the different stages of development of a particular cell lineage. For example, members of the GATA family of transcription factors are involved in both primitive and definitive erythropoiesis. 2-4 Different phenotypic features seen after ablation of either the GATA1 or the GATA2 gene in mice clearly demonstrate that these factors have different but overlapping functions. The Krüppel-like factors (KLFs) are a family of DNA-binding proteins named after the Drosophila Krüppel protein. KLFs have 3 C2/H2 zinc finger domains and share conserved residues located primarily within these zinc fingers. 5,6 Several of the KLFs are expressed in erythroid cells starting early in development. Erythroid Krüppel-like factor (EKLF or KLF1) was the first of 16 KLFs to be identified. EKLF Ϫ/Ϫ mice develop fatal anemia during fetal liver erythropoiesis. 7,8 EKLF is responsible for positively regulating the adult ␤-globin gene, but it is not required for embryonic/fetal globin gene expression. [9][10][11] Other KLF family members may be involved in the developmental control of the embryonic and fetal globin genes. A few of the KLFs, namely KLF2 and KLF5, 12 and KLF11 and KLF13,13,14 have been shown to activate the fetal ␥-globin gene in transient transfection assays in human erythroleukemia cell lines. So far none of these studies have been replicated in vivo. In a recent study, KLF11 (fetal Krüppel like factor, FKLF1)-null mice were found to be fertile, with normal hematopoiesis at all stages of development. There was no effe...

Mouse fetal liver was studied ultrastructurally to identify and characterize the developing hepatic parenchyma or prehepatocyte which may be responsible for producing the liver hemopoietic environment. It was observed that as the liver develops, there is close association of endodermal and mesenchymal cells in the region of the septum transversum. Numerous intercellular adhesions were observed between endodermal cells and mesenchymal cells. Twenty-four after endodermal and mesenchymal cells first intermingle, the liver extravascular space consisted of spherical hemopoietic cells dispersed among a heterogenous population of dark and light cells. The reticulum of prehepatocytes formed a three-dimensional cellular network which structurally supported the hemopoietic cells residing in the liver. By 12 days of gestation, prehepatocytes were a homogenous population of dark, stellate cells joined together by numerous intercellular adhesions. Broad areas of intercellular association were noted between processes and prehepatocytes and hemopoietic cells; however, no intercellular junctions between these two disparate cell populations were observed at this or any stage in development. Characteristics reflecting a cell population capable of synthesis and secretion of proteinaceous substances, namely, dilated Golgi apparati, increased numbers of polyribosomes and profiles of rough endoplasmic reticulum (RER), two types of vacuoles and/or vesicles, and intercellular microvillus-lined spaces, were observed in the prehepatocytes between 12 and 17 days gestation. By day 17 of gestation, glycogen accumulation, biliary channel development, appearance of a subendothelial microvillus surface, nuclear shape and chromatin pattern, and arrangement of cytoplasmic organelles reflected the maturation of prehepatocytes into hepatocytes, the adult liver parenchyma.