In vitro differentiation of embryonic stem cells: immunophenotypic analysis of cultured embryoid bodies

Marrow-mindedness: a perspective on neuropoiesis

There are pluripotent stem cells in the adult brain that might not be very different from those found in bone marrow. Recent and profound advances in the field of neuropoiesis, which often rely on insights from studies of hematopoiesis and in some instances use cross-paradigms with this field, have already revealed that bone marrow has much in common with so-called 'brain marrow'. Proliferative primogenitors and developmentally regulated molecules are hallmarks of both neuropoiesis and hematopoiesis. This article will focus on recent advances in neuropoiesis within a central core of the mature brain that is referred to as brain marrow, discussing its pluripotency and proliferative capacity, in vitro and molecular assays used in its study, and markers of neuropoietic stem/progenitor cells. As hematopoiesis research has led to the discovery of numerous morphogenetic factors, it is anticipated that studies of neuropoiesis should also uncover many new factors and genes that affect the growth and differentiation of neural cells. Recent breakthroughs in the stem-cell field prompt an inclusion of rationale for the persistence of normal stem/progenitor cells even in the aged brain. By analogy with hematopoiesis research, a thorough investigation of brain marrow should provide basic insights into developmental and persistent neurogenesis while anticipating cell-transplant and gene therapies for debilitating neurological diseases.

The molecular characterization of the fetal stem cell marker AA4

We have identified and characterized the stem cell antigen AA4. This molecule is a type I transmembrane protein whose overall structure suggests a role in cell adhesion. During fetal ontogeny (days 9-14 of development), AA4 is expressed in three major cell types: vascular endothelial cells, aorta-associated hematopoietic clusters, and primitive fetal liver hematopoietic progenitors. In the adult, AA4 is abundant in lung, heart, and whole bone marrow. In the adult hematopoietic compartment, aa4 transcripts are present in bone marrow CD34(-/lo) Lin- Sca-1+ c-Kit+ and CD34hi Lin- Sca-1+ c-Kit+ stem and progenitor cell subsets. Our observations suggest that AA4 plays a role in cell-cell interactions during hematopoietic and vascular development.

Developmental platelet endothelial cell adhesion molecule expression suggests multiple roles for a vascular adhesion molecule

Platelet endothelial cell adhesion molecule (PECAM) is used extensively as a murine vascular marker. PECAM interactions have been implicated in both vasculogenesis and angiogenesis. To better understand the role of PECAM in mammalian development, PECAM expression was investigated during differentiation of murine embryonic stem (ES) cells and in early mouse embryos. Undifferentiated ES cells express PECAM, and as in vitro differentiation proceeds previously unidentified PECAM-positive cells that are distinct from vascular endothelial cells appear. PECAM expression is gradually restricted to endothelial cells and some hematopoietic cells of differentiated blood islands. In embryos, the preimplantation blastocyst contains PECAM-positive cells. PECAM expression is next documented in the postimplantation embryonic yolk sac, where clumps of mesodermal cells express PECAM before the development of mature blood islands. The patterns of PECAM expression suggest that undifferentiated cells, a prevascular cell type, and vascular endothelial cells express this marker during murine development. PECAM expression in blastocysts and by ES cells suggests that PECAM may function outside the vascular/hematopoietic lineage.

Embryonic stem cells and embryoid bodies express lymphocyte costimulatory molecules

Despite the importance of the costimulatory proteins B7-1 (CD80), B7-2 (CD86), and their counterreceptors CD28 and CTLA-4 (CD154) in the regulation of T cell proliferation in the adult immunological system, the initial appearance of these proteins during embryonic development has not been investigated. Using in vitro cultures of undifferentiated mouse embryonic stem (ES) cells and differentiating embryoid bodies as a model of very early embryonic development, we examined these cells for the presence of mRNA and protein corresponding to the B7 and CD28 families of costimulatory molecules. By flow cytometry, a stochastically regulated subpopulation of B7-1+ cells comprising 33% of total cells was detected in ES cell cultures, while negligible staining was found for B7-2, CTLA-4, and CD28. When ES cells were differentiated into embryoid bodies for 12 days, a CD45+ subpopulation of embryoid body cells were found to stain positively for B7-1, B7-2, and CD28. RT-PCR confirmed cell staining data by revealing amplification products corresponding to B7-1, B7-2, and CD28 in corresponding samples. Very low levels of CTLA-4 amplification products were found in all samples; however, surface staining of CTLA-4 was never detected. The functional capacity of ES cell B7-1 to bind its ligand was verified by the ability of the soluble fusion protein CTLA-4-Ig to bind ES cells and the ability of this reagent to block anti-B7-1 antibody binding in cell based competition assays. These results demonstrate that expression of costimulatory molecules arises very early during in vitro development and suggests that the early embryonic environment may utilize cellular signaling systems analogous to those seen in the immune system.

Embryonic stem cells and hematopoietic stem cell biology

Two advances in murine embryonic stem (ES) cell technology and their applications for the study of hematopoietic stem cells (HSCs) are discussed in this article. First, ES cells induced to differentiate in vitro form hematopoietic lineages in a fashion that recapitulates the ontogeny of blood formation in the embryo. This system offers a unique opportunity to isolate, examine, and manipulate the most primitive hematopoietic progenitors. Second, targeted gene ablation (knockout) studies in ES cells have identified several genes that are required for normal hematopoiesis and may function in the formation, maintenance, and differentiation of HSCs. Insights into murine hematopoiesis gained through the study of ES cells generally should be applicable to other vertebrates, including humans.