Human Embryonic Stem Cells

Human embryonic stem cells hold great promise in furthering our treatment of disease and increasing our understanding of early development. This chapter describes protocols for the derivation and maintenance of human embryonic stem cells. In addition, it summarizes briefly several alternative methods for the culture of human embryonic stem cells. Thus, this chapter provides a good starting point for researchers interested in harnessing the potential of human embryonic stem cells.

Nuclear reprogramming of somatic cells after fusion with human embryonic stem cells

We have explored the use of embryonic stem cells as an alternative to oocytes for reprogramming human somatic nuclei. Human embryonic stem (hES) cells were fused with human fibroblasts, resulting in hybrid cells that maintain a stable tetraploid DNA content and have morphology, growth rate, and antigen expression patterns characteristic of hES cells. Differentiation of hybrid cells in vitro and in vivo yielded cell types from each embryonic germ layer. Analysis of genome-wide transcriptional activity, reporter gene activation, allele-specific gene expression, and DNA methylation showed that the somatic genome was reprogrammed to an embryonic state. These results establish that hES cells can reprogram the transcriptional state of somatic nuclei and provide a system for investigating the underlying mechanisms.

Gene expression in the preimplantation embryo: in-vitro developmental changes

The regulation of early embryo development and the mechanism of implantation remains poorly understood, due to the large number of genes and the complexity of the systems involved. The effect of in-vitro culture on embryos also remains unclear, which raises concerns about the safety of assisted reproductive technology. Changes in the expression of several individual genes in cultured embryos have been reported previously, but a large-scale comparison has not yet been performed to investigate the effects of in-vitro culture systems on embryo development. This study investigated established gene expression profiles of more that 20,000 genes from in-vitro cultured mouse embryos at eight different stages (oocytes, zygote, 2-, 4-, 8-cell embryo, compacting embryo, morula and blastocyst) using microarray technology, and compared these profiles with in-vivo embryos. In most stages of development there was little significant difference in overall expression patterns between in-vitro and in-vivo embryos. In addition, the expression patterns of developmentally important genes from several different categories, such as apoptosis, glycolysis, adhesion and methylation, were examined and compared between in-vitro and in-vivo embryos. Among the genes examined, DNA methyltransferase 1 (DNMT1) shows a significantly higher (P < 0.05) expression level in cultured embryos. Cadherin-11 also demonstrates a slightly different pattern, although the difference is not statistically significant. All the other genes have remarkably similar expression patterns between in-vitro and in-vivo embryos throughout preimplantation stages.

Ephrin-B2 reverse signaling is required for axon pathfinding and cardiac valve formation but not early vascular development

Vascular development begins with the formation of a primary vascular plexus that is rapidly remodeled by angiogenesis into the interconnected branched patterns characteristic of mature vasculature. Several receptor tyrosine kinases and their ligands have been implicated to control early development of the vascular system. These include the vascular endothelial growth factor receptors (VEGFR-1 and VEGFR-2) that bind VEGF, the Tie-1 and Tie-2 receptors that bind the angiopoietins, and the EphB4 receptor that binds the membrane-anchored ligand ephrin-B2. Targeted mutations in the mouse germline have revealed essential functions for these molecules in vascular development. In particular, protein-null mutations that delete either EphB4 or ephrin-B2 from the mouse have been shown to result in early embryonic lethality due to failed angiogenic remodeling. The venous expression of EphB4 and arterial expression of ephrin-B2 has lead to the speculation that the interaction of these two molecules leads to bidirectional signaling into both the receptor-expressing cell and the ligand-expressing cell, and that both forward and reverse signals are required for proper development of blood vessels in the embryo. Indeed, targeted removal of the ephrin-B2 carboxy-terminal cytoplasmic tail by another group was shown to perturb vascular development and result in the same early embryonic lethality as the null mutation, leading the authors to propose that ephrin-B2 reverse signaling directs early angiogenic remodeling of the primary vascular plexus [Cell 104 (2001) 57]. However, we show here that the carboxy-terminal cytoplasmic domain of ephrin-B2, and hence reverse signaling, is not required during early vascular development, but it is necessary for neonatal survival and functions later in cardiovascular development in the maturation of cardiac valve leaflets. We further show that ephrin-B2 reverse signaling is required for the pathfinding of axons that form the posterior tract of the anterior commissure. Our results thus indicate that ephrin-B2 functions in the early embryo as a typical instructive ligand to stimulate EphB4 receptor forward signaling during angiogenic remodeling and that later in embryonic development ephrin-B2 functions as a receptor to transduce reverse signals involved in cardiac valve maturation and axon pathfinding.

Bidirectional signaling mediated by ephrin-B2 and EphB2 controls urorectal development

Incomplete urethral tubularization (hypospadias) and anorectal abnormalities are two common and poorly understood birth defects that affect the extreme caudal midline of the human embryo. We now show that cell surface molecules essential for proper axon pathfinding in the developing nervous system, namely ephrin-B2 and the ephrin receptors EphB2 and EphB3, also play major roles in cell adhesion events that tubularize the urethra and partition the urinary and alimentary tracts. Mice carrying mutations which disrupt the bidirectional signals that these molecules transduce develop with variably penetrant severe hypospadias and incomplete midline fusion of the primitive cloaca. We further show that animals completely lacking ephrin-B2 reverse signaling present a fully penetrant failure in cloacal septation. This results in severe anorectal malformations characterized by an absence of the terminal-most hindgut (rectum) and formation of a fistula that aberrantly connects the intestines to the urethra at the base of the bladder. Consistent with an apparent requisite for both forward and reverse signaling in these caudal remodeling events, EphB2 and ephrin-B2 are coexpressed at the midline in the fusing urethral/cloacal endoderm and underlying lateral mesoderm of the urorectal septum that migrates toward the caudal midline as the cloaca septates. Our data thus indicate that B-subclass Eph and ephrin molecules play an important role in these clinically significant midline cell-cell adhesion and fusion events.

The Src family of tyrosine kinases is important for embryonic stem cell self-renewal

cYes, a member of the Src family of non-receptor tyrosine kinases, is highly expressed in mouse and human embryonic stem (ES) cells. We demonstrate that cYes kinase activity is regulated by leukemia inhibitory factor (LIF) and serum and is down-regulated when cells differentiate. Moreover, selective chemical inhibition of Src family kinases decreases growth and expression of stem cell genes that mark the undifferentiated state, including Oct3/4, alkaline phosphatase, fibroblast growth factor 4, and Nanog. A synergistic effect on differentiation is observed when ES cells are cultured with an Src family inhibitor and low levels of retinoic acid. Src family kinase inhibition does not interfere with LIF-induced JAK/STAT3 (Janus-associated tyrosine kinases/signal transducer and activator of transcription 3) or p42/p44 MAPK (mitogen-activated protein kinase) phosphorylation. Together the results suggest that the activation of the Src family is important for maintaining mouse and human ES in an undifferentiated state and may represent a third, independent pathway, downstream of LIF in mouse ES cells.

Ephrins in reverse, park and drive

Eph receptors and their membrane-anchored ephrin ligands are thought to orchestrate cell movements by transducing bidirectional tyrosine-kinase-mediated signals into both cells expressing the receptors and cells expressing the ligands. Whether the resulting event is repulsion of an axonal growth cone, directing the orderly segmentation of hindbrain rhombomere cells or controlling angiogenic remodelling, such elaborate and diverse cell movements require intricate changes in the actin cytoskeleton, as well as precise regulation of cellular adhesion. Recent work by several groups has begun to link ephrin reverse signals to intracellular pathways that regulate actin dynamics and might help to explain how these ligands function as receptors to direct cell movement, adhesion and de-adhesion events.

Crystal structure of an Eph receptor-ephrin complex

The Eph family of receptor tyrosine kinases and their membrane-anchored ephrin ligands are important in regulating cell-cell interactions as they initiate a unique bidirectional signal transduction cascade whereby information is communicated into both the Eph-expressing and the ephrin-expressing cells. Initially identified as regulators of axon pathfinding and neuronal cell migration, Ephs and ephrins are now known to have roles in many other cell-cell interactions, including those of vascular endothelial cells and specialized epithelia. Here we report the crystal structure of the complex formed between EphB2 and ephrin-B2, determined at 2.7 A resolution. Each Eph receptor binds an ephrin ligand through an expansive dimerization interface dominated by the insertion of an extended ephrin loop into a channel at the surface of the receptor. Two Eph-Ephrin dimers then join to form a tetramer, in which each ligand interacts with two receptors and each receptor interacts with two ligands. The Eph and ephrin molecules are precisely positioned and orientated in these complexes, promoting higher-order clustering and the initiation of bidirectional signalling.

The SH2/SH3 adaptor Grb4 transduces B-ephrin reverse signals

Bidirectional signals mediated by membrane-anchored ephrins and Eph receptor tyrosine kinases have important functions in cell-cell recognition events, including those that occur during axon pathfinding and hindbrain segmentation. The reverse signal that is transduced into B-ephrin-expressing cells is thought to involve tyrosine phosphorylation of the signal's short, conserved carboxy-terminal cytoplasmic domain. The Src-homology-2 (SH2) domain proteins that associate with activated tyrosine-phosphorylated B-subclass ephrins have not been identified, nor has a defined cellular response to reverse signals been described. Here we show that the SH2/SH3 domain adaptor protein Grb4 binds to the cytoplasmic domain of B ephrins in a phosphotyrosine-dependent manner. In response to B-ephrin reverse signalling, cells increase FAK catalytic activity, redistribute paxillin, lose focal adhesions, round up, and disassemble F-actin-containing stress fibres. These cellular responses can be blocked in a dominant-negative fashion by expression of the isolated Grb4 SH2 domain. The Grb4 SH3 domains bind a unique set of other proteins that are implicated in cytoskeletal regulation, including the Cbl-associated protein (CAP/ponsin), the Abl-interacting protein-1 (Abi-1), dynamin, PAK1, hnRNPK and axin. These data provide a biochemical pathway whereby cytoskeletal regulators are recruited to Eph-ephrin bidirectional signalling complexes.

Forward signaling mediated by ephrin-B3 prevents contralateral corticospinal axons from recrossing the spinal cord midline

To investigate Eph-ephrin bidirectional signaling, a series of mutations were generated in the ephrin-B3 locus. The absence of both forward and reverse signaling resulted in mice with mirror movements as typified by a hopping locomotion. The corticospinal tract was defective as axons failed to respect the midline boundary of the spinal cord and bilaterally innervated both contralateral and ipsilateral motor neuron populations. A second mutation that expresses a truncated ephrin-B3 protein lacking its cytoplasmic domain did not lead to hopping, indicating that reverse signaling is not required for corticospinal innervation. Ephrin-B3 is concentrated at the spinal cord midline, while one of its receptors, EphA4, is expressed in postnatal corticospinal neurons as their fibers pathfind down the contralateral spinal cord. Our data indicate ephrin-B3 functions as a midline-anchored repellent to stimulate forward signaling in EphA4-expressing axons.

National health expenditures, 1999

The health care spending share of gross domestic product (GDP) remained steady between 1993 and 1999 as moderate-to-strong economic growth coincided with a rapid shift to managed care. This shift, along with decelerating growth in Medicare spending, appears to have generated a mostly one-time saving that lowered aggregate health expenditure growth.

EphB2 guides axons at the midline and is necessary for normal vestibular function

Mice lacking the EphB2 receptor tyrosine kinase display a cell-autonomous, strain-specific circling behavior that is associated with vestibular phenotypes. In mutant embryos, the contralateral inner ear efferent growth cones exhibit inappropriate pathway selection at the midline, while in mutant adults, the endolymph-filled lumen of the semicircular canals is severely reduced. EphB2 is expressed in the endolymph-producing dark cells in the inner ear epithelium, and these cells show ultrastructural defects in the mutants. A molecular link to fluid regulation is provided by demonstrating that PDZ domain-containing proteins that bind the C termini of EphB2 and B-ephrins can also recognize the cytoplasmic tails of anion exchangers and aquaporins. This suggests EphB2 may regulate ionic homeostasis and endolymph fluid production through macromolecular associations with membrane channels that transport chloride, bicarbonate, and water.

Kinase independent function of EphB receptors in retinal axon pathfinding to the optic disc from dorsal but not ventral retina

Optic nerve formation requires precise retinal ganglion cell (RGC) axon pathfinding within the retina to the optic disc, the molecular basis of which is not well understood. At CNS targets, interactions between Eph receptor tyrosine kinases on RGC axons and ephrin ligands on target cells have been implicated in formation of topographic maps. However, studies in chick and mouse have shown that both Eph receptors and ephrins are also expressed within the retina itself, raising the possibility that this receptor-ligand family mediates aspects of retinal development. Here, we more fully document the presence of specific EphB receptors and B-ephrins in embryonic mouse retina and provide evidence that EphB receptors are involved in RGC axon pathfinding to the optic disc. We find that as RGC axons begin this pathfinding process, EphB receptors are uniformly expressed along the dorsal-ventral retinal axis. This is in contrast to the previously reported high ventral-low dorsal gradient of EphB receptors later in development when RGC axons map to CNS targets. We show that mice lacking both EphB2 and EphB3 receptor tyrosine kinases, but not each alone, exhibit increased frequency of RGC axon guidance errors to the optic disc. In these animals, major aspects of retinal development and cellular organization appear normal, as do the expression of other RGC guidance cues netrin, DCC, and L1. Unexpectedly, errors occur in dorsal but not ventral retina despite early uniform or later high ventral expression of EphB2 and EphB3. Furthermore, embryos lacking EphB3 and the kinase domain of EphB2 do not show increased errors, consistent with a guidance role for the EphB2 extracellular domain. Thus, while Eph kinase function is involved in RGC axon mapping in the brain, RGC axon pathfinding within the retina is partially mediated by EphB receptors acting in a kinase-independent manner.

National health expenditures, 1998

In 1998, national health care expenditures reached $1.1 trillion, an increase of 5.6 percent from the previous year. This marked the fifth consecutive year of spending growth under 6 percent. Underlying the stability of the overall growth, major changes began taking place within the Nation's health care system. Public payers felt the initial effects of the Balanced Budget Act of 1997 (BBA), and private payers experienced increased health care costs and increased premium growth.

National health expenditures, 1997

In 1997 health spending in the United States increased just 4.8 percent to $1.1 trillion. As a share of gross domestic product (GDP), national health expenditures (NHE) absorbed 13.5 percent of the country's output in 1997--a share that has remained relatively constant for 5 years. Despite the relative stability in recent years, signs of changing trends are emerging.

National health expenditures, 1996

The national health expenditures (NHE) series presented in this report for 1960-96 provides a view of the economic history of health care in the United States through spending for health care services and the sources financing that care. In 1996 NHE topped $1 trillion. At the same time, spending grew at the slowest rate, 4.4 percent, ever recorded in the current series. For the first time, this article presents estimates of Medicare managed care payments by type of service, as well as nursing home and home health spending in hospital-based facilities.

Business, households, and government: health care spending, 1995

For the period 1990-95, we will present data on health care spending by business, households, and government. In addition, we will measure the relative impact of these expenditures on each sector's ability to pay. In 1994 and 1995, health care costs experienced the slowest growth in 3 decades. Combined with healthy revenue growth, slow cost growth helped ease or stabilize the financing burden faced by business, households and government.

[Leukocyte count of puerperal sows]

147 blood samples of postparturient sows of a secluded housing were taken. The samples were conserved with ACD-solution. The influence of the number and week of the lactation and the health of the sow, determined by puerperal diseases was studied. Hematological values of healthy postparturient sows are: leucocytes 12.6 +/- 2.2 G/l; basophile granulocytes 0.1 +/- 0.1 G/l, eosinophile granulocytes 0.5 +/- 0.4 G/l; banded neutrophile granulocytes 1.3 +/- 0.6 G/l, segmented neutrophile granulocytes 5.2 +/- 1.4 G/l; lymphocytes 5.5 +/- 1.4 G/l, monocytes 0.3 +/- 0.3 G/l. The leucocyte number is lower in the investigated herd compared with quotations in the literature. This is based on the good health conditions in the herd. Changes due to the number and week of the lactation have no clinical relevance. Health status, here described by puerperal diseases is the significant influencing factor of the leucocyte number. The severity of puerperal diseases is significant. Due to puerperal diseases the leucocyte number rises quickly after a short drop about 2 G/l. The number of the neutrophile granulocytes increases, but the lymphocyte number is reduced at the beginning of the illness. The application of ACD-solution for stabilizing of great amounts of blood samples under practical conditions is demonstrated. It is possible to stabilize pigs blood well.

Business, households, and government: health spending, 1994

During the 1990s, growth in health care costs slowed considerably, helping to lessen the spending strain on business, government, and households. Although cost growth has slowed, the Federal Government continues to pay an ever-increasing share of the total health care bill. This article reviews important health care spending trends, and for the first time, provides separate estimates of the employer and employee share of the premium costs for employer-sponsored private health insurance. This article also highlights some of the emerging trends in the employer-sponsored insurance market, including managed care, cost-sharing, and employment shifts.

National health expenditures, 1994

This article presents data on health care spending for the United States, covering expenditures for various types of medical services and products and their sources of funding from 1960 to 1994. Although these statistics for 1994 show the slowest growth in more than three decades, health spending continued to grow faster than the overall economy. The Federal Government continued to fund an increasing share of health care expenditures in 1994, offset by a falling share from out-of-pocket sources. Shares paid by State and local governments and by other private payers including private health insurance remained unchanged from 1993.

National health expenditures, 1995

This article presents data on health care spending for the United States, covering expenditures for various types of medical services and products and their sources of funding from 1960 to 1995. In 1995, $988.5 billion was spent to purchase health care in the United States, up 5.5 percent from 1994. Growth in spending between 1993 and 1995 was the slowest in more than three decades, primarily because of slow growth in private health insurance and out-of-pocket spending. As a result, the share of health spending funded by private sources fell, reflecting the influence of increased enrollment in managed care plans.

State health expenditure accounts: building blocks for state health spending analysis

The dynamics of financing health care among various levels of government and the private sector are rapidly changing; structural relationships among health care providers are also being altered. These changes are placing increased importance on State-level expenditure estimates that will be instrumental in measuring the differential impact of Federal policies and State-specific initiatives on individual States. This article presents personal health care expenditures (PHCE) for 1980-93. Statistics show wide variation in level and rate of growth of regional spending per person. These statistics also quantify differences in both the percent of health care costs in each State borne by Medicare and Medicaid and in the proportion of each State's economy devoted to the provision of health care.