Search

The mammalian heart arises from various populations of Mesp1-expressing cardiovascular progenitors (CPs) that are specified during the early stages of gastrulation. Mesp1 is a transcription factor that acts as a master regulator of CP specification and differentiation. However, how Mesp1 regulates the chromatin landscape of nascent mesodermal cells to define the temporal and spatial patterning of the distinct populations of CPs remains unknown. Here, by combining ChIP-seq, RNA-seq and ATAC-seq during mouse pluripotent stem cell differentiation, we defined the dynamic remodelling of the chromatin landscape mediated by Mesp1. We identified different enhancers that are temporally regulated to erase the pluripotent state and specify the pools of CPs that mediate heart development. We identified Zic2 and Zic3 as essential cofactors that act with Mesp1 to regulate its transcription-factor activity at key mesodermal enhancers, thereby regulating the chromatin remodelling and gene expression associated with the specification of the different populations of CPs in vivo. Our study identifies the dynamics of the chromatin landscape and enhancer remodelling associated with temporal patterning of early mesodermal cells into the distinct populations of CPs that mediate heart development.

Funder: Medical Research Council (UK) Career Development Award (G1100312/1), Funder: Medical Research Council (UK) MR/M011089/1 Wellcome Trust-Medical Research Council core funding to the Wellcome-MRC Cambridge Stem Cell Institute, Studies of mechanical signalling are typically performed by comparing cells cultured on soft and stiff hydrogel-based substrates. However, it is challenging to independently and robustly control both substrate stiffness and extracellular matrix tethering to substrates, making matrix tethering a potentially confounding variable in mechanical signalling investigations. Moreover, unstable matrix tethering can lead to poor cell attachment and weak engagement of cell adhesions. To address this, we developed StemBond hydrogels, a hydrogel in which matrix tethering is robust and can be varied independently of stiffness. We validate StemBond hydrogels by showing that they provide an optimal system for culturing mouse and human pluripotent stem cells. We further show how soft StemBond hydrogels modulate stem cell function, partly through stiffness-sensitive ERK signalling. Our findings underline how substrate mechanics impact mechanosensitive signalling pathways regulating self-renewal and differentiation, indicating that optimising the complete mechanical microenvironment will offer greater control over stem cell fate specification.

Significance Mammalian DNA is widely modified by methylation of cytosine residues. This modification is added to DNA during early development. If methylation is prevented, the embryo dies by midgestation with multiple abnormalities. In this study we found that stem cells lacking the DNA methylation enzymes do not differentiate efficiently into cell types of the embryo and are diverted into producing placental cells. This switch in cell fate is driven by a transcription factor, Ascl2, which should only be produced in the placenta. In the absence of DNA methylation, the Ascl2 gene is misexpressed. Removing Ascl2 redirects embryonic fate but not full differentiation potential, suggesting that methylation acts at multiple developmental transitions to restrict activation of disruptive genes., Genome remethylation is essential for mammalian development but specific reasons are unclear. Here we examined embryonic stem (ES) cell fate in the absence of de novo DNA methyltransferases. We observed that ES cells deficient for both Dnmt3a and Dnmt3b are rapidly eliminated from chimeras. On further investigation we found that in vivo and in vitro the formative pluripotency transition is derailed toward production of trophoblast. This aberrant trajectory is associated with failure to suppress activation of Ascl2. Ascl2 encodes a bHLH transcription factor expressed in the placenta. Misexpression of Ascl2 in ES cells provokes transdifferentiation to trophoblast-like cells. Conversely, Ascl2 deletion rescues formative transition of Dnmt3a/b mutants and improves contribution to chimeric epiblast. Thus, de novo DNA methylation safeguards against ectopic activation of Ascl2. However, Dnmt3a/b-deficient cells remain defective in ongoing embryogenesis. We surmise that multiple developmental transitions may be secured by DNA methylation silencing potentially disruptive genes.

Genome remethylation is essential for mammalian development but specific reasons are unclear. Here we examined embryonic stem (ES) cell fate in the absence of de novo DNA methyltransferases. We observed that ES cells deficient for both Dnmt3a and Dnmt3b are rapidly eliminated from chimaeras. On further investigation we found that in vivo and in vitro the formative pluripotency transition is derailed towards production of trophoblast. This aberrant trajectory is associated with failure to suppress activation of Ascl2. Ascl2 encodes a bHLH transcription factor expressed in placenta. Misexpression of Ascl2 in ES cells provokes transdifferentiation to trophoblast-like cells. Conversely, Ascl2 deletion rescues formative transition of Dnmt3a/b mutants and improves contribution to chimaeric epiblast. Thus, de novo DNA methylation safeguards against ectopic activation of Ascl2. However, Dnmt3a/b-deficient cells remain defective in ongoing embryogenesis. We surmise that multiple developmental transitions may be secured by DNA methylation silencing potentially disruptive genes.SIGNIFICANCE STATEMENTMammalian DNA is widely modified by methylation of cytosine residues. This modification is added to DNA during early development. If methylation is prevented, the embryo dies by mid-gestation with multiple abnormalities. In this study we found that stem cells lacking the DNA methylation enzymes do not differentiate efficiently into cell types of the embryo and are diverted into producing placental cells. This switch in cell fate is driven by a transcription factor, Ascl2, which should only be produced in placenta. In the absence of DNA methylation, the Ascl2 gene is mis-expressed. Removing Ascl2 redirects embryonic fate but not full differentiation potential, suggesting that methylation acts at multiple developmental transitions to restrict activation of disruptive genes.

Summary Mammalian embryogenesis is a paradigm of regulative development as mouse embryos show plasticity in the regulation of cell fate, cell number, and tissue morphogenesis. However, the mechanisms behind embryo plasticity remain largely unknown. Here, we determine how mouse embryos respond to an increase in cell numbers to regulate the timing and mechanism of embryonic morphogenesis, leading to the formation of the pro-amniotic cavity. Using embryos and embryonic stem cell aggregates of different size, we show that while pro-amniotic cavity formation in normal-sized embryos is achieved through basement membrane-induced polarization and exocytosis, cavity formation of increased-size embryos is delayed and achieved through apoptosis of cells that lack contact with the basement membrane. Importantly, blocking apoptosis, both genetically and pharmacologically, alters pro-amniotic cavity formation but does not affect size regulation in enlarged embryos. We conclude that the regulation of embryonic size and morphogenesis, albeit concomitant, have distinct molecular underpinnings., Graphical abstract, Highlights • Increased-size embryos present a delay in embryonic morphogenesis • Embryo size determines the mechanism of pro-amniotic cavity formation • Apoptosis is required for the formation of a single cavity in enlarged embryos • Cavity formation and size regulation have distinct molecular underpinnings, In this article Shahbazi, Zernicka-Goetz, and colleagues explore how increasing embryo size affects the mechanism and timing of embryonic morphogenesis. Enlarged embryos present a delay in pro-amniotic cavity formation, an essential morphogenetic event that happens concomitantly with the regulation of embryo size. However, whereas pro-amniotic cavity formation requires apoptosis of embryonic cells lacking basement membrane contact, size regulation occurs independently of cell death.

No abstract available. Article truncated after first page. A 46-year-old man presented after a motor vehicle collision. He suffered abdominal injuries (liver laceration, avulsed gall bladder) which were successfully managed non-operatively. The patient remained intubated on mechanical ventilation and remained hypotensive after the injuries resolved. The patient required norepinephrine at low doses to maintain a normal blood pressure. It was noted the patient had a history of remote tricuspid valve replacement. A bedside echocardiogram was then performed to determine the etiology of the patient’s persistent hypotension after hypovolemia had been excluded. What tricuspid pathology do the following videos and images demonstrate? 1. Mobile vegetation; 2. Tricuspid Regurgitation; 3. Tricuspid Stenosis; 4. All of the above.

Studies of mechanical signalling are typically performed by comparing cells cultured on soft and stiff hydrogel-based substrates. However, it is challenging to independently and robustly control both substrate stiffness and tethering of extracellular matrix (ECM) to substrates, making ECM tethering a potentially confounding variable in mechanical signalling investigations. Moreover, poor ECM tethering can lead to weak cell attachment. To address this, we developed StemBond hydrogels, a hydrogel formulation in which ECM tethering is stable and can be varied independently of stiffness. We show that soft StemBond hydrogels provide an optimal format for culturing embryonic stem (ES) cells. We find that soft StemBond substrates improve the homogeneity of ES cell populations, boost their self-renewal, and increase the efficiency of cellular reprogramming. Our findings underline how soft microenvironments impact mechanosensitive signalling pathways regulating self-renewal and differentiation, indicating that optimising the complete mechanical microenvironment will offer greater control over stem cell fate specification.

Summary Pluripotent cells emerge as a naive founder population in the blastocyst, acquire capacity for germline and soma formation, and then undergo lineage priming. Mouse embryonic stem cells (ESCs) and epiblast-derived stem cells (EpiSCs) represent the initial naive and final primed phases of pluripotency, respectively. Here, we investigate the intermediate formative stage. Using minimal exposure to specification cues, we derive stem cells from formative mouse epiblast. Unlike ESCs or EpiSCs, formative stem (FS) cells respond directly to germ cell induction. They colonize somatic tissues and germline in chimeras. Whole-transcriptome analyses show similarity to pre-gastrulation formative epiblast. Signal responsiveness and chromatin accessibility features reflect lineage capacitation. Furthermore, FS cells show distinct transcription factor dependencies, relying critically on Otx2. Finally, FS cell culture conditions applied to human naive cells or embryos support expansion of similar stem cells, consistent with a conserved staging post on the trajectory of mammalian pluripotency., Graphical abstract, Highlights • Derivation of mouse formative pluripotent stem cells from pre-gastrulation epiblast • Formative stem cells are competent for germline induction • Formative stem cells can contribute to chimeras • Candidate human FS cells derived using similar culture conditions to mouse, Three stages of pluripotency have been proposed: naive, formative, and primed. Kinoshita and colleagues derived stem cells with properties anticipated for formative pluripotency by culturing mouse epiblast under conditions of low growth factor stimulation. Application to human embryos resulted in propagation of similar stem cells.

CTLA-4 is a critical “checkpoint” regulator in autoimmunity. Variation in CTLA-4 isoform expression has been linked to type 1 diabetes development in human and NOD mouse studies. In the NOD mouse, a causative link between increased expression of the minor isoform ligand-independent CTLA-4 and a reduction in diabetes has become widely accepted. Altered splicing of CTLA-4 has been attributed to a single nucleotide polymorphism (SNP) in Ctla4 exon2 (e2_77A/G). To investigate this link, we have used NOD embryonic stem (ES) cells to generate a novel NOD transgenic line with the 77A/G SNP. This strain phenocopies the increase in splicing toward the liCTLA4 isoform seen in B10 Idd5.1 mice. Crucially, the SNP does not alter the spontaneous incidence of diabetes, the incidence of cyclophosphamide-induced diabetes, or the activation of diabetogenic T-cell receptor transgenic CD4+ T cells after adoptive transfer. Our results show that one or more of the many other linked genetic variants between the B10 and NOD genome are required for the diabetes protection conferred by Idd5.1. With the NOD mouse model closely mimicking the human disease, our data demonstrate that knock-in transgenic mice on the NOD background can test causative mutations relevant in human diabetes.

Naïve pluripotent embryonic stem (ESCs) cells and embryonic germ (EGCs) cells are derived from the preimplantation epiblast and primordial germ cells (PGCs), respectively. We investigated whether differences exist between ESCs and EGCs in view of their distinct developmental origins. PGCs are programmed to undergo global DNA demethylation; however we find that EGCs exhibit equivalent levels of global DNA methylation to ESCs. Importantly, inhibition of MEK and Gsk3b by 2i conditions leads to a pronounced reduction in DNA methylation in both cell types. This is driven by Prdm14 and is associated with downregulation of Dnmt3a and Dnmt3b. However, genomic imprints are maintained in 2i and we report derivation of EGCs with intact genomic imprints. Collectively, our findings establish that culture in 2i instils a naïve pluripotent state with a distinctive epigenetic configuration that parallels molecular features observed in both the preimplantation epiblast and nascent PGCs.

SummaryEmbryonic stem cell (ESC) pluripotency is dependent on an intrinsic gene regulatory network centered on Oct4. Propagation of the pluripotent state is stimulated by the cytokine leukemia inhibitory factor (LIF) acting through the transcriptional regulator Stat3. Here, we show that this extrinsic stimulus converges with the intrinsic circuitry in Krüppel-factor activation. Oct4 primarily induces Klf2 while LIF/Stat3 selectively enhances Klf4 expression. Overexpression of either factor reduces LIF dependence, but with quantitative and qualitative differences. Unlike Klf4, Klf2 increases ESC clonogenicity, maintains undifferentiated ESCs in the genetic absence of Stat3, and confers resistance to BMP-induced differentiation. ESCs expanded with Klf2 remain capable of contributing to adult chimeras. Postimplantation-embryo-derived EpiSCs lack both Klf2 and Klf4 and expression of either can reinstate naive pluripotency. These findings indicate that Oct4 and Stat3 intersect in directing expression of Klf transcriptional regulators with overlapping properties that additively reinforce ground-state ESC pluripotency, identity, and self-renewal.

Mouse embryonic stem (ES) cells derived from pluripotent early epiblast contribute functionally differentiated progeny to all foetal lineages of chimaeras. By contrast, epistem cell (EpiSC) lines from post-implantation epithelialised epiblast are unable to colonise the embryo even though they express the core pluripotency genes Oct4, Sox2 and Nanog. We examined interconversion between these two cell types. ES cells can readily become EpiSCs in response to growth factor cues. By contrast, EpiSCs do not change into ES cells. We exploited PiggyBac transposition to introduce a single reprogramming factor, Klf4, into EpiSCs. No effect was apparent in EpiSC culture conditions, but in ground state ES cell conditions a fraction of cells formed undifferentiated colonies. These EpiSC-derived induced pluripotent stem (Epi-iPS) cells activated expression of ES cell-specific transcripts including endogenous Klf4, and downregulated markers of lineage specification. X chromosome silencing in female cells, a feature of the EpiSC state, was erased in Epi-iPS cells. They produced high-contribution chimaeras that yielded germline transmission. These properties were maintained after Cre-mediated deletion of the Klf4 transgene, formally demonstrating complete and stable reprogramming of developmental phenotype. Thus, re-expression of Klf4 in an appropriate environment can regenerate the naïve ground state from EpiSCs. Reprogramming is dependent on suppression of extrinsic growth factor stimuli and proceeds to completion in less than 1% of cells. This substantiates the argument that EpiSCs are developmentally, epigenetically and functionally differentiated from ES cells. However, because a single transgene is the minimum requirement to attain the ground state, EpiSCs offer an attractive opportunity for screening for unknown components of the reprogramming process.

Background: Immunofluorescence (IF) is one of the most important techniques where fluorochromes conjugated to antibodies are used to detect specific proteins or antigens. In tissue sections, autofluorescence (AF) can lead to poor quality images that impair assessment. The placenta is a pivotal extra-embryonic organ in embryo development, where trophoblasts make up a large proportion of the cells. Teratoma formation is one of the critical assays for pluripotent stem cells. Methods: We tested whether ultraviolet (UV), ammonia (NH3), copper (II) sulfate (CuSO4), Trypan Blue (TB), Sudan Black B (SB), TrueBlack™ Lipofusin Autofluorescence Quencher (TLAQ) and combinations of these treatments could reduce AF in paraffin and frozen sections of placenta and teratoma in FITC, Texas Red and Cy5.5 channels. Results: We found that UV, NH3, TB and CuSO4 quenched AF to some extent in different tissue and filters, but increased AF in Texas Red or Cy5.5 channels in some cases. SB and TLQA exhibited the most consistent effects on decreasing AF, though TLQA reduced the overall IF signal in placenta sections. Not all combined treatments further reduced AF in both placenta and teratoma sections. Conclusions: SB and TLAQ can effectively quench AF in placenta and teratoma IF.

Summary Current human pluripotent stem cells lack the transcription factor circuitry that governs the ground state of mouse embryonic stem cells (ESC). Here, we report that short-term expression of two components, NANOG and KLF2, is sufficient to ignite other elements of the network and reset the human pluripotent state. Inhibition of ERK and protein kinase C sustains a transgene-independent rewired state. Reset cells self-renew continuously without ERK signaling, are phenotypically stable, and are karyotypically intact. They differentiate in vitro and form teratomas in vivo. Metabolism is reprogrammed with activation of mitochondrial respiration as in ESC. DNA methylation is dramatically reduced and transcriptome state is globally realigned across multiple cell lines. Depletion of ground-state transcription factors, TFCP2L1 or KLF4, has marginal impact on conventional human pluripotent stem cells but collapses the reset state. These findings demonstrate feasibility of installing and propagating functional control circuitry for ground-state pluripotency in human cells., Graphical Abstract, Highlights • Transcription factor circuitry is rewired in human pluripotent stem cells • Transcriptome and metabolism are similar to mouse ground-state embryonic stem cells • Genome-wide hypomethylation in reset cells indicates global epigenetic erasure • Reset human cells can incorporate into mouse preimplantation epiblast, Invoking the transcription factor circuitry that defines mouse embryonic stem cell identity converts human pluripotent cells to a more naive state, characterized by altered transcriptional and metabolic activity, absence of lineage priming, and global DNA hypomethylation.

Induced pluripotent stem cells (iPSCs) have the potential to generate patient-specific tissues for disease modeling and regenerative medicine applications. However, before iPSC technology can progress to the translational phase, several obstacles must be overcome. These include uncertainty regarding the ideal somatic cell type for reprogramming, the low kinetics and efficiency of reprogramming, and karyotype discrepancies between iPSCs and their somatic precursors. Here we describe the use of late-outgrowth endothelial progenitor cells (L-EPCs), which possess several favorable characteristics, as a cellular substrate for the generation of iPSCs. We have developed a protocol that allows the reliable isolation of L-EPCs from peripheral blood mononuclear cell preparations, including frozen samples. As a proof-of-principle for clinical applications we generated EPC-iPSCs from both healthy individuals and patients with heritable and idiopathic forms of pulmonary arterial hypertension. L-EPCs grew clonally; were highly proliferative, passageable, and bankable; and displayed higher reprogramming kinetics and efficiencies compared with dermal fibroblasts. Unlike fibroblasts, the high efficiency of L-EPC reprogramming allowed for the reliable generation of iPSCs in a 96-well format, which is compatible with high-throughput platforms. Array comparative genome hybridization analysis of L-EPCs versus donor-matched circulating monocytes demonstrated that L-EPCs have normal karyotypes compared with their subject's reference genome. In addition, >80% of EPC-iPSC lines tested did not acquire any copy number variations during reprogramming compared with their parent L-EPC line. This work identifies L-EPCs as a practical and efficient cellular substrate for iPSC generation, with the potential to address many of the factors currently limiting the translation of this technology.

Summary Transcriptional heterogeneity within embryonic stem cell (ESC) populations has been suggested as a mechanism by which a seemingly homogeneous cell population can initiate differentiation into an array of different cell types. Chromatin remodeling proteins have been shown to control transcriptional variability in yeast and to be important for mammalian ESC lineage commitment. Here we show that the Nucleosome Remodeling and Deacetylation (NuRD) complex, which is required for ESC lineage commitment, modulates both transcriptional heterogeneity and the dynamic range of a set of pluripotency genes in ESCs. In self-renewing conditions, the influence of NuRD at these genes is balanced by the opposing action of self-renewal factors. Upon loss of self-renewal factors, the action of NuRD is sufficient to silence transcription of these pluripotency genes, allowing cells to exit self-renewal. We propose that modulation of transcription levels by NuRD is key to maintaining the differentiation responsiveness of pluripotent cells., Graphical Abstract Highlights ► NuRD directly regulates the transcription of pluripotency genes ► The repressive activity of NuRD is required for ESC differentiation ► NuRD mediates transcriptional heterogeneity in ESC populations ► Transcription levels are determined by balanced activation and silencing

Nonobese diabetic (NOD) mice provide an excellent model of type 1 diabetes. The genetic contribution to this disease is complex, with more than 20 loci implicated in diabetes onset. One of the challenges for researchers using the NOD mouse model (and, indeed, other models of spontaneous autoimmune disease) has been the high density of sequence variation within candidate chromosomal segments. Furthermore, the scope for analyzing many putative disease loci via gene targeting has been hampered by the lack of NOD embryonic stem (ES) cells. We describe here the derivation of NOD ES cell lines capable of generating chimeric mice after stable genetic modification. These NOD ES cell lines also show efficient germline transmission, with offspring developing diabetes. The availability of these cells will not only enable the dissection of closely linked loci and the role they have in the onset of type 1 diabetes but also facilitate the generation of new transgenics.

The mouse limb deformity (ld) mutations cause limb malformations by disrupting epithelial–mesenchymal signaling between the polarizing region and the apical ectodermal ridge. Formin was proposed as the relevant gene because three of the five ld alleles disrupt its C-terminal domain. In contrast, our studies establish that the two other ld alleles directly disrupt the neighboring Gremlin gene, corroborating the requirement of this BMP antagonist for limb morphogenesis. Further doubts concerning an involvement of Formin in the ld limb phenotype are cast, as a targeted mutation removing the C-terminal Formin domain by frame shift does not affect embryogenesis. In contrast, the deletion of the corresponding genomic region reproduces the ld limb phenotype and is allelic to mutations in Gremlin. We resolve these conflicting results by identifying a cis-regulatory region within the deletion that is required for Gremlin activation in the limb bud mesenchyme. This distant cis-regulatory region within Formin is also altered by three of the ld mutations. Therefore, the ld limb bud patterning defects are not caused by disruption of Formin, but by alteration of a global control region (GCR) required for Gremlin transcription. Our studies reveal the large genomic landscape harboring this GCR, which is required for tissue-specific coexpression of two structurally and functionally unrelated genes.

The tsunami which follows a strong local earthquake occurs within a few minutes of the origin time. This requires that any warning system for such local tsunamis be decentralized, as in the system used by the Japan Meterological Agency. Thus, decisions are being made by local officials rather than in a central office. This usually means that the level of training, the education, and the preparation is not as intense in such a local office as in a central office. Therefore, the decision making should be assisted by instrumentation that discriminates tsunamigenic earthquakes from non‐tsunamigenic earthquakes. This is not yet possible in real time; however, since only very large earthquakes (more than 6.5) generate significant tsunamis, an instrument to inform the local official of the tsunami prospects can be implemented. An instrument for assisting the local decision maker has been developed. This instrument consists of an analog computer (an inverted pendulum having a period of 0.75 sec and damping about ...

A series of paired explosions in a salt mine near Winnfield, Louisiana, has been conducted to test a theory by A. L. Latter concerning seismic decoupling by underground cavities. The theory predicted a decoupling of 130. Free‐field and surface measurements from an explosion in either a 6‐ft‐ or a 15‐ft‐radius spherical cavity were compared with similar measurements from a completely tamped explosion of equal size. Shot sizes were from 20 pounds to a ton. Surface measurements were made out to 100 km and covered the frequency range from 0.05 to 100 cps. The experiment confirmed that decoupling does occur. For explosions that produce an average cavity pressure up to one‐fifth and possibly more of the lithostatic overburden pressure, seismic waves were decoupled by more than 100, i.e., two orders of magnitude. Even for explosions producing an average cavity pressure of six times the lithostatic overburden pressure, the seismic waves were decoupled by 20—more than a full order of magnitude. Minimum decoupling factors as a function of frequency are presented.

n/a

In a former paper1 from this laboratory giving the results of a study of bacteria of the colon group occurring in milk, three principal points were made: (1) The gas ratio and volume under uniform conditions are quite constant; (2) on the basis of the gas ratio and volume the collection of cultures under observation was divided into two distinct groups, one of which was probably a mixture of two or more varieties, while the other, giving a CO2 to H2 ratio of approximately 1.1, was distinct and homogeneous; (3) the fermentation of carbohydrates and alcohols is closely correlated with the gas ratio, indicating that this marks the lines of natural relationship within the group. We found in the milk samples one group holding to the type with great uniformity and distinguished by the comparatively small volume of gas, a CO, to H2 ratio of 1.1, and by a somewhat limited ability to ferment carbohydrates and alcohols. A considerable percentage of this group, however, fermented the resistant alcohol dulcite. About 50 per cent. of the cultures isolated from milk gave a ratio varying from 1.5 to 2.8, a large volume of gas, and, in general, the ability to ferment all of the test substances used with the exception of dulcite. A small number liquefied gelatin, a character apparently correlated with the fermentation of glycerin. The low ratio group evidently included those members of the colon group usually designated as Bacillus coli communis and Bacillus coli communior, while in the high ratio group we may recognize Bacillus lactis akrogenes, Bacillus acidi lactici and possibly Bacillus cloacae. In determining the sanitary quality of both water and milk, the origin of bacteria of the colon type and the directness with which they

It would be a large task to compile all the recorded instances in which the reaction of a medium has influenced the morphology, pigment formation, proteolytic activity, rate of fermentation, or even the mere viability of bacteria. In 1895 Fuller1 had collected "over a hundred" such cases; to-day they occur so frequently in the literature that there is no need to emphasize the importance of the subject. Adjustments in the reaction of bacteriologic culture media are generally accomplished by titrations. General use of such methods dates from the suggestions made by Schultz2 (1891), Fuller1 (1895) and others in the early nineties. Founded upon these methods are the "scales of reaction." They are eminently useful, and, now that a vast deal of laboriously-gained experience has dictated in terms of these scalies the proper reaction of important media, they are not likely to be easily displaced. But the theory of titration has undergone a fundamental change since the origination of these methods, and we are now able to gain a clearer conception of many points which were of necessity inadequately treated by the early writers. Furthermore, the physiologic importance of "titratable acidity" is dwindling before the broader conceptions of modern physiologic and physical chemistry, and it may be seriously suggested that too strong an adherence to the old titration methods is a hindrance to progress. It has long been recognized that acidity and alkalinity are best defined in terms of hydrogen ion concentration. Such a conception certainly correlates a vast number of facts. The rate of enzyme action, the stability of colloidal structures upon which cellular life depends, the solubility of many physiologically important compounds

n/a