Validation of the Postimplantation Rat Whole-embryo Culture Test in the International ECVAM Validation Study on Three In Vitro Embryotoxicity Tests

A detailed report is presented on the performance of the postimplantation rat whole-embryo culture (WEC) test in a European Centre for the Validation of Alternative Methods (ECVAM)-sponsored formal validation study on three in vitro tests for embryotoxicity. Twenty coded test chemicals, classified as non-embryotoxic, weakly embryotoxic or strongly embryotoxic on the basis of their in vivo effects in animals and/or humans, were tested in four laboratories. The outcome showed that the WEC test can be considered to be a scientifically validated test, which is ready for consideration for use in assessing the embryotoxic potentials of chemicals for regulatory purposes.

Right ventricular myocardium derives from the anterior heart field

The mammalian heart develops from a primary heart tube, which is formed by fusion of bilateral cardiac territories in which myocardial and endothelial cells have already begun to differentiate from splanchnic mesoderm. A population of myocardial precursors has been identified in pharyngeal mesoderm, anterior to the early heart tube. Cell labeling studies have indicated that this novel territory, called the anterior heart field (AHF), gives rise to the myocardial wall of the outflow tract. We now report that not only the myocardium of the outflow tract but also myocardial cells of the embryonic right ventricle are derived from this source. Explants of pharyngeal mesoderm or of the early heart tube were cultured from transgenic mice in which transgene expression marks different regions of the heart. Pharyngeal mesoderm from 5 to 7 somite embryos gives rise to cardiomyocytes with right ventricular and outflow tract identities, whereas the heart tube as this stage has an essentially left ventricular identity. DiI labeling confirms that the early heart tube is destined to contribute to the embryonic left ventricle and indicates that right ventricular myocardium is added from extracardiac mesoderm. Retrospective clonal analysis of the heart at embryonic day (E) 10.5 reveals the existence of a clonal boundary in the interventricular region, which appears during ventricular septation, underlining different origins of the two ventricular compartments. This study demonstrates the differences in the embryological origin of right and left ventricular myocardium, which has important implications for congenital heart disease.

Salicylidene salicylhydrazide, a selective inhibitor of beta 1-containing GABAA receptors

1. A high-throughput assay utilizing the voltage/ion probe reader (VIPR) technology identified salicylidene salicylhydrazide (SCS) as being a potent selective inhibitor of alpha2beta1gamma1 GABA(A) receptors with a maximum inhibition of 56+/-5% and an IC(50) of 32 (23, 45) nm. 2. Evaluation of this compound using patch-clamp electrophysiological techniques demonstrated that the compound behaved in a manner selective for receptors containing the beta1 subunit (e.g. maximum inhibition of 68.1+/-2.7% and IC(50) value of 5.3 (4.4, 6.5) nm on alpha2beta1gamma1 receptors). The presence of a beta1 subunit was paramount for the inhibition with changes between alpha1 and alpha2, gamma1 and gamma2, and the presence of a subunit having little effect. 3. On all subtypes, SCS produced incomplete inhibition with the greatest level of inhibition at alpha1beta1gamma1 receptors (74.3+/-1.4%). SCS displayed no use or voltage dependence, suggesting that it does not bind within the channel region. Concentration - response curves to GABA in the presence of SCS revealed a reduction in the maximum response with no change in the EC(50) or Hill coefficient. In addition, SCS inhibited pentobarbitone-induced currents. 4. Threonine 255, located within transmembrane domain (TM) 1, and isoleucine 308, located extracellularly just prior to TM3, were required for inhibition by SCS. 5. SCS did not compete with the known allosteric modulators, picrotoxin, pregnenolone sulphate, dehydroepiandrosterone 3-sulphate, bicuculline, loreclezole or mefenamic acid. Neither was the inhibition by SCS influenced by the benzodiazepine site antagonist flumazenil. 6. In conclusion, SCS is unique in selectively inhibiting GABA(A) receptors containing the beta1 subunit via an allosteric mechanism. The importance of threonine 255 and isoleucine 308 within the beta1 subunit and the lack of interaction with a range of GABA(A) receptor modulators suggests that SCS is interacting at a previously unidentified site.

Pitx2cpatterns anterior myocardium and aortic arch vessels and is required for local cell movement into atrioventricular cushions

Inactivation of the left-right asymmetry gene Pitx2 has been shown, in mice, to result in right isomerism with associated defects that are similar to that found in humans. We show that the Pitx2c isoform is expressed asymmetrically in a presumptive secondary heart field within the branchial arch and splanchnic mesoderm that contributes to the aortic sac and conotruncal myocardium. Pitx2c was expressed in left aortic sac mesothelium and in left splanchnic and branchial arch mesoderm near the junction of the aortic sac and branchial arch arteries. Mice with an isoform-specific deletion of Pitx2c had defects in asymmetric remodeling of the aortic arch vessels. Fatemapping studies using a Pitx2 cre recombinase knock-in allele showed that daughters ofPitx2-expressing cells populated the right and left ventricles,atrioventricular cushions and valves and pulmonary veins. In Pitx2mutant embryos, descendents of Pitx2-expressing cells failed to contribute to the atrioventricular cushions and valves and the pulmonary vein,resulting in abnormal morphogenesis of these structures. Our data provide functional evidence that the presumptive secondary heart field, derived from branchial arch and splanchnic mesoderm, patterns the forming outflow tract and reveal a role for Pitx2c in aortic arch remodeling. Moreover, our findings suggest that a major function of the Pitx2-mediated left right asymmetry pathway is to pattern the aortic arches, outflow tract and atrioventricular valves and cushions.

Expression ofPeg1 (Mest) in the developing mouse heart: Involvement in trabeculation

Peg1 (Mest) is an imprinted gene of unknown function widely expressed in the mouse embryo, predominantly in cells of the mesodermal lineage. We have revealed a restricted expression pattern within the developing heart. Initial uniform expression throughout the linear heart tube subsequently becomes restricted, primarily to the developing myocardial trabeculae of both the atria and ventricles, where it persists into late development. Expression in the atrial appendage myocardium precedes the emergence of trabeculae (pectinate muscles), and occurs earlier and to a greater extent on the right than on the left, reflecting the spatial and temporal pattern of trabeculation. Analysis of myocardial morphology in mice lacking the Peg1 gene, which are viable and appear grossly normal, reveals a subtle alteration in the pattern of trabeculation: an increase in thickness and reduction in density of the compact myocardium, similar to that seen in the human cardiomyopathy ventricular noncompaction.

Selection of test chemicals for the ECVAM international validation study on in vitro embryotoxicity tests. European Centre for the Validation of Alternative Methods

The European Centre for the Validation of Alternative Methods (ECVAM) has sponsored a large international prevalidation and validation study of three embryotoxicity tests, involving embryonic stem cells, limb bud micromass cultures, and post-implantation whole-embryo cultures. The main objective of the study was to assess the performance of these in vitro tests in discriminating between non-embryotoxic, weakly embryotoxic and strongly embryotoxic compounds. An initial part of the study was to select 20 test substances for the formal validation trial, conducted under blind conditions. A database of in vivo and in vitro developmental toxicity test results was complied on 310 chemicals that had been used in previous validation studies, or suggested for such use, or that had good quality "segment II"-type in vivo data, or for which there were human data. From this database, a shortlist of about 30 candidates was constructed. Because the ECVAM study would not include metabolic activation, chemicals known to require activation for their developmental effects were excluded as candidates, although some known stable metabolites were included. Attempts were made: to include substances of diverse mechanism; to avoid overemphasis on pharmaceuticals; to avoid biologically inert substances as non-embryotoxicants; and to make the list different from those used previously. The candidates were of three categories: Class 3, strongly embryotoxic, was defined as developmentally toxic in all species tested, inducing multiple developmental effects, and with a high A/D ratio. Class 1, non-embryotoxic, was defined as not developmentally toxic at maternally toxic exposures, but which may show some minor embryo/fetal toxicity, which cannot be separated from maternal toxicity. Class 2, weakly embryotoxic, were chemicals of intermediate activity. From this candidate list, chemicals of known receptor (androgen, oestrogen, glucocorticoid, aryl hydrocarbon) mechanisms were excluded, on the basis that simple tests for such activity are already available. In addition, chemicals not freely available were excluded, and an emphasis on human data was applied. The final list of 20 chemicals was: Class 3--6-aminonicotinamide, 5-bromo- 2'-deoxyuridine, hydroxyurea, methylmercury chloride, methotrexate, all-trans-retinoic acid; Class 2--boric acid, dimethadione, lithium chloride, methoxyacetic acid, valproic acid (VPA), 2-propyl-4-pentynoic acid (4-yn-VPA), salicylic acid sodium salt; and Class 1--acrylamide, D-(+)-camphor, dimethyl phthalate, diphenhydramine hydrochloride, 2-ethyl-4- methylpentanoic acid (isobutyl-ethyl-VPA), Penicillin G sodium salt, saccharin sodium hydrate.

Decreasing fibrogenesis: an immunohistochemical study of paired liver biopsies following lamivudine therapy for chronic hepatitis B

BACKGROUND

Activation of hepatic stellate cells is the earliest step in fibrogenesis. Alpha-smooth muscle actin (alpha-SMA), expressed by activated hepatic stellate cells, and C-terminal procollagen alpha1(III) propeptide (PIIICP) are early markers of fibrogenesis and should precede fibrosis.

AIM

Determine if suppression of hepatitis B virus replication with lamivudine would decrease fibrogenesis as measured by immunohistochemical markers.

METHODS

Paired liver biopsies from patients with hepatitis B before and after therapy with lamivudine (n=47) or placebo (n=33) were studied. alpha-SMA and PIIICP were detected in paraffin-embedded tissue by immunohistochemistry and quantified in a blinded manner by video imaging analysis.

RESULTS

Liver biopsies from patients treated with lamivudine showed a significant decrease in alpha-SMA expression (1.06+/-0.23 vs. 0.58+/-0.11, pre vs. post, P<0.05). Placebo recipients had increased levels of alpha-SMA (0.82+/-0.14 vs. 1.32+/-0.21, P<0.05). PIIICP was similarly decreased after lamivudine. Among subjects whose Histologic Activity Index fibrosis score was unchanged or worsened, the mean change in alpha-SMA expression was significantly decreased in the lamivudine group compared with placebo.

CONCLUSIONS

Lamivudine decreased markers of hepatic stellate cell activation and collagen synthesis. Immunohistochemical techniques are sensitive for assessing fibrogenesis and will be useful in trials of antiviral and antifibrotic agents.

Development of the human pulmonary vein and its incorporation in the morphologically left atrium

OBJECTIVE

Using a newly acquired archive of previously prepared material, we sought to re-examine the origin of the pulmonary vein in the human heart, aiming to determine whether it originates from the systemic venous sinus ("sinus venosus"), or appears as a new structure draining to the left atrium. In addition, we examined the temporal sequence of incorporation of the initially solitary pulmonary vein to the stage at which four venous orifices opened to the left atrium.

METHODS

We studied 26 normal human embryos, ranging from 3.8 mm to 112 mm crown-rump length, and representing the period from the 12th Carnegie stage to 15 weeks of gestation.

RESULTS

The pulmonary vein canalised as a solitary vessel within the mediastinal tissues so as to connect the intraparenchymal pulmonary venous networks to the heart, using the regressing dorsal mesocardium as its portal of cardiac entry. The vein was always distinct from the tributaries of the embryonic systemic venous sinus. The orifice of the solitary vein became committed to the left atrium by growth of the vestibular spine. During development, a marked disparity was seen between the temporal and morphological patterns of incorporation of the left-sided and right-sided veins into the left atrium. The pattern of the primary bifurcation was asymmetrical, a much longer tributary being formed on the left than on the right. Contact between the atrial wall and the venous tributary on the left initially produced a shelf, which became effaced with incorporation of the two left-sided veins into the atrium.

CONCLUSIONS

The initial process of formation of the human pulmonary vein is very similar to that seen in animal models. The walls of the initially solitary vein in humans become incorporated by a morphologically asymmetric process so that four pulmonary veins eventually drain independently into the left atrium. Failure of incorporation on the left side may provide the substrate for congenital division of the left atrium.

Septation and valvar formation in the outflow tract of the embryonic chick heart

There is no agreement, in the chick, about the number of the endocardial cushions within the outflow tract or their pattern of fusion. Also, little is known of their relative contributions to the formation of the arterial valves, the subpulmonary infundibulum, and the arterial valvar sinuses. As the chick heart is an important model for studying septation of the outflow tract, our objective was to clarify these issues. Normal septation of the outflow tract was studied in a series of 60 staged chick hearts, by using stained whole-mount preparations, serial sections, and scanning electron microscopy. A further six hearts were examined subsequent to hatching. At stage 21, two pairs of endocardial cushions were seen within the developing outflow tract. One pair was positioned proximally, with the other pair located distally. By stage 25, a third distal cushion had developed. This finding was before the appearance of two further, intercalated, endocardial cushions, also distally positioned, which were first seen at stage 29. In the arterial segment, the aortic and pulmonary channels were separated by the structure known as the aortopulmonary septum. The dorsal limb of this septum penetrated the distal dorsal cushion, whereas the ventral limb grew between the remaining two distal cushions, both of which were positioned ventrally. The three distal endocardial cushions, and the two intercalated endocardial cushions, contributed to the formation of the leaflets and sinuses of the arterial roots. The two proximal cushions gave rise to a transient septum, which later became transformed into the muscular component of the subpulmonary infundibulum. Concomitant with these changes, an extracardiac tissue plane was formed which separated this newly formed structure from the sinuses of the aortic root. Our study confirms that three endocardial cushions are positioned distally, and two proximally, within the developing outflow tract of the chick. The pattern of the distal cushions, and the position of the ventral limb of the aortopulmonary septum, differs significantly from that seen in mammals.

The arterial pole of the mouse heart forms from Fgf10-expressing cells in pharyngeal mesoderm

Development of the arterial pole of the heart is a critical step in cardiogenesis, yet its embryological origin remains obscure. We have analyzed a transgenic mouse line in which beta-galactosidase activity is observed in the embryonic right ventricle and outflow tract of the heart and in contiguous splanchnic and pharyngeal mesoderm. The nlacZ transgene has integrated upstream of the fibroblast growth factor 10 (Fgf10) gene and comparison with the expression pattern of Fgf10 in pharyngeal mesoderm indicates transgene control by Fgf10 regulatory sequences. Dil labeling shows a progressive movement of cells from the pharyngeal arch region into the growing heart tube between embryonic days 8.25 and 10.5. These data suggest that arterial pole myocardium originates outside the classical heart field.

Misexpression of noggin leads to septal defects in the outflow tract of the chick heart

BMP-2 and BMP-4 are known to be involved in the early events which specify the cardiac lineage. Their later patterns of expression in the developing mouse and chick heart, in the myocardium overlying the atrioventricular canal (AV) and outflow tract (OFT) cushions, also suggest that they may play a role in valvoseptal development. In this study, we have used a recombinant retrovirus expressing noggin to inhibit the function of BMP-2/4 in the developing chick heart. This procedure resulted in abnormal development of the OFT and the ventricular septum. A spectrum of abnormalities was seen ranging from common arterial trunk to double outlet right ventricle. In hearts infected with noggin virus, where the neural crest cells have been labelled, the results show that BMP-2/4 function is required for the migration of neural crest cells into the developing OFT to form the aortopulmonary septum. Prior to septation, misexpression of noggin also leads to a decrease in the number of proliferating mesenchymal cells within the proximal cushions of the outflow tract. These results suggest that BMP-2/4 function may mediate several key events during cardiac development.

Regulation of left-right asymmetry by thresholds of Pitx2c activity

Although much progress has been made in understanding the molecular mechanisms regulating left-right asymmetry, the final events of asymmetric organ morphogenesis remain poorly understood. The phenotypes of human heterotaxia syndromes, in which organ morphogenesis is uncoupled, have suggested that the early and late events of left-right asymmetry are separable. The Pitx2 homeobox gene plays an important role in the final stages of asymmetry. We have used two new Pitx2 alleles that encode progressively higher levels of Pitx2c in the absence of Pitx2a and Pitx2b, to show that different organs have distinct requirements for Pitx2c dosage. The cardiac atria required low Pitx2c levels, while the duodenum and lungs used higher Pitx2c doses for normal development. As Pitx2c levels were elevated, the duodenum progressed from arrested rotation to randomization, reversal and finally normal morphogenesis. In addition, abnormal duodenal morphogenesis was correlated with bilateral expression of Pitx2c. These data reveal an organ-intrinsic mechanism, dependent upon dosage of Pitx2c, that governs asymmetric organ morphogenesis. They also provide insight into the molecular events that lead to the discordant organ morphogenesis of heterotaxia.

Posterior subcapsular and nuclear cataract after vitrectomy

PURPOSE

To examine the morphological changes in the postvitrectomy lens and to monitor the development of these changes over time.

SETTING

Oxford Eye Hospital, Oxford, United Kingdom.

METHODS

In this prospective study, 33 consecutive phakic patients having pars plana vitrectomy were recruited. Cataract development was quantified by clinical grading and digital Scheimpflug image analysis. Slitlamp biomicroscopy and photography were used to document the morphological appearance. The main outcome measures were the incidence, morphology, and development of posterior subcapsular and nuclear cataract.

RESULTS

A characteristic, transient posterior subcapsular cataract (PSC) was present in 89% (17 of 19) of tamponade patients within 24 hours of surgery. Of the patients who had vitrectomy without tamponade, 9% (1 of 11) developed similar changes. Nuclear opacity developed in 61% (11 of 18) of tamponade patients and in 50% (3 of 6) of nontamponade patients. A longer term retrospective review of the same patients' case notes revealed nuclear cataract in 67% (12 of 18) of tamponade cases and 30% (3 of 10) of nontamponade cases. Eighteen percent (2 of 11) of nontamponade cases and 67% (14 of 21) of tamponade cases had cataract surgery after a 10.7 month and a 12.4 month follow-up, respectively.

CONCLUSIONS

Vitrectomy and tamponade produced a characteristic transient PSC in the immediate postoperative period. Disruption of fluid balance in the region of the posterior lens was suggested by the morphological appearance. The acute changes resolved but were followed by accelerated nuclear opacification.

[Results of the first phase of the ECVAM project "prevalidation and validation of three in vitro embryotoxicity tests"]

The results of the experimental prevalidation of three in vitro embryotoxicity tests in an ECVAM study are reported: the rat whole embryo culture assay (WEC), the micromass assay (MM) and the embryonic stem cell test (EST). Three test chemicals assigned to the three classes of embryotoxicity, non-, moderate- and strong embryotoxic, and also a positive and a negative control chemical were repeatedly tested in each test in two laboratories. The data obtained were evaluated independently by a biostatistician. It could be shown that the standard protocols (SOPs) for the three in vitro embryotoxicity tests could be transferred to a second laboratory and that reproducible results were obtained. The three methods were able to discriminate the test chemicals according to their embryotoxic potential. In addition, within the scope of this study, a test chemical database for in vitro embryotoxicity testing was established that will be used in the following formal validation of the three in vitro tests.

MLC3F transgene expression iniv mutant mice reveals the importance of left-right signalling pathways for the acquisition of left and right atrial but not ventricular compartment identity

Abstract Transcriptional differences between left and right cardiac chambers are revealed by an nlacZ reporter transgene controlled by regulatory sequences of the MLC3F gene, which is expressed in the left ventricle (LV), atrioventricular canal (AVC), and right atrium (RA). To examine the role of left-right signalling in the acquisition of left and right chamber identity, we have investigated MLC3F transgene expression in iv mutant mice. iv/iv mice exhibit randomised direction of heart looping and an elevated frequency of associated laterality defects, including atrial isomerism. At fetal stages, 3F-nlacZ-2E transgene expression remains confined to the morphological LV, AVC, and RA in L-loop hearts, although these appear on the opposite side of the body. In cases of morphologically distinguishable right atrial appendage isomerism, both atrial appendages show strong transgene expression. Conversely, specimens with morphological left atrial appendage isomerism show only weak expression in both atrial appendages. The earliest left-right atrial differences in the expression of the 3F-nlacZ-2E transgene are observed at E8.5. DiI labelling experiments confirmed that transcriptional regionalisation of the 3F-nlacZ-2E transgene at this stage reflects future atrial chamber identity. In some iv/iv embryos at E8.5, the asymmetry of 3F-nlacZ-2E expression was lost, suggesting atrial isomerism at the transcriptional level prior to chamber formation. These data suggest that molecular specification of left and right atrial but not ventricular chambers is dependent on left-right axial cues.

Spinal malformations in the fetuses of HIV infected women receiving combination antiretroviral therapy and co-trimoxazole

HIV positive women of reproductive age are increasingly treated with a combination of antiretroviral agents, with effects on the developing human fetus that are largely unknown. We report two cases of severe spinal malformations in the fetuses of women treated with combination antiretroviral therapy and co-trimoxazole.

A review of the developmental and reproductive toxicity of styrene

The reproductive and developmental toxicity of styrene has been studied in animals and humans. The animal studies on styrene have diverse study designs and conclusions. Developmental or reproductive toxicity studies have been conducted in rats, mice, rabbits, and hamsters. In most cases, high doses are required to elicit effects, and the effects are not unique to reproduction or development. In a number of the reports, either the experimental designs are limited or the descriptions of the designs and the endpoints measured are insufficient to draw conclusions about the toxicity of styrene. The more complete and better-reported studies show that styrene does not cause developmental toxicity at dose levels that are not maternally toxic. Some neurochemical or neurobehavioral effects have been reported at high exposures. Styrene does not affect fertility or reproductive function. Considerable animal toxicity data on styrene support the conclusion that styrene is neither an endocrine-active substance nor an endocrine disrupter. Human studies often suffer from either inadequate exposure data or exposure to a wide variety of materials, so that attribution of effects to styrene exposure is impossible. Furthermore, investigators often have failed to account for other exposures in the workplace or for other potentially confounding factors in their studies. Menstrual cycle irregularities and congenital abnormalities were initially reported; however, the better and more recent reports do not show that styrene causes developmental or reproductive effects in humans. Human studies also support the conclusion that styrene is not an endocrine disrupter. Although some study authors have concluded that styrene is either a human or an animal reproductive or developmental toxicant, careful review demonstrates that such conclusions are not justified.

A governing relationship for repetitive muscular contraction

During repetitive contractions, muscular work has been shown to exhibit complex relationships with muscle strain length, cycle frequency, and muscle shortening velocity. Those complex relationships make it difficult to predict muscular performance for any specific set of movement parameters. We hypothesized that the relationship of impulse with cyclic velocity (the product of shortening velocity and cycle frequency) would be independent of strain length and that impulse-cyclic velocity relationships for maximal cycling would be similar to those of in situ muscle performing repetitive contraction. Impulse and power were measured during maximal cycle ergometry with five cycle-crank lengths (120-220mm). Kinematic data were recorded to determine the relationship of pedal speed with joint angular velocity. Previously reported in situ data for rat plantaris were used to calculate values for impulse and cyclic velocity. Kinematic data indicated that pedal speed was highly correlated with joint angular velocity at the hip, knee, and ankle and was, therefore, considered a valid indicator of muscle shortening velocity. Cycling impulse-cyclic velocity relationships for each crank length were closely approximated by a rectangular hyperbola. Data for all crank lengths were also closely approximated by a single hyperbola, however, impulse produced on the 120mm cranks differed significantly from that on all other cranks. In situ impulse-cyclic velocity relationships exhibited similar characteristics to those of cycling. The convergence of the impulse-cyclic velocity relationships from most crank and strain lengths suggests that impulse-cyclic velocity represents a governing relationship for repetitive muscular contraction and thus a single equation can predict muscle performance for a wide range of functional activities. The similarity of characteristics exhibited by cycling and in situ muscle suggests that cycling can serve as a window though which to observe basic muscle function and that investigators can examine similar questions with in vivo and in situ models.

Atrial development in the human heart: an immunohistochemical study with emphasis on the role of mesenchymal tissues

The development of the atrial chambers in the human heart was investigated immunohistochemically using a set of previously described antibodies. This set included the monoclonal antibody 249-9G9, which enabled us to discriminate the endocardial cushion-derived mesenchymal tissues from those derived from extracardiac splanchnic mesoderm, and a monoclonal antibody recognizing the B isoform of creatine kinase, which allowed us to distinguish the right atrial myocardium from the left. The expression patterns obtained with these antibodies, combined with additional histological information derived from the serial sections, permitted us to describe in detail the morphogenetic events involved in the development of the primary atrial septum (septum primum) and the pulmonary vein in human embryos from Carnegie stage 14 onward. The level of expression of creatine kinase B (CK-B) was found to be consistently higher in the left atrial myocardium than in the right, with a sharp boundary between high and low expression located between the primary septum and the left venous valve indicating that the primary septum is part of the left atrial gene-expression domain. This expression pattern of CK-B is reminiscent of that of the homeobox gene Pitx2, which has recently been shown to be important for atrial septation in the mouse. This study also demonstrates a poorly appreciated role of the dorsal mesocardium in cardiac development. From the earliest stage investigated onward, the mesenchyme of the dorsal mesocardium protrudes into the dorsal wall of the primary atrial segment. This dorsal mesenchymal protrusion is continuous with a mesenchymal cap on the leading edge of the primary atrial septum. Neither the mesenchymal tissues of the dorsal protrusion nor the mesenchymal cap on the edge of the primary septum expressed the endocardial tissue antigen recognized by 249-9G9 at any of the stages investigated. The developing pulmonary vein uses the dorsal mesocardium as a conduit to reach the primary atrial segment. Initially, the pulmonary pit, which will becomes the portal of entry for the pulmonary vein, is located along the midline, flanked by two myocardial ridges. As development progresses, tissue remodeling results in the incorporation of the portal of entry of the pulmonary vein in left atrial myocardium, which is recognized because of its high level of creatine. Closure of the primary atrial foramen by the primary atrial septum occurs as a consequence of the fusion of these mesenchymal structures.

Five years' experience with the Telemedicine Information Exchange

The Telemedicine Information Exchange (TIE) has provided comprehensive telemedicine information on the World Wide Web (Web) since early 1995. It received major funding from the National Library of Medicine in 1997. Among other things, the TIE contains six major databases: literature citations; active telemedicine programmes; a 'what's new in telemedicine' column; funding opportunities; forthcoming conferences; and a list of vendors of telemedicine equipment and services. Recent additions include a document delivery service, inaugurated in early 1999. More than 1000 other Web sites link to the TIE, and we have 5000 visitors per month from several countries. Given its relative longevity on the Web, TIE researchers have been in a unique position to observe trends in telemedicine.

Relationship in the chick of the developing pulmonary vein to the embryonic systemic venous sinus

Previous studies have shown that the relationship of the systemic venous sinus (sinus venosus) to the developing pulmonary vein are very similar in mice, rats, and man, with the pulmonary vein gaining access to the heart through a persisting segment of the dorsal mesocardium. It has been suggested that this process differs in avian development, with the pulmonary vein being connected to the systemic venous sinus with subsequent transfer to the left atrium. Here we have investigated the anatomical sequence of events in the chick, using serial histological sections and microdissection followed by scanning electron microscopy. We examined a temporal series of chick embryos, ranging from Hamburger and Hamilton stage 15 to stage 30. Although there are some differences in detail, the development of the pulmonary venous connections in the chick was found to be directly comparable to that already described in eutherian mammals. In both mammals and the chick, the dorsal mesocardial connection, which connects the primitive atrium to the posterior thoracic wall, forms a fixed point through which the pulmonary vein gains access to the atrial compartment of the heart, only varying if the connection itself is anomalous. The tributaries of the systemic venous sinus and the primary atrial septal structures develop around the dorsal connection.

Cloning and expression of the Wnt antagonists Sfrp-2 and Frzb during chick development

The Wnt genes are known to play fundamental roles during patterning and development of a number of embryonic structures. Receptors for Wnts are members of the Frizzled family of proteins containing a cysteine-rich domain (CRD) that binds the Wnt protein. Recently several secreted frizzled-related proteins (Sfrps) that also contain a CRD have been identified and some of these can both bind and antagonise Wnt proteins. In this paper we report the expression patterns of the chick homologues of Frzb, a known Wnt antagonist, and Sfrp-2. Both genes are expressed in areas where Wnts are known to play a role in development, including the neural tube, myotome, cartilage, and sites of epithelial-mesenchymal interactions. Initially, Sfrp-2 and Frzb are expressed in overlapping areas in the neural plate and neural tube, whereas later, they have distinct patterns. In particular Sfrp-2 is associated with myogenesis while Frzb is associated with chondrogenesis, suggesting that they play different roles during development. Finally, we have used the early Xenopus embryo as an in vivo assay to show that Sfrp-2, like Frzb, is a Wnt antagonist. These results suggest that Sfrp-2 and Frzb may function in the developing embryo by modulating Wnt signalling.

Analysis of sediments from Shetland Island voes for polycyclic aromatic hydrocarbons, steranes and triterpanes

A few days after the grounding of the oil tanker Braer on 5 January 1993, an Exclusion Zone was designated by Order under the Food and Environment Protection Act 1985, prohibiting the harvesting of farmed or wild shellfish within the Zone to prevent contaminated products reaching the market place. The order was progressively lifted for species that were found to be free of petrogenic taint and for which the polycyclic aromatic hydrocarbon (PAH) levels were within the range for reference samples. This Order, however, still remains in place for mussels (Mytilus edulis) as the PAH levels are higher than in reference mussels. To investigate the possible source of PAHs found in these mussels, sediments were collected from three reference and three Zone sites and their hydrocarbon compositions studied using the n-alkane composition and concentration, PAH composition and concentration and the sterane and triterpane composition. The reference site at Olna Firth was found to have the highest levels of 2-6-ring parent and branched PAHs, the highest concentration in one of the pooled sediments being 4,530 ng g(-1) dry weight. Values in the other two reference sites (Vaila Sound and Mangaster Voe) ranged from 248.7 to 902.2 ng g(-1) dry weight. PAH concentrations at the Zone sites (Sandsound Voe, Stromness Voe and Punds Voe) ranged from 641.0 to 2,766 ng g(-1) dry weight. The PAH data were normalised to the percentage of organic carbon and log-transformed prior to being analysed using principal component analysis. The mean total PAH concentrations for Zone sites were found not to be significantly different from the reference sites. The PAH concentration ratios were consistent with the main source of PAHs being pyrolysis. However, there was a petrogenic contribution, suggested by the presence of alkylated PAHs, with Punds Voe having the largest petrogenic hydrocarbon content. This was supported by the triterpane profiles and the presence of a UCM in the aliphatic chromatograms from Punds Voe sediments.

Quantitation of hepatitis B viremia and emergence of YMDD variants in patients with chronic hepatitis B treated with lamivudine

Hepatitis B viremia and emergence of hepatitis B virus (HBV) YMDD variants with reduced susceptibility to lamivudine were analyzed in patient sera from a phase II study of extended lamivudine therapy. Within 12 weeks, all patients exhibited a marked virologic response to lamivudine: >99% reduction (median 5 log decrease) in serum HBV DNA levels. Virus remained at >104 genomes/mL in 11 patients and decreased to <104 genomes/mL in the remaining 12 patients. In 10 patients, detectable YMDD variants emerged during the course of treatment. Six patients, including 3 with YMDD variants, experienced hepatitis B e antigen seroconversion while on lamivudine therapy or soon after its discontinuation. No patients with HBV DNA levels >104 genomes/mL seroconverted. Thus, patients who respond to lamivudine therapy with dramatic reductions in viral DNA level (to <104 genomes/mL) appear more likely to seroconvert than patients who do not achieve this level of HBV clearance.

Lamivudine as initial treatment for chronic hepatitis B in the United States

BACKGROUND AND METHODS

Although the nucleoside analogue lamivudine has shown promise in patients with chronic hepatitis B, long-term data on patients from the United States are lacking. We randomly assigned previously untreated patients with chronic hepatitis B to receive either 100 mg of oral lamivudine or placebo daily for 52 weeks. We then followed them for an additional 16 weeks to evaluate post-treatment safety and the durability of responses. The primary end point with respect to efficacy was a reduction of at least 2 points in the score on the Histologic Activity Index. On this scale, scores can range from 0 (normal) to 22 (most severe abnormalities).

RESULTS

Of the 143 randomized patients, 137 were included in the efficacy analysis: 66 in the lamivudine group and 71 in the placebo group. The other six patients were excluded at the base-line visit because of the absence of a documented history of hepatitis B surface antigen for at least six months. After 52 weeks of treatment, lamivudine recipients were more likely than placebo recipients to have a histologic response (52 percent vs. 23 percent, P<0.001), loss of hepatitis B e antigen (HBeAg) in serum (32 percent vs. 11 percent, P=0.003), sustained suppression of serum hepatitis B virus (HBV) DNA to undetectable levels (44 percent vs. 16 percent, P<0.001), and sustained normalization of serum alanine aminotransferase levels (41 percent vs. 7 percent, P<0.001), and they were less likely to have increased hepatic fibrosis (5 percent vs. 20 percent, P=0.01). Lamivudine recipients were also more likely to undergo HBeAg seroconversion, defined as the loss of HBeAg, undetectable levels of serum HBV DNA, and the appearance of antibodies against HBeAg (17 percent vs. 6 percent, P=0.04). HBeAg responses persisted in most patients for 16 weeks after the discontinuation of treatment. Lamivudine was well tolerated. Self-limited post-treatment elevations in serum alanine aminotransferase were more common in lamivudine recipients: 25 percent had serum alanine aminotransferase levels that were at least three times base-line levels, as compared with 8 percent of placebo recipients (P=0.01). The clinical condition of all patients remained stable during the study.

CONCLUSIONS

In U.S. patients with previously untreated chronic hepatitis B, one year of lamivudine therapy had favorable effects on histologic, virologic, and biochemical features of the disease and was well tolerated. HBeAg responses were generally sustained after treatment.

Clinical anatomy of the atrial septum with reference to its developmental components

Knowledge of development is of crucial importance and can help clarify mechanisms of maldevelopment, but it must be properly validated. Concepts of development must be consistent with the anatomy seen in postnatal life. Such consistency is not always achieved. We have reviewed new and old accounts of cardiac embryology with regard to the definitive structure of the atrial septum. The key to understanding is to distinguish between folds of the atrial wall and true interatrial partitions. The flap valve of the oval foramen, and its inferior rim, are true septal structures, whereas the other rims, particularly the antero-superior rim, are infoldings enclosing extracardiac fat. During embryonic life, the systemic venous tributaries must achieve entrance only to the right side of the primary atrium. Development of the pulmonary venous component is a late event, with the canalizing vein using the dorsal mesocardium to gain access to the left side of the atrium. Once the systemic venous tributaries have achieved their rightward shift, the primary septum, together with the mesenchymal cap, grows between the systemic and pulmonary venous orifices. Closure of the primary foramen is achieved by fusion of the mesenchymal cap of the primary septum with the atrioventricular endocardial cushions and the vestibular spine (an additional mesenchymal structure carried on the right side of the pulmonary venous orifice). The superior margin of the newly formed secondary foramen is produced by an infolding of the atrial walls. Historically these mechanisms received appropriate recognition, but not all receive their proper due in current writings.

Mechanisms of deficient cardiac septation in the mouse with trisomy 16

It used to be thought that the atrioventricular septum was predominantly the product of the atrioventricular endocardial cushions. In a previous study, we have shown that multiple developmental primordia are of importance in its formation. With this in mind, we have evaluated cardiac morphogenesis in the mouse with trisomy 16, an animal model with a high incidence of atrioventricular septal defects. Normal and trisomic fetuses from an Rb(11.16)2H/Rb(16.17)7Bnr x C57BL/6J cross were collected on days 10 to 15 of gestation and examined by scanning electron microscopy and histological serial sectioning. No evidence was found to suggest that atrioventricular septal defect could be explained simply on the basis of "failure of fusion" between the atrioventricular endocardial cushions. Rather, our findings supported two other developmental elements as being important in the genesis of atrioventricular septal defect. The first is an alteration in the configuration of the heart tube, with inadequate remodeling of the inner heart curvature. This resulted in the failure of the atrioventricular junction to expand to the right, with subsequent malalignment of the atrioventricular endocardial cushions with the proximal outflow cushions. The second is a variability in the connection of the primary atrial cardiac segment to the body of the embryo, the so-called dorsal mesocardium, which influences its relationship to the extracardiac mediastinal mesoderm. There appeared little difference in the connection between normal and trisomic embryos at the stage of 20 to 25 somites, but the area subsequently showed marked changes. In most trisomic embryos, the connection with the mediastinal mesoderm of the body was over a larger area than seen in normal embryos. As this area of attachment encloses the pulmonary pit, the entry point of the pulmonary vein, this gives potential for variation in the connection of the pulmonary vein. In addition, in the majority of trisomic embryos, the right pulmonary ridge (the spina vestibuli) did not accumulate extracardiac mesoderm, nor did it undergo the pronounced forward growth seen in normal embryos of equivalent stages. Consequently, the trisomic embryos show incomplete formation of both the atrial and the atrioventricular septal structures.

Mechanism of hyperthermia effects on CNS development: rostral gene expression domains remain, despite severe head truncation; and the hindbrain/otocyst relationship is altered

To study the mechanism of hyperthermia on the development of the rostral neural tube, we used a model in which closely-staged presomite 9.5-day rat embryos were exposed in culture to 43 degrees C for 13 min, and then cultured further for 12-48 hr. This treatment had little effect on the development of the rest of the embryo, but resulted in a spectrum of brain defects, the most severe being a lack of all forebrain and midbrain structures. Whole-mount in situ hybridisation was used to monitor the expression domains of Otx2, Emx2, Krox20, and hoxb1. These showed that there were no ectopic expression patterns, for any gene at any stage examined. Even in those embryos which apparently lacked all forebrain and midbrain structures, there were expression domains of Otx2 and Emx2 in the most rostral neural tissue, and these retained their nested dorso-ventral boundaries, showing that cells fated to form rostral brain were not wholly eliminated. Thus, heat-induced rostral neural tube truncation is of a quite different mechanism from the respecification proposed for retinoic acid, despite their very similar phenotypes. In the hindbrain region of treated embryos, we observed decreased intensity of Krox20, staining and an abnormal relationship developed between the position of hoxb1 expression and the otocyst and pharyngeal arches. In the most extreme cases, this domain was shifted to be more caudal than the rostral edge of the otocyst, while the otocyst retained its normal position relative to the pharyngeal arches. We interpret this as a growth imbalance between neuroepithelium and overlying tissues, perhaps due to a disruption of signals from the midbrain/hindbrain boundary.

Embryonic asymmetry: the left side gets all the best genes

The origin of left-right developmental asymmetry is a continuing puzzle, but some recent results provide new insights into the steps leading to organ asymmetry - implicating the homeobox protein Pitz-2 in one key step - and others support a model of symmetry-breaking that involves the chirality of microtubules.

The role of the brachyury gene in heart development and left-right specification in the mouse

The midline has a theoretical role in the development of left-right asymmetry, and this is supported by both genetic analyses and experimental manipulation of midline structures in vertebrates. The mouse brachyury (T) gene encodes a transcription factor which is expressed in the developing notochord and is required for its development. T/T mice lack a mature notochord and have a dorsalised neural tube. We have examined the hearts of T/T mice and have found consistent morphological abnormalities, resulting in ventrally displaced ventricular loops, and a 50% incidence of inverted heart situs. Three TGF-beta related genes, lefty-1, lefty-2 and nodal, are expressed asymmetrically in mouse embryos, and are implicated in the development of situs. We find that nodal, which is normally expressed around the node and in left lateral plate mesoderm in early somite embryos, is completely absent at this stage in T/T embryos. In contrast, lefty-1 and lefty-2, which are normally expressed in the left half of prospective floorplate and left lateral plate mesoderm, respectively, are both expressed in T/T embryos only in a broad patch of ventral cells in, and just rostral to, the node region. These results implicate the node as a source of instructive signals driving expression of nodal and lefty-2 in the left lateral plate mesoderm, and being required for normal looping and situs of the heart.

Defective lateralisation in children with congenitally malformed hearts

Defects in lateralization can be studied from the stance of populations, the individual, or the systems of organs within each individual. Unfortunately, and confusingly, the same terms are being applied to each of these situations, but inevitably with different meanings. Thus, there is presently no consensus on how we should use terms such as "heterotaxy" and "situs ambiguus". By far the least ambiguous use of these words is encountered when they are applied to the organs. In fact, each system of organs can accurately and simply be described in terms of its left-right morphology. All those organs which are paired then can be described, when interpreted on the basis of their intrinsic morphology, as being usually arranged, mirror-imaged, or as showing left or right isomerism. Within the heart, these changes are seen only in the atrial segment. The criterion for distinction of rightness or leftness within the atrial segment is the extent of the pectinate muscles relative to the atrioventricular junction. Application of this criterion permits unequivocal recognition of symmetry as opposed to lateralization. The same holds good for the other organs. Within any individual organ, therefore, the situation is neither ambiguous nor heterotaxic. Instead, it is lateralised or symmetrical. Within the individual, in contrast, there may well be discrepancies in the expected disposition of the systems of organs which produces potential ambiguity. To dispel this ambiguity, it is necessary to provide a full catalogue. For example, persons with otherwise normally arranged organs may have left bronchial isomerism. Other persons may have discordance between the thoracic organs, which are usually arranged, and the abdominal organs, which are mirror-imaged, but no evidence of isomerism. Within the population, however, we are unaware of any genetically or environmentally induced syndrome in which all individuals show evidence of mirror-imagery, or of isomerism, or of specific discordance between the systems. In fact, all known syndromes encompass all types of defective lateralization. When attempting to identify the genetic mechanisms for production of the syndromes, therefore, it could be positively misleading to attempt to separate isomerism from other perceived forms of"heterotaxy". Our preference is to consider any deviation from the usual arrangement as heterotaxy, and to specify the specific arrangement of the organs within each malformed individual.

Waves of mouse Lunatic fringe expression, in four-hour cycles at two-hour intervals, precede somite boundary formation

During somitogenesis, cells are recruited to the caudal presomitic mesoderm (PSM) from the primitive streak (and later the tail bud), while somites separate from the rostral end as epithelial cubes. This is a regular process, one somite forming every 2 hours in the mouse, that can be simulated by clock and wavefront models. The chick basic helix-loop-helix transcription factor encoded by c-hairy1 is expressed in dynamic waves in the PSM, undergoing one cycle for each somite formed. This is compatible with an underlying oscillating molecular clock. We have shown here that Lunatic fringe (L-fng) expression is indicative of it being one of the implementing outputs of this clock. Fringe genes regulate the Notch signalling pathway in boundary formation. Of the known mouse genes, only L-fng is expressed in PSM and it is required for somite segmentation and patterning. We have now shown that L-fng is expressed as dynamic, repetitive and complex waves within the mouse PSM. A wave takes 4 hours to complete one cycle and terminates immediately at, and prior to, somite boundary formation. Consecutive waves are temporally but not spatially overlapping, being initiated in the caudal PSM every 2 hours, so offset by one half-cycle. Waves of expression are not associated with cell movement and do not require cell contact for propagation, so appear to reflect a cell-autonomous clock that is synchronous in all PSM cells.

The telemedicine information exchange: an online resource

Telemedicine is the use of telecommunications to provide health care services at a distance. It has grown from mostly government-subsidized research initiatives into a fledgling industry. This growth was fueled by decreasing costs of telecommunications and information technologies and fanned by rising health care costs. The telemedicine information exchange (TIE) (http:(/)/tie.telemed.org), developed by the Telemedicine Research Center in Portland, OR, is both a product and a vehicle of this rapid growth. It facilitates collection of telemedicine information from many sources, providing an easy-to-use hypertext format. This article describes the TIE's development, advantages and disadvantages of a web-based online library, and it's codevelopment with a rapidly expanding industry.

Human D2 and D4 dopamine receptors couple through betagamma G-protein subunits to inwardly rectifying K+ channels (GIRK1) in a Xenopus oocyte expression system: selective antagonism by L-741,626 and L-745,870 respectively

To examine the effects of a novel selective D4 receptor ligand, L-745,870 (3-[4-(4-chlorophenyl)piperazin-1-yl]methyl-1H-pyrrolo[2,3-b]pyrid ine), on human dopamine receptor function, the ability of this ligand to antagonise G-protein gated inwardly rectifying K+ (GIRK/Kir3) currents activated by cloned human D2 and D4 receptors expressed in Xenopus oocytes was examined using voltage-clamp recording. Its effects were also contrasted with that of a selective D2 receptor antagonist L-741,626. L-745,870 had no detectable agonist activity on human D4 receptors and selectively blocked currents activated by D4 but not D2 receptors. The role of G-protein subunits in dopamine receptor modulation of GIRK currents was also examined by co-expression of beta1 and/or gamma2 subunits on spontaneously active and receptor-activated currents. Currents activated by both D2 and D4 receptors were occluded by direct activation of GIRK currents following co-transfection with the cDNA encoding G-protein betagamma subunits. These data demonstrate that L-745,870 and L-741,626 act as antagonists on human D4 and D2 receptors respectively, and that activation of GIRK channels by these dopamine receptors can be disrupted by direct stimulation of K+ currents by G-protein betagamma subunits.

Hyperthermia in the chick embryo: HSP and possible mechanisms of developmental defects

Although hyperthermia is an established teratogen in all species studied and the cellular heat shock response is well known, the mechanisms of developmental deviation remain obscure. We have used a chick model system in which fertilized eggs containing embryos at presomite and/or early somite stages (HH 4-10) were exposed to 45 degrees C for 180 min. Six hours following treatment we did not observe any overt morphological disturbance, but at twelve hours following exposure (when controls reached HH 11-13) embryos exposed at late streak stages (HH 4-6) exhibited severe malformation of the head. Embryos exposed later (HH 6-9) manifested spina bifida at the thoracic and lumbosacral levels. Mirror image heart looping was also observed in 20% of these embryos. Paraxial mesoderm was apparently unaffected. Changes in cell proliferation and induced cell death preceded morphological changes. We used acridine orange and confocal laser microscopy to demonstrate that hyperthermia induced cell death in neural folds starting 6 h following treatment. To assess cell proliferation, we used BrdU incorporation for 4 h. Immunodetection on paraffin sections demonstrated that proliferation was inhibited 6 h after treatment. Heat-exposed embryos exhibited the heat shock response, with protein expression reaching a maximum 4-6 h following heat treatment. Malformed embryos showed an intense heat shock response for a further 6 h. The levels of induced heat shock proteins were similar in the affected neural tube and in the heart, where neither induced cell death nor malformations were observed.

The mouse with trisomy 16 as a model of human hearts with common atrioventricular junction

OBJECTIVE

To establish if the mouse with trisomy 16 is a suitable animal model with which to elucidate the development of a common atrioventricular junction.

METHODS

The junctional morphologies in the normal human heart and those with a common atrioventricular junction are compared and contrasted. These are then related to observations made in normal mice and those with trisomy 16. So as better to understand development, a full description is given first of the normal atrioventricular junctions. Developmental implications are discussed because failure of fusion of the endocardial cushions cannot account for all the anomalies found in RXR alpha knockout, and in iv/iv mice.

RESULTS

Mice with trisomy 16 showed evidence of deficiencies of atrioventricular septation and possessed a common atrioventricular junction, but the valvar orifices were not balanced between the ventricles as is the case in humans. Whilst some mice showed affinities with human tricuspid atresia, other cardiac malformations in the mice had no counterparts in human cardiac pathology. In humans both "partial" and "complete" forms of "atrioventricular canal malformations" share a basically common muscular junctional morphology, the differences being due exclusively to the way the bridging leaflets are fused to each other and/or the septum.

CONCLUSIONS

It is simplistic to use the mouse with trisomy 16 as a model for cardiac abnormalities seen in humans. A spectrum more comparable to humans is found in RXR knockout mice. Study of the iv/iv mouse may help elucidate the genetic steps involved in normal and abnormal atrioventricular septation.

Nutrition supplements and the eye

PURPOSE

A review of the role of vitamins, minerals, carotenoids and essential fatty acids in relation to eye health. The mode of action may be directly on the eye or by promoting bodily health on which the eye depends.

RESULTS

The lens and retina suffer oxidative damage and the anti-oxidant vitamins A, C and E are implicated as protective. Studies in man give indifferent support to the role of nutrition in the development of cataract. In the elderly, vitamin intake may be inadequate, so that a vitamin supplement may be reasonable. Zinc has a role in retinal metabolism and may be beneficial in macular degeneration. Selenium has an anti-oxidant role. Other minerals including copper have a less defined role. Carotenoids are concentrated at the macula and have an anti-oxidant role. A reduced risk of macular degeneration is found in relation to a high serum level. The essential fatty acid, gamma-linolenic acid (GLA), is useful in Sjögren's syndrome and may help in other dry eye conditions. Omega-3 fatty acids are important in retinal development and have a role in preventing cardiovascular disease.

CONCLUSION

All persons should be encouraged to maintain healthy nutrition. Middle-aged and elderly patients may benefit from a supplement. An intake in excess of the recommended daily intake may be beneficial, but this is not proven. Further clinical trials are indicated to define the advisability of vitamin, mineral and other supplements. Dosages for recommended intake and for supplements are given.

Effects of pulsed ultrasound on embryonic development: an in vitro study

Whole-embryo culture was used as the model system to study the effects of pulsed ultrasound on embryonic development. Rat embryos (9.5 days old) were exposed to a wide range of ultrasound levels at ultrasound frequencies between 1-4 MHz for 30 min in vitro. After 48 h in culture, absolute control, sham and treatment embryos were assessed for viability, morphology, growth and development. At an ambient temperature of 37 degrees C, no significant effects were observed for spatial peak temporal average intensities below 4 W/cm2 or peak negative pressures below 1.9 MPa. At higher acoustic levels, there was a significant increase in the number of nonviable embryos and the number of morphological abnormalities in viable embryos increased. Abnormal cephalocaudal flexion and abnormal head development were the most common gross morphological abnormalities. Both thermal and nonthermal bioeffect mechanisms are involved.

Formation of the atrioventricular septal structures in the normal mouse

It is sometimes thought that formation of the atrioventricular septum is equated with fusion of the endocardial cushions and that failure of fusion can explain all deficiencies of atrioventricular septation. Clearly, this is simplistic, but the exact contribution of different primordia to atrioventricular septation is not well understood. To clarify this, we studied normal mouse embryos (days 10 to 15 of gestation), which were serially sectioned and examined by light microscopy. Another group of embryos was examined by scanning electron microscopy after microdissection. Our results show that development of the atrioventricular septal area is highly complex. Proper formation requires the following: remodeling of the inner heart curvature, rotation of the horns of the systemic venous sinus around the pulmonary portal, expansion of the right atrioventricular junction, formation of the muscular atrial and ventricular septa, bridging by the dextrodorsal outflow ridge and the superior endocardial cushion, fusion with the inferior margins of the venous valves, and formation of the mouth of the coronary sinus from the cranial muscular wall of the left sinus horn. Multiple primordia contribute to a central mesenchymal mass (the "septum intermedium"), including the mesenchyme on the leading edge of the primary atrial septum, the atrioventricular endocardial cushions, and the cap of mesenchyme on the spina vestibuli. Fusion of these components closes the ostium primum, completing atrial and atrioventricular septation. Additionally, the spina vestibuli has a mesodermal core, which contributes to the muscularization of the lower margin of the oval fossa. This contrasts with the formation of the upper rim, which occurs as a result of an infolding of the atrial wall itself.