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.