Myogenic cytodifferentiation of the precardiac mesoderm in the rat

Nfatc1 directs the endocardial progenitor cells to make heart valve primordium

Heart valves arise from the cardiac endocardial cushions located at the atrioventricular canal (AVC) and cardiac outflow tract (OFT) during development. A subpopulation of cushion endocardial cells undergoes endocardial to mesenchymal transformation (EMT) and generates the cushion mesenchyme, which is then remodeled into the interstitial tissue of the mature valves. The cushion endocardial cells that do not undertake EMT proliferate to elongate valve leaflets. During EMT and the post-EMT valve remodeling, endocardial cells at the cushions highly express nuclear factor in activated T cell, cytoplasmic 1 (Nfatc1), a transcription factor required for valve formation in mice. In this review, we present the current knowledge of Nfatc1 roles in the ontogeny of heart valves with a focus on the fate decision of the endocardial cells in the processes of EMT and valve remodeling.

What chick and mouse models have taught us about the role of the endocardium in congenital heart disease

Specific cell and tissue interactions drive the formation and function of the vertebrate cardiovascular system. Although much attention has been focused on the muscular components of the developing heart, the endocardium plays a key role in the formation of a functioning heart. Endocardial cells exhibit heterogeneity that allows them to participate in events such as the formation of the valves, septation of the outflow tract, and trabeculation. Here we review, the contributions of the endocardium to cardiovascular development and outline useful approaches developed in the chick and mouse that have revealed endocardial cell heterogeneity, the signaling molecules that direct endocardial cell behavior, and how these insights have contributed to our understanding of cardiovascular development and disease.

Dynamic positional fate map of the primary heart-forming region

Here we show the temporal-spatial orchestration of early heart morphogenesis at cellular level resolution, in vivo, and reconcile conflicting positional fate mapping data regarding the primary heart-forming field(s). We determined the positional fates of precardiac cells using a precision electroporation approach in combination with wide-field time-lapse microscopy in the quail embryo, a warm-blooded vertebrate (HH Stages 4 through 10). Contrary to previous studies, the results demonstrate the existence of a "continuous" circle-shaped heart field that spans the midline, appearing at HH Stage 4, which then expands to form a wide arc of progenitors at HH Stages 5-7. Our time-resolved image data show that a subset of these cardiac progenitor cells do not overlap with the expression of common cardiogenic factors, Nkx-2.5 and Bmp-2, until HH Stage 10, when a tubular heart has formed, calling into question when cardiac fate is specified and by which key factors. Sub-groups and anatomical bands (cohorts) of heart precursor cells dramatically change their relative positions in a process largely driven by endodermal folding and other large-scale tissue deformations. Thus, our novel dynamic positional fate maps resolve the origin of cardiac progenitor cells in amniotes. The data also establish the concept that tissue motion contributes significantly to cellular position fate - i.e., much of the cellular displacement that occurs during assembly of a midline heart tube (HH Stage 9) is NOT due to "migration" (autonomous motility), a commonly held belief. Computational analysis of our time-resolved data lays the foundation for more precise analyses of how cardiac gene regulatory networks correlate with early heart tissue morphogenesis in birds and mammals.

VCAM-1 inhibits TGFβ stimulated epithelial–mesenchymal transformation by modulating Rho activity and stabilizing intercellular adhesion in epicardial mesothelial cells

Regulation of epithelial-mesenchymal transformation (EMT) is of central importance both in normal development and in disease. During heart development, cells of the superficial epicardial mesothelium undergo EMT to give rise to precursor cells of the coronary vasculature and cardiac fibroblasts. Here we report that the alpha(4)beta(1) integrin ligand, VCAM-1, inhibits EMT of chick epicardial mesothelial cells stimulated by TGFbeta isoforms. We further investigated the molecular basis of this inhibition using cultured chick embryonic and rat adult epicardial mesothelial cells. We observed that VCAM-1 increased cortical actin filaments at intercellular junctions and reduced stress fibers across epicardial cells. VCAM-1 inhibited stress fiber formation by TGFbeta1, TGFbeta2, TGFbeta3 and lysophosphatidic acid and altered Rho activity stimulated by TGFbeta3. This was accompanied by an increase in tyrosine phosphorylation of p190RhoGAP. All three TGFbeta isoforms weakened intercellular adhesion, reduced membrane localization of beta-catenin and E-cadherin and stimulated epicardial EMT in chick hearts. Each of these effects was restricted by simultaneous VCAM-1 treatment. Our data support the hypothesis that VCAM-1 can alter epicardial EMT at two key points: it limits Rho-dependent events such as stress fiber formation and it maintains the association of beta-catenin and E-cadherin with the adherens junction.

Abnormal development of the sinuatrial venous valve and posterior hindbrain may contribute to late fetal resorption of vitamin A-deficient rat embryos

BACKGROUND

Normal embryonic development and survival in utero is dependent on an adequate supply of vitamin A. Embryos from vitamin A-deficient (VAD) pregnant rats fed an inadequate amount of all-trans retinoic acid (atRA; 12 microg per g of diet or approximately 230 microg per rat per day) exhibit severe developmental abnormalities of the anterior cardinal vein and hindbrain by embryonic day (E) 12.5 and die shortly thereafter.

METHODS

In the present study, we sought to determine whether supplementation of VAD-RA supported (12 microg per g of diet) pregnant rats with retinol (ROL) at the late-gastrula (presomite or rat E9.5) or early somite stages (E10.5), or provision of higher levels of atRA throughout this period could prevent abnormalities in the developing cardiovascular and nervous systems.

RESULTS

A newly described defect in the sinuatrial venus valve along with enlarged anterior cardinal veins and nervous system abnormalities and the later death of embryos are prevented by supplementing pregnant animals with ROL on the morning of E9.5. If ROL supplementation is delayed by 1 day (E10.5), most embryos are abnormal and die by E18.5. Supplementation of VAD rats with atRA (250 microg per g of diet) between E8.5 and E10.5 also prevents the cardiovascular and nervous system abnormalities and a significant number of these embryos survive to parturition. Thus, high levels of atRA can obviate the need for ROL between E9.5 and E10.5.

CONCLUSIONS

These results support an essential role for retinoid signaling between the late gastrula and early somite stages in the rat embryo for normal morphogenesis of the primitive heart tube and the posterior hindbrain. Further, these results suggest that embryonic death occurring at midgestation in the VAD rat may be linked to the abnormal development of one or both of these embryonic structures.

Evidence that translation of smooth muscle alpha-actin mRNA is delayed in the chick promyocardium until fusion of the bilateral heart-forming regions

Heart development in the chick embryo proceeds from bilateral mesodermal primordia established during gastrulation. These primordia migrate to the midline and fuse into a single heart trough. During their migration as a cohesive sheet, the cells of the paired heart fields become epithelial and undergo cardiac differentiation, exhibiting organized myofibrils and rhythmic contractions near the time of their fusion. Between the stages of cardiomyoblast commitment and overt differentiation of cardiomyocytes, a significant time interval exists. Using a new riboprobe (usmaar) for whole-mount in situ hybridization in chick embryos, we report the earliest phases of smooth muscle alpha-actin (smaa) mRNA distribution during the precontractile developmental window. We show that ingressed heart-forming regions express smaa by the head-process stage (Hamburger and Hamilton stage 5). In addition, we used usmaar to study the formation and early morphogenesis of the heart. Consistent with fate mapping studies (Garcia-Martinez and Schoenwolf [1993] Dev. Biol. 159:706-719; Schoenwolf and Garcia-Martinez [1995] Cell Mol. Biol. Res. 41:233-240; Garcia-Martinez et al., in preparation), our results with this probe, combined with detailed histological and SEM analyses of the so-called cardiac crescent, demonstrate unequivocally that the heart arises from separated and paired heart rudiments, rather than from a single crescent-shaped rudiment (that is, prior to fusion of the paired heart rudiments to establish the straight-heart tube, the rostral midline of the cardiac crescent lacks mesodermal cells and consequently fails to label with usmaar). Smaa is also expressed in the splanchnic and somatic mesoderm, marking the earliest step in coelom formation. Consequently, we also used usmaar to describe formation of the pericardium. Finally, we provide evidence of a post-transcriptional level of control of smaa gene expression in the heart fields. Our results suggest that the expression of smaa may mark a primitive mesodermal state from which definitive cell types can be derived through inductive events.

The transcription factor NF-ATc is essential for cardiac valve formation

Nuclear factor of activated T cells (NF-AT) is the name of a family of four related transcription factors that may be needed for cytokine gene expression in activated lymphocytes. Here we report that mice with a targeted disruption of the NF-ATc gene show an unexpected and dramatic defect in cardiac morphogenesis, with selective absence of the aortic and pulmonary valves, leading to death in utero from congestive heart failure at days 13.5-17.5 of gestation. In contrast, tricuspid and mitral valve morphogenesis is normal. NF-ATc is the first transcription factor known to be expressed only in the endothelial cells of the heart. As in T cells, nuclear translocation of NF-ATc in cardiac endothelial cells is controlled by the calcium-regulated phosphatase calcineurin: NF-ATc remains cytoplasmic in normal embryos cultured with cyclosporin A, an inhibitor of calcineurin. Abnormal development of the cardiac valves and septae is the most frequent form of birth defect, yet few molecular regulators of valve formation are known. Our results indicate that NF-ATc may play a critical role in signal-transduction processes required for normal cardiac valve formation.

Expression of the smooth-muscle proteins alpha-smooth-muscle actin and calponin, and of the intermediate filament protein desmin are parameters of cardiomyocyte maturation in the prenatal rat heart

BACKGROUND

Coexpression of alpha- and beta-myosin heavy chain (MHC) is a characteristic of the primary myocardial tube. To establish if the smooth-muscle proteins alpha-smooth-muscle actin (alpha-SMA) and calponin, and the intermediate filament protein, desmin, contribute to the specific functional properties of these early cardiomyocytes, we studied their spatiotemporal expression pattern.

METHODS

Sections of prenatal and neonatal Wistar rats were stained with antibodies against alpha- and beta-MHC, alpha-SMA, calponin, and desmin.

RESULTS

The expression of alpha-SMA and calponin in embryonic cardiomyocytes increases to reach its highest level at ED14. Subsequently, these proteins gradually disappear, beginning in the interventricular septum (IVS) and followed successively by the compact myocardium of the left ventricle, the right ventricle, and the central atrium. Expression of alpha-SMA persists longer in the ventricular conduction system, making it a convenient marker for the ventricular conduction system of the fetal rat. Desmin becomes expressed one day later than alpha-SMA, but also reaches its peak at ED14, suggesting that a relatively high concentration is required to form mature sarcomeres.

CONCLUSIONS

The results indicate that alpha-SMA, calponin, and desmin are involved in the myofibrillar development in rat heart. The presence of spatiotemporal differences in the expression of these proteins reveals regional differences in the developmental timing of cardiomyocyte maturation. The maturation process extends from the compact myocardium in the IVS to the left and right ventricular free walls, whereas the atrioventricular junction, the ventricular trabeculae, and developing ventricular conduction system show a relatively slow maturation. Smooth-muscle proteins may contribute to the slow shortening speed that is characteristic of the embryonic myocardium.

Fibronectins Are Essential for Heart and Blood Vessel Morphogenesis But Are Dispensable for Initial Specification of Precursor Cells

The underlying mechanisms of lethal cardiovascular defects associated with the fibronectin-null (FN.null) mutation in mouse embryos were investigated by lineage analysis of myocardial, endocardial, and endothelial cells. A wide variation in phenotype was observed on two genetic backgrounds. In the less severe class (C57/BL6 background), FN.null embryos display a defective heart. Myocardial cells express the specific marker MF-20 and are correctly localized in the anterior trunk region, but myocardial organization is disrupted, resulting in a bulbous heart tube. Endocardial cells express the specific marker platelet-endothelial cell adhesion molecule-1 (PECAM-1) and are localized within the myocardium, but the endocardium appears collapsed. Endothelial cells of two vascular beds are specified, but the aortae are distended and lack contact with the surrounding mesenchyme, while no vessels form in the yolk sac. Defects in the more severe class suggest that FNs are essential earlier in development on the 129/Sv background. Myocardial and endocardial cells are specified, but morphogenesis of the myocardium and endocardium does not occur. Aortic endothelial cells are specified and localized normally, but remain scattered. Yolk sac endothelial cells resemble those of the less severe class. We conclude that FNs are essential for organization of heart and blood vessels, but are dispensable for cellular specification in the appropriate regions within the embryo.

Smooth and striated muscle development in the intrinsic urethral sphincter

PURPOSE

The intrinsic urethral sphincter is composed of adjacent striated and smooth muscle. We studied the sequential expression of smooth and striated muscle proteins to gain insight into the ontogeny of intrinsic sphincter development.

MATERIALS AND METHODS

The intrinsic urethral sphincters of timed Fischer 344 rat embryos at 14, 16 and 18 days of gestation, neonates on postnatal day 1 and adult animals were examined. Serial sections of the urethra and adjacent levator ani muscles were studied histologically with hematoxylin and eosin, anti-alpha-smooth muscle actin, anti-alpha-sarcomeric actin and antistriated muscle myosin heavy chain antibodies.

RESULTS

The intrinsic urethral sphincter was identified within the periurethral mesenchyma as early as day 14 of gestation. Although striated myotubules were identified within the urethra by hematoxylin and eosin staining starting on postnatal day 1, striated muscle myosin heavy chain protein was absent in the embryonic and neonatal development of the sphincter, and it was expressed only in the mature myotubule of adults. alpha-Smooth muscle actin was expressed throughout the urethral sphincter of embryonic and neonatal animals. In adults alpha-smooth muscle actin was confined to the smooth muscle component of the urethra. Co-expression of alpha-smooth and alpha-sarcomeric muscle actin by the striated sphincter myotubule was noted only in neonates.

CONCLUSIONS

Development of the intrinsic urethral sphincter is characterized by sequential expression of well characterized muscle marker proteins. The co-expression of smooth and striated muscle markers by developing sphincter myotubule suggests the possibility that trans-differentiation of smooth to striated muscle occurs in the developing genitourinary tract.

The allocation of epiblast cells to the embryonic heart and other mesodermal lineages: the role of ingression and tissue movement during gastrulation

The cardiogenic potency of cells in the epiblast of the early primitive-streak stage (early PS) embryo was tested by heterotopic transplantation. The results of this study show that cells in the anterior and posterior epiblast of the early PS-stage embryos have similar cardiogenic potency, and that they differentiated to heart cells after they were transplanted directly to the heart field of the late PS embryo. That the epiblast cells can acquire a cardiac fate without any prior act of ingression through the primitive streak or movement within the mesoderm suggests that neither morphogenetic event is critical for the specification of the cardiogenic fate. The mesodermal cells that have recently ingressed through the primitive streak can express a broad cell fate that is characteristic of the pre-ingressed cells in the host when they were returned to the epiblast. However, mesoderm cells that have ingressed through the primitive streak did not contribute to the lateral plate mesoderm after transplantation back to the epiblast, implying that some restriction of lineage potency may have occurred during ingression. Early PS stage epiblast cells that were transplanted to the epiblast of the mid PS host embryos colonised the embryonic mesoderm but not the extraembryonic mesoderm. This departure from the normal cell fate indicates that the allocation of epiblast cells to the mesodermal lineages is dependent on the timing of their recruitment to the primitive streak and the morphogenetic options that are available to the ingressing cells at that instance.

Relationship between fibronectin expression during gastrulation and heart formation in the rat embryo

By utilizing myosin immunostaining, we were able to identify early rat myocardium as a thin epithelial sheet and realized that its cohesive movement toward the midline leads to the straight heart tube formation. Localization study of fibronectin mRNA and protein was, therefore, carried out to investigate its tissue origin and possible roles in facilitating mesoderm migration and heart formation. Fibronectin mRNAs were first detected throughout the mesoderm during the early primitive streak stage, suggesting that the mesoderm is the source of fibronectin. By pre-head fold (pre-somite) and head fold (early somite) stages, the mesoderm became largely down-regulated for fibronectin mRNAs, while it was also at these stages when myosin-positive myocardium formed itself into the epithelium and was subsequently folding toward the midline. Thus, there appears to be little fibronectin synthesis during and directly relevant to early heart tube formation. Later, during the early straight heart tube stage (5 somite and older), endocardium became highly positive for fibronectin mRNAs, suggesting that the endocardium is the major source of fibronectin for the cardiac jelly. Based on the results, we present a map for the early mammalian heart in which the heart is a single crescentic band lying in front of the prechordal plate. We also suggest a process for heart tube formation based on the cohesive movement of the myocardial epithelium. During heart tube formation, fibronectin protein had been deposited previously by the mesoderm and was found uniformly in the ECM and not newly produced by any adjacent tissue. The data contradict the endodermal guidance of heart migration by fibronectin gradient and suggest, instead, a permissive role for the fibronectin substrate.

Inhibition of transcription factor GATA-4 expression blocks in vitro cardiac muscle differentiation

Commitment of mesodermal cells to the cardiac lineage is a very early event that occurs during gastrulation, and differentiation of cardiac muscle cells begins in the presomite stage prior to formation of the beating heart tube. However, the molecular events, including gene products that are required for differentiation of cardiac muscle cells, remain essentially unknown. GATA-4 is a recently characterized cardiac muscle-restricted transcription factor whose properties suggest an important regulatory role in heart development. We tested the role of GATA-4 in cardiac differentiation, using the pluripotent P19 embryonal carcinoma cells, which can be differentiated into beating cardiac muscle cells. In this system, GATA-4 transcripts and protein are restricted to cells committed to the cardiac lineage, and induction of GATA-4 precedes expression of cardiac marker genes and appearance of beating cells. Inhibition of GATA-4 expression by antisense transcripts blocks development of beating cardiac muscle cells and interferes with expression of cardiac muscle markers. These data indicate that GATA-4 is necessary for development of cardiac muscle cells and identify for the first time a tissue-specific transcription factor that may be crucial for early steps of mammalian cardiogenesis.

Gradient of integrin alpha 6A distribution in the myocardium during early heart development

The interactions of cells with extracellular matrices (ECM)1 are likely to be key determinants of embryonic development. Integrin adhesion receptors are ideally positioned to mediate some of these interactions since, in addition to mechanical adhesion, they transduce signals affecting cell proliferation and differentiation. We investigated expression of the integrin alpha 6 beta 1, a receptor for the ECM component, laminin in the early mouse embryo. An intriguing feature of this integrin is the existence of alpha 6 subunit isoforms. The A and B isoforms, which differ in the cytoplasmic tails, are expressed in cell-type specific fashion, and are likely to implement distinct cellular interactions with laminin. By RT-PCR, alpha 6B but not alpha 6A mRNA was detectable in embryo extracts from fertilized oocytes to 6.5 d.p.c. In subsequent stages, up to 11.5 d.p.c., alpha 6A mRNA was observed in mRNA extracts from whole embryos, but still in significantly lower amounts than alpha 6B. However, in extracts from isolated heart (9.5 to 11.5 d.p.c.), alpha 6A was the predominant alpha 6 isoform, while in extracts from other embryo parts no alpha 6A mRNA was detectable. At the protein level, immunostaining with specific antibodies showed alpha 6A protein in myocardial cells, at the early stage of heart tube development (8.5 d.p.c.). Localization to the myocardium was tightly restricted, since other structures of the embryonic heart, e.g., endocardium, or of the remaining embryo did not stain with anti-alpha 6A antibody. In the ventricular myocardium, expression of alpha 6A appeared more intense than in the subendocardial layer. Quantitation by confocal microscopy unveiled a gradient of expression of alpha 6A, increasing from the outer to the inner layers of the myocardium. This is the first demonstration of a gradient distribution of integrin molecules in a tissue, which appears to be directly connected with the process of organogenesis. The mechanism underlying our observations is not the turning on of a gene, rather it is the activation of a splicing mechanism that substitutes the cytoplasmic domain of a laminin receptor. Because integrin cytoplasmic domains are thought to be an important functional end of the molecule, this may be a mechanism to modulate cellular responses to laminin.

Cardiac myosin heavy chain expression during heart development in Xenopus laevis

Muscle-specific gene expression in the heart during Xenopus development was investigated using reverse transcription-polymerase chain reaction (RT-PCR) and whole-mount in situ hybridization to detect transcripts of the gene for the cardiac myosin heavy chain (CMHC). RT-PCR analysis determined that CMHC transcripts are present in the cardiac mesoderm at state 13, demonstrating that muscle-specific gene expression in the primitive myocardium has begun by the early neurula stage, approximately 30 h before the heart beat begins. Xenopus, therefore, is similar to amniotes and mammals in that cardiac precursor cells begin to express muscle-specific gene transcripts soon after commitment to the cardiac myocyte lineage. The earliest CMHC gene transcripts can be detected in the heart using whole-mount in situ hybridization is early tailbud stage 28, which coincides with the onset of heart tube morphogenesis. CMHC gene expression was also detected in skeletal muscle: RT-PCR analysis determined that CMHC transcripts are transiently expressed in the somite during the initial phases of skeletal muscle differentiation. Furthermore, CMHC mRNAs are expressed in a subset of head muscles of the feeding tadpole. CMHC gene expression is induced in ectodermal cells of the animal cap in blastula-stage embryos injected with synthetic MyoD or Myf5 RNA, suggesting that the CMHC gene contains regulatory elements that are responsive to the activity of those skeletal-muscle-specific transcription factors.

Patterns of expression of sarcoplasmic reticulum Ca(2+)-ATPase and phospholamban mRNAs during rat heart development

This study reports the clonal analysis and sequence of rat phospholamban (PLB) cDNA clones and the temporal appearance and patterns of distribution of the mRNAs encoding sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA2) and PLB in the developing rat heart determined by in situ hybridization. Both proteins play a critical role in the contraction-relaxation cycle of the heart. SERCA2 mRNA is already abundantly present in the first stage studied, in the cardiogenic plate of the 9-day-old presomite embryo, before the occurrence of the first contractions. This very early expression makes it an excellent marker for the study of early heart development. Subsequently, SERCA2 mRNA becomes expressed in a craniocaudal gradient, being highest at the venous pole and decreasing in concentration toward the arterial pole of the heart. PLB mRNA can be detected in hearts from 12 days of development onward in a virtually opposite gradient. In essence, these patterns do not change during further development. PLB mRNA levels remain highest in the ventricle and outflow tract, whereas SERCA2 mRNA prevails in the inflow tract and atrium, although the difference between atrium and ventricle becomes less pronounced. These observations are compatible with a model in which the upstream part of the heart (inflow tract and atrium) would have a greater capacity to clear calcium and hence would have a longer duration of the diastole than the downstream compartments (atrioventricular canal, ventricle, and outflow tract), similar to the observed pattern of contraction of the embryonic heart. The sinoatrial and atrioventricular nodes do not reveal an expression pattern of SERCA2 and PLB mRNA that allows one to distinguish them from the surrounding atrial working myocardium. However, the ventricular part of the conduction system, comprising atrioventricular bundle and bundle branches, are almost devoid of SERCA2 mRNA.

Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31): alternatively spliced, functionally distinct isoforms expressed during mammalian cardiovascular development

The establishment of the cardiovascular system represents an early, critical event essential for normal embryonic development. An important component of vascular ontogeny is the differentiation and development of the endothelial and endocardial cell populations. This involves, at least in part, the expression and function of specific cell surface receptors required to mediate cell-cell and cell-matrix adhesion. Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) may well serve such a function. It is a member of the immunoglobulin superfamily expressed by the entire vascular endothelium in the adult. It is capable of mediating adhesion by a heterophilic mechanism requiring glycosaminoglycans, as well as by a homophilic, glycosaminoglycan independent, mechanism. It has been shown to regulate the expression of other adhesion molecules on naive T cells. This report documents by RT-PCR and immunohistochemical analysis the expression of PECAM-1 during early post implantation mouse embryo development. PECAM-1 was expressed by early endothelial precursors first within the yolk sac and subsequently within the embryo itself. Interestingly, embryonic PECAM-1 was expressed as multiple isoforms in which one or more clusters of polypeptides were missing from the cytoplasmic domain. The sequence and location of the deleted polypeptides corresponded to exons found in the human PECAM-1 gene. The alternatively spliced isoforms were capable of mediating cell-cell adhesion when transfected into L-cells. The isoforms differed, however, in their sensitivity to a panel of anti-PECAM-1 monoclonal antibodies. These data suggest that changes in the cytoplasmic domain of PECAM-1 may affect its function during cardiovascular development, and are consistent with our earlier report that systematic truncation of the cytoplasmic domain of human PECAM-1 resulted in changes in its ligand specificity, divalent cation and glycosaminoglycan dependence, as well as its susceptibility to adhesion blocking monoclonal antibodies. This is the first report of naturally occurring alternatively spliced forms of PECAM-1 having possible functional implications.

Type II collagen is transiently expressed during avian cardiac valve morphogenesis

We present new evidence of the temporal and spatial expression of type II collagen in the embryonic chick heart during the very early stages of its development. In particular, we emphasize the distribution of its mRNA and protein during valve formation. Type II collagen as well as several other fibrillar collagens (types I, III, and V) are present in stage 18 endocardial cushion mesenchymal cells. At stage 23, alpha 1 (II) collagen transcripts and the cognate polypeptide colocalize in the atrioventricular valves. As development proceeds, the relative abundance of alpha 1 (II) collagen transcripts decreases during the stages studied (stages 22 to 45; day 3.5 to day 19) as assayed by RNA blotting of extracts of whole hearts. Type II collagen protein was immunologically undetectable in stage 38 (day 12) hearts, although collagens I, III, and V persisted and localize in the valve regions, in the endothelial lining of the heart, and in the epicardium. In keeping with other observations of type II collagen expression in non-chondrogenic regions of a variety of vertebrate embryos, the avian heart also exhibits transient type II collagen expression.

Expression of sarcomeric myosin in the presumptive myocardium of chicken embryos occurs within six hours of myocyte commitment

The distribution of sarcomeric myosin heavy chain (MyHC) has been examined immunocytochemically in the presumptive myocardial cells of chicken embryos (stages 6-10) prior to the onset of the heart beat. Embryos were stained with monoclonal antibody MF20, a reagent which recognizes all chicken sarcomeric MyHCs (Bader et al., 1982), and then examined both in whole mount by immunofluorescence and in semithin, plastic-embedded sections following immunoperoxidase labeling. We observed that myosin could be detected as early as stage 7 (0-2 pairs of somites) in 29% of the 31 embryos examined, and by stage 8 (4 pairs of somites) more than 80% of the embryos were MF20+. Every embryo with 5 pairs of somites (stage 8+) labeled strongly with MF20. Labeling was first detected at stage 7 to 7+ as a diffuse fluorescent signal within pleomorphic cells of the splanchnic mesoderm located in two crescent-shaped regions bordering each side of the anterior intestinal portal (AIP). With progressive development, the two crescent-shaped regions merged at the apex of the AIP, and as the two heart tubes began fusion at stage 9, the MyHC+ regions extended cranially and medially. By somite stages 9-10, the myosin-positive cells completely encircled the heart tube. From stages 7 to 9 the myosin signal had no sarcomeric distribution; i.e., there were no MyHC striations nor periodic repeats evident in the presumptive myocytes until late stage 9 and stage 10. Semithin sections revealed that myosin was first distributed in apical regions of the myocytes, adjacent to the pericardial coelom. The implications of these findings for myocyte determination, differentiation and morphogenesis are discussed.