Morphology of the developing muscularis externa in the mouse esophagus

Muscularis externa of mouse esophagus is composed of two skeletal muscle layers in the adult. But less attention is paid to the histogenesis of the muscularis externa of the esophagus, and controversies still exist about the developmental process and the spatio-temporal expression characteristics of muscle-specific proteins during the development of esophageal muscularis externa. To further probe into the developmental pattern of muscularis externa of the mouse esophagus and the expression characteristics of different muscle-specific proteins, immunohistochemical and terminal deoxyribonucleotidyl transferase-mediated deoxyuridine triphosphate (dUTP)-digoxigenin nick-end labeling apoptotic staining methods are used to investigate the expression patterns of different muscle-specific proteins and to elucidate the relationship of these protein expressions with the development of muscularis externa of the mouse esophagus. Thus, an understanding of the developing esophageal muscularis externa may be important for developing therapeutic strategies for the treatment of human esophagus diseases. Serial sections of mouse embryos from embryonic day (ED) 12 to ED18, and full-length esophagi from postnatal first to 5th day were stained with monoclonal antibodies against α-smooth muscle actin (α-SMA), α-sarcomerical actin (α-SCA), desmin, and monoclonal anti-skeletal myosin (MHC), while apoptosis was determined using the terminal deoxyribonucleotidyl transferase-mediated dUTP-digoxigenin nick-end labeling assay. The expression of α-SMA was started at ED12. During the development of ED14-ED15, α-SMA positive cells were seen extending from the walls of left three, four, and six arch arteries toward the dorsal wall of esophagus. Stronger expression of α-SCA and desmin could be detected at ED14 and ED15, expression intensity in caudal segment and inner layer was stained stronger than that of cranial segment and outer layer, but after ED16, strong expression of α-SCA and desmin was found in the outer layer of muscularis externa. Expression of MHC was first detected in the outer layer of cranial segment of muscularis externa at ED17. At ED18, MHC had extended to the level of thyroid gland, staining intensity in the outer layer and cranial segment was stronger than that of inner layer and caudal segment. One to five days after birth, the thickness of the esophageal muscle layer was obviously increased. Most of the muscle cells in the cranial segment of esophagus showed strong expression of α-SCA and clear cross striations at higher magnification. With progression toward the caudal segment, expression intensity of α-SCA became weaker, but the expression intensity of desmin was the same at different levels of esophagus. The muscle fibers were arranged densely with high expression of MHC in the cranial segment. During the development of esophageal muscularis externa, few apoptotic cells were observed. α-SMA, α-SCA, desmin, and MHC show different expression patterns. The differentiation of outer layer of esophageal muscularis externa is quicker than that of inner layer, and the caudal segment is quicker than that of the cranial segment. Besides, apoptosis may not participate in the development of esophageal muscularis externa. The smooth muscle cells from arch arteries may participate in the development of esophageal muscularis externa.

Myosin light chain 2a and 2v identifies the embryonic outflow tract myocardium in the developing rodent heart

The embryonic heart consists of five segments comprising the fast-conducting atrial and ventricular segments flanked by slow-conducting segments, i.e. inflow tract, atrioventricular canal and outflow tract. Although the incorporation of the flanking segments into the definitive atrial and ventricular chambers with development is generally accepted now, the contribution of the outflow tract myocardium to the definitive ventricles remained controversial mainly due to the lack of appropriate markers. For that reason we performed a detailed study of the pattern of expression of myosin light chain (MLC) 2a and 2v by in situ hybridization and immunohistochemistry during rat and mouse heart development. Expression of MLC2a mRNA displays a postero-anterior gradient in the tubular heart. In the embryonic heart it is down-regulated in the ventricular compartment and remains high in the outflow tract, atrioventricular canal, atria and inflow tract myocardium. MLC2v is strongly expressed in the ventricular myocardium and distinctly lower in the outflow tract and atrioventricular canal. The co-expression of MLC2a and MLC2v in the outflow tract and atrioventricular canal, together with the single expression in the atrial (MLC2a) and ventricular (MLC2v) myocardium, permits the delineation of their boundaries. With development, myocardial cells are observed in the lower endocardial ridges that share MLC2a and MLC2v expression with the myocardial cells of the outflow tract. In neonates, MLC2a continues to be expressed around both right and left semilunar valves, the outlet septum and the non-trabeculated right ventricular outlet. These findings demonstrate the contribution of the outflow tract to the definitive ventricles and demonstrate that the outlet septum is derived from outflow tract myocardium.

Sox4-deficiency syndrome in mice is an animal model for common trunk

Embryonic mice lacking functional Sox4 transcription factor die from cardiac failure at embryonic day (ED) 14. Heart morphogenesis in these embryos was analyzed in hematoxylin-azophlochsin or immunohistochemically stained, 3-dimensionally reconstructed serial sections between ED12 and ED14. Although Sox4 is expressed in the endocardially derived tissue of both the outflow tract and atrioventricular canal, Sox4-deficient hearts only suffer from defective transformation of the endocardial ridges into semilunar valves and from lack of fusion of these ridges, usually resulting in common trunk, although the least affected hearts should be classified as having a large infundibular septal defect. The more serious cases are, in addition, characterized by an abnormal number and position of the semilunar valve-leaflet anlagen, a configuration of the ridges typical for transposition of the great arteries (with linear rather than spiral course of both ridges and posterior position of the pulmonary trunk at the level of the valve), and variable size of the aorta relative to the pulmonary trunk. The coronary arteries always originated from the aorta, irrespective of its position relative to the pulmonary trunk. The restriction of the malformations to the arterial pole implies that the interaction between the endocardially derived tissue of the outflow tract and the neural crest-derived myofibroblasts determines proper development of the arterial pole.

Normal development of the outflow tract in the rat

The outflow tract (OFT) provides the structural components forming the ventriculoarterial connection. The prevailing concept that this junction "rotates" to acquire its definitive topography also requires a concept of "counterrotation" and is difficult to reconcile with cell-marking studies. Rats between 10 embryonic days (EDs) and 2 postnatal days were stained immunohistochemically and by in situ hybridization. DNA replication was determined by incorporation of bromodeoxyuridine and apoptosis by the annexin V binding and terminal deoxynucleotidyl transferase-mediated dUTP-X nick end labeling (TUNEL) assays. Starting at ED12, cardiomyocytes in the distal (truncal) part of the OFT begin to shed their myocardial phenotype without proceeding into apoptosis, suggesting transdifferentiation. Myocardial regression is most pronounced on the dextroposterior side and continues until after birth, as revealed by the disappearance of the myocardial cuff surrounding the coronary roots and semilunar sinuses and by the establishment of fibrous continuity between mitral and aortic semilunar valves. Fusion of the endocardial ridges of the truncus on late ED13 is accompanied by the organization of alpha-smooth muscle actin-and nonmuscle myosin heavy chain-positive myofibroblasts into a central whorl and the appearance of the semilunar valve anlagen at their definitive topographical position within the proximal portion of the truncus. After fusion of the proximal (conal) portion of the endocardial ridges, many of the resident myofibroblasts undergo apoptosis and are replaced by cardiomyocytes. The distal myocardial boundary of the OFT is not a stable landmark but moves proximally over the spiraling course of the aortic and pulmonary routes, so that the semilunar valves develop at their definitive topographic position. After septation, the distal boundary of the OFT continues to regress, particularly in its subaortic portion. The myocardializing conus septum, on the other hand, becomes largely incorporated into the right ventricle. These opposite developments account for the pronounced asymmetry of the subaortic and subpulmonary outlets in the formed heart.

Heart defects in connexin43-deficient mice

Cardiac malformation in connexin43 (CX43)-disrupted mice is restricted to the junction between right ventricle and outflow tract, even though CX43 is also expressed abundantly elsewhere. We analyzed cardiac morphogenesis in immunohistochemically and hybridohistochemically stained and three-dimensionally reconstructed serial sections of CX43-deficient embryos between embryonic day (ED) 10 and birth. The establishment of the D configuration in the ascending loop of CX43-deficient hearts is markedly retarded, so that the right ventricle retains a craniomedial position and is connected with the outflow tract by a more acute bend in ED10 and ED11 embryos. Because of the subsequent growth of the right ventricle, this condition usually evolves into a D loop, but when it persists, a "crisscross" configuration develops, with the atrioventricular cushions rotated 90 degrees, a horizontal muscular ventricular septum, and a parallel course of the endocardial ridges of the outflow tract. After ED12, large intertrabecular pouches develop at the ventricular side of both shelflike myocardial structures that support the endocardial ridges of the outflow tract, ie, at the location that was earlier characterized by the acute bend between the right ventricle and the outflow tract and that subsequently develops into the anterosuperior leaflet of the tricuspid valve. Retarded development of the D configuration in the ascending loop of the embryonic heart predisposes the myocardium at the junction of the right ventricle and outflow tract to excessive development of intertrabecular pouches during subsequent development.

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.

Animal models of congenital defects in the ventriculoarterial connection of the heart

The embryonic heart functions as a pump without one-way valves. To accomplish this, a long, slowly conducting myocardial structure, the outflow tract, functions as a sphincter at the arterial pole of the heart. During subsequent development tissue remodeling in the outflow tract and immigrating cells of the neural crest are responsible for connecting the right ventricle with the pulmonary trunk and the left ventricle with the aorta, that is, for the developmental formation of the ventriculoarterial junction. Most congenital malformations of the ventriculoarterial junction stem from disturbances that result in developmental arrest or in abnormal pattern formation ("real" teratology). Abnormal pattern formation can in turn originate from problems with laterality or from aberrant or incomplete formation of structural elements. Genetically modified animals with well-defined gene deficiencies are beginning to provide insight in the signal-transduction pathways and structural elements that are responsible for normal development.

Defects in cardiac outflow tract formation and pro-B-lymphocyte expansion in mice lacking Sox-4

A striking example of the relationship between regulation of transcription and phenotype is the central role of the Y-chromosomal gene Sry in mammalian sex determination. Sry is the founding member of a large family of so-called Sox genes. During murine embryogenesis, the transcriptional activator Sox-4 is expressed at several sites, but in adult mice expression is restricted to immature B and T lymphocytes. Using targeted gene distruption, we have found that SOX-4(-/-) embryos succumb to circulatory failure at day E14. This was a result of impaired development of the endocardial ridges (a specific site of Sox-4 expression) into the semilunar valves and the outlet portion of the muscular ventricular septum. The observed range of septation defects is known as 'common arterial trunk' in man. We studied haemopoiesis in lethally irradiated mice reconstituted with SOX-4(-/-) fetal liver cells and found that a specific block occurred in B-cell development at the pro-B cell stage. In line with this, the frequency and proliferative capacity of IL-7-responsive B cell progenitors in fetal liver were severely decreased in vitro.