Laterality defect of the heart in non-teleost fish

Dextrocardia is a rare congenital malformation in humans in which most of the heart mass is positioned in the right hemithorax rather than on the left. The heart itself may be normal and dextrocardia is sometimes diagnosed during non-related explorations. A few reports have documented atypical positions of the cardiac chambers in farmed teleost fish. Here, we report the casual finding of a left-right mirrored heart in an 85 cm long wild-caught spiny dogfish (Squalus acanthias) with several organ malformations. Macroscopic observations showed an outflow tract originating from the left side of the ventricular mass, rather than from the right. Internal inspection revealed the expected structures and a looped cavity. The inner curvature of the loop comprised a large trabeculation, the bulboventricular fold, as expected. The junction between the sinus venosus and the atrium appeared normal, only mirrored. MRI data acquired at 0.7 mm isotropic resolution and subsequent 3D-modeling revealed the atrioventricular canal was to the right of the bulboventricular fold, rather than on the left. Spurred by the finding of dextrocardia in the shark, we revisit our previously published material on farmed Adriatic sturgeon (Acipenser naccarii), a non-teleost bony fish. We found several alevins with inverted (left-loop) hearts, amounting to an approximate incidence of 1%-2%. Additionally, an adult sturgeon measuring 90 cm in length showed abnormal topology of the cardiac chambers, but normal position of the abdominal organs. In conclusion, left-right mirrored hearts, a setting that resembles human dextrocardia, can occur in both farmed and wild non-teleost fish.

The gas bladder of Heterotis niloticus (Cuvier, 1829)

This work reports on the structural characteristics of the respiratory gas bladder of the osteoglossiform fish Heterotis niloticus. The bladder-vertebrae relationships are also analysed. A slit-shaped orifice in the mediodorsal pharyngeal wall is surrounded by a muscle sphincter and serves as a glottis-like opening to the gas bladder. The dorsolateral internal surface of the gas bladder is lined by a parenchyma of highly vascularized trabeculae and septa displaying an alveolar-like structure. The trabeculae contain, in addition to vessels, numerous eosinophils probably involved in immune responses. The air spaces are endowed with a thin exchange barrier indicating a good potential for respiratory gas exchange. The ventral wall of the gas bladder is a well-vascularized membrane that exhibits an exchange barrier in the luminal face and an inner structure dominated by the presence of a layer of richly-innervated smooth muscle. This is suggestive of an autonomous adjustability of the gas bladder ventral wall. - The trunk vertebrae show large transverse processes (parapophyses) and numerous surface openings that lead into intravertebral spaces that become invaded by the bladder parenchyma. Curiously, the caudal vertebrae show a regular teleost morphology with neural and haemal arches, but have similar surface openings and intravertebral pneumatic spaces. The African arowana hence rivals the freshwater butterfly fish Pantodon in its exceptional role of displaying postcranial skeletal pneumaticity outside of Archosauria. The possible significance of these findings is discussed. This article is protected by copyright. All rights reserved.

The gas bladder of Pantodon buchholzi: Structure and relationships with the vertebrae

We report here on the histological and structural characteristics of the gas bladder, the vertebral morphology, and the bladder-vertebra relationships of the butterfly fish, Pantodon buchholzi. The bladder opens at the boundary between the pharynx and the esophagus by a middle slit. A pneumatic duct is absent. The bladder shows a dorsolateral wall that adapts to the anfractuosities of the coelomic cavity and a ventral wall in contact with the abdominal organs. The vertebral bodies are formed by an hourglass shaped autocentrum, and by an arcocentrum reduced to several longitudinal ridges. The transverse processes adopt the structure of a cage whose walls are formed by bone trabeculae of variable size and distribution pattern. The dorsolateral wall of the bladder is a membrane that covers the kidney, adapts to the irregular shape of the vertebrae, and invades the transverse processes at several points before extending laterally. However, invasion of the vertebral bodies, the presence of a labyrinth, or the formation of respiratory parenchyma were not observed. The luminal surface of this wall is a thin respiratory barrier containing a single epithelial cell type. In addition, the wall contains numerous eosinophils that may be implicated in immune defense. The bladder ventral wall is a membrane rich in collagen, vessels, smooth muscle, and nerves that lacks a respiratory barrier. Its luminal surface contains ciliated and nonciliated cells. The two cell types appear implicated in surfactant production.

Lungs and gas bladders: Morphological insights

This paper summarizes the main morphological tracts exhibited by lungs and gas bladders in fishes. The origin and organ location, the presence of a glottal region, the inner architecture, the characteristics of the exchange barrier and the presence of pulmonary arteries have been reviewed in the two types of air-breathing organs. With the exception of the dorsal (bladders) or ventral (lungs) origin from the posterior pharynx, none of the morphological traits analyzed can be considered specific for either lungs or gas bladders. This is exemplified by analysis of the morphology of the lung of the Dipnoii and Polypteriformes and of the bladder of the Lepisosteiformes. All of them are obligate air-breathers and show a lung-like (pulmonoid) air-breathing organ. However, while the lungfish lung and the bladder of the Lepisosteiformes occupy a dorsal position and are highly trabeculated, the polypterid lung occupies a ventral position and shows a smooth inner surface. Structural and ultrastructural differences are also highlighted. Noticeably, a large part of the inner surface area of the lung of the Australian lungfish is covered by a ciliated epithelium. A restricted respiratory surface area may help to explain the incapability of this species to aestivate. The respiratory bladder of basal teleosts displays a more complex morphology than that observed in more primitive species. The bladder of basal teleosts may appear divided into respiratory and non-respiratory portions, exhibit intricate shapes, invade adjacent structures and gain additional functions. The increase in morphological and functional complexity appears to prelude the loss of the respiratory functions.

Acinetobacter baumannii maintains its virulence after long-time starvation

Acinetobacter baumannii is a cause of healthcare-associated infections. Although A. baumannii is an opportunistic pathogen, its infections are notoriously difficult to treat due to intrinsic and acquired antimicrobial resistance, often limiting effective therapeutic options. A. baumannii can survive for long periods in the hospital environment, particularly on inanimate surfaces. Such environments may act as a reservoir for cross-colonization and infection outbreaks and should be considered a substantial factor in infection control practices. Moreover, clothing of healthcare personnel and gadgets may play a role in the spread of nosocomial bacteria. A link between contamination of hospital surfaces and A. baumannii infections or between its persistence in the environment and its virulence has not yet been established. Bacteria under stress (i.e., long-term desiccation in hospital setting) could conserve factors that favor infection. To investigate whether desiccation and/or starvation may be involved in the ability of certain strains of A. baumannii to retain virulence factors, we have studied five well-characterized clinical isolates of A. baumannii for which survival times were determined under simulated hospital conditions. Despite a considerable reduction in the culturability over time (up to 88% depending on strain and the condition tested), some A. baumannii strains were able to maintain their ability to form biofilms after rehydration, addition of nutrients, and changing temperature. Also, after long-term desiccation, several clinical strains were able to grow in the presence of non-immune human serum as fine as their non-stressed homologs. Furthermore, we also show that the ability of bacterial strains to kill Galleria mellonella larvae does not change although A. baumannii cells were stressed by long-term starvation (up to 60 days). This means that A. baumannii can undergo a rapid adaptation to both the temperature shift and nutrients availability, conditions that can be easily found by bacteria in a new patient in the hospital setting.

Morphological analysis of the hagfish heart. II. The venous pole and the pericardium

The morphological characteristics of the venous pole and pericardium of the heart were examined in three hagfish species, Myxine glutinosa, Eptatretus stoutii, and Eptatretus cirrhatus. In these species, the atrioventricular (AV) canal is long, funnel-shaped and contains small amounts of myocardium. The AV valve is formed by two pocket-like leaflets that lack a papillary system. The atrial wall is formed by interconnected muscle trabeculae and a well-defined collagenous system. The sinus venosus (SV) shows a collagenous wall and is connected to the left side of the atrium. An abrupt collagen-muscle boundary marks the SV-atrium transition. It is hypothesized that the SV is not homologous to that of other vertebrates which could have important implications for understanding heart evolution. In M. glutinosa and E. stoutii, the pericardium is a closed bag that hangs from the tissues dorsal to the heart and encloses both the heart and the ventral aorta. In contrast, the pericardium is continuous with the loose periaortic tissue in E. cirrhatus. In all three species, the pericardium ends at the level of the SV excluding most of the atrium from the pericardial cavity. In M. glutinosa and E. stoutii, connective bridges extend between the base of the aorta and the ventricular wall. In E. cirrhatus, the connections between the periaortic tissue and the ventricle may carry blood vessels that reach the ventricular base. A further difference specific to E. cirrhatus is that the adipose tissue associated with the pericardium contains thyroid follicles. J. Morphol. 277:853-865, 2016. © 2016 Wiley Periodicals, Inc.

Structural analysis of chordae tendineae in degenerative disease of the mitral valve

BACKGROUND

Degenerative disease of the mitral valve (DDMV) is always accompanied by lengthening and/or rupture of chordae tendineae. However, the mechanisms and the mode of chordal rupture remain controversial, and the pathologic anatomy of the apparently healthy chordae has mostly been overlooked. We analyze the structural aspects of both ruptured and intact chordae tendineae in DDMV.

METHODS AND RESULTS

Structural and ultrastructural microscopic analyses indicate that both the extracellular matrix and the interstitial cells are severely affected. Degenerative chordae show alterations in the synthesis and deposition of collagen and elastin, disorganization of collagen bundles and rupture of collagen fibres, accumulation of proteoglycans and of cellular and vesicular remnants, and cell transformation into a myofibroblast phenotype. Structural disruption makes the spongiosa and the dense collagenous core separate and break. Degeneration of the chordae is segmental, affecting both chordae that are clearly abnormal, and chordae that appear healthy on visual inspection.

CONCLUSIONS

Changes in both matrix synthesis and degradation disturb the ordered collagen arrangement and modify the structural and physical properties of the chordae. Progressive structural disruption of the diseased chordae is the cause of chordal rupture. Mitral surgery corrects the damage, but the underlying causes of DDMV are not corrected. Thus, progression of the disease and affectation of additional chordae may be at the basis of the late complications and the recurrent mitral regurgitation which occurs several years after surgery. Our results indicate that a more aggressive approach to surgery may be needed.

The development of the epicardium in the sturgeon Acipenser naccarii

This article reports on the development of the epicardium in alevins of the sturgeon Acipenser naccarii, aged 4-25 days post-hatching (dph). Epicardial development starts at 4 dph with formation of the proepicardium (PE) that arises as a bilateral structure at the boundary between the sinus venosus and the duct of Cuvier. The PE later becomes a midline organ arising from the wall of the sinus venosus and ending at the junction between the liver, the sinus venosus and the transverse septum. This relative displacement appears related to venous reorganization at the caudal pole of the heart. The mode and time of epicardium formation is different in the various heart chambers. The conus epicardium develops through migration of a cohesive epithelium from the PE villi, and is completed through bleb-like aggregates detached from the PE. The ventricular epicardium develops a little later, and mostly through bleb-like aggregates. The bulbus epicardium appears to derive from the mesothelium located at the junction between the outflow tract and the pericardial cavity. Strikingly, formation of the epicardium of the atrium and the sinus venosus is a very late event occurring after the third month of development. Associated to the PE, a sino-ventricular ligament develops as a permanent connection. This ligament contains venous vessels that communicate the subepicardial coronary plexus and the sinus venosus, and carries part of the heart innervation. The development of the sturgeon epicardium shares many features with that of other vertebrate groups. This speaks in favour of conservative mechanisms across the evolutionary scale.

The structural characteristics of the heart ventricle of the African lungfish Protopterus dolloi: freshwater and aestivation

This paper reports on the structure and ultrastructure of the ventricular myocardium of the African lungfish Protopterus dolloi in freshwater (FW), in aestivation (AE), and after the AE period. The myocardium shows a conventional myofibrillar structure. All the myocytes contain large intracytoplasmic spaces occupied by a pale material that could contain glycosaminoglycans and/or glycogen, which may be used as food and water reservoirs. In FW, the myocytes in the trabeculae associated with the free ventricular wall show structural signs of low transcriptional and metabolic activity (heterochromatin, mitochondria of the dense type). These signs are partially reversed during the AE period (euchromatin, mitochondria with a light matrix), with a return to the FW appearance after arousal. The myocytes in the septum show, in FW conditions, nuclear polymorphism (heterochromatin, euchromatin), and two types (colliquative and coagulative) of necrosis. In AE, all the septal myocytes show euchromatin, and the number of necrotic cells increases greatly. Cell necrosis appears to be related to the septal architecture. After arousal, the septal myocytes exhibit a heterochromatin pattern, the number of necrotic cells decreases, cell debris accumulates under the endocardium, and phagocytosis takes place. Despite being a morphologic continuum, the trabeculae associated with the free ventricular wall appear to constitute a different compartment from that formed by the trabeculae in the ventricular septum. Paradoxically, AE appears to trigger an increase in transcriptional and synthetic myocardial activities, especially at the level of the ventricular septum. This activity may be involved in mechanisms of autocrine/paracrine regulation. Aestivation cannot be regarded as the result of a general depression of all cellular and organic activities. Rather, it is a much more complex state in which the interplay between upregulation and downregulation of diverse cell activities appears to play a fundamental role.

Dorsoventral transposition of the heart chambers in sturgeon Acipenser naccarii alevins

This is the first description of a dorsoventral transposition of the heart chambers in sturgeons Acipenser naccarii. The affected individuals were 2 farmed alevins aged 9 and 10 d posthatching, respectively. One was examined by light microscopy and the other by scanning electron microscopy. In both cases, the atrium and sinus venosus occupied a left ventrolateral position, the ventricle, conus arteriosus and bulbus arteriosus were located dorsally, and the transverse septum was incomplete. The anomalous heart examined by light microscopy did not differ histologically from normal hearts of similar developmental stages. The abnormal dorsoventral arrangement of the heart chambers was presumably due to a distortion of the morphogenetic movements that bring the ventricle to the ventral and the atrium to the dorsal position. The present findings, together with genetic data reported in the literature, suggest that the defective cardiac phenotype detected in the present specimens might result from a mutation affecting the sturgeon ortholog of the zebrafish overlooped (olp) gene.

The coronary arteries of the C57BL/6 mouse strains: implications for comparison with mutant models

There are few detailed descriptions of the coronary arterial patterns in the mouse. Some recent reports on coronary anomalies in mutant mouse models have uncovered the importance of several genes (i.e. iv and connexin43) in coronary morphogenesis. These mutations spontaneously appeared (iv) or were generated (connexin43) in a C57BL/6 background, which is widely used for the development of mutant mice. We have studied the origin and course of the main coronary arteries of two C57BL/6 mouse strains. Unusual anatomical coronary arterial patterns were found, including: solitary ostium in aorta, accessory ostium, high take-off, aortic intramural course, slit-like ostium, sinus-like ostium and origin of a septal artery from the left coronary artery. In humans, some of these conditions are clinically relevant. Most of these patterns, which differ from those observed in wild mice and Swiss albino mice, coincide with those previously found in iv/iv and connexin43 knockout mice. The results indicate that there is variability in the coronary arterial arrangement of the laboratory mouse. Care should be taken when analysing coronary phenotypes of mutant mouse models.

Basement membrane heterogeneity during chick development as shown by tomato (Lycopersicon esculentum) lectin binding

Basement membranes (BMs) constitute a distinct compartment of the extracellular matrix (ECM). All BMs show a similar structural appearance but differ in molecular composition. These variations have critical functional implications. The aim of this study is to establish the pattern of the tomato lectin (Lycopersicon esculentum agglutinin--LEA) binding sites in the BMs of the developing chick embryo (stages 4-21, Hamburger and Hamilton, 1951) in order to achieve a better understanding of the molecular heterogeneity of BMs. The study was performed with transmission electron microscopy (TEM) histochemistry, and confocal laser microscopy. TEM showed that LEA bound to the lamina densa and to the lamina fibroreticularis of the BMs. Through the period studied, most of the LEA binding appeared in the ectodermal BM and its derivatives. In the limb bud, LEA binding to the ectoderm BM was more intense in the ventral half than in the dorsal half. Furthermore, LEA allowed the early (HH16) detection of the transverse fibrillar tracts. In the lens and in the inner ear primordium, the BMs were LEA positive through the placode and cup stages. The binding was progressively reduced through the vesicle stage. The BMs of the olfactory primordium, and of the Rathke's pouch were positive. In contrast, the BMs of the developing central nervous system were negative. The BMs of both the paraxial and the lateral plates of the mesoderm were negative, whereas the notochord and the BM of the Wolffian duct were positive. The endodermal BM and its derivatives were negative. The ECM located between the fusing endocardial tubes, and the BM of the fusion zone of the paired aortae, were positive. This suggested an active role of the LEA-positive glycoproteins in the fusion of endothelia. Our results show the heterogeneity of the chick embryo BMs during development. In addition, LEA constitutes an excellent marker for the primordial germ cells.

Conus arteriosus of the teleost heart: Dismissed, but not missed

The heart outflow tract (OFT) of primitive fish is formed by two portions: a proximal conus arteriosus and a distal bulbus arteriosus. The OFT of modern teleosts is considered to be formed by a single component, the bulbus, the conus having been lost through evolution. This article challenges the concept of the disappearance of the conus arteriosus in the teleost heart. A total of 28 teleost species belonging to 19 families and 10 orders were analyzed. The hearts were divided into two large groups: those having entirely trabeculated ventricles, and those possessing a compacta. In the hearts having entirely trabeculated ventricles, the conus arteriosus appears as a distinct segment interposed between the ventricle and the bulbus arteriosus, being formed by compact vascularized myocardium. However, the conus of several species lacks vessels. In these cases, the conus presents large intercellular spaces bounded by collagen. In the hearts possessing a ventricular compacta, the conus either appears as a muscular ring of variable length connecting the ventricle and the bulbus or forms a crown or ring of myocardium apposed to the ventricular base. In all the teleosts studied, the conus can be recognized as an anatomic entity different from the ventricle. Furthermore, the conus appears as a distinct heart segment in the developing fish. Therefore, the conus arteriosus has not been lost in evolution and constitutes a fundamental part of the teleost OFT. In all the species studied, the conus supports the OFT valves, which should properly be named conus valves.

Microanatomy and ultrastructure of the kidney of the African lungfishProtopterus dolloi

The Dipnoi (lungfishes) have developed true lungs, having the ability to take oxygen from both the gills and the lungs. During the tropical dry season, many lungfish estivate on land, breathing only air. The estivation period is accompanied by profound functional modifications, including the suppression of urine. Thus, the lungfish kidney must be designed to cope with these dramatic cyclic changes in renal function. We study here the microanatomy and the structure of the kidney of the African lungfish Protopterus dolloi, maintained under controlled freshwater conditions. Chemical microdissection, light microscopy, and scanning and transmission electron microscopy have been used. The nephrons of P. dolloi are composed of a renal corpuscle (RC) and of a renal tubule that appears divided into five morphologically distinct segments: neck segment (NS), proximal tubule (PT), intermediate segment (IS), distal tubule (DT), and collecting tubule (CT). Paired CTs abut into a collecting duct, the latter emptying into an archinephric duct. The RCs lie in the mid-zone of the kidney, between the PTs and the convoluted DTs. The spatial distribution of these elements allows recognition of a kidney zonation. The RCs group into clusters (3-4 RCs per cluster) that are supplied by a single arteriole surrounded by pericytes. Each cluster appears to represent a functional unit with a common hemodynamic regulatory mechanism. The major processes of the podocytes form flattened networks that appear to constitute an integrated system due to the presence of gap junctions. The existence of mesangial cells with large cell processes, and of mesangial cells with a dendritic appearance, suggests a complex functional role (contractile and phagocytic) for the mesangium. The NS and the IS are the narrowest nephron segments, formed only by multiciliated cells. The PT and the DT can be subdivided, based on the tubular morphology and on cell composition, into portions I and II: PTI is formed only by brush border (BB) cells, while PTII contains BB and multiciliated cells. The DTI is formed by segment-specific cells, while the DTII contains segment-specific and a small number of flask cells. The CT contains principal and flask cells in a 5:1 ratio. The flask cells adopt two different configurations (with a narrow canaliculus or with a large cavity). The main goal of this study was to disclose specific kidney features that could be related to function, phylogeny, and habitat. In addition, the present results constitute the basis for a study of the morphologic changes that should occur in the kidney of P. dolloi during estivation.

Ventricle and outflow tract of the African lungfish Protopterus dolloi

We report a morphologic study of the heart ventricle and outflow tract of the African lungfish Protopterus dolloi. The ventricle is saccular and appears attached to the anterior pericardial wall by a thick tendon. An incomplete septum divides the ventricle into two chambers. Both the free ventricular wall and the incomplete ventricular septum are entirely trabeculated. Only a thin rim of myocardium separates the trabecular system from the subepicardial space. The outflow tract consists of proximal, middle, and distal portions, separated by two flexures, proximal and distal. The proximal outflow tract portion is endowed with a layer of compact, well-vascularized myocardium. This portion is homologous to the conus arteriosus observed in the heart of most vertebrates. The middle and distal outflow tract portions are arterial-like, thus being homologous to the bulbus arteriosus. However, the separation between the muscular and arterial portions of the outflow tract is not complete in the lungfish. A thin layer of myocardium covers the arterial tissue, and a thin layer of elastic tissue underlies the conus myocardium. Two unequal ridges composed of loose connective tissue, the spiral and bulbar folds, run the length of the outflow tract. They form an incomplete division of the outflow tract, but fuse at the distal end. The two folds are covered by endocardium and contain collagen, elastin, and fibroblast-like cells. They appear to be homologous to the dextro-dorsal and sinistro-ventral ridges observed during the development of the avian and mammalian heart. Two to three rows of vestigial arterial-like valves appear in the dorsal and ventral aspects of the conus. These valves are unlikely to have a functional role. The possible functional significance of the "gubernaculum cordis," the thick tendon extending between the anterior ventricular surface and the pericardium, is discussed.

The development of the sturgeon heart

This paper presents a sequential analysis of the development of the sturgeon (Acipenser naccarii) heart from the end of gastrulation to the early juvenile stages. At late neurulation, the heart appears as a straight, short tube located over the endoderm that forms the wall of the yolk sac, in front of the developing head. The heart axis is aligned with the axis of the developing head. Subsequently, the heart elongates and adopts a C-shape, and its axis becomes perpendicular to that of the head. Around the time of hatching, the heart loses the loop and appears as a mostly straight tube with the chambers arranged in a craniocaudal sequence: outflow tract, ventricle, atrium, and a small sinus venosus. During the first 4 days post-hatching (dph), the heart starts looping again, adopts a C-shape, and undergoes a counterclockwise movement that brings the atrium to the left of the outflow tract and the ventricle to a caudal position. Thus, a primary and a secondary cardiac loop occur in the sturgeon. Later, the atria come to occupy a middle position behind the outflow tract, and the sinus venosus shifts from a caudal to a dorsal position. A morphological arrangement similar to that found in adult sturgeons is attained in all specimens at days 7-9 dph. The external changes are accompanied by a series of internal modifications that include trabeculation (3-4 dph), development of endocardial cushions in the atrioventricular canal (4 dph) and in the conus arteriosus (3-4 dph), conus (22-24 dph) and atrioventricular (18-20 dph) valve formation, and development of the epicardium (4 dph) and the coronary vessels (10 dph). The main developmental features of the heart have been registered, and a basic body of information, which should be very useful in future developmental studies, has been established. Similarities and dissimilarities between the development of the sturgeon heart and that of other vertebrates are underscored.

Cardiac morphodynamic remodelling in the growing eel (Anguilla anguilla L.)

The morphodynamic changes occurring during growth were evaluated in the eel(Anguilla anguilla L.) heart. Using an in vitro working heart preparation, cardiac performance of small (body mass 96.76±27.49 g; mean ± s.d.) and large (body mass 656±12 g; mean± s.d.) eels was compared under basal conditions and under loading (i.e. preload and afterload) challenges. A parallel morphometric evaluation of the ventricle was made using light and transmission electron microscope images.

The small eel hearts show a basal cardiac output lower than their large counterparts (heart rate fh, 38.93±2.82 and 52.7±1.8 beats min–1, respectively; stroke volume Vs, 0.27±0.017 and 0.37±0.016 ml kg–1, respectively; means ± s.e.m.). The two groups show similar responses at increasing preload, but differ remarkably at increasing afterload. Small eel hearts decreased Vs at afterload greater than 3 kPa, in contrast to larger hearts, which maintained constant Vs up to 6 kPa. These changes in mechanical performance are related to structural differences.

Compared with the small eels, the large eels show an increase in the compacta thickness and in the diameter of the trabeculae in the spongiosa,together with reduction of the lacunary spaces. The increased compacta thickness is attained by enlargements of both the muscular and vascular compartments and reduction of the interstitium; consequently, this layer appears more compacted. Both compacta and spongiosa show higher number of myocytes together with reduced cross-sectional area and myofibrillar compartment. The compacta also shows an increased mitochondrial compartment. Our results document a cardiac morphodynamic remodelling in the growing eel.

Differentiation of the cardiac outflow tract components in alevins of the sturgeonAcipenser naccarii(Osteichthyes, Acipenseriformes): Implications for heart evolution

Previous work showed that in the adult sturgeon an intrapericardial, nonmyocardial segment is interposed between the conus arteriosus of the heart and the ventral aorta. The present report illustrates the ontogeny of this intermediate segment in Acipenser naccarii. The sample studied consisted of 178 alevins between 1 and 24 days posthatching. They were examined using light and electron microscopy. Our observations indicate that the entire cardiac outflow tract displays a myocardial character during early development. Between the fourth and sixth days posthatching, the distal portion of the cardiac outflow tract undergoes a phenotypical transition, from a myocardial to a smooth muscle-like phenotype. The length of this region with regard to the whole outflow tract increases only moderately during subsequent developmental stages, becoming more and more cellularized. The cells soon organize into a pattern that resembles that of the arterial wall. Elastin appears at this site by the seventh day posthatching. Therefore, two distinct components, proximal and distal, can be recognized from the fourth day posthatching in the cardiac outflow tract of A. naccarii. The proximal component is the conus arteriosus, characterized by its myocardial nature and the presence of endocardial cushions. The distal component transforms into the intrapericardial, nonmyocardial segment mentioned above, which is unequivocally of cardiac origin. We propose to designate this segment the "bulbus arteriosus" because it is morphogenetically equivalent to the bulbus arteriosus of teleosts. The present findings, together with data from the literature, point to the possibility that cells from the cardiac neural crest are involved in the phenotypical transition that takes place at the distal portion of the cardiac outflow tract, resulting in the appearance of the bulbus arteriosus. Moreover, they suggest that the cardiac outflow tract came to be formed by a bulbus arteriosus and a conus arteriosus from an early period of the vertebrate evolutionary story. Finally, we hypothesize that the embryonic truncus of birds and mammals is homologous to the bulbus arteriosus of fish.

Heart inflow tract of the African lungfishProtopterus dolloi

We report a morphologic study of the heart inflow tract of the African lungfish Protopterus dolloi. Attention was paid to the atrium, the sinus venosus, the pulmonary vein, and the atrioventricular (AV) plug, and to the relationships between all these structures. The atrium is divided caudally into two lobes, has a common part above the sinus venosus, and appears attached to the dorsal wall of the ventricle and outflow tract through connective tissue covered by the visceral pericardium. The pulmonary vein enters the sinus venosus and runs longitudinally toward the AV plug. Then it fuses with the pulmonalis fold and disappears as an anatomic entity. However, the oxygenated blood is directly conveyed into the left atrium by the formation of a pulmonary channel. This channel is formed cranially by the pulmonalis fold, ventrally by the AV plug, and caudally and dorsally by the atrial wall. The pulmonalis fold appears as a wide membranous fold which arises from the left side of the AV plug and extends dorsally to form the roof of the pulmonary channel. The pulmonalis fold also forms the right side of the pulmonary channel and sequesters the upper left corner of the sinus venosus from the main circulatory return. The AV plug is a large structure, firmly attached to the ventricular septum, which contains a hyaline cartilaginous core surrounded by connective tissue. The atrium is partially divided into two chambers by the presence of numerous pectinate muscles extended between the dorsal wall of the atrium and the roof of the pulmonary channel. Thus, partial atrial division is both internal and external, precluding the more complete division seen in amphibians. The present report, our own unpublished observations on other Protopterus, and a survey of the literature indicate that not only the Protopterus, but also other lungfish share many morphologic traits.

The conus valves of the adult gilthead seabream (Sparus auratus)

The conus (bulbo-ventricular) valves of teleosts perform a key function in the control of blood backflow during ventricular diastole. However, the structural characteristics of these valves are almost unknown. This paper presents a systematic anatomical, histological and structural study of the conus valves of the adult gilthead seabream (Sparus auratus). S. auratus shows two major left and right valves consisting of the leaflet and the supporting sinus. Each valvar leaflet can be divided into a stout proximal body and a flap-like distal region. The proximal body is structured into three layers: a luminal fibrosa, a dense cellular core and a parietal fibrosa. The luminal fibrosa is a collagenous structure extending the entire length of the leaflet, while the parietal fibrosa is restricted to the most proximal area. The dense cellular core consists of fibroblastic cells and a matrix rich in glycoconjugates, collagen and elastin. The histochemical and structural data suggest that the luminal fibrosa bears most of the force associated with valvar closure, while the cellular core acts as a cushion dampening vibrations and absorbing the elastic recoil. The sinus wall is a fibrous layer which shows proximal-distal differences in thickness. It also shows compositional differences that can be related to mechanical function. We describe the presence of a fibrous cylinder formed by the sinus wall, the fibrous interleaflet triangles and the fibrous layer that covers the inner surface of the conus myocardium. This fibrous cylinder constitutes the structural nexus between the ventricle, the conus and the bulbus arteriosus, provides support for the conus valves and separates the valvar complex from the surrounding tissues. The structure of the conus valves in S. auratus is different from that found in other vertebrates. Anatomical similarities between the conus valves and the mammalian arterial valves are emphasized. Each phyletic group appears to have developed specific structures in order to perform similar functions.

Renal corpuscle of the sturgeon kidney: an ultrastructural, chemical dissection, and lectin-binding study

The sturgeon is an ancient species of fish that thrives in a wide range of ecological environments, from freshwater to seawater. Basic in this process of adaptation is the ability of the kidney to control fluid filtration and urine formation. However, the morphological basis of this process is mostly unknown. The aim of the present study was to use microdissection techniques (scanning electron microscopy (SEM), transmission electron microscopy (TEM), and lectin-binding histochemistry) to examine the structure of the renal corpuscle of the sturgeon Acipenser nacarii in order to reveal morphologic features that could be related to function, phylogeny, and habitat. The renal corpuscles are aligned along the intrarrenal arteries. The urinary pole shows a siphon-like neck segment (NS) in 92% of the nephrons, whose structural characteristics are different from those of other fish. The podocytes have cuboidal cellular bodies, intercellular contacts, and poorly developed cell processes. The podocyte glycocalyx contains N-acetylglucosamine and lacks sialic acid. The structural and lectin-binding patterns are similar to those found in the immature mammalian kidney. The glomerular basement membrane (GBM) is very thick and consists of three layers: a lamina rara externa, a lamina densa, and a thick subendothelial lamina. The latter contains tubular microfibrils, collagen fibers, and long mesangial cell processes. Frequently, the podocyte bodies attach directly to the GBM, and the area occupied by the filtration slits is very small. Furthermore, the GBM shows a glycosylation pattern different from that observed in most vertebrates. Contrary to what would be expected in sturgeons living in freshwater, the A. nacarii renal corpuscle morphology suggests a low glomerular filtration rate.

The structure of the conus arteriosus of the sturgeon (Acipenser naccarii) heart: II. The myocardium, the subepicardium, and the conus-aorta transition

Sturgeons constitute a family of living "fossil" fish whose heart is related to that of other ancient fish and the elasmobranches. We have undertaken a systematic study of the structure of the sturgeon heart aimed at unraveling the relationship between the heart structure and the adaptive evolutionary changes. In a related paper, data were presented on the conus valves and the subendocardium. Here, the structure of the conus myocardium, the subepicardial tissue, and the conus-aorta transition were studied by conventional light, transmission, and scanning electron microscopy. In addition, actin localization by fluorescent phalloidin was used. The conus myocardium is organized into bundles whose spatial organization changes along the conus length. The variable orientation of the myocardial cell bundles may be effective in emptying the conus lumen during contraction and in preventing reflux of blood. Myocardial cell bundles are separated by loose connective tissue that contains collagen and elastin fibers, vessels, and extremely flat cells separating the cell bundles and enclosing blood vessels and collagen fibers. The ultrastructure of the myocardial cells was found to be very similar to that of other fish groups, suggesting that it is largely conservative. The subepicardium is characterized by the presence of nodular structures that contain lympho-hemopoietic (thymus-like) tissue in the young sturgeons and a large number of lymphocytes after the sturgeons reach sexual maturity. This tissue is likely implicated in the establishment and maintenance of the immune responses. The intrapericardial ventral aorta shows a middle layer of circumferentially oriented cells and internal and external layers with cells oriented longitudinally. Elastin fibers completely surround each smooth muscle cell, and the spaces between the different layers are occupied by randomly arranged collagen bundles. The intrapericardial segment of the ventral aorta is a true transitional segment whose structural characteristics are different from those of both the conus subendocardium and the rest of the ventral aorta.

Cleared extrachromosomal domain (CED): a nuclear domain enriched in nuclear matrix filaments is a common structure in sturgeon podocytes

The cell nucleus is divided into chromosome territories and the extrachromosomal domain. The latter includes several structural and functional compartments involved in RNA processing and transport. Morphological and cytochemical analyses of the cell nucleus in sturgeon podocytes revealed the existence of a previously uncharacterised nuclear compartment. It appears as a cleared nucleoplasmic area of variable size within the extrachromosomal domain. Conventional light and electron microscopy revealed that this compartment, here referred to as cleared extrachromosomal domain (CED), appears free of chromatin and RNA-containing organelles and is closely surrounded by heterochromatin masses. Cytochemical and immunogold electron microscope studies indicated that CEDs lack DNA, RNA and glycoconjugates. The ultrastructural examination of Lowicryl-embedded sections showed that CEDs are formed by a fibrillar network. In resinless preparations, this network appears as a dense mesh of crosslinked nuclear matrix filaments. The density of nuclear matrix filaments within the CED is remarkably higher than that observed in the rest of the nucleus. Our results indicate that the CED is a single, distinct extrachromosomal domain of the nuclear matrix of sturgeon podocytes. The absence in the CED of detectable DNA and RNA, as well as the lack of chromatin and RNA-containing structures, suggests that transcription and RNA processing do not occur in this novel nuclear compartment. The volume occupied by the CED may preserve the volume-to-cytoplasm ratio in the podocyte and contribute to modulate the spatial organisation and the volume occupied by the chromosomal territories.

The conus arteriosus of the adult gilthead seabream (Sparus auratus)

This paper reports on the presence of the conus arteriosus in the heart of the adult gilthead seabream, Sparus auratus (Perciformes, Teleostei). The junctional region between the single ventricle and the bulbus arteriosus has been studied by conventional light microscopy, and by scanning and transmission electron microscopy. In addition, fluorescent phalloidin and antibodies against the muscle myosin heavy chains, laminin and collagen type IV have been used. The conus arteriosus is a distinct muscular segment interposed between the ventricle and the bulbus arteriosus. It is clearly different from the bulbus arteriosus due to its myocardial nature. It can also be distinguished from the ventricular myocardium because: (1) it has a conus shape; (2) it is formed by compact, well-vascularized myocardium; (3) it is surrounded on its inner and outer faces by fibrous layers rich in collagen and elastin; (4) it constitutes the anatomical support of the so-termed conus valves; (5) it shows intense staining for laminin and type-IV collagen; and (6) the myocardial cells located close to the inner fibrous layer are helicoidally arranged. By contrast, the ventricular myocardium is highly trabecular, lacks a compacta, shows no vessels, and presents barely detectable amounts of laminin and collagen type IV. The presence of a distinct conus arteriosus in the heart of an evolutionary advanced teleost species indicates that the conus is not a vestigial segment from the evolutionary or embryological points of view. The characteristic spatial arrangement of the conus myocytes strongly suggests that the conus is implicated in the mechanical performance of the conus valves.

Structure of the conus arteriosus of the sturgeon (Acipenser naccarii) heart. I: the conus valves and the subendocardium

Sturgeons are bony fish that retain structural traits typical of the more primitive Chondrostei. From an evolutionary viewpoint, sturgeons are considered relic fish. However, they show remarkable ecological plasticity and are well adapted to contemporary environmental conditions. Although development of the cardiovascular system is critical for all organs and systems, and is affected by evolutionary changes, the structure of the sturgeon heart has been mostly overlooked. This is also true for the conus arteriosus, which, as in Chondrostei, is endowed with several rows of valves and a layer of contractile myocardium. This work reports on the structure of the valves, the endocardium, and the subendocardium of the conus arteriosus of the sturgeon (Acipenser naccarii) heart. It is part of a broader study that aims to cover the entire structure of the sturgeon heart. The conus arteriosus of 15 A. naccarii hearts, ranging in age from juveniles to sexually-differentiated adults, has been studied by conventional light, transmission (TEM), and scanning electron microscopy (SEM). In addition, maceration of the soft tissues with NaOH, and actin localization by fluorescent phalloidin has been used. The conus is a tubular chamber that arises from the right ventricular side and presents two constrictions at the conus-ventricle and conus-aorta junctions. The conus is endowed with three rows of valves: one distal and two proximal. The segment of the conus located between the distal and the two proximal rows is devoid of valvular structures. The distal row has four leaflets, while the two proximal rows show the greatest variation in leaflet number, size, and shape. All leaflets have collagenous chordae tendineae arising from the free border and from the parietal side of the leaflets. The endocardium is a flat endothelium which shows a thick, irregular basement membrane. The leaflet body is formed by a loose connective tissue which blends with the subendocardium. The subendocardium is a connective tissue consisting of myofibroblasts, collagen, and elastin. It is divided into two distinct areas: one proximal, which shows little elastin and poorly organized collagen; and one distal, which is rich in elastin, with cells and extracellular fibers organized into layers that are oriented in alternative circumferential and longitudinal directions. The present report is the first systematic analysis of the structure of the sturgeon conus. Descriptions of the conus valves should recognize the existence of three valve rows only. The variability in valve morphology, and the loose structure of the leaflet tissue make it unlikely that the valves play an effective role in preventing blood backflow. In this regard, the ventricle-conus constriction may act as a sphincter. The subendocardium is an elastic coat capable of actively sustaining the tissue deformation that accompanies the heart contractile cycle. Further comparative studies are needed to provide deeper insight into the structural changes that accompany phyletic diversification.

Origin and course of the coronary arteries in normal mice and in iv/iv mice

This paper reports on the origin and distribution of the coronary arteries in normal mice and in mice of the iv/iv strain, which show situs inversus and heterotaxia. The coronary arteries were studied by direct observation of the aortic sinuses with the scanning electron microscope, and by examination of vascular corrosion casts. In the normal mouse, the left and right coronaries (LC, RC) arise from the respective Valsalva sinus and course along the ventricular borders to reach the heart apex. Along this course the coronary arteries give off small branches at perpendicular or acute angles to supply the ventricles. The ventricular septum is supplied by the septal artery, which arises as a main branch from the right coronary. Conus arteries arise from the main coronary trunks, from the septal artery and/or directly from the Valsalva sinus. The vascular casts demonstrate the presence of intercoronary anastomoses. The origin of the coronary arteries was found to be abnormal in 84% of the iv/iv mice. These anomalies included double origin, high take-off, slit-like openings and the presence of a single coronary orifice. These anomalies occurred singly or in any combination, and were independent of heart situs. The septal artery originated from RC in most cases of situs solitus but originated predominantly from LC in situs inversus hearts. Except for this anomalous origin no statistical correlation was found between the coronary anomalies and heart situs or a particular mode of heterotaxia. The coronary anomalies observed in the iv/iv mice are similar to those found in human hearts. Most coronary anomalies appear to be due to defective connections between the aortic root and the developing coronaries. iv/iv mice may therefore constitute a good model to study the development of similar anomalies in the human heart.

Pitx2 expression defines a left cardiac lineage of cells: evidence for atrial and ventricular molecular isomerism in the iv/iv mice

The homeobox gene Pitx2 has been characterized as a mediator of left-right signaling in heart, gut, and lung morphogenesis. However, the relationship between the developmental role of Pitx2 and its expression pattern at the organ level has not been explored. In this study we focus on the role of Pitx2 in heart morphogenesis. Chicken Pitx2 transcripts are present in the left portion of the cardiac crescent and in the left side of the heart tube. Through looping Pitx2 is present in the left atrium, in the ventral portion of the ventricles and in the left-ventral part of the outflow tract. Mouse Pitx2 shows a similar developmental profile of expression. To test whether Pitx2 represents a lineage marker we have tagged the left portion of the chicken cardiac tube with fluorescent DiD. Labeled cells were found at HH16 in the left atrium and in the ventral region of the ventricles and the outflow tract. In the iv/iv mouse model of cardiac heterotaxia Pitx2 was abnormally expressed in the atrial and in the ventricular chambers. Furthermore, altered Pitx2 expression correlated with the occurrence of DORV. Our data reveal the existence of molecular isomerism not only in the atrial, but also in the ventricular compartment of the heart.

Molecular characterization of the ventricular conduction system in the developing mouse heart: topographical correlation in normal and congenitally malformed hearts

OBJECTIVES

Within the adult heart, it is convention to distinguish the conduction system and working (atrial and ventricular) myocardium. The adult conduction system (CS) comprises the sinoatrial (SAN), and atrioventricular (AVN) nodes, the atrioventricular bundle (AVB), the bundle branches and the peripheral Purkinje fibers, each of which display distinct functional properties and distinct profile of gene expression. Characterization of the mouse cardiac conduction system during development is rudimentary at present, even though genetically-modified mice are an increasing source of information regarding cardiac function and embryonic heart development.

METHODS

We have performed a detailed study of the pattern of expression of myosin heavy chain (MHC), myosin light chain (MLC), troponin I (TnI) isoforms, connexin 43 (Cx43), desmin and alpha-smooth muscle actin (alpha-SMA), in the ventricular conduction system of normal and congenitally malformed mouse hearts (iv background) from embryonic day 14.5 to 19.5.

RESULTS

The AVN is characterized by co-expression of MHC and MLC isoforms and no detectable expression of Cx43, desmin or alpha-SMA. The AVB expresses betaMHC and MLC2v, but no alphaMHC, MLC2a, Cx43, desmin or alpha-SMA. The right and left bundle branches display enhanced expression of desmin and alpha-SMA but no Cx43. The normal expression profile is maintained in congenitally malformed hearts such as double-outlet right ventricle and common atrioventricular canal. Three-dimensional reconstruction of the conduction system shows normal arrangement of the bundle branches in congenitally malformed hearts, but abnormal location and/or extension of the AVN.

CONCLUSIONS

Molecular characterization allows to follow the development of the CS in both, normal and malformed mouse hearts. Normal phenotypic expression of the CS is independent of heart situs but shows minor modifications in the presence of heart malformations. It is concluded that the AVN derives from the atrioventricular canal myocardium, the bundle of His from the ventricular myocardium, and the bundle branches from the ventricular trabeculations. Our results do not provide evidence to support an extra-cardiac origin of the ventricular CS.

Light and electron microscopy of the bulbus arteriosus of the European eel (Anguilla anguilla)

The bulbus arteriosus of teleost fish acts as an elastic reservoir that dilates during ventricular systole to store a large part of the cardiac stroke volume. Despite its functional importance, the knowledge of the structure of the bulbus wall is still fragmentary. We have undertaken a series of studies in order to establish a general morphological plan of the teleost bulbus. The bulbus arteriosus of the European eel is studied here by means of conventional light, and transmission and scanning electron microscopy. The inner surface of the bulbus wall is irregular due to the presence of branching ridges that flatten and disappear toward the ventral aorta. The ridge surface is covered by flattened endocardial cells that show moderately dense bodies. In the ridge tissue, cells near the endocardium are mostly undifferentiated and appear isolated in a loose filamentous matrix. Ridge cells progressively cluster toward the middle layer, become surrounded by a dense matrix, and adopt characteristics typical of smooth muscle cells. This suggests the existence of a differentiation gradient. The middle layer is formed by typical smooth muscle cells embedded in a meshwork matrix that contains thin and thick filaments. Stretching of this meshwork suggests an active role of smooth muscle cells in bulbus wall dynamics. Furthermore, large areas of the extracellular space are occupied by elastin-like material. The amount of this material decreases toward the external layer. Collagen is demonstrated across the entire thickness of the bulbus wall, its amount and organization increasing from the inner toward the outer bulbus surface. The existence of matrix gradients should progressively increase wall strength, maintaining bulbus dilation within safe physiological parameters. The epicardium is formed by flattened cells that contain numerous pinocytotic vesicles, suggesting an active interchange of solutes with the pericardial cavity.

Bulbus arteriosus of the Antarctic teleosts. II. The red-blooded Trematomus bernacchii

The structure of the bulbus arteriosus of the Antarctic teleost, Trematomus bernacchii, has been studied by light, scanning, and transmission electron microscopy. The wall of the bulbus arteriosus is divided into endocardial, subendocardial, middle and external layers. The endocardial endothelium covers the inner surface of the bulbus wall and invaginates into the subendocardium to form solid epithelial cords that show secretory activity. The subendocardial tissue is divided into finger-like ridges. Ridge cells located under the endocardium appear in niches limited by collagen fibers and thin cell extensions. Away from the endocardium ridge cells cluster into small groups, show some of the characteristics of smooth muscle cells, and appear enmeshed in a filamentous meshwork that lacks collagen and elastin fibers. The middle bulbus layer is formed by typical smooth muscle cells that are enmeshed in a filamentous meshwork similar to that observed in the ridges. The ridges and the middle layer appear to be formed by the same cell type, smooth muscle, with a gradient of differentiation from the endocardium toward the middle layer. In the absence of elastin fibers the filamentous meshwork should confer elastic properties to the bulbus wall. The stretching of the meshwork along the main axis of the middle layer cells, and between different cellular layers, suggests the existence of tensile stress and, hence, the involvement of smooth muscle cells in bulbus wall dynamics. The external layer is formed by numerous cellular types embedded in a collagenous matrix. Among these cellular types, myofibroblasts, macrophages, granulocytes, lymphocytes, dendrite-like cells, degenerating cells, and plasma cells can be recognized. The subepicardial tissue appears to be a specialized site involved in the production of the humoral immune response and displays many of the morphological characteristics of a germinal center. The outer limiting layer of the bulbus, the visceral pericardium, is formed by epithelial cells that show desmosomes and tight junctions. This suggests a close control of permeability with respect to the pericardial fluid.

Bulbus arteriosus of the antarctic teleosts. I. The white-blooded Chionodraco hamatus

The bulbus arteriosus of teleost fish is a thick-walled chamber that extends between the single ventricle and the ventral aorta. The functional importance of the bulbus resides in the fact that it maintains a steady blood flow into the gill system through heart contraction. Despite of this, a thorough study of the structure of the bulbus in teleost fish is still lacking. We have undertaken a morphologic study of the bulbus arteriosus in the stenothermal teleosts of the Antarctic sea. The structural organization of the bulbus arteriosus of the icefish Chionodraco hamatus has been studied here by conventional light, scanning, and transmission electron microscopy. The inner surface of the bulbus shows a festooned appearance due to the presence of longitudinal, unbranched ridges that extend between the ventricle and the arterial trunk. The wall of the bulbus is divided into endocardial, subendocardial, middle, and external layers. Endocardial cells show a large number of moderately-dense bodies. The endocardium invaginates into the subendocardium forming solid epithelial cords that contain numerous secretory vacuoles. Cells in the subendocardium group into small domains, have some of the morphological characteristics of smooth muscle cells, and appear enmeshed in a three-dimensional network of matrix filaments. Cells in the middle layer are typical smooth muscle cells. They appear arranged into layers and are surrounded by a filamentous meshwork that excludes collagen fibers. Orientation of this meshwork occurs in the vicinity of the smooth muscle cells. Elastin fibers are never observed. The external layer is formed by wavy collagen bundles and fibroblast-like cells. This layer lacks blood vessels and nerve fibers. The endocardium and the endocardium-derived cords are secretory epithelia that may be involved in the formation ofmucins or glycosaminoglycans. These mucins may have a protecting effect on the endocardium. The subendocardium and the middle layer appear to be formed by the same cell type, smooth muscle, with a gradient of differentiation from the secretory (subendocardium) to the contractile (middle layer) phenotype. Despite the absence of elastin fibers, the filamentous matrix could maintain the elastic properties of the bulbus wall. Smooth muscle cells appear to be actively involved in bulbus wall dynamics. The restriction of collagen to the external layer suggests that it may control wall dilatation and bulbus compliance. When comparison was possible, structural differences between C. hamatus and temperate teleosts seemed to be not species-related, but of phenotypic adaptative significance. This is remarkable since Antarctic fishes have lived isolated in freezing waters for the last two million years.

Collagenous skeleton of the human mitral papillary muscle

The papillary muscles (PM) of the heart have been the subject of numerous structural and functional studies. However, despite the importance of the collagenous compartment of the heart in the mechanical and electrical properties of the myocardium, little information is available on the structural organization of collagen within the PM. We study here the structural organization of collagen within the mitral papillary muscles (PM) of the human heart. Fragments of human mitral PM from normal and hypertensive subjects were macerated in NaOH to eliminate the cellular components. Macerated and nonmacerated samples were then studied with the scanning electron microscope (SEM). SEM shows that cardiac myocytes and endomysial capillaries are ensheathed in a layer of collagenous tissue. The myocyte sheath wall is formed by thin collagen fibers oriented at right angles to the main cell axis. These sheaths are open structures, collagen fibers continuing into adjacent sheaths at the points of lateral communications. Thick perimysial septa do not divide the PM tissue into separate compartments. Hypertensive hearts show perivascular and interstitial fibrosis. In addition, the lumen of the coronary vessels is reduced or obliterated, and large areas of the myocardium are substituted by densely packed collagen. Endomysial sheaths constitute a continuous collagenous layer that replicates the myocyte network. The endomysium should play a complex role in myocardial mechanics, assuring the equal distribution of force during the cardiac cycle. The absence of insulating boundaries should facilitate lateral propagation of excitation. Fibrosis in hypertensive hearts appears to be both reactive and reparative. The increase in the amount of collagen should greatly impair contractile capabilities and electrical conductance, severely compromise heart function, and contribute to development of heart failure.

Pitx2 participates in the late phase of the pathway controlling left-right asymmetry

Pitx2, a member of the bicoid-related family of homeobox-containing genes, is asymmetrically expressed in the left lateral plate mesoderm and derived tissues during chick and mouse development. Modifications of Pitx2 pattern of expression in the iv mouse mutation correlate with the situs alterations characteristic of the mutation. Misexpression experiments demonstrate that Shh and nodal positively regulate Pitx2 expression. Our results are compatible with a Pitx2 function in the late phase of the gene cascade controlling laterality.

Developmental biology of the vertebrate heart

This paper summarizes the development of the heart from the formation of the heart mesoderm to cardiac septation. A brief account of morphological changes is provided, but attention is focused on mechanisms rather than on morphologic descriptions. Heart induction and differentiation, and the expression of cardiac specific proteins, are reviewed. New developments in these areas include the possible role of cell surface proteins and peptide growth factors in the segregation of the splanchnic mesoderm and in cardiac commitment. Past and recent experiments indicate that the heart morphogenetic information is engraved in the precardiac mesoderm. In spite of this, specific differentiative signals can be overriden experimentally demonstrating the unstability of the cardiac phenotype at the early heart tube stage. The relationship between differentiation and morphogenesis is analyzed. While cardiac differentiation appears to be a prerequisite for morphogenesis, a number of experiments indicate that differentiation can proceed in the absence of any morphogenesis. Formation of the heart loop is separated into two different components; looping itself and the acquisition of handedness. Late heart morphogenesis is explained in terms of differential tissue growth and tissue remodeling. This not only includes morphogenetic changes intrinsic to the heart but the addition of new cell types (neural crest, epicardium, vessels, nerves) that become integrated into the developing heart. The contribution of specific mechanisms to our understanding of heart development, such as cell death and hemodynamics is also analyzed.

Atrioventricular valves of the mouse: III. Collagenous skeleton and myotendinous junction

BACKGROUND

The leaflet tissue of the mouse atrioventricular (AV) valves contains a system of wavy collagen bundles that organize like tendons, orientate along lines of tension, and constitute an essential component of the valve tissue. The organization of these bundles is different in the two AV valves, reflecting differences in the anatomy of the entire valvular complex. Further insights into this kind of organization are needed to gain a complete understanding of the functional anatomy of the mouse AV valves.

METHODS

The endocardial covering of the mouse AV valves (from 21 days to 1 year of age) was eliminated by the sonication or the maceration method. This allowed us to study in situ the organization of the collagenous valve skeleton, as well as the structure of the myotendinous junction.

RESULTS

The leaflets of the two AV valves are formed by a fibrous layer (on the ventricular side) and a spongy layer (on the atrial side). The fibrosa is formed by undulating collagen bundles that organize and orientate differently on the right and left sides. The spongiosa is formed, on both sides, by a loose network of thin collagen fibers with no apparent orientation. Myocardial cells in the papillary muscles of the tricuspid valve are elongated and show cone-shaped tips. Collagen fibers attach to the myocyte surface. Collagen struts and thin septa can also be recognized. On the other hand, the collagenous components of the mitral leaflets attach tangentially to the mitral papillary muscles. On the two sides, the myocytes appear to be ensheathed in a layer of collagenous tissue. The sheaths are formed by circularly arranged fibers and appear to be tightly interconnected.

CONCLUSIONS

The differences in the collagenous organization between the two AV valves reflect differences in the gross anatomy of the valves. The attachment of collagen to the papillary myocytes in the tricuspid valve resembles that of a typical myotendinous junction. However, the collagen-muscle junction in the mitral valve is more similar to the structure of a pennate muscle. The collagen matrix of the heart has been divided into endomysial, perimysial, and epimysial components. The presence of sheaths housing individual myocytes and capillaries, struts, and thin septa, corresponds to the endomysium. The absence of perimysial septa, which aggregate myocytes into groups, is striking, but this may just be a species difference. The appropriateness of the term epimysium, as applied to the heart, is discussed.

Atrioventricular valves of the mouse: II. Light and transmission electron microscopy

BACKGROUND

Mouse atrioventricular (AV) valves present a number of conspicuous morphologic differences with human AV valves. Given the existence of these differences, it is important to know the structural organization of mouse AV valves. Since the mouse is often considered to be a good animal model for developmental and anatomical studies, the presence of significant differences in structure may render comparative studies difficult. In addition, we wished to learn about the existence of structural changes in the mouse AV valves with age.

METHODS

The structural organization of mouse AV valves from 21 days to 1 year of age was studied by polarizing microscopy and by conventional light and transmission electron microscopy.

RESULTS

Polarizing microscopy reveals the presence of a system of birefringent fibers that consist of collagen bundles that organize like tendons. The spatial organization of these fibers is different in the two AV valves, reflecting differences in the anatomy of the entire valvular complex. Interstitial cells (IC) are of two different phenotypes: some are typical fibroblasts, while some others share smooth muscle cell characteristics. In addition, small areas of fibrocartilage are also observed. The compactness and thickness of the collagen bundles increase with age. Also with age, the basement membranes become thickened or multilayered, and matrix vesicles and deposits of amyloid P can be observed.

CONCLUSIONS

The collagenous birefringent fibers form an internal skeleton that should transmit the cycling stress evenly over the entire leaflets. IC should help to maintain the structure and deformability of the valve tissue and appear actively involved in the synthesis and renewal of extracellular material. The cartilaginous foci appear to be a normal component of the valve tissue. The structural changes observed in old animals appear to be related to the degenerative processes which take place in normal valvular tissues with age. Despite the structural differences, age changes appear to be similar in the AV valves of mouse and man.

Lectin-binding sites during postnatal differentiation of normal and cystic rabbit renal corpuscles

Fluorochrome-labeled lectins were used to study the expression of glycoconjugates during the postnatal differentiation of normal and cystic rabbit renal corpuscles. Glomerular cysts (GC) are induced in the rabbit by a single injection of corticoids. The Bowman's capsule of these cysts is exclusively formed of podocytes (parietal podocytes). During normal development, the cell coat of the podocytes is intensely positive for wheat germ agglutinin (WGA) and Maclura pomifera agglutinin (MPA). This reaction decreases considerably during maturation, in parallel with an increase in the number of binding sites masked by terminal sialylation. Throughout the stages studied, the podocyte coat is peanut agglutinin (PNA)-negative, but it becomes intensely positive after neuraminidase treatment. Visceral and parietal podocytes in the glomerular cysts show the same pattern of glycosylation as the normal podocytes. In contrast, normal parietal cells only transiently expressed a weak reactivity to WGA and MPA during the first stages of differentiation, and did not express cryptic binding sites at any stage. The glomerular basement membrane (GBM) is positive to WGA, to succinylated WGA, and to MPA, in all the stages studied. Maturation of the GBM is characterized by expression of cryptic MPA-binding sites, and by a considerable increase in the number of cryptic PNA-binding sites. The basement membrane of the parietal layer of the cystic Bowman's capsule shows the same pattern of glycosylation, despite the fact that this epithelial layer is solely formed of podocytes and lacks endothelial cells. In contrast, the normal parietal basement membrane does not express PNA or MPA cryptic sites at any stage.(ABSTRACT TRUNCATED AT 250 WORDS)

The atrioventricular valves of the mouse. I. A scanning electron microscope study

This paper reports a scanning electron microscope study of the morphology of the atrioventricular (AV) valves in the mouse. The leaflet tissue of the 2 AV valves consists of a continuous veil that shows no commissures or clefts. In all instances, the chordae that arise from the papillary system merge with the free border of the leaflet tissue. No distinct terminations of chordae were observed on the ventricular face of the valves. The leaflet tissue of the right AV valve can be divided into parietal and septal components on the basis of the insertion into the ventricular wall and of the papillary system. While the septal component is similar in shape, location and tension apparatus to the septal tricuspid leaflet in man, the parietal component appears to correspond to the anterior and posterior human leaflets. This segment of the valve is served by 3 papillary muscles that arise from the septal wall. The right AV valve is not a tricuspid structure from the morphological standpoint, but appears to function as such because of the particular attachment of the papillary muscles. The leaflet tissue of the mitral valve is served by 2 papillary muscles, anterior and posterior, which consist of muscular trabeculae extending from the heart apex to the base of the valve. These muscles remain associated with the ventricular wall. The leaflet tissue attaches directly to these papillary muscles, which give rise to a very small number of slender chordae. There are thus several important differences between the AV valves of the mouse and man.(ABSTRACT TRUNCATED AT 250 WORDS)