Distribution of mucosubstances in the developing rat heart

The State of Cardiovascular Developmental Biology is Strong - Honoring Dr. Roger Markwald and his Seminal Contributions to the Field

In August 2017, the Cardiovascular Developmental Biology Center (CDBC), together with the "Department of Regenerative Medicine and Cell Biology (RMCB) at the Medical University of South Carolina (MUSC), organized their 13th Annual CDBC Symposium. During this special event, which was organized in collaboration with The Anatomical Record, the unique and important contributions of Dr. Roger Markwald (known to all of us as Roger) to the field of cardiovascular research were celebrated. Fifteen leading investigators in the field presented their ideas and reported results of their studies to an audience that included many familiar faces from Roger's past and present. This group consisted of established investigators from around the world as well as young and upcoming scientists from local institutions. In their presentations, the platform speakers emphasized the significance of Roger's scientific contributions and advice to their professional development and career. In this Special Issue of The Anatomical Record, we assembled a collection of invited papers written by several attendees of the symposium. The issue also contains a number of articles written by colleagues who, for one reason or the other, were not able to attend the meeting, but expressed their desire to contribute to this special "festschrift" of The Anatomical Record in honor and recognition of Roger's amazing career. Anat Rec, 302:14-18, 2019. © 2018 Wiley Periodicals, Inc.

The pathogenesis of atrial and atrioventricular septal defects with special emphasis on the role of the dorsal mesenchymal protrusion

Partitioning of the four-chambered heart requires the proper formation, interaction and fusion of several mesenchymal tissues derived from different precursor populations that together form the atrioventricular mesenchymal complex. This includes the major endocardial cushions and the mesenchymal cap of the septum primum, which are of endocardial origin, and the dorsal mesenchymal protrusion (DMP), which is derived from the Second Heart Field. Failure of these structures to develop and/or fully mature results in atrial septal defects (ASDs) and atrioventricular septal defects (AVSD). AVSDs are congenital malformations in which the atria are permitted to communicate due to defective septation between the inferior margin of the septum primum and the atrial surface of the common atrioventricular valve. The clinical presentation of AVSDs is variable and depends on both the size and/or type of defect; less severe defects may be asymptomatic while the most severe defect, if untreated, results in infantile heart failure. For many years, maldevelopment of the endocardial cushions was thought to be the sole etiology of AVSDs. More recent work, however, has demonstrated that perturbation of DMP development also results in AVSD. Here, we discuss in detail the formation of the DMP, its contribution to cardiac septation and describe the morphological features as well as potential etiologies of ASDs and AVSDs.

Atrioventricular valve development: new perspectives on an old theme

Atrioventricular valve development commences with an EMT event whereby endocardial cells transform into mesenchyme. The molecular events that induce this phenotypic change are well understood and include many growth factors, signaling components, and transcription factors. Besides their clear importance in valve development, the role of these transformed mesenchyme and the function they serve in the developing prevalve leaflets is less understood. Indeed, we know that these cells migrate, but how and why do they migrate? We also know that they undergo a transition to a mature, committed cell, largely defined as an interstitial fibroblast due to their ability to secrete various matrix components including collagen type I. However, we have yet to uncover mechanisms by which the matrix is synthesized, how it is secreted, and how it is organized. As valve disease is largely characterized by altered cell number, cell activation, and matrix disorganization, answering questions of how the valves are built will likely provide us with information of real clinical relevance. Although expression profiling and descriptive or correlative analyses are insightful, to advance the field, we must now move past the simplicity of these assays and ask fundamental, mechanistic based questions aimed at understanding how valves are "built". Herein we review current understandings of atrioventricular valve development and present what is known and what isn't known. In most cases, basic, biological questions and hypotheses that were presented decades ago on valve development still are yet to be answered but likely hold keys to uncovering new discoveries with relevance to both embryonic development and the developmental basis of adult heart valve diseases. Thus, the goal of this review is to remind us of these questions and provide new perspectives on an old theme of valve development.

Expression of the familial cardiac valvular dystrophy gene, filamin-A, during heart morphogenesis

Myxoid degeneration of the cardiac valves is a common feature in a heterogeneous group of disorders that includes Marfan syndrome and isolated valvular diseases. Mitral valve prolapse is the most common outcome of these and remains one of the most common indications for valvular surgery. While the etiology of the disease is unknown, recent genetic studies have demonstrated that an X-linked form of familial cardiac valvular dystrophy can be attributed to mutations in the Filamin-A gene. Since these inheritable mutations are present from conception, we hypothesize that filamin-A mutations present at the time of valve morphogenesis lead to dysfunction that progresses postnatally to clinically relevant disease. Therefore, by carefully evaluating genetic factors (such as filamin-A) that play a substantial role in MVP, we can elucidate relevant developmental pathways that contribute to its pathogenesis. In order to understand how developmental expression of a mutant protein can lead to valve disease, the spatio-temporal distribution of filamin-A during cardiac morphogenesis must first be characterized. Although previously thought of as a ubiquitously expressed gene, we demonstrate that filamin-A is robustly expressed in non-myocyte cells throughout cardiac morphogenesis including epicardial and endocardial cells, and mesenchymal cells derived by EMT from these two epithelia, as well as mesenchyme of neural crest origin. In postnatal hearts, expression of filamin-A is significantly decreased in the atrioventricular and outflow tract valve leaflets and their suspensory apparatus. Characterization of the temporal and spatial expression pattern of filamin-A during cardiac morphogenesis is a crucial first step in our understanding of how mutations in filamin-A result in clinically relevant valve disease.

Cardiac cushions modulate action potential phenotype during heart development [corrected]

The extracellular matrix plays an important role in cardiac function. Its role in the generation and modulation of electrical activity in the early stages of heart development has not been studied extensively. Our study demonstrates that the extracellular matrix in cardiac cushions can alter the action potential phenotype by direct contact with cardiomyocytes from different regions of the heart. We also demonstrate that fibronectin, an important and abundant component of the cardiac extracellular matrix, partially mimics the effects of the cushion tissue in altering the changes in action potential. Fibronectin increases I(Ca) (2+) and acutely increases cytosolic calcium. These findings suggest that the composition of the cardiac extracellular matrix during development plays an important role in defining patterns of electrical activity in the developing heart.

Periostin regulates atrioventricular valve maturation

Cardiac valve leaflets develop from rudimentary structures termed endocardial cushions. These pre-valve tissues arise from a complex interplay of signals between the myocardium and endocardium whereby secreted cues induce the endothelial cells to transform into migratory mesenchyme through an endothelial to mesenchymal transformation (EMT). Even though much is currently known regarding the initial EMT process, the mechanisms by which these undifferentiated cushion mesenchymal tissues are remodeled "post-EMT" into mature fibrous valve leaflets remains one of the major, unsolved questions in heart development. Expression analyses, presented in this report, demonstrate that periostin, a component of the extracellular matrix, is predominantly expressed in post-EMT valve tissues and their supporting apparatus from embryonic to adult life. Analyses of periostin gene targeted mice demonstrate that it is within these regions that significant defects are observed. Periostin null mice exhibit atrial septal defects, structural abnormalities of the AV valves and their supporting tensile apparatus, and aberrant differentiation of AV cushion mesenchyme. Rescue experiments further demonstrate that periostin functions as a hierarchical molecular switch that can promote the differentiation of mesenchymal cells into a fibroblastic lineage while repressing their transformation into other mesodermal cell lineages (e.g. myocytes). This is the first report of an extracellular matrix protein directly regulating post-EMT AV valve differentiation, a process foundational and indispensable for the morphogenesis of a cushion into a leaflet.

Cell Biology of Cardiac Cushion Development

The valves of the heart develop in the embryo from precursor structures called endocardial cushions. After cardiac looping, endocardial cushion swellings form and become populated by valve precursor cells formed by an epithelial-mesenchymal transition (EMT). Endocardial cushions subsequently undergo directed growth and remodeling to form the valvular structures and the membranous septa of the mature heart. The developmental processes that mediate cushion formation include many prototypic cellular actions including adhesion, signaling, migration, secretion, replication, differentiation, and apoptosis. Cushion morphogenesis is unique in that these cellular possesses occur in a functioning organ where the cushions act as valves even while developing into definitive valvular structures. Cardiovascular defects are the most common congenital defects, and one of the most common causes of death during infancy. Thus, there is significant interest in understanding the mechanisms that underlie this complex developmental process. In this regard, substantial progress has been made by incorporating an understanding of cardiac morphology and cell biology with the rapidly expanding repertoire of molecular mechanisms gained through human genetics and research using animal models. This article reviews cardiac morphogenesis as it relates to heart valve formation and highlights selected growth factors, intracellular signaling mediators, and extracellular matrix components involved in the creation and remodeling of endocardial cushions into mature cardiac structures.

Dynamic expression of a native chondroitin sulfate epitope reveals microheterogeneity of extracellular matrix organization in the embryonic chick heart

TC2 is a novel monoclonal antibody produced by in vitro immunization of splenocytes with a peanut agglutinin-positive fraction from extracts of prechondrogenic micromass cultures of chick limb mesenchyme. ELISA results demonstrated TC2 reactivity with a native epitope on a glycosaminoglycan (GAG) enriched in chondroitin-4-sulfate and with multiple intact proteoglycans, but not with other GAGs tested. TC2 immunohistochemical reactivity was abolished by pretreatment of sections with chondroitinase AC or preadsorption with chondroitin-4-sulfate GAG. Strong TC2 localization occurred throughout the developing heart at stage 9. As looping ensued, a graded reactivity was observed from lowest in the atrium to highest in the conotruncus that correlated well with versican localization. The superior atrioventricular cushion stained preferentially with TC2 as compared to the inferior cushion at stages 16-18. At these later stages TC2 patterns did not agree completely with anti-versican reactivity. By stage 23 there was a marked reduction in TC2 localization in the heart, however, strong reactivity remained at certain sites, including the conotruncus and in subcompartments of both atrioventricular cushions. A heterogeneous distribution of other native chondroitin sulfate glycosaminoglycan epitopes recognized by monoclonal antibodies d1C4 and CS-56 was observed as well. The distribution of the TC2 epitope usually did not overlap with d1C4 or CS-56 localization at the stages examined. Overall, the spatiotemporal characteristics of TC2 reactivity in the developing chick heart appear to correlate with subdomains of the endocardial cushions as well as with trabecular and atrial septal formation.

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.

Origin of the pulmonary venous orifice in the mouse and its relation to the morphogenesis of the sinus venosus, extracardiac mesenchyme (spina vestibuli), and atrium

BACKGROUND

Human embryology textbooks indicate that the trunks of the pulmonary vein and artery originate from the left atrium and aortic sac, respectively, based on histological analyses of limited human specimens. However, our studies show that the pulmonary venous trunk in the mouse as in other nonhuman vertebrates originates from a vascular "sac" at the venous pole, the sinus venosus.

METHODS

Mouse embryos of 9-11 days gestation were obtained and staged according to Theiler's criteria and fixed in Carnoy's solution. Samples were embedded in paraffin and serial sections were prepared.

RESULTS

Histological analysis showed that at day 9.5 the pulmonary venous rudiment was initially observed along the left margin in the extracardiac mesenchyme that separated the venous pole of the heart from the lung buds. The endothelium of the pulmonary vein was continuous, with a vascular sac we identified as sinus venosus based on its location immediately posterior to the left sinoatrial fold. The sinus venosus became incorporated into the left atrium (days 10-10.5) to form part of the posterior atrial wall. Similarly, the pulmonary vein and associated extracardiac mesenchyme were "drawn" into the atrium. This extracardiac mesenchyme of the venous pole, also called "spina vestibuli" and containing the pulmonary vein at its left margin, formed a wedge-shaped invagination within the atrium that contributed nonmuscular tissue to the primary atrial septum.

CONCLUSIONS

We propose that the orifice of the pulmonary vein establishes a link with the left side of the atrium as a consequence of a venous sac, the sinus venosus, and its associated mesenchyme (in which the root of the pulmonary vein is embedded) being incorporated into the atrium.

The extracellular matrix during heart development

The embryonic extracellular matrix, which is comprised of glycosaminoglycans, glycoproteins, collagens, and proteoglycans, is believed to play multiple roles during heart morphogenesis. Some of these ECM components appear throughout development, however, certain molecules exhibit an interesting transient spatial and temporal distribution. Due to significant new data that have been gathered predominantly in the past 10 years, a comprehensive review of the literature is needed. The intent of this review is to highlight work that addresses mechanisms by which extracellular matrix influences vertebrate heart development.

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.

Hyaluronate degradation affects ventricular function of the early postlooped embryonic rat heart in situ

Hyaluronic acid is the major glycosaminoglycan of the early cardiac extracellular matrix or "cardiac jelly," yet little is known about its role in the ontogeny of early ventricular performance. To investigate the in situ effect of hyaluronate degradation on ventricular function, whole rat embryos were cultured in rat serum alone (control embryos) or rat serum plus 20 TRU/mL of Streptomyces hyaluronidase (treatment embryos) from gestational day 9.5 (before formation of the heart tube) through initial looping of the heart. Cardiac function was measured before looping (24 hours in culture) and immediately after looping (36 hours in culture) by video motion analysis of the external wall motion of the bulbus cordis and primitive ventricle. Degradation of hyaluronic acid in the treated embryos was confirmed by Alcian blue staining at pH 2.5. Significant increases in heart rate, circumferential shortening fraction, maximum velocity of circumferential contraction, and maximum velocity of circumferential relaxation were observed with looping in both control and treatment embryos. Although there was minimal difference in ventricular performance between control and treatment embryos before looping, there was a significant increase in all parameters of ventricular performance in the hyaluronidase-treated embryos immediately after looping of the heart. Endocardial cushions were absent in hyaluronidase-treated embryos, and an additional group of embryos cultured in the presence of Streptomyces hyaluronidase for 48 to 72 hours failed to develop endocardial cushions. These experiments are the first to (1) document a quantifiable increase in ventricular performance during early cardiac looping and (2) demonstrate that hyaluronate degradation results in abnormal endocardial cushion formation and altered ventricular performance of the postlooped heart.(ABSTRACT TRUNCATED AT 250 WORDS)

Degradation of hyaluronic acid does not prevent looping of the mammalian heart in situ

It is currently proposed that accumulation of hyaluronic acid (HA) and subsequent hydration of the cardiac extracellular matrix is required for normal looping of the vertebrate heart. To test this hypothesis, we cultured Wistar rat embryos (Gestational Day 9.5) in rat serum plus 20 TRU/ml of Streptomyces hyaluronidase (treated embryos) or rat serum alone (control embryos). Despite degradation of HA as documented by Alcian blue staining at pH 2.5, 57 of 59 treated embryos developed normally looped hearts after 36 hr in culture. These experiments suggest that the accumulation of HA is not required for normal looping of the mammalian heart in situ.

Distribution of laminin, collagen type IV, collagen type I, and fibronectin in chicken cardiac jelly/basement membrane

Light microscopic immunolabeling studies were designed to identify and locate structural components within the cell-free extracellular matrix which lies between the embryonic endocardial and myocardial tubes. Affinity-purified antibodies were used to examine stage 15-22 embryonic chicken hearts. Specimens were immunolabeled by using three different methodologies: 1) postembedding labeling of 10 microns cryostat sections, 2) preembedding labeling (en bloc) of whole hearts, and 3) postembedding labeling of ethanol/acetic acid-fixed paraffin sections. Our results establish the spatial distribution of collagen type I and demonstrate for the first time the presence of collagen type IV and laminin in the myocardial-basement-membrane/cardiac jelly.

Glycoconjugates in normal wound tissue matrices during the initiation phase of limb regeneration in adult Ambystoma

The present study identifies, localizes, and reports the relative composition of specific glycosaminoglycans within tissue matrices during the initiation phase of limb regeneration. The regenerate tissues were harvested and assayed morphologically, histochemically, and chemically. We observed 1) a population of cells interspersed among the cells of the dermis, epimysium, perimysium, perichondrium, and periosteum. 2) This population was distinguishable by a unique pattern of glycoconjugate staining, i.e., intracellular and pericellular heparan sulfate and glycoproteins and extracellularly associated hyaluronate and glycoproteins. 3) Cells with these staining characteristics aggregated to a position directly beneath the apical epidermal cap. 4) Extracellular hyaluronate and glycoproteins colocalized with undifferentiated tissues. And 5) extracellular chondroitin sulfate, dermatan sulfate, and keratan sulfate glycosaminoglycans colocalized with differentiated tissues. The correlations of distinct glycoconjugate compositions with specific regeneration morphologies suggest the possibility that these components may be related to the phenotypic expression of tissues during regeneration.

Protein extracts from early embryonic hearts initiate cardiac endothelial cytodifferentiation

Prior to the formation of multiple chambers, the embryonic heart consists of two epithelial tubes, one within the other. As development proceeds, portions of the inner epithelium, i.e., the endothelium, undergo a morphological transformation into a migrating mesenchymal cell population. Our results show that this transformation is affected by proteins secreted by the outer epithelium, i.e., the myocardium, into the extracellular matrix between these two tissues. This conclusion is based on tissue autoradiographic studies of whole embryo cultures with 3H-amino acids. Continuous labeling conditions generated an apparent gradient of proteins extending away from the myocardium and contacting the endothelium just prior to the formation of mesenchyme, i.e., activation of the transformation sequence. Pulse/chase studies confirmed this directional movement of matrix protein. By performing sequential extractions of preactivation staged embryonic hearts with EDTA and testicular hyaluronidase followed by ammonium sulfate precipitation we obtained an enriched preparation of cardiac extracellular matrix. This fraction was capable of eliciting several of the events characteristic of endothelial activation in vitro. These events included: (i) cell-cell separation, (ii) lateral cell mobility, and (iii) hypertrophy and polarization of intracellular PAS staining (Golgi apparati). The biological activity of the extract was sensitive to heat denaturation: a homogenate of the remaining extracted tissue would not substitute for the matrix extract. Morphologically the extracted hearts appeared intact, however, the extracellular matrix space was significantly diminished. No more than 6% of the total lactic dehydrogenase activity, a cytosolic enzyme, was found in the extract. Preliminary electrophoretic characterization of the extract (metabolically labeled with 14C-amino acids) indicated that it may contain as many as 35 proteins or subunits. The relationship of ECM to endothelial differentiation in cardiac morphogenesis is discussed as a model for other developmental systems.

Histological analysis of limb regeneration in postmetamorphic adult Ambystoma

Previous investigation into the regenerative ability of postmetamorphic adult land phase Ambystoma has revealed that these species have the capacity to completely regenerate a limb, given optimal environmental conditions, and the gross morphological characteristics of limb regeneration in these species compared favorably with the external regeneration morphology of aquatic phase forms. The present study concerns a histological and histochemical examination of the regenerating limb tissues and their respective extracellular and intracellular tissue matrices. Postmetamorphic adult Ambystoma were amputated through the forearm, placed within optimal environmental conditions, and allowed to regenerate. The tissues were harvested at designated intervals after amputation and prepared for light microscopic examination. The limb tissues were assayed histologically for similarities to and differences from previously established regeneration morphologies. It was noted that specific correlations (i.e., apical epidermal cap formation, but outgrowth and elongation, palette formation, and digit formation) existed between regeneration histologies in these species and those previously reported for the aquatic urodeles, newt, axolotl, and larval salamander. By utilizing the histological and histochemical characteristics of the tissue, the regenerate limb was divided into five tissue units: epidermal, blastemal, soft, hard, and neuro/vascular. Based on the unique morphology of their extracellular matrices and respective histochemical staining patterns, four distinct blastemal regions were delineated within the blastemal units: subregenerate epidermal blastema, soft-tissue blastema, hard-tissue blastema, and core blastema. Histochemically, changing patterns of highly sulfated, weakly sulfated, and carboxylated polysaccharides and glycosylated compounds were located within both the extra- and intracellular stump and regenerate tissue matrices during regeneration. In addition, these patterns of intra- and extracellular macromolecular material correlated to previous reports of similar-type compounds assayed during regeneration in aquatic urodeles. With this in mind, the adult land phase Ambystoma can be considered an appropriate model system for studies concerning normal limb regeneration.

Hyaluronidase activity in embryonic chick heart muscle and cushion tissue and cells

Hyaluronidase activity was compared in embryonic chick cardiac cushion and noncushion segments, as well as in cultures of mesenchyme derived from cardiac cushion endocardium (cushion tissue-enriched cultures) and in cultures of myocardial cells at stages critical to heart valve and septum development. Enzyme levels were higher in both heart tissue regions at periods of active cushion tissue mesenchyme migration than after migration ceases, and higher in the cushion region than in the noncushion region at both periods. Hyaluronidase was measured in cells and medium in both types of cultures, with five times greater activity found in the myocardial cultures. The cardiac hyaluronidase from cells and medium of both culture types had an estimated molecular weight of 41,000 to 44,000 and degraded hyaluronate and, to a lesser degree, chondroitin sulfate, at an acidic pH optimum. Ion-exchange chromatography demonstrated that in both culture types, a proportion of the secreted enzyme was more acidic than that found in the cell layer. These studies indicate the potential for hyaluronate degradation by the major cell types present in the developing heart at early stages and that the enzyme responsible is probably a lysosomal enzyme. Therefore, hyaluronate internalization is a likely requirement for degradation, and thus, the turnover of hyaluronate in developing heart valves is more complex than the extracellular degradative process suggested by histochemical data.

Hyaluronate binding and degradation by cultured embryonic chick cardiac cushion and myocardial cells

Cultured cells obtained from developing chick heart valvular and septal primordial tissues (cardiac cushions) and myocardium were tested for their capacity to bind, internalize, and degrade hyaluronate. A presumptive lysosomal hyaluronidase capable of hyaluronate degradation has been previously isolated and partially characterized from cultures enriched in either cushion tissue cells or myocardial cells (D. H. Bernanke and R. W. Orkin, 1984, Dev. Biol. 106, 351-359). In this study, both types of cultures were found to bind hyaluronate, but only the myocardial cultures could degrade the hyaluronate substrate. The lack of hyaluronate degradative capacity in the mesenchymal cushion tissue cells appears to result from their inability to internalize the macromolecule, thus failing to make it available to the lysosomal hyaluronidase. The data suggest that hyaluronate clearance from the extracellular matrix of the developing cushion is a complex process, involving more than simple extracellular degradation adjacent to the migrating mesenchymal cushion tissue cells. Instead, a sequence of events may be indicated which includes binding of hyaluronate to the cushion tissue cell surfaces and its transport by these cells across the cushion matrix toward the myocardium. The myocardium may be involved in the ultimate removal of hyaluronate from the cardiac jelly.

Invasion of mesenchyme into three-dimensional collagen gels: a regional and temporal analysis of interaction in embryonic heart tissue

In normal heart development the endothelium of the atrioventricular canal, but not the ventricle, produces mesenchymal cells which seed (invade) into the intervening extracellular matrix toward the myocardium at around 64-69 hr of development. We have utilized three-dimensional collagen substrates to examine the initiation of seeding by atrioventricular canal endothelia in vitro and to compare and contrast the responses of the ventricular endothelia. Explants of atrioventricular canals and ventricles from staged embryos were placed on the surfaces of collagen gels prior to the onset of seeding in situ. At varied intervals of incubation, the explant was removed, leaving behind a monolayer on the surface of the gel which consisted of endothelial cells. Subsequently, the endothelial outgrowths were examined for seeded cells. The results confirm the regional endothelial differences seen in vivo. They also show that invasion of the collagen gels is due to an alteration in phenotype mediated by interaction with other components of embryonic heart explant. Lastly, the time course of this tissue interaction in vitro mimics the onset of seeding in vivo.

A histochemical analysis of polyanoinic compounds found in the extracellular matrix encountered by migrating cephalic neural crest cells

Neural crest cells destined to form craniofacial primordia initially are "seeded" into and subsequently migrate through the extracellular matrix (ECM) of a cell free space (CFS) between the surface ectoderm and the underlying mesoderm. Utilizing histochemical procedures for polyanionic compounds, we have demonstrated that both sulfated and nonsulfated glycosaminoglycans (GAG) are present in the CFS of the cephalic region of the chick embryo and that their distribution and structural organization vary with the passage of neural crest or mesodermally derived (MD) mesenchymal cells through it. In stages 7 and 8 embryos a predominance of fine filamentous strands composed primarily on nonsulfated, carboxyl-rich GAG is seen spanning intercellular spaces between adjacent tissues and MD mesenchymal cells. In older embryos (stages 9 and 10) much of the filamentous material is replaced by coarse fibrillar strands or amorphous material which coats the surfaces of MD mesenchymal and neural crest cells as they invade the CFS. Using enzymatic digestions (Streptomyces and testicular hyaluronidase) and the critical electrolyte concentration procedure, data suggest that the fine filamentous matrix onto which the neural crest cells migrate consists mainly of hyaluronate with lesser amounts of chondroitin and some sulfated GAG present. The coarse fibrillar matrix that appears after passage of either neural crest or MD mesenchymal cells through the original CFS contains strongly sulfated polyanionic material, predominantly chondroitin sulfates A, C. Since GAG is located ubiquitously within the ECM of embryos at various stages, the role of GAG, if any, in the transfer of developmental information may be of a general nature (ie. stimulus of motility) rather than of specific morphogenetic cues (for specific differentiation into craniofacial primordia).

Morphogenesis of the truncus arteriosus of the chick embryo heart: the formation and migration of mesenchymal tissue

The appearance and migration of mesenchymal cushion tissue within the truncus arteriosus of the normal 2.5 to 6-day chick embryo heart was surveyed systemically with the light microscope. Series of cross-sections taken from replicate hearts at successive developmental stages allowed comparison of the following qualitative and quantitative aspects of early truncal morphogenesis. Mesenchyme within the truncus was derived from two distinct sources. The first mesenchyme appeared to migrate caudally into the cardiac jelly of the distal truncus from the nearby aortic arch region, coincident with slowing of the anterior elongation of the heart tube (Hamburger-Hamilton Stage 17-18). A second, separate mesenchymal population, derived from endocardium, began to fill the conus and proximal truncus in a radial direction, coicident with expansion of the bulbs cordis (Stage 12-19). The measured kinetics of relative cell numbers, distributions, and mitotic indices suggest substantial contributions from both sources. By Stage 26, the conotruncal region was filled with mesenchyme, which then condensed to form the anlagen of three future structures: the semilunar valves, the aorticopulmonary septum, and the tunica media of the great arteries.

Teratogenic action of bis(dichloroacetyl)diamine on rats: patterns of malformations produced in high incidence at time-limited periods of development

Win 18,446, a bis(dichloroacetyl)diamine, is a drug that was previously shown to suppress spermatogenesis and to be an effective oral abortifacient in rats. The present study shows that the drug is capable of producing characteristic congenital malformations in high incidence, by a single treatment, and with high survival of fetuses to day 21. Gestation day 11 is the most sensitive time. The teratologies obtained after various schedules of treatment include malformations of the snout (100%), septal heart defects (100%) diaphragmatic hernias (100%), cryptorchism (100%), cervical pockets (100%) and absent or small irregular thymus (92%). Some of these data, namely of the heart, face and thymus, cluster in patterns that indicate that the action of the drug is upon a time-resistricted developmental process. These periods of sensitivity are subsiding or have ended before primordia of these structures appear, but they coincide with the proliferation and migration of those mesenchyme cells that will eventually form or contribute to the structures affected. It is postulated that the drug acts on these mesenchyme cells, or on the extracellular matrix that provides the necessary framework for their dispersal.

Structural development of endocardial cushions

Development of chick and rat endocardial cushions (cardiac mesenchyme) was studied histologically (using Nomarski differential interference optics on living and unfixed tissue), ultrastructurally (scanning and transmission electron microscopy), cytochemically (using acidified dialyzed iron as a visual probe for polyanionic material) and autoradiographically (using 35S) to elucidate the origin of the mesenchyme, the morphologic sequences leading to cushion formation and secretion of sulfated glycosaminoglycans, if any, by migrating mesenchymal cells. Cushion formation was similar for both species. Mesenchymal cells appeared initially, in 16- to 18-somite embryos, beneath the endothelium (which lacked a basal lamina) of the future atrioventricular canal and outflow tract. The cytoplasm of cushion mesenchymal cells was structurally similar to the ensothelium; probably these cells arose by proliferation of the endothelium. Mitotic figures among the "seeded" cells were also numerous. Cushion cells were initially attached to the endothelium by desmosomes but acquired motile apparatus (pseudopodia and filopodia containing microtubules and microfilamentous bundles). Serial sectioning of successively-aged embryos (20-44 somites) indicated a centrifugal migratory direction. Interaction of the cell processes with extracellular matrix suggested that the latter was used as a migratory substrate. Contact of the advancing wedge of cushion cells with the myocardium produced no alteration in cell structure or mitotic activity. Localization of hyaluronidase-sensitive, dialyzed iron (DI) precipitates in 250-nm Golgi vacuoles and hyaluronidase-sensitive 35S-endangendered silver grains over cushion cells indicated that this tissue contributed sulfated macromolecules to the matrix. Localization of hyaluronidase-labile, DI material in coated, endocytic-like vesicles and caveolae also suggested potential modification or conditioning of the matrix by migrating mesenchymal cells. Altogether, the study established loci in developing cushions where disruption where disruption of the developmental sequence could engender valvular or septal defects.