Early development of macrophages in intact and organ cultured hearts of rat embryos

Macrophage lineages in heart development and regeneration

During development, macrophage subpopulations derived from hematopoietic progenitors take up residence in the developing heart. Embryonic macrophages are detectable at the early stages of heart formation in the nascent myocardium, valves and coronary vasculature. The specific subtypes of macrophages present in the developing heart reflect the generation of hematopoietic progenitors in the yolk sac, aorta-gonad-mesonephros, fetal liver, and postnatal bone marrow. Ablation studies have demonstrated specific requirements for embryonic macrophages in valve remodeling, coronary and lymphatic vessel development, specialized conduction system maturation, and myocardial regeneration after neonatal injury. The developmental origins of macrophage lineages change over time, with embryonic lineages having more reparative and remodeling functions in comparison to the bone marrow derived myeloid lineages of adults. Here we review the contributions and functions of cardiac macrophages in the developing heart with potential regenerative and reparative implications for cardiovascular disease.

Tissue-resident macrophages regulate lymphatic vessel growth and patterning in the developing heart

Macrophages are components of the innate immune system with key roles in tissue inflammation and repair. It is now evident that macrophages also support organogenesis, but few studies have characterized their identity, ontogeny and function during heart development. Here, we show that the distribution and prevalence of resident macrophages in the subepicardial compartment of the developing heart coincides with the emergence of new lymphatics, and that macrophages interact closely with the nascent lymphatic capillaries. Consequently, global macrophage deficiency led to extensive vessel disruption, with mutant hearts exhibiting shortened and mis-patterned lymphatics. The origin of cardiac macrophages was linked to the yolk sac and foetal liver. Moreover, the Cx3cr1⁺ myeloid lineage was found to play essential functions in the remodelling of the lymphatic endothelium. Mechanistically, macrophage hyaluronan was required for lymphatic sprouting by mediating direct macrophage-lymphatic endothelial cell interactions. Together, these findings reveal insight into the role of macrophages as indispensable mediators of lymphatic growth during the development of the mammalian cardiac vasculature.

Highlighted Article: Tissue-resident macrophages are indispensable mediators of lymphatic vessel formation during heart development, and function to remodel the vascular plexus.

Reciprocal analyses in zebrafish and medaka reveal that harnessing the immune response promotes cardiac regeneration

Zebrafish display a distinct ability to regenerate their heart following injury. However, this ability is not shared by another teleost, the medaka. In order to identify cellular and molecular bases for this difference, we performed comparative transcriptomic analyses following cardiac cryoinjury. This comparison points to major differences in immune cell dynamics between these models. Upon closer examination, we observed delayed and reduced macrophage recruitment in medaka, along with delayed neutrophil clearance. To investigate the role of immune responses in cardiac regeneration, we delayed macrophage recruitment in zebrafish and observed compromised neovascularization, neutrophil clearance, cardiomyocyte proliferation and scar resolution. In contrast, stimulating Toll-like receptor signaling in medaka enhanced immune cell dynamics and promoted neovascularization, neutrophil clearance, cardiomyocyte proliferation and scar resolution. Altogether, these data provide further insight into the complex role of the immune response during regeneration, and serve as a platform to identify and test additional regulators of cardiac repair.

DOI:http://dx.doi.org/10.7554/eLife.25605.001

In Vitro Murine Posterior Frontal Suture Fate Is Age-Dependent:

In CD-1 mice, the posterior frontal suture (analogous to the human metopic suture) fuses while all other cranial sutures remain patent. In an in vitro organ culture model, the authors previously demonstrated that posterior frontal sutures explanted immediately before the onset of suture fusion (at 25 days old) mimic in vivo physiologic fusion. In the first portion of this study, the authors defined how early in development the posterior frontal suture fuses in their tension-free, serum-free organ culture system by serially analyzing posterior frontal suture fusion from calvariae explanted at different stages of postnatal development. Their results revealed a divergence of suture fate leading to abnormal patency or physiologic fusion between the first and second weeks of life, respectively, despite viability and continued growth of the calvarial explants in vitro. From these data, the authors postulated that the gene expression patterns present in the suture complex at the time of explant may determine whether the posterior frontal suture fuses or remains patent in organ culture. Therefore, to elucidate potentially important differences in gene expression within this "window of opportunity," they performed a cDNA microarray analysis on 5-day-old and 15-day-old posterior frontal and sagittal whole suture complexes corresponding to the age ranges for unsuccessful (1 to 7 days old) and successful (14 to 21 days old) in vitro posterior frontal suture fusion. Overall, their microarray results reveal interesting differential expression patterns of candidate genes in different categories, including angiogenic cytokines and mechanosensitive genes potentially important in cranial suture biology.

Tumor necrosis factor modulates apoptosis of monocytes in areas of developmentally regulated bone remodeling

Tooth eruption is characterized by spatially segregated bone resorption along the path of eruption and bone formation in the opposite direction. Monocyte recruitment occurs in two distinct peaks in both areas of resorption and formation. Without such recruitment tooth eruption does not occur. The signals that regulate this recruitment are thought to involve the expression of cytokines and chemokines. One such cytokine is tumor necrosis factor (TNF), which can affect monocyte recruitment through the induction of chemokines and adhesion molecules and increase their lifespan by acting as antiapoptotic cell survival signals. We examined the latter by studying mice with targeted deletions of TNF receptors p55 and p75 (TNFRp55/p75). The results indicate that mice that lack functional TNF receptors have a significantly reduced number of monocytes in the apical area associated with bone formation. The reduced number of monocytes in this area can be accounted for by an increase in apoptosis in TNFRp55-/-/p75-/-. In contrast, the number of monocytes, the rate of monocyte apoptosis, and the formation of osteoclasts in the occlusal area associated with bone resorption occurred independently of TNF activity. These results suggest that TNF receptor signaling can affect tooth eruption by acting as a monocyte survival signal in some but not all areas of bone undergoing developmentally regulated remodeling.

Apoptosis in the pattern formation of the ventricular wall during mouse heart organogenesis

Apoptosis is an important mechanism in organogenesis, but its role in heart development has been poorly characterized. We have here studied apoptosis in the developing ventricular wall of mouse embryonic heart. Developing mice hearts on days 11 to 16 of gestation were studied using in situ end-labeling of degraded DNA (TUNEL), immunocytochemistry of regulatory genes Bcl-2 and Bax, and light and electron microscopy. TUNEL end-labeled apoptotic cells were found in the ventricular wall on days 11 to 16 of gestation. The proportions of apoptotic cells of all cells in the ventricular wall differed between the trabecular and compact regions (P = 0.003) and between the days of gestation (P = 0.0001), the calculated apoptotic index was greater in the compact region at all ages except day 14. Ultrastructural analysis showed typical apoptotic shrinkage, chromatin degradation, and apoptotic bodies in several myoblastic and myocardial endothelial cells which were also positive by DNA end-labeling. Immunocytochemical reaction for the apoptosis checkpoint proteins in the ventricular wall showed clearly more Bcl-2 positive cells than Bax positive cells. The numerical densities of all cells in the compact and trabecular regions remained always higher in the compact region (P = 0.04) despite the fact that apoptosis was present in both areas at the same time. In conclusion, apoptosis takes place in the developing myocardial muscle as well as the myocardial endothelium during ventricular morphogenesis on days 11 through 16 and decreases clearly on day 16. We suggest that apoptosis and its regulatory factors are closely involved in the morphogenesis of the ventricular wall of the mammalian heart.

Myocardialization of the cardiac outflow tract

During development, the single-circuited cardiac tube transforms into a double-circuited four-chambered heart by a complex process of remodeling, differential growth, and septation. In this process the endocardial cushion tissues of the atrioventricular junction and outflow tract (OFT) play a crucial role as they contribute to the mesenchymal components of the developing septa and valves in the developing heart. After fusion, the endocardial ridges in the proximal portion of the OFT initially form a mesenchymal outlet septum. In the adult heart, however, this outlet septum is basically a muscular structure. Hence, the mesenchyme of the proximal outlet septum has to be replaced by cardiomyocytes. We have dubbed this process "myocardialization." Our immunohistochemical analysis of staged chicken hearts demonstrates that myocardialization takes place by ingrowth of existing myocardium into the mesenchymal outlet septum. Compared to other events in cardiac septation, it is a relatively late process, being initialized around stage H/H28 and being basically completed around stage H/H38. To unravel the molecular mechanisms that are responsible for the induction and regulation of myocardialization, an in vitro culture system in which myocardialization could be mimicked and manipulated was developed. Using this in vitro myocardialization assay it was observed that under the standard culture conditions (i) whole OFT explants from stage H/H20 and younger did not spontaneously myocardialize the collagen matrix, (ii) explants from stage H/H21 and older spontaneously formed extensive myocardial networks, (iii) the myocardium of the OFT could be induced to myocardialize and was therefore "myocardialization-competent" at all stages tested (H/H16-30), (iv) myocardialization was induced by factors produced by, most likely, the nonmyocardial component of the outflow tract, (v) at none of the embryonic stages analyzed was ventricular myocardium myocardialization-competent, and finally, (vi) ventricular myocardium did not produce factors capable of supporting myocardialization.

Pax3 is required for cardiac neural crest migration in the mouse: evidence from the splotch (Sp2H) mutant

Neural crest cells originating in the occipital region of the avian embryo are known to play a vital role in formation of the septum of the cardiac outflow tract and to contribute cells to the aortic arches, thymus, thyroid and parathyroids. This ‘cardiac’ neural crest sub-population is assumed to exist in mammals, but without direct evidence. In this paper we demonstrate, using RT-PCR and in situ hybridisation, that Pax3 expression can serve as a marker of cardiac neural crest cells in the mouse embryo. Cells of this lineage were traced from the occipital neural tube, via branchial arches 3, 4 and 6, into the aortic sac and aorto-pulmonary outflow tract. Confirmation that these Pax3-positive cells are indeed cardiac neural crest is provided by experiments in which hearts were deprived of a source of colonising neural crest, by organ culture in vitro, with consequent lack of up-regulation of Pax3. Occipital neural crest cell outgrowths in vitro were also shown to express Pax3. Mutation of Pax3, as occurs in the splotch (Sp2H) mouse, results in development of conotruncal heart defects including persistent truncus arteriosus. Homozygotes also exhibit defects of the aortic arches, thymus, thyroid and parathyroids. Pax3-positive neural crest cells were found to emigrate from the occipital neural tube of Sp2H/Sp2H embryos in a relatively normal fashion, but there was a marked deficiency or absence of neural crest cells traversing branchial arches 3, 4 and 6, and entering the cardiac outflow tract. This decreased expression of Pax3 in Sp2H/Sp2H embryos was not due to down-regulation of Pax3 in neural crest cells, as use of independent neural crest markers, Hoxa-3, CrabpI, Prx1, Prx2 and c-met also revealed a deficiency of migrating cardiac neural crest cells in homozygous embryos. This work demonstrates the essential role of the cardiac neural crest in formation of the heart and great vessels in the mouse and, furthermore, shows that Pax3 function is required for the cardiac neural crest to complete its migration to the developing heart.

Studies in cranial suture biology: in vitro cranial suture fusion

The biology underlying craniosynostosis remains unknown. Previous studies have shown that the underlying dura mater, not the suture itself, signals a suture to fuse. The purpose of this study was to develop an in vitro model for cranial-suture fusion that would still allow for suture-dura interaction, but without the influence of tensional forces transmitted from the cranial base. This was accomplished by demonstrating that the posterior frontal mouse cranial suture, known to be the only cranial suture that fuses in vivo, fuses when plated with its dura in an organ-culture system. In such an organ-culture system, the sutures are free from both the influence of dural forces transmitted from the cranial base and from hormonal influences only available in a perfused system. For the cranial-suture fusion in vitro model study, the sagittal sutures (controls that remain patent in vivo) and posterior frontal sutures (that fuse in vivo) with the underlying dura were excised from 24-day-old euthanized mice, cut into 5 x 4 x 2-mm specimens, and cultured in a chemically defined, serum-free media. One hundred sutures were harvested at the day of sacrifice, then every 2 days thereafter until 30 days in culture, stained with H & E, and analyzed. A subsequent cranial-suture without dura in vitro study was performed in a similar fashion to the first study, but only the calvariae with the posterior frontal or sagittal sutures (without the underlying dura) were cultured. Results from the cranial-suture fusion in vitro model study showed that all sagittal sutures placed in organ culture with the underlying dura remained patent. More importantly, the posterior frontal sutures with the underlying dura, which were plated-down as patent at 24 days of age, demonstrated fusion after various growth periods in organ culture. In vitro posterior frontal mouse-suture fusion occurred in an anterior-to-posterior direction but in a delayed fashion, 4 to 7 days later than in vivo posterior frontal mouse-suture fusion. In contrast, the subsequent cranial-suture without dura in vitro study showed patency of all sutures, including the posterior frontal suture. These data from in vitro experiments indicate that: (1) mouse calvariae, sutures, and the underlying dura survive and grow in organ-culture systems for 30 days; (2) the local dura, free from external influences transmitted from the cranial base and hormones from distant sites, influences the cells of its overlying suture to cause fusion; and (3) without dura influence, all in vitro cranial sutures remained patent. By first identifying the factors involved in dural-suture signaling and then regulating these factors and their receptors, the biologic basis of suture fusion and craniosynostosis may be unraveled and used in the future to manipulate pathologic (premature) suture fusion.

Precursors of macrophages in embryonic rat lungs fail to exhibit granulocyte-forming potential

BACKGROUND

Mesenchyme-like macrophage (M) precursors called angular cells are present in rat lungs on the thirteenth day of gestation and by then can differentiate into outright macrophages. Based on studies of bone marrow-derived cells, it is widely believed that the macrophage line necessarily proceeds from a colony-forming unit with dual granulocyte-macrophage potential (CFU-GM). In embryos this seems doubtful since macrophages are already scattered throughout the body before the first granulocytes appear. We examined the question in organ cultured 14 day prenatal rat lungs after having shown earlier that the macrophage population developed in explants is increased by exposure to M- and GM-colony-stimulating factors (CSFs) but is unaffected by multi (IL-3)- or granulocyte (G)-CSF. Reportedly retinoic acid (RA) shifts CFU-GM strongly towards granulocytic differentiation and inhibits mitosis of unipotential macrophage precursors but not differentiated cells. Transforming growth factor beta 1 (TGF) inhibits multipotential blood progenitors but allows proliferation of committed precursors, and TGF together with GM-CSF induces granulocytopoiesis from CFU-GM.

METHODS

Lung pairs were grown on a serum-containing medium or one supplemented either by RA, TGF, or TGF/GM-CSF to form a control and three experimental groups. A fourth experiment compared responses to M-CSF exposure and M-CSF/TGF. Macrophage population growth was estimated by measuring the areas of coronas formed by macrophages emerged from the explants. F-actin was stained with fluorescein-labeled phalloidin.

RESULTS

In all experiments macrophages were produced unmixed with granulocytes. By +8 days they had largely emerged to form coronas about the lungs. In cultures exposed to RA, macrophages were less intensely stained for actin and slower to emerge than controls. At +8 days, however, coronal areas were not significantly different from controls, as was also true for the TGF group. In contrast, coronal areas of cultures grown with TGF/GM-CSF were much larger. At +17 days, mean coronal area of TGF cultures was about half that of controls (P < 0.05), whereas mean coronal area of the TGF/GM-CSF group was 5.4 times greater (P < 0.001). Macrophages from control and TGF-exposed cultures responded to M-CSF by an increase in coronal area which was greater among cultures given M-CSF alone than those given TGF + M-CSF (both P < 0.005).

CONCLUSIONS

Macrophage precursors in embryonic lungs are distinct from CFU-GM.

Exogenous cytokines enhance survival of macrophages from organ cultured embryonic rat tissues

BACKGROUND

Macrophage precursors are present in embryonic rats shortly after the onset of hematopoiesis. During organogenesis they soon establish residency in many parts of the body and become convertible into phagocytes, at first gaining morphological characteristics of macrophages and later a range of surface antigens used to characterize subpopulations in adults. Nonetheless, it is uncertain whether representatives of this fetal lineage continue to exist past birth. We investigated the question indirectly by seeing if such cells can be made to survive in vitro to an age equivalent to adulthood and by examining underlying conditions that favor this outcome.

METHODS

Fourteen-day embryonic lungs, hearts, and limb buds were organ cultured on a firm serum-containing medium. Fetal macrophages developed within all explants and then migrated out to form a corona of cells surrounding each explant. The lung cultures were selected for subsequent work which mainly used coronal area as the measure of macrophage population size in experimental and control groups. Baseline growth and survival of macrophages were established for cultures grown on standard medium, then effects of the following were examined: indomethacin (10(-6) M) as it influences initial production of macrophages from precursors and later survival of differentiated cells; and macrophage colony-stimulating factor (M-CSF), used alone at moderate dosage (50-100 U), and combined with granulocyte-macrophage CSF (both 200 U), for its importance to long-term survival of the population. Mitogenic influence of M-CSF on differentiated macrophages was demonstrated by uptake of 5-bromo-2'-deoxyuridine.

RESULTS

Indomethacin inhibited the formation of macrophages from precursors but enhanced the survival of differentiated cells. M-CSF increased BrdU uptake of differentiated macrophages and permitted coronal growth to continue long past the approximately 30 day limit of controls. Beyond this interval, M-CSF was essential for macrophage survival, since coronas quickly shrank after the cytokine was withdrawn. Administration of the M-CSF/GM-CSF mixture to the 2 oldest M-CSF-exposed cultures between 98 and 127 days in vitro resulted in an increase in the number of coronal macrophages (P < 0.001); withdrawal between 129 and 140 days led to a decrease (P < 0.005). Ultimately a few cells were still surviving at 183 days.

CONCLUSIONS

Intrinsic factors promote early formation of macrophages within the explants, but the availability of factors is lessened by the anti-inflammatory action of indomethacin. Its later promotion of macrophage survival may be based on suppression of autogenous prostaglandin (PGE2) synthesis. M-CSF greatly promotes macrophage survival; in context this is sufficient to show that the fetal macrophage line has a clear potential to survive well into adulthood.