In this study we present data on the spatial relationship between neural crest-derived cells (NCC... more In this study we present data on the spatial relationship between neural crest-derived cells (NCC) and the specialized cardiac conduction system (CCS) in the developing murine heart. Using Wnt1-Cre/R26R conditional reporter mice that express β-galactosidase from ROSA26 upon Cre-mediated recombination, two populations of NCC are seen: one migrates through the arterial pole and contributes to the bundle branches, whereas the second population enters by way of the venous pole and provides cells to the sinoatrial and atrioventricular node areas. The CCS/lacZ construct is found in the myocardium of the early embryonic heart and afterward only persists in the definitive CCS and is acknowledged as a reporter for the developing conduction system. The contiguous expression of both reporters is suggestive for a potential role of cardiac NCC in the induction of the final differentiation of the CCS.
The morphogenetic process underlying the remodelling of the embryonic mammalian pharyngeal arch a... more The morphogenetic process underlying the remodelling of the embryonic mammalian pharyngeal arch artery system is unknown. Within this process, the right sixth, carotid ducts and the distal part of the dorsal aorta (right alpha-segment) regress. In order to unravel the underlying mechanism we studied the role of apoptosis in the normal regression of pharyngeal arch artery segments and in a mouse model that develops aortic arch malformations. Normal remodelling was studied in wild-type Swiss (CPBS) and altered remodelling in the Tgfbeta2-/- compared to the Tgfbeta2+/+ (Swiss/Bl6) strain using immunohistochemistry and morphometric analysis. During normal remodelling, apoptosis occurs in the mesenchyme surrounding pharyngeal arch arteries before regression starts. With the onset of regression, apoptosis spreads from the mesenchyme to the media. Morphometric evaluation confirms the increase in apoptosis in the actin-positive media of the disappearing segments. In Tgfbeta2-/-, aberrant apoptosis was found in both fourth arch arteries, whereas the right dorsal aorta lacks apoptosis associated with normal regression. Fourth arch hypoplasia is the main arch abnormality. In the most severe case, the fourth arch is interrupted and the right dorsal aorta alpha-segment persists, giving rise to aortic arch interruption type-B and an aberrant right subclavian artery. We have shown for the first time that specific vascular apoptosis patterns accompany normal regression and that the incidence of apoptosis is selectively altered in the case of arch artery abnormalities in Tgfbeta2 knock-out mice.
To examine the roles of TGFβ isoforms on corneal morphogenesis, the eyes of mice that lack TGFβs ... more To examine the roles of TGFβ isoforms on corneal morphogenesis, the eyes of mice that lack TGFβs were analyzed at different developmental stages for cell proliferation, migration and apoptosis, and for expression patterns of keratin 12, lumican, keratocan and collagen I. Among the three Tgfb−/− mice, only Tgfb2−/− mice have abnormal ocular morphogenesis characterized by thin corneal stroma, absence of corneal endothelium, fusion of cornea to lens (a Peters'-like anomaly phenotype), and accumulation of hyaline cells in vitreous. In Tgfb2−/− mice, fewer keratocytes were found in stroma that has a decreased accumulation of ECM; for example, lumican, keratocan and collagen I were greatly diminished. The absence of TGFβ2 did not compromise cell proliferation, nor enhance apoptosis. The thinner stroma resulting from decreased ECM synthesis may account for the decreased cell number in the stroma of Tgfb2 null mice. Keratin 12 expression was not altered in Tgfb2−/− mice, implicating normal corneal type epithelial differentiation. Delayed appearance of macrophages in ocular tissues was observed in Tgfb2−/− mice. Malfunctioning macrophages may account for accumulation of cell mass in vitreous of Tgfb2 null mice.
Molecular processes underlying the remodeling of the symmetric system and the already muscularize... more Molecular processes underlying the remodeling of the symmetric system and the already muscularized pharyngeal arch arteries into an asymmetric aortic arch are slowly becoming unraveled. In normal arch remodeling, selective apoptosis of part of the right dorsal aorta and sixth pharyngeal arch is seen, whereas in the common arch malformations in Tgfbeta2 knockout mice, comprising type B interruption and an aberrant right subclavian artery, selective upregulation of apoptosis is additionally found in the left and/or right fourth artery segment. All pharyngeal arch arteries derive transforming growth factor beta2-expressing smooth muscle cells from the neural crest. The marked high vulnerability of specifically the fourth arch arteries can be linked to a localized reduced vascular SMAD2 signaling and related expression of fibronectin and neural cell adhesion molecule, providing a link between disturbed arteriogenesis and innervation. The marked correlation between intracardiac and aortic arch malformations in Tgfbeta2 mutants can also be understood as a combination of neural crest and flow-related mechanisms. A relation between apoptosis and flow is postulated, in which a role for genes with a shear stress-responsive element, including the endothelially expressed transforming growth factor beta1, is implied.
In this study we present data on the spatial relationship between neural crest-derived cells (NCC... more In this study we present data on the spatial relationship between neural crest-derived cells (NCC) and the specialized cardiac conduction system (CCS) in the developing murine heart. Using Wnt1-Cre/R26R conditional reporter mice that express β-galactosidase from ROSA26 upon Cre-mediated recombination, two populations of NCC are seen: one migrates through the arterial pole and contributes to the bundle branches, whereas the second population enters by way of the venous pole and provides cells to the sinoatrial and atrioventricular node areas. The CCS/lacZ construct is found in the myocardium of the early embryonic heart and afterward only persists in the definitive CCS and is acknowledged as a reporter for the developing conduction system. The contiguous expression of both reporters is suggestive for a potential role of cardiac NCC in the induction of the final differentiation of the CCS.
The morphogenetic process underlying the remodelling of the embryonic mammalian pharyngeal arch a... more The morphogenetic process underlying the remodelling of the embryonic mammalian pharyngeal arch artery system is unknown. Within this process, the right sixth, carotid ducts and the distal part of the dorsal aorta (right alpha-segment) regress. In order to unravel the underlying mechanism we studied the role of apoptosis in the normal regression of pharyngeal arch artery segments and in a mouse model that develops aortic arch malformations. Normal remodelling was studied in wild-type Swiss (CPBS) and altered remodelling in the Tgfbeta2-/- compared to the Tgfbeta2+/+ (Swiss/Bl6) strain using immunohistochemistry and morphometric analysis. During normal remodelling, apoptosis occurs in the mesenchyme surrounding pharyngeal arch arteries before regression starts. With the onset of regression, apoptosis spreads from the mesenchyme to the media. Morphometric evaluation confirms the increase in apoptosis in the actin-positive media of the disappearing segments. In Tgfbeta2-/-, aberrant apoptosis was found in both fourth arch arteries, whereas the right dorsal aorta lacks apoptosis associated with normal regression. Fourth arch hypoplasia is the main arch abnormality. In the most severe case, the fourth arch is interrupted and the right dorsal aorta alpha-segment persists, giving rise to aortic arch interruption type-B and an aberrant right subclavian artery. We have shown for the first time that specific vascular apoptosis patterns accompany normal regression and that the incidence of apoptosis is selectively altered in the case of arch artery abnormalities in Tgfbeta2 knock-out mice.
To examine the roles of TGFβ isoforms on corneal morphogenesis, the eyes of mice that lack TGFβs ... more To examine the roles of TGFβ isoforms on corneal morphogenesis, the eyes of mice that lack TGFβs were analyzed at different developmental stages for cell proliferation, migration and apoptosis, and for expression patterns of keratin 12, lumican, keratocan and collagen I. Among the three Tgfb−/− mice, only Tgfb2−/− mice have abnormal ocular morphogenesis characterized by thin corneal stroma, absence of corneal endothelium, fusion of cornea to lens (a Peters'-like anomaly phenotype), and accumulation of hyaline cells in vitreous. In Tgfb2−/− mice, fewer keratocytes were found in stroma that has a decreased accumulation of ECM; for example, lumican, keratocan and collagen I were greatly diminished. The absence of TGFβ2 did not compromise cell proliferation, nor enhance apoptosis. The thinner stroma resulting from decreased ECM synthesis may account for the decreased cell number in the stroma of Tgfb2 null mice. Keratin 12 expression was not altered in Tgfb2−/− mice, implicating normal corneal type epithelial differentiation. Delayed appearance of macrophages in ocular tissues was observed in Tgfb2−/− mice. Malfunctioning macrophages may account for accumulation of cell mass in vitreous of Tgfb2 null mice.
Molecular processes underlying the remodeling of the symmetric system and the already muscularize... more Molecular processes underlying the remodeling of the symmetric system and the already muscularized pharyngeal arch arteries into an asymmetric aortic arch are slowly becoming unraveled. In normal arch remodeling, selective apoptosis of part of the right dorsal aorta and sixth pharyngeal arch is seen, whereas in the common arch malformations in Tgfbeta2 knockout mice, comprising type B interruption and an aberrant right subclavian artery, selective upregulation of apoptosis is additionally found in the left and/or right fourth artery segment. All pharyngeal arch arteries derive transforming growth factor beta2-expressing smooth muscle cells from the neural crest. The marked high vulnerability of specifically the fourth arch arteries can be linked to a localized reduced vascular SMAD2 signaling and related expression of fibronectin and neural cell adhesion molecule, providing a link between disturbed arteriogenesis and innervation. The marked correlation between intracardiac and aortic arch malformations in Tgfbeta2 mutants can also be understood as a combination of neural crest and flow-related mechanisms. A relation between apoptosis and flow is postulated, in which a role for genes with a shear stress-responsive element, including the endothelially expressed transforming growth factor beta1, is implied.
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Papers by Mohamad Azhar