Transitions in cell organization and in expression of contractile and extracellular matrix proteins during development of chicken aortic smooth muscle: evidence for a complex spatial and temporal differentiation program

Postnatal development alters functional compartmentalization of myosin light chain kinase in ovine carotid arteries

The rate-limiting enzyme for vascular contraction, myosin light chain kinase (MLCK), phosphorylates regulatory myosin light chain (MLC20) at rates that appear faster despite lower MLCK abundance in fetal compared with adult arteries. This study explores the hypothesis that greater apparent tissue activity of MLCK in fetal arteries is due to age-dependent differences in intracellular distribution of MLCK in relation to MLC20. Under optimal conditions, common carotid artery homogenates from nonpregnant adult female sheep and near-term fetuses exhibited similar values of Vmax and Km for MLCK. A custom-designed, computer-controlled apparatus enabled electrical stimulation and high-speed freezing of arterial segments at exactly 0, 1, 2, and 3 s, calculation of in situ rates of MLC20 phosphorylation, and measurement of time-dependent colocalization between MLCK and MLC20. The in situ rate of MLC20 phosphorylation divided by total MLCK abundance averaged to values 147% greater in fetal (1.06 ± 0.28) than adult (0.43 ± 0.08) arteries, which corresponded, respectively, to 43 ± 10% and 31 ± 3% of the Vmax values measured in homogenates. Confocal colocalization analysis revealed in fetal and adult arteries that 33 ± 6% and 20 ± 5% of total MLCK colocalized with pMLC20, and that MLCK activation was greater in periluminal than periadventitial regions over the time course of electrical stimulation in both age groups. Together, these results demonstrate that the catalytic activity of MLCK is similar in fetal and adult arteries, but that the fraction of total MLCK in the functional compartment involved in contraction is significantly greater in fetal than adult arteries.

Pericyte Ontogeny: The Use of Chimeras to Track a Cell Lineage of Diverse Germ Line Origins

The goal of lineage tracing is to understand body formation over time by discovering which cells are the progeny of a specific, identified, ancestral progenitor. Subsidiary questions include unequivocal identification of what they have become, how many descendants develop, whether they live or die, and where they are located in the tissue or body at the end of the window examined. A classical approach in experimental embryology, lineage tracing continues to be used in developmental biology and stem cell and cancer research, wherever cellular potential and behavior need to be studied in multiple dimensions, of which one is time. Each technical approach has its advantages and drawbacks. This chapter, with some previously unpublished data, will concentrate nonexclusively on the use of interspecies chimeras to explore the origins of perivascular (or mural) cells, of which those adjacent to the vascular endothelium are termed pericytes for this purpose. These studies laid the groundwork for our understanding that pericytes derive from progenitor mesenchymal pools of multiple origins in the vertebrate embryo, some of which persist into adulthood. The results obtained through xenografting, like in the methodology described here, complement those obtained through genetic lineage-tracing techniques within a given species.

Hypoxic modulation of fetal vascular MLCK abundance, localization, and function

Changes in vascular contractility are among the most important physiological effects of acute and chronic fetal hypoxia. Given the essential role of myosin light-chain kinase (MLCK) in smooth muscle contractility and its heterogeneous distribution, this study explores the hypothesis that subcellular changes in MLCK distribution contribute to hypoxic modulation of fetal carotid artery contractility. Relative to common carotid arteries from normoxic term fetal lambs (FN), carotids from fetal lambs gestated at high altitude (3,802 m) (FH) exhibited depressed contractility without changes in MLCK mRNA or protein abundance. Patterns of confocal colocalization of MLCK with α-actin and 20-kDa regulatory myosin light chain (MLC20) enabled calculation of subcellular MLCK fractions: 1) colocalized with the contractile apparatus, 2) colocalized with α-actin distant from the contractile apparatus, and 3) not colocalized with α-actin. Chronic hypoxia did not affect MLCK abundance in the contractile fraction, despite a concurrent decrease in contractility. Organ culture for 72 h under 1% O2 decreased total MLCK abundance in FN and FH carotid arteries, but decreased the contractile MLCK abundance only in FH carotid arteries. Correspondingly, culture under 1% O2 depressed contractility more in FH than FN carotid arteries. In addition, hypoxia appeared to attenuate ubiquitin-independent proteasomal degradation of MLCK, as reported for other proteins. In aggregate, these results demonstrate that the combination of chronic hypoxia followed by hypoxic culture can induce MLCK translocation among at least three subcellular fractions with possible influences on contractility, indicating that changes in MLCK distribution are a significant component of fetal vascular responses to hypoxia.

An update on clonality: what smooth muscle cell type makes up the atherosclerotic plaque?

Almost 50 years ago, Earl Benditt and his son John described the clonality of the atherosclerotic plaque. This led Benditt to propose that the atherosclerotic lesion was a smooth muscle neoplasm, similar to the leiomyomata seen in the uterus of most women. Although the observation of clonality has been confirmed many times, interest in the idea that atherosclerosis might be a form of neoplasia waned because of the clinical success of treatments for hyperlipemia and because animal models have made great progress in understanding how lipid accumulates in the plaque and may lead to plaque rupture. Four advances have made it important to reconsider Benditt's observations. First, we now know that clonality is a property of normal tissue development. Second, this is even true in the vessel wall, where we now know that formation of clonal patches in that wall is part of the development of smooth muscle cells that make up the tunica media of arteries. Third, we know that the intima, the "soil" for development of the human atherosclerotic lesion, develops before the fatty lesions appear. Fourth, while the cells comprising this intima have been called "smooth muscle cells", we do not have a clear definition of cell type nor do we know if the initial accumulation is clonal. As a result, Benditt's hypothesis needs to be revisited in terms of changes in how we define smooth muscle cells and the quite distinct developmental origins of the cells that comprise the muscular coats of all arterial walls. Finally, since clonality of the lesions is real, the obvious questions are do these human tumors precede the development of atherosclerosis, how do the clones develop, what cell type gives rise to the clones, and in what ways do the clones provide the soil for development and natural history of atherosclerosis?

Isolation, Culture, and Immunostaining of Skeletal Muscle Myofibers from Wildtype and Nestin-GFP Mice as a Means to Analyze Satellite Cell

Multinucleated myofibers, the functional contractile units of adult skeletal muscle, harbor mononuclear Pax7+ myogenic progenitors on their surface between the myofiber basal lamina and plasmalemma. These progenitors, known as satellite cells, are the primary myogenic stem cells in adult muscle. This chapter describes our laboratory protocols for isolating, culturing, and immunostaining intact myofibers from mouse skeletal muscle as a means for studying satellite cell dynamics. The first protocol discusses myofiber isolation from the flexor digitorum brevis (FDB) muscle. These short myofibers are plated in dishes coated with PureCol collagen (formerly known as Vitrogen) and maintained in a mitogen-poor medium (± supplemental growth factors). Employing such conditions, satellite cells remain at the surface of the parent myofiber while synchronously undergoing a limited number of proliferative cycles and rapidly differentiate. The second protocol discusses the isolation of longer myofibers from the extensor digitorum longus (EDL) muscle. These EDL myofibers are routinely plated individually as adherent myofibers in wells coated with Matrigel and maintained in a mitogen-rich medium, conditions in which satellite cells migrate away from the parent myofiber, proliferate extensively, and generate numerous differentiating progeny. Alternatively, these EDL myofibers can be plated as non-adherent myofibers in uncoated wells and maintained in a mitogen-poor medium (± supplemental growth factors), conditions that retain satellite cell progeny at the myofiber niche similar to the FDB myofiber cultures. However, the adherent myofiber format is our preferred choice for monitoring satellite cells in freshly isolated (Time 0) myofibers. We conclude this chapter by promoting the Nestin-GFP transgenic mouse as an efficient tool for direct analysis of satellite cells in isolated myofibers. While satellite cells have been often detected by their expression of the Pax7 protein or the Myf5nLacZ knockin reporter (approaches that are also detailed herein), the Nestin-GFP reporter distinctively permits quantification of satellite cells in live myofibers, which enables linking initial Time 0 numbers and subsequent performance upon culturing. We additionally point out to the implementation of the Nestin-GFP transgene for monitoring other selective cell lineages as illustrated by GFP expression in capillaries, endothelial tubes and neuronal cells. Myofibers from other types of muscles, such as diaphragm, masseter, and extraocular, can also be isolated and analyzed using protocols described herein. Collectively, this chapter provides essential tools for studying satellite cells in their native position and their interplay with the parent myofiber.

Isolation of Mouse Periocular Tissue for Histological and Immunostaining Analyses of the Extraocular Muscles and Their Satellite Cells

The extraocular muscles (EOMs) comprise a group of highly specialized skeletal muscles controlling eye movements. Although a number of unique features of EOMs including their sparing in Duchenne muscular dystrophy have drawn a continuous interest, knowledge about these hard to reach muscles is still limited. The goal of this chapter is to provide detailed methods for the isolation and histological analysis of mouse EOMs. We first introduce in brief the basic anatomy and established nomenclature of the extraocular primary and accessory muscles. We then provide a detailed description with step-by-step images of our procedure for isolating (and subsequently cryosectioning) EOMs while preserving the integrity of their original structural organization. Next, we present several useful histological protocols frequently used by us, including: (1) a method for highlighting the general organization of periocular tissue, using the MyoD(Cre) × R26(mTmG) reporter mouse that elegantly distinguishes muscle (MyoD(Cre)-driven GFP(+)) from the non-myogenic constituents (Tomato(+)); (2) analysis by H&E staining, allowing for example, detection of the pathological features of the dystrophin-null phenotype in affected limb and diaphragm muscles that are absent in EOMs; (3) detection of the myogenic progenitors (i.e., satellite cells) in their native position underneath the myofiber basal lamina using Pax7/laminin double immunostaining. The EOM tissue harvesting procedure described here can also be adapted for isolating and studying satellite cells and other cell types. Overall, the methods described in this chapter should provide investigators the necessary tools for entering the EOM research field and contribute to a better understanding of this highly specialized muscle group and its complex micro-anatomy.

VEGF receptors mediate hypoxic remodeling of adult ovine carotid arteries

Recent studies suggest that VEGF contributes to hypoxic remodeling of arterial smooth muscle, although hypoxia produces only transient increases in VEGF that return to normoxic levels despite sustained changes in arterial structure and function. To explore how VEGF might contribute to long-term hypoxic vascular remodeling, this study explores the hypothesis that chronic hypoxia produces sustained increases in smooth muscle VEGF receptor density that mediate long-term vascular effects of hypoxia. Carotid arteries from adult sheep maintained at sea level or altitude (3,820 m) for 110 days were harvested and denuded of endothelium. VEGF levels were similar in chronically hypoxic and normoxic arteries, as determined by immunoblotting. In contrast, VEGF receptor levels were significantly increased by 107% (VEGF-R1) and 156% (VEGF-R2) in hypoxic compared with normoxic arteries. In arteries that were organ cultured 24 h with 3 nM VEGF, VEGF replicated effects of hypoxia on abundances of smooth muscle α actin (SMαA), myosin light chain kinase (MLCK), and MLC20 and the effects of hypoxia on colocalization of MLC20 with SMαA, as measured via confocal microscopy. VEGF did not replicate the effects of chronic hypoxia on colocalization of MLCK with SMαA or MLCK with MLC20, suggesting that VEGF's role in hypoxic remodeling is highly protein specific, particularly for contractile protein organization. VEGF effects in organ culture were inhibited by VEGF receptor blockers vatalinib (240 nM) and dasatinib (6.3 nM). These findings support the hypothesis that long-term upregulation of VEGF receptors help mediate sustained effects of hypoxia on the abundance and colocalization of contractile proteins in arterial smooth muscle.

Contribution of increased VEGF receptors to hypoxic changes in fetal ovine carotid artery contractile proteins

Recent studies suggest that vascular endothelial growth factor (VEGF) can modulate smooth muscle phenotype and, consequently, the composition and function of arteries upstream from the microcirculation, where angiogenesis occurs. Given that hypoxia potently induces VEGF, the present study explores the hypothesis that, in fetal arteries, VEGF contributes to hypoxic vascular remodeling through changes in abundance, organization, and function of contractile proteins. Pregnant ewes were acclimatized at sea level or at altitude (3,820 m) for the final 110 days of gestation. Endothelium-denuded carotid arteries from full-term fetuses were used fresh or after 24 h of organ culture in a physiological concentration (3 ng/ml) of VEGF. After 110 days, hypoxia had no effect on VEGF abundance but markedly increased abundance of the Flk-1 (171%) and Flt-1 (786%) VEGF receptors. Hypoxia had no effect on smooth muscle α-actin (SMαA), decreased myosin light chain (MLC) kinase (MLCK), and increased 20-kDa regulatory MLC (MLC(20)) abundances. Hypoxia also increased MLCK-SMαA, MLC(20)-SMαA, and MLCK-MLC(20) colocalization. Compared with hypoxia, organ culture with VEGF produced the same pattern of changes in contractile protein abundance and colocalization. Effects of VEGF on colocalization were blocked by the VEGF receptor antagonists vatalanib (240 nM) and dasatinib (6.3 nM). Thus, through increases in VEGF receptor density, hypoxia can recruit VEGF to help mediate remodeling of fetal arteries upstream from the microcirculation. The results support the hypothesis that VEGF contributes to hypoxic vascular remodeling through changes in abundance, organization, and function of contractile proteins.

Chronic hypoxia and VEGF differentially modulate abundance and organization of myosin heavy chain isoforms in fetal and adult ovine arteries

Chronic hypoxia increases vascular endothelial growth factor (VEGF) and thereby promotes angiogenesis. The present study explores the hypothesis that hypoxic increases in VEGF also remodel artery wall structure and contractility through phenotypic transformation of smooth muscle. Pregnant and nonpregnant ewes were maintained at sea level (normoxia) or 3,820 m (hypoxia) for the final 110 days of gestation. Common carotid arteries harvested from term fetal lambs and nonpregnant adults were denuded of endothelium and studied in vitro. Stretch-dependent contractile stresses were 32 and 77% of normoxic values in hypoxic fetal and adult arteries. Hypoxic hypocontractility was coupled with increased abundance of nonmuscle myosin heavy chain (NM-MHC) in fetal (+37%) and adult (+119%) arteries. Conversely, hypoxia decreased smooth muscle MHC (SM-MHC) abundance by 40% in fetal arteries but increased it 123% in adult arteries. Hypoxia decreased colocalization of NM-MHC with smooth muscle α-actin (SM-αA) in fetal arteries and decreased colocalization of SM-MHC with SM-αA in adult arteries. Organ culture with physiological concentrations (3 ng/ml) of VEGF-A(165) similarly depressed stretch-dependent stresses to 37 and 49% of control fetal and adult values. The VEGF receptor antagonist vatalanib ablated VEGF's effects in adult but not fetal arteries, suggesting age-dependent VEGF receptor signaling. VEGF replicated hypoxic decreases in colocalization of NM-MHC with SM-αA in fetal arteries and decreases in colocalization of SM-MHC with SM-αA in adult arteries. These results suggest that hypoxic increases in VEGF not only promote angiogenesis but may also help mediate hypoxic arterial remodeling through age-dependent changes in smooth muscle phenotype and contractility.

Nestin-GFP reporter expression defines the quiescent state of skeletal muscle satellite cells

Repair of adult skeletal muscle depends on satellite cells, quiescent myogenic stem cells located beneath the myofiber basal lamina. Satellite cell numbers and performance decline with age and disease, yet the intrinsic molecular changes accompanying these conditions are unknown. We identified expression of GFP driven by regulatory elements of the nestin (NES) gene within mouse satellite cells, which permitted characterization of these cells in their niche. Sorted NES-GFP+ cells exclusively acquired a myogenic fate, even when supplemented with media supporting non-myogenic development. Mutual and unique gene expression by NES-GFP+ cells from hindlimb and diaphragm muscles demonstrated intra- and inter-muscular heterogeneity of satellite cells. NES-GFP expression declined following satellite cell activation and was reacquired in late stage myogenic cultures by non-proliferating Pax7+ progeny. The dynamics of this expression pattern reflect the cycle of satellite cell self-renewal. The NES-GFP model reveals unique transcriptional activity within quiescent satellite cells and permits novel insight into the heterogeneity of their molecular signatures.

Secondary heart field contributes myocardium and smooth muscle to the arterial pole of the developing heart

The arterial pole of the heart is the region where the ventricular myocardium continues as the vascular smooth muscle tunics of the aorta and pulmonary trunk. It has been shown that the arterial pole myocardium derives from the secondary heart field and the smooth muscle tunic of the aorta and pulmonary trunk derives from neural crest. However, this neural crest-derived smooth muscle does not extend to the arterial pole myocardium leaving a region at the base of the aorta and pulmonary trunk that is invested by vascular smooth muscle of unknown origin. Using tissue marking and vascular smooth muscle markers, we show that the secondary heart field, in addition to providing myocardium to the cardiac outflow tract, also generates prospective smooth muscle that forms the proximal walls of the aorta and pulmonary trunk. As a result, there are two seams in the arterial pole: first, the myocardial junction with secondary heart field-derived smooth muscle; second, the secondary heart field-derived smooth muscle with the neural crest-derived smooth muscle. Both of these seams are points where aortic dissection frequently occurs in Marfan's and other syndromes.

An ex vivo model for functional studies of myofibroblasts

Migration, proliferation and invasive growth of myofibroblasts are key cellular events during formation of granulation tissue in situations of wound healing, arteriosclerosis and tumor growth. To study the invasive phenotype of myofibroblasts, we established an assay where arterial tissue from chicken embryos was embedded in fibrin gels and stimulated with growth factors. Addition of serum, PDGF-BB and FGF-2, but not VEGF-A, resulted in an outgrowth of cellular sprouts with a pattern that was similar to the organization of cells invading a provisional matrix in an in vivo model of wound healing using the chicken chorioallantoic membrane. Sprouting cells were defined as myofibroblasts based on being alpha-smooth muscle actin-positive but desmin-negative. There was no contribution of endothelial cells in outgrowing sprouts. The acquired myofibroblastic phenotype was stable since sprout-derived cells resumed sprouting in a growth factor-independent manner when re-embedded as spheroids in a fibrin matrix. Invasive growth and sprouting of vascular smooth muscle cells was not limited to chicken cells since a similar response was seen when spheroids composed of purified primary human aortic smooth muscle cells were embedded in fibrin. Finally, a technique for flat visualization of the three-dimensional sprouting and a quantification method is described. This ex vivo model allows quantitative analysis of invasive growth and differentiation of vascular smooth muscle cells and fibroblasts into myofibroblasts.

Pattern of Pax7 expression during myogenesis in the posthatch chicken establishes a model for satellite cell differentiation and renewal

The paired-box transcription factor Pax7 plays a critical role in the specification of satellite cells in mouse skeletal muscle. In the present study, the position and number of Pax7-expressing cells found in muscles of growing and adult chickens confirm the presence of this protein in avian satellite cells. The expression pattern of Pax7 protein, along with the muscle regulatory proteins MyoD and myogenin, was additionally elucidated in myogenic cultures and in whole muscle from posthatch chickens. In cultures progressing from proliferation to differentiation, the expression of Pax7 in MyoD+ cells declined as the cells began expressing myogenin, suggesting Pax7 as an early marker for proliferating myoblasts. At all time points, some Pax7+ cells were negative for MyoD, resembling the reserve cell phenotype. Clonal analysis of muscle cell preparations demonstrated that single progenitors can give rise to both differentiating and reserve cells. In muscle tissues, Pax7 protein expression was the strongest by 1 day posthatch, declining on days 3 and 6 to a similar level. In contrast, myogenin expression peaked on day 3 and then dramatically declined. This finding was accompanied by a robust growth in fiber diameter between day 3 and 6. The distinctions in Pax7 and myogenin expression patterns, both in culture and in vivo, indicate that while some of the myoblasts differentiate and fuse into myofibers during early stages of posthatch growth, others retain their reserve cell capacity.

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.

Formation of cartilage in aortic valves of Syrian hamsters

The formation of cartilage in aortic valves of Syrian hamsters was studied using histological, histochemical and immunohistochemical techniques. The sample consisted of 281 specimens aged 0-363 days, all of which had a normal (tricuspid) aortic valve. The first sign of valvular chondrogenesis is the presence of small groups of cells embedded in a type II collagen-positive matrix. These groups of cells, which can appear as early as one day after birth, increase in size and differentiate into hyaline cartilage or fibrocartilage. From the fourth day of life, all hamsters examined displayed cartilaginous foci in the aortic valve. They were located along the fibrous attachments of the valve leaflets to their respective sinuses, including the valve commissures. A considerable proportion (76%) of cartilages formed within the first 40 days of life, that is during the period of time in which the histogenesis of the valve takes place. The present observations are consistent with the assumption that in mammals, the precursors of the aortic valve chondrocytes are neural crest-derived cells. Results of a statistical analysis substantiate that the incidence is significantly higher in (1) the territory that comprises the collagenous condensation of the ventral commissure and the ventro-lateral and proximal fibrous attachments of the right leaflet to its sinus, and (2) the proximal fibrous attachment of dorsal leaflet to its sinus. These findings together with data in the literature concerning the distribution of stress in each leaflet-sinus assembly of the valve during the cardiac cycle, suggest that mechanical action might play an inductive role in the formation of the cartilaginous tissue in the aortic valve of mammals. In addition, they point to the possibility that locally intense mechanical stimulation is responsible for the differentiation of the anticipated cartilaginous tissue into hyaline cartilage.

Inactivation of Src family kinases inhibits angiogenesis in vivo: implications for a mechanism involving organization of the actin cytoskeleton

Inhibition of angiogenesis could be a treatment strategy for diseases such as cancer, rheumatoid arthritis, and diabetic retinopathy. PP2 is a pharmacological inhibitor of Src family kinases and was found to inhibit FGF-2 induced angiogenesis in vivo. Experiments in vitro showed that PP2 inhibited invasive growth and sprouting of both endothelial and vascular smooth muscle cells into a fibrin matrix. PP2 inhibited the formation of lamellopodia and expression of kinase inactive c-Src reduced phosphorylation of cortactin and paxillin, suggesting a model in which Src kinases are involved in organization of the actin cytoskeleton. Consequently, endothelial cells expressing kinase inactive c-Src failed to spread and form cord-like structures on a collagen matrix. These data suggest that pharmacological inactivation of Src family kinases inhibits FGF-2 stimulated angiogenesis by interference with organization of the actin cytoskeleton in both endothelial and vascular smooth muscle cells, which affects cell migration.

Formation of cartilage in the heart of the Spanish terrapin, Mauremys leprosa (Reptilia, Chelonia)

Cartilaginous deposits are regularly present in the heart of several reptilian, avian, and mammalian species. The formation of these extraskeletal cartilages has been studied in birds and mammals, but not in reptiles. The aim here was to elucidate this question in the Spanish terrapin. Hearts from 23 embryos belonging to Yntema (1968) developmental stages 17 to 26 and eight terrapins age 3 months to 10 years were examined using histological, histochemical, and immunohistochemical techniques. In the heart of the Spanish terrapin (Mauremys leprosa), chondrogenesis can start during embryonic life. Cartilaginous tissue develops from a mesenchymal cellular condensation that extends along the aorticopulmonary septum and the incipient pars fibrosa of the ventricular horizontal septum. This cellular condensation, which is smooth muscle alpha-actin (SMalpha-actin)-negative and type II collagen-negative during stages 17 to 22, acts as a prechondrogenic condensation. In stage 23, production of type II collagen begins in the central core of the condensation and gradually spreads toward its periphery. The type II collagen-positive (chondrogenic) cellular condensation remains devoid of perichondrium prior to birth. Thereafter, it converts into hyaline cartilage that extends along the proximal part of the aorticopulmonary septum and the pars fibrosa of the horizontal septum. Our findings are consistent with the assumption that, as in birds and mammals, the precursors of the cardiac chondrocytes in chelonians are neural crest-derived cells of nonmuscular nature. In addition, they point to the possibility that cells from the neural crest populate the embryonic pars fibrosa of the horizontal septum, thereby contributing to its alignment with the aorticopulmonary septum. In the present species, a second cartilaginous deposit of a hyaline nature extends along the sinus wall of the right semilunar valve of the right aorta, penetrating the fibrous cushion that constitutes the proximal support of the corresponding valve leaflet. This cartilage develops after birth, between the third and eighteenth month of life; its morphogenetic origin is unclear. The cartilaginous foci occurring in hearts of Spanish terrapin appear to act as pivots resisting mechanical tensions generated during the cardiac cycle. In the specimens examined there was no sign of replacement of the cardiac cartilages by bone tissue.

Melatonin potentiates phenylephrine-stimulated intracellular Ca2+ transient in smooth muscle cell of large arteries of chick embryo

In the current study we first characterized the properties of the 2-[125I]iodomelatonin binding in pulmonary artery and aorta of chick embryo, and then determined the location of the binding site with autoradiography. Receptor binding assays were used using 2-[125I]iodomelatonin as ligand. The binding was stable, saturable, specific and reversible. Scatchard anlaysis revealed an equilibrium binding constant (Kd) of 27.12 +/- 1.34 pM and a maximum binding capacity (Bmax) of 1.93 +/- 0.19 fmol/mg protein (n = 5). Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) increased the Kd, but reduced the Bmax, indicating the binding being coupled to a G-protein. Autoradiography revealed the binding sites in the smooth muscle layer. To delineate the physiological function of melatonin in the large artery of the chick embryo, we determined the intracellular calcium ([Ca2+]i) in smooth muscle cells of the aorta with spectrofluorometry, using fura 2-AM as calcium indicator. Melatonin at 10(-8) to 10(-5), which itself had no effect, potentiated the stimulating effect of 0.1 microM phenylephrine, a selective agonist of alpha -adrenoceptor, on [Ca2+]i. In conclusion, specific binding of 2-[125I]iodomelatonin and physiological response to melatonin suggest the existence of melatonin receptor in the smooth muscle of large arteries of the chick embryo. Melatonin potentiates the effects of alpha1-adrenergic stimulation.

Maturation-dependent neointima formation in fowl aorta

Fowl show spontaneous elevation of blood pressure (BP) and neointimal plaque formation in the abdominal aorta at young ages. Maturation/age-dependent modulation of vascular lesions and a causal relationship between elevated BP and neointima formation, however, have not been clarified. We therefore intended to characterize, first, maturation/age-dependent neointimal plaque formation and vascular lesions and, second, their relationship to BP elevation. The BP measured in conscious domestic fowl, Gallus gallus, White Leghorn breed, DeKalb strain, via an indwelling catheter inserted into the ischiadic artery, increased with maturation in males; and at plateau level, BP (mmHg) was significantly (P<0.01) higher in males (194.0+/-4.6, n=11) than in females (169.3+/-3.1, n=10). Neointimal plaques consisting of neointimal cells and abundant extracellular matrix appeared initially in the distal segment of the abdominal aorta (lesion-prone area) of chicks as early as 6 weeks old. The area (size) of neointimal plaques right above the ischiadic bifurcation increased with maturation, whereas the plaque area became smaller with some degenerative changes in adult birds. In some birds, diffuse subendothelial hyperplasia and more extensive plaque formation at the branching points of the aorta were observed. The plaque area appears to be larger in birds, particularly in males that have higher BP (r=0.68). The width of aortic smooth muscle (SM) layers, measured in regions with no plaque, increased with age, whereas the number of cells per unit of area decreased, suggesting that hypertrophy of vascular SM occurs in response to exposure of the vascular wall to high BP. The number of cells was significantly (P<0.01) higher in the plaque than in underlying aortic SM layers or in layers with no plaque formation. Both neointimal plaques and underlying SM layers are immunohistochemically positive for alpha SM actin, suggesting that neointimal cells are modulated SM cells, whereas the staining with SM myosin heavy chain antibody is low in neointimal plaques. Furthermore, plasma arginine levels dropped in accordance with the time of neointimal plaque formation, whereas plasma cholesterol levels showed an age-dependent increase. The results suggest that spontaneous development of neointimal plaques may be a consequence of exposure to high BP and associated local hemodynamic changes.

Cartilage in pulmonary valves of Syrian hamsters

The presence of cartilage in the pulmonary valve has been reported in birds, but not in mammals. We describe here the occurrence of cartilaginous tissue in the pulmonary valves of 40 (11.4%) of 351 Syrian hamsters examined using histological, histochemical and/or immunohistochemical techniques. The cartilaginous deposits were located along the fibrous attachments of the valve leaflets to the wall of the pulmonary artery trunk. Our findings indicate that the proximal attachments of the leaflets to their respective sinuses, and especially that of the ventral leaflet, are the most prone valvular regions to develop cartilaginous foci. Nonetheless, the possible function of these foci remains an open question. Formation of cartilage in the pulmonary valve starts within the first month of life, that is during the period in which the valve reaches histological maturation. The earliest evidence of chondrogenesis is the presence of small groups of cells embedded in a type II collagen-positive extracellular matrix. These groups of cells, which can appear as early as one day after birth, increase moderately in size and differentiate into hyaline cartilaginous tissue. The precursors of the cartilaginous cells are presumed to be neural crest-derived elements. However, the factor or factors involved in the differentiation of these precursors into chondrocytes are still unknown. In this regard, our observations cast doubt on the hypothesis that the formation of cardiac cartilages is primarily due to locally intense mechanical stimulation.

Physiology of angiogenesis

Angiogenesis is a key prerequisite for growth in all vertebrate embryos and in many tumors. Rapid growth requires efficient transport of oxygen and metabolites. Hence, for a better understanding of tissue growth, biophysical properties of vascular systems, in addition to their molecular mechanisms, need to be investigated. The purpose of this article is twofold: (1) to discuss the biophysics of growing and perfused vascular systems in general, emphasizing non-sprouting angiogenesis and remodeling of vascular plexuses; and (2) to report on cellular details of sprouting angiogenesis in the initially non-perfused embryonic brain and spinal cord. It is concluded that (1) evolutionary optimization of the circulatory system corresponds to highly conserved vascular patterns and angiogenetic mechanisms; (2) deterministic and random processes contribute to both extraembryonic and central nervous system vascularization; (3) endothelial cells interact with a variety of periendothelial cells during angiogenesis and remodeling; and that (4) mathematical models integrating molecular, morphological and biophysical expertise improve our understanding of normal and pathological angiogenesis and account for allometric relations.

Vascular smooth muscle cells spontaneously adopt a skeletal muscle phenotype: a unique Myf5(-)/MyoD(+) myogenic program

Smooth and skeletal muscle tissues are composed of distinct cell types that express related but distinct isoforms of the structural genes used for contraction. These two muscle cell types are also believed to have distinct embryological origins. Nevertheless, the phenomenon of a phenotypic switch from smooth to skeletal muscle has been demonstrated in several in vivo studies. This switch has been minimally analyzed at the cellular level, and the mechanism driving it is unknown. We used immunofluorescence and RT-PCR to demonstrate the expression of the skeletal muscle-specific regulatory genes MyoD and myogenin, and of several skeletal muscle-specific structural genes in cultures of the established rat smooth muscle cell lines PAC1, A10, and A7r5. The skeletal muscle regulatory gene Myf5 was not detected in these three cell lines. We further isolated clonal sublines from PAC1 cultures that homogeneously express smooth muscle characteristics at low density and undergo a coordinated increase in skeletal muscle-specific gene expression at high density. In some of these PAC1 sublines, this process culminates in the high-frequency formation of myotubes. As in the PAC1 parental line, Myf5 was not expressed in the PAC1 sublines. We show that the PAC1 sublines that undergo a more robust transition into the skeletal muscle phenotype also express significantly higher levels of the insulin-like growth factor (IGF1 and IGF2) genes and of FGF receptor 4 (FGFR4) gene. Our results suggest that MyoD expression in itself is not a sufficient condition to promote a coordinated program of skeletal myogenesis in the smooth muscle cells. Insulin administered at a high concentration to PAC1 cell populations with a poor capacity to undergo skeletal muscle differentiation enhances the number of cells displaying the skeletal muscle differentiated phenotype. The findings raise the possibility that the IGF signaling system is involved in the phenotypic switch from smooth to skeletal muscle. The gene expression program described here can now be used to investigate the mechanisms that may underlie the propensity of certain smooth muscle cells to adopt a skeletal muscle identity.(J Histochem Cytochem 48:1173-1193, 2000)

Gene expression patterns of the fibroblast growth factors and their receptors during myogenesis of rat satellite cells

Satellite cells are the myogenic precursors in postnatal muscle and are situated beneath the myofiber basement membrane. We previously showed that fibroblast growth factor 2 (FGF2, basic FGF) stimulates a greater number of satellite cells to enter the cell cycle but does not modify the overall schedule of a short proliferative phase and a rapid transition to the differentiated state as the satellite cells undergo myogenesis in isolated myofibers. In this study we investigated whether other members of the FGF family can maintain the proliferative state of the satellite cells in rat myofiber cultures. We show that FGF1, FGF4, and FGF6 (as well as hepatocyte growth factor, HGF) enhance satellite cell proliferation to a similar degree as that seen with FGF2, whereas FGF5 and FGF7 are ineffective. None of the growth factors prolongs the proliferative phase or delays the transition of the satellite cells to the differentiating, myogenin(+) state. However, FGF6 retards the rapid exit of the cells from the myogenin(+) state that routinely occurs in myofiber cultures. To determine which of the above growth factors might be involved in regulating satellite cells in vivo, we examined their mRNA expression patterns in cultured rat myofibers using RT-PCR. The expression of all growth factors, excluding FGF4, was confirmed. Only FGF6 was expressed at a higher level in the isolated myofibers and not in the connective tissue cells surrounding the myofibers or in satellite cells dissociated away from the muscle. By Western blot analysis, we also demonstrated the presence of FGF6 protein in the skeletal musle tissue. Our studies therefore suggest that the myofibers serve as the main source for the muscle FGF6 in vivo. We also used RT-PCR to analyze the expression patterns of the four tyrosine kinase FGF receptors (FGFR1-FGFR4) and of the HGF receptor (c-met) in the myofiber cultures. Depending on the time in culture, expression of all receptors was detected, with FGFR2 and FGFR3 expressed only at a low level. Only FGFR4 was expressed at a higher level in the myofibers but not the connective tissue cell cultures. FGFR4 was also expressed at a higher level in satellite cells compared to the nonmyogenic cells when the two cell populations were released from the muscle tissue and fractionated by Percoll density centrifugation. The unique localization patterns of FGF6 and FGFR4 may reflect specific roles for these members of the FGF signaling complex during myogenesis in adult skeletal muscle.

Cytochemical evidence for the presence of actin in the nucleus of the voodoo lily appendix

Immunoflorescence microscopy of sections of the voodoo lily Sauromatum guttatum appendix stained with monoclonal antibodies against alpha-smooth muscle actin and cytoplasmic actin revealed different staining intensity of different parts of the cell. The anti-cytoplasmic-actin recognized antigens present mainly in the cytoplasm, and the anti-alpha-smooth muscle-actin recognized more intensively antigens present in the nuclei. A positive staining of the nucleus was also obtained with FITC-phalloidin confirming the presence of actin in its filamenous form in the nucleus. The presence of a nuclear alpha-smooth muscle-actin-like protein was further confirmed by confocal laser microscopy. On Western blots, the two anti-actins labelled a protein band that comigrated with standard actin at the approximate molecular weight of 43 kDa. Several other proteins interacted with the two antibodies to a different degree. The monoclonal antibodies against beta-tubulin subunit stained only the periphery of the cytoplasm and anti-pan cytoplasmic myosin stained the cytoplasm weakly. On a Western blot, anti-beta-tubulin subunit primarily recognized a protein band at the appropriate molecular weight of 50 kDa. This is the first cytochemical evidence for the presence of alpha-smooth muscle-actin-like protein in the plant nucleus.

Formation of cartilage in cardiac semilunar valves of chick and quail

The development of cartilage in the aortic and pulmonary valves of chick and quail was studied using histological, histochemical and immunohistochemical techniques. In both species, the earliest evidence of chondrogenesis is the formation of smooth muscle alpha-actin-negative prechondrogenic (type II collagen-negative) cellular condensations in the tunica media of the proximal aorta and pulmonary trunk, in front of or slightly distal to the valvular commissures. Such condensations are present as early as stage 37 of Hamburger-Hamilton in the aortic and pulmonary valves of the chick. In quail embryos, they form somewhat later, namely, at stage 38 in the aortic valves and stage 39 in the pulmonary valves. In the chick, synthesis of type II collagen starts in the central core of the aortic cellular condensations at stage 38. In the pulmonary valves of chick and aortic and pulmonary valves of quail, production of type II collagen does not begin until stage 40. This production then gradually increases toward the periphery of the condensations, which remain devoid of perichondrium prior to hatching. After birth, the condensations become transformed into hyaline cartilaginous foci. In the aortic valves of some chickens and quails, more or less extensive deposits of hyaline cartilage or fibrocartilage form along the attachments of the leaflets to their supporting sinuses. They develop later than the commissural cartilages. The present findings, together with previous data from the literature, suggest that the aortic and pulmonary valve cartilages differentiate from neural crest-derived nonmuscular cells.