Temporal and spatial development of intestinal smooth muscle layers of human embryos and fetuses.

The sequential occurrence of three layers of smooth muscle layers (SML) in human embryos and fetus is not known. Here, we investigated the process of gut SML development in human embryos and fetuses and compared the morphology of SML in fetuses and neonates. The H&E, Masson trichrome staining, and Immunohistochemistry were conducted on 6-12 gestation week human embryos and fetuses and on normal neonatal intestine. We showed that no lumen was seen in 6-7th gestation week embryonic gut, neither gut wall nor SML was developed in this period. In 8-9th gestation week embryonic and fetal gut, primitive inner circular SML (IC-SML) was identified in a narrow and discontinuous gut lumen with some vacuoles. In 10th gestation week fetal gut, the outer longitudinal SML (OL-SML) in gut wall was clearly identifiable, both the inner and outer SML expressed α-SMA. In 11-12th gestation week fetal gut, in addition to the IC-SML and OL-SML, the muscularis mucosae started to develop as revealed by α-SMA immune-reactivity beneath the developing mucosal epithelial layer. Comparing with the gut of fetuses of 11-12th week of gestation, the muscularis mucosae, IC-SML, and OL-SML of neonatal intestine displayed different morphology, including branching into glands of lamina propria in mucosa and increased thickness. In conclusions, in the human developing gut between week-8 to week-12 of gestation, the IC-SML develops and forms at week-8, followed by the formation of OL-SML at week-10, and the muscularis mucosae develops and forms last at week-12.

The origin and mechanisms of smooth muscle cell development in vertebrates.

Smooth muscle cells (SMCs) represent a major structural and functional component of many organs during embryonic development and adulthood. These cells are a crucial component of vertebrate structure and physiology, and an updated overview of the developmental and functional process of smooth muscle during organogenesis is desirable. Here, we describe the developmental origin of SMCs within different tissues by comparing their specification and differentiation with other organs, including the cardiovascular, respiratory and intestinal systems. We then discuss the instructive roles of smooth muscle in the development of such organs through signaling and mechanical feedback mechanisms. By understanding SMC development, we hope to advance therapeutic approaches related to tissue regeneration and other smooth muscle-related diseases.

A mechanoresponsive PINCH-1-Notch2 interaction regulates smooth muscle differentiation of human placental mesenchymal stem cells.

Extracellular matrix (ECM) stiffness plays an important role in the decision making process of smooth muscle differentiation of mesenchymal stem cells (MSCs) but the underlying mechanisms are incompletely understood. Here we show that a signaling axis consisting of PINCH-1 and Notch2 is critically involved in mediating the effect of ECM stiffness on smooth muscle differentiation of MSCs. Notch2 level is markedly increased in ECM stiffness-induced smooth muscle differentiation of human placental MSCs. Knockdown of Notch2 from human placental MSCs effectively inhibits ECM stiffness-induced smooth muscle differentiation, whereas overexpression of North intracellular domain (NICD2) is sufficient to drive human placental MSC differentiation toward smooth muscle cells. At the molecular level, Notch2 directly interacts with PINCH-1. The interaction of Notch2 with PINCH-1 is significantly increased in response to ECM stiffness favoring smooth muscle differentiation. Furthermore, depletion of PINCH-1 from human placental MSCs reduces Notch2 level and consequently suppresses ECM stiffness-induced smooth muscle differentiation. Re-expression of PINCH-1, but not that of a Notch2-binding defective PINCH-1 mutant, in PINCH-1 knockdown human placental MSCs restores smooth muscle differentiation. Finally, overexpression of NICD2 is sufficient to override PINCH-1 deficiency-induced defect in smooth muscle differentiation. Our results identify an ECM stiffness-responsive PINCH-1-Notch2 interaction that is critically involved in ECM stiffness-induced smooth muscle differentiation of human placental MSCs.

Functions of Muscarinic Receptor Subtypes in Gastrointestinal Smooth Muscle: A Review of Studies with Receptor-Knockout Mice.

Parasympathetic signalling via muscarinic acetylcholine receptors (mAChRs) regulates gastrointestinal smooth muscle function. In most instances, the mAChR population in smooth muscle consists mainly of M2 and M3 subtypes in a roughly 80% to 20% mixture. Stimulation of these mAChRs triggers a complex array of biochemical and electrical events in the cell via associated G proteins, leading to smooth muscle contraction and facilitating gastrointestinal motility. Major signalling events induced by mAChRs include adenylyl cyclase inhibition, phosphoinositide hydrolysis, intracellular Ca2+ mobilisation, myofilament Ca2+ sensitisation, generation of non-selective cationic and chloride currents, K+ current modulation, inhibition or potentiation of voltage-dependent Ca2+ currents and membrane depolarisation. A lack of ligands with a high degree of receptor subtype selectivity and the frequent contribution of multiple receptor subtypes to responses in the same cell type have hampered studies on the signal transduction mechanisms and functions of individual mAChR subtypes. Therefore, novel strategies such as genetic manipulation are required to elucidate both the contributions of specific AChR subtypes to smooth muscle function and the underlying molecular mechanisms. In this article, we review recent studies on muscarinic function in gastrointestinal smooth muscle using mAChR subtype-knockout mice.

Changes in urethral smooth muscle and external urethral sphincter function with age in rats.

To confirm changes in urethral activity with age, both intravesical pressure and urethral perfusion pressure (UPP) were recorded and external urethral sphincter electromyography (EUS-EMG) was performed. A total of 33 female Sprague Dawley rats aged 3 months (young rats), 12 months (middle-aged rats), and 24 months (aged rats) were used. Bladder activity was evaluated using continuous cystometry. Urethral activity was evaluated by simultaneously recording intravesical pressure and UPP in isovolumetric conditions under urethane anesthesia in each group. Additionally, EUS-EMG activity was monitored under the same conditions. In continuous cystometry, the amplitude of bladder contractions was not different among the three groups; nevertheless, residual urine volume was significantly increased in middle-aged and aged rats, as compared in young rats. With respect to UPP, the change in UPP was significantly smaller in aged rats (60%) and middle-aged rats (64%) than in young rats. Furthermore, the mean amplitude of high-frequency oscillations of the EUS was significantly lower in aged (61%) and middle-aged rats (70%) than in young rats. EUS-EMG revealed EUS bursting activity during voiding with clear active and silent phases in young rats but unclear active and silent phases in aged rats. Masson's trichrome staining of the urethra showed EUS atrophy in aged rats compared to young and middle-aged rats. The results indicate that aging induces two urethral dysfunctions in the urethral smooth muscle and EUS, which may lead to dyscoordination between the urinary bladder and urethra.

Mechanical and signaling mechanisms that guide pre-implantation embryo movement.

How a mammalian embryo determines and arrives at its attachment site has been studied for decades, but our understanding of this process is far from complete. Using confocal imaging and image analysis, we evaluate embryo location along the longitudinal oviductal-cervical axis of murine uteri. Our analysis reveals three distinct pre-implantation phases: embryo entry, unidirectional movement of embryo clusters and bidirectional scattering and spacing of embryos. We show that unidirectional clustered movement is facilitated by a mechanical stimulus of the embryo and is regulated by adrenergic uterine smooth muscle contractions. Embryo scattering, on the other hand, depends on embryo-uterine communication reliant on the LPAR3 signaling pathway and is independent of adrenergic muscle contractions. Finally, we demonstrate that uterine implantation sites in mice are neither random nor predetermined but are guided by the number of embryos entering the uterine lumen. These studies have implications for understanding how embryo-uterine communication is key to determining an optimal implantation site necessary for the success of a pregnancy.

Developmental mechanisms regulating the formation of smooth muscle layers in the mouse uterus†.

Uterine smooth muscle cells differentiate from mesenchymal cells, and gap junctions connect the muscle cells in the myometrium. At the neonatal stage, a uterine smooth muscle layer is situated away from the epithelium when smooth muscle cells are grafted near the epithelium, suggesting that the epithelium plays an important role in differentiation, proliferation, and/or migration of smooth muscle cells. In this study, developmental mechanisms regulating the formation of the smooth muscle layers in the mouse uterus were analyzed using an in vitro culture model. Differentiation of smooth muscle cells occurs at a neonatal stage because ACTA2 gene expression was increased at the outer layer, and GJA1 was not expressed in cellular membranes of uterine smooth muscle cells by postnatal day 15. To analyze the effects of the epithelium on the differentiation of smooth muscle cells, a bulk uterine mesenchymal cell line was established from p53-/- mice at postnatal day 3 (P3US cells). Co-culture with Müllerian ductal epithelial cells (E1 cells) induced repulsive migration of ACTA2-positive cells among bulk P3US cells from E1 cells, but it had no effects on the migration of any of 100% ACTA2-positive or negative smooth muscle cell lines cloned from P3US cells. Thus, uterine epithelial cells indirectly affected the repulsive migration of smooth muscle cells via mesenchymal cells. Conditioned medium by E1 cells inhibited differentiation into smooth muscle cells of clonal cells established from P3US cells. Therefore, the uterine epithelium inhibits the differentiation of stem-like progenitor mesenchymal cells adjacent to the epithelium into smooth muscle cells.

High levels of androgens cause chondrocyte accumulation and loss of smooth muscle in the mouse penile body†.

Androgens are essential for penile development and for maintaining penile structural and functional integrity. Loss of androgen levels or function results in a decrease in smooth muscle content, accumulation of adipocytes in the corpora cavernosa, and inhibition of erectile function. Our previous studies with a mouse model (KiLHRD582G) of constitutive luteinizing hormone receptor activity also showed structural abnormalities in the penis caused by a decrease in smooth muscle content, accumulation of chondrocytes, and sexual dysfunction. As KiLHRD582G mice exhibit very high levels of testosterone at all postnatal ages, the goal of this study was to determine if the elevated androgen levels were responsible for the morphological changes in the penis. Implantation of testosterone capsules in wild-type mice at neonatal (2 weeks) and postpubertal (5 weeks) ages resulted in the accumulation of chondrocytes in the corpora cavernosa of the adult animals. Mice implanted with testosterone capsules at 2 weeks of age exhibited a 4-fold increase in serum testosterone with a 1.5-fold loss of smooth muscle at 24 weeks of age. Collagen content was unchanged. Only 57% of testosterone implanted mice were fertile at 24 weeks of age. Mice implanted with testosterone capsules at 5 weeks of age showed no decrease in smooth muscle content at 24 weeks, although serum testosterone levels were elevated 5-fold. Implantation with dihydrotestosterone also resulted in chondrocyte accumulation and a 2-fold loss in smooth muscle content. Together, these studies demonstrate that supraphysiological levels of androgens cause structural changes in the penile corpora cavernosa and impair fertility.

Predifferentiated Smooth Muscle-Like Adipose-Derived Stem Cells for Bladder Engineering.

Introduction: All organs of human body are a conglomerate of various cell types with multidirectional interplay between the different cells and the surrounding microenvironment, leading to a stable tissue formation, homeostasis, and function. To develop a functional smooth muscle tissue, we need to simulate and create a multicellular microenvironment. The multilineage adipose-derived stem cells (ADSCs), which can be easily harvested in large numbers, may provide an alternative cell source for the replacement of smooth muscle cells (SMCs) in cell-based detrusor bioengineering therapeutic approaches. The aim of this study was to investigate whether predifferentiated smooth muscle-like ADSC (pADSC) can support SMCs to generate stable smooth muscle tissue through remodeling of extracellular matrix (ECM) and factor secretion. Methods: Rat SMC and pADSC were mono- and cocultured in the cell ratios 1:1, 1:2, 1:3, and 1:5 (SMC-pADSC) and grown for up to 2 weeks in vitro. The expression of the SMC-specific markers alpha-smooth muscle actin, calponin, myosin heavy chain 11 (MyH11), and smoothelin was assessed, and cell proliferation and contractility were analyzed. Proteomic analysis of the secretome (cell-cell contact was compared with a noncontact transwell 1:1 coculture) and the cell pellets was performed, with the focus on ECM deposition and remodeling, integrin expression and growth factor secretion. Results: SMC and pADSC were strongly positive for all smooth muscle markers. After 1 and 2 weeks of culture, the 1:1 cell ratio developed a significantly higher number of smooth muscle organoids and improved contractility. These organoids were highly structured, consisting of an SMC core surrounded by a pADSC layer. The deposition of various EMC proteins, such as collagens 1a1, 1a2, 2a1, 3a1, 5a2, 6a2, 12a1, and fibrillin 1, was significantly increased. A decreased matrix metalloproteinase 3 (MMP3), MMP9 and MMP13 secretion, as well as increased tissue inhibitors of metalloproteinase 1 (TIMP1) and TIMP2 secretion were found in the contact coculture compared with the monoculture controls. Conclusion: SMC-pADSC 1:1 cocultures exhibit an improved cell proliferation, contractility, and organoid formation compared with all other ratios and monoculture, while retaining a stable phenotype that is comparable with the SMC monoculture. These effects are mediated by increased ECM deposition and tight ECM remodeling by the secreted MMP and TIMP. Impact statement Harvesting smooth muscle cells (SMCs) from diseased bladders represents a significant limitation for clinical translation of bladder Tissue Engineering. Our results suggest that autologous predifferentiated smooth muscle-like adipose-derived stem cell can substitute SMCs, and may be used in combination with SMCs to generate contractile detrusor muscle tissue for patients suffering from end-stage bladder diseases. We demonstrate a beneficial effect when using these cells in a 1:1 ratio with improved deposition of extracellular matrix (ECM) molecules and superior remodeling of the ECM by matrix metalloproteinases and decreased tissue inhibitors of metalloproteinase activity.

Genetic and Mechanical Regulation of Intestinal Smooth Muscle Development.

The gastrointestinal tract is enveloped by concentric and orthogonally aligned layers of smooth muscle; however, an understanding of the mechanisms by which these muscles become patterned and aligned in the embryo has been lacking. We find that Hedgehog acts through Bmp to delineate the position of the circumferentially oriented inner muscle layer, whereas localized Bmp inhibition is critical for allowing formation of the later-forming, longitudinally oriented outer layer. Because the layers form at different developmental stages, the muscle cells are exposed to unique mechanical stimuli that direct their alignments. Differential growth within the early gut tube generates residual strains that orient the first layer circumferentially, and when formed, the spontaneous contractions of this layer align the second layer longitudinally. Our data link morphogen-based patterning to mechanically controlled smooth muscle cell alignment and provide a mechanistic context for potentially understanding smooth muscle organization in a wide variety of tubular organs.

Neurochemical characterization of myenteric neurons in the juvenile gilthead sea bream (Sparus aurata) intestine.

We evaluated the chemical coding of the myenteric plexus in the proximal and distal intestine of gilthead sea bream (Sparus aurata), which represents one of the most farmed fish in the Mediterranean area. The presence of nitric oxide (NO), acetylcholine (ACh), serotonin (5-HT), calcitonin-gene-related peptide (CGRP), substance P (SP) and vasoactive intestinal peptide (VIP) containing neurons, was investigated in intestinal whole mount preparations of the longitudinal muscle with attached the myenteric plexus (LMMP) by means of immunohistochemical fluorescence staining. The main excitatory and inhibitory neurochemicals identified in intestinal smooth muscle were ACh, SP, 5HT, and NO, VIP, CGRP. Some neurons displayed morphological features of ascending and descending interneurons and of putative sensory neurons. The expression of these pathways in the two intestinal regions is largely superimposable, although some differences emerged, which may be relevant to the morphological properties of each region. The most important variances are the higher neuronal density and soma size in the proximal intestine, which may depend on the volume of the target tissue. Since in the fish gut the submucosal plexus is less developed, myenteric neurons substantially innervate also the submucosal and epithelial layers, which display a major thickness and surface in the proximal intestine. In addition, myenteric neurons containing ACh and SP, which mainly represent excitatory motor neurons and interneurons innervating the smooth muscle were more numerous in the distal intestine, possibly to sustain motility in the thicker smooth muscle coat. Overall, this study expands our knowledge of the intrinsic innervation that regulates intestinal secretion, absorption and motility in gilthead sea bream and provides useful background information for rational design of functional feeds aimed at improving fish gut health.

Age Related Differences in Responsiveness to Sildenafil and Tamsulosin are due to Myogenic Smooth Muscle Tone in the Human Prostate.

Lower urinary tract symptoms (LUTS) due to Benign Prostatic Hyperplasia (BPH) are highly prevalent in older men, having a profound impact on patient quality of life. Current therapeutics for BPH/LUTS target neurogenic smooth muscle tone, but response is unpredictable and many patients fail to respond. Spontaneous myogenic tone is another component of smooth muscle contractility that is uncharacterized in human prostate. To better understand and improve the predictability of patient response, we defined myogenic contractility using human prostate specimens and examined the effect of existing therapeutics. We show that myogenic activity is present in the human prostate with the frequency of contractions in transition zone (TZ) specimens from BPH diagnosed patients approximately 160% greater than matched controls. α1-adrenoreceptor antagonists (Tamsulosin) and PDE5 inhibitors (Sildenafil) both significantly reduced myogenic contractile parameters, including frequency, with notable interpatient variability. Tamsulosin was more effective in older patients (R2 = 0.36, p < 0.01) and men with larger prostate volumes (R2 = 0.41, p < 0.05), while Sildenafil was more effective in younger men (R2 = 0.45, p < 0.05). As myogenic tone is significantly increased in BPH, therapeutics targeting this mechanism used with reference to patient characteristics could improve clinical outcomes and better predict patient response.

Bladder Reconstruction with Human Amniotic Membrane in a Xenograft Rat Model: A Preclinical Study.

Background: Human amniotic membranes (HAMs) are assumed to have a number of unique characteristics including durability, hypoallergenic and anti-inflammatory properties. Materials and Methods: Multilayer HAMs from caesarian sections were applied to repair defined bladder defects in male Sprague-Dawley rats. The animals were sacrificed at 7, 21 and 42 days after implantation. Bladder volume capacity after grafting was measured. Histological analyses were performed to asses a number of parameters including HAM degradation, inflammatory reaction, graft rejection and smooth muscle ingrowth. Results: One rat died from sepsis in the treated group. No severe complications or signs of leakage were observed. Bladder capacity did not change over time. The initially increased inflammation in the HAM group diminished significantly over time (p<0.05). No signs of HAM degradation were observed and smooth muscle staining increased over time. Conclusions: HAMs appear to be durable and hypoallergenic grafts. The assumed suitability for the reconstruction of urinary tract justifies further research on detailed immunological process in larger grafts.

Developmental changes in contractile responses to cholinergic stimuli: role of calcium sensitization and related pathways.

This study was performed to analyze the developmental changes in bladder response to cholinergic stimulation in detail, highlighting calcium sensitization (CS) and its related pathways. Rats were divided into three groups in accordance with reported time of developmental milestones (newborns, days 1-4; youngsters, days 5-14; and grown-ups, days 15-28). Following cholinergic stimulation (carbachol, 5 µM), the contractile response to detrusor was analyzed with respect to three phases (initial phasic, tonic, and superimposed phasic contractions). Contractile responses were analyzed by their dynamic and kinetic aspects. The responses were further compared in varying external calcium concentrations and in the presence of inhibitors of protein kinase C (PKC) and Rho kinase (ROCK), which are involved in CS. The responses of newborns contrasted with the others by their short and brisk initial phasic contractions, prominent tonic contractions, and delayed participation of irregular superimposed phasic contractions. With development, phasic contractions became prominent, and tonic contractions diminished. These developmental changes in phasic contractions were reproduced when exposed to increasing calcium concentrations. Application of specific inhibitors and molecular phasic analysis revealed that PKC was functional in tonic contractions of the newborns, whereas ROCK took over its role with development. Within a few days of birth, rats' bladders experienced drastic changes in contractile mechanisms. This included dominance of phasic contractions over tonic contractions due to increased calcium dependence and the maturational shift of the calcium sensitivity mechanism from PKC to ROCK.

Bioaccumulation of Heavy Metals in Various Tissues of Some Fish Species and Green Tiger Shrimp (Penaeus semisulcatus) from Iskenderun Bay, Turkey, and Risk Assessment for Human Health.

In this study, concentrations of heavy metals were determined by ICP-MS in the edible tissues of common sole (Solea solea Linnaeus, 1758), whiting (Merlangius merlangus Linnaeus, 1758), silver sillago (Sillago sihama Forsskål, 1775) and muscle-exoskeleton of green tiger shrimp (Penaeus semisulcatus De Haan, 1844), from the seas of Iskenderun Bay, Eastern Mediterranean, Turkey, in January-February, 2016. The lowest and highest mean concentrations of Mn, Cr, Cd, Ni, Hg, As, Pb, and Co in fish and shrimp' muscles were found, respectively, as follows: 0.166-0.382, 0.134-0.336, 0.005-0.008, 0.091-0.140, 0.026-0.228, 1.741-29.254, 0.087-0.110, and <0.0005-0.027 mg kg-1; in the skin and exoskeleton, the values were found, respectively, as 0.103-15.819, 0.301-0.778, 0.007-0.026, 0.115-0.513, 0.021-0.243, 1.548-17.930, 0.148-0.295, and <0.0005-0.140 mg kg-1. According to the results, mean concentrations of all metals in the muscles of fish, except for arsenic and chromium, were found to be below certain legal limit values, especially arsenic levels in shrimp that were found to be above all the legal limit values. Also, the hazard quotients (HQ) of individual heavy metals in fish and shrimp, except for As, revealed safe levels for human consumption. However, the HQ values of estimated inorganic As exceeded 1 in the muscles of shrimp (P. semisulcatus), which may constitute a risk to public health.

Epigallocatechin-3-gallate ameliorates erectile function in aged rats via regulation of PRMT1/DDAH/ADMA/NOS metabolism pathway.

Aging-related ED is predominantly attributed to neurovascular dysfunction mediated by NO suppression and increased oxidative stress in penis. The alterations of protein arginine methyltransferases 1 (PRMT1)/dimethylarginine dimethylaminohydrolase (DDAH)/asymmetrical dimethylarginine (ADMA)/NO synthase (NOS) pathway regulate NO production in the vascular endothelium. Epigallocatechin-3-gallate (EGCG) is one of the most abundant and antioxidative ingredients isolated from green tea. In the present study, 40 Sprague-Dawley rats were randomly distributed into four groups: one young rat group and three aged rat groups treated with daily gavage feedings of EGCG at doses of 0, 10 mg kg-1 and 100 mg kg-1 for 12 weeks, respectively. Erectile function was assessed by electrical stimulation of the cavernous nerves with intracavernous pressure (ICP) measurement. After euthanasia, penile tissue was investigated using Western blot and ELISA to assess the PRMT1/DDAH/ADMA/NOS metabolism pathway. Superoxide dismutase (SOD) and malondialdehyde (MDA) levels were detected by colorimetry. We also evaluated smooth muscle contents. The ratio of maximal ICP and mean systemic arterial pressure (MAP) was markedly higher in EGCG-treated aged rats than in untreated aged rats. We found that DDAH1 and DDAH2 were expressed in cavernosal tissue, and they were downregulated in corpora of aged rats. The administration of EGCG upregulated the expression and activity of DDAH. In contrast, EGCG treatment downregulated the expression of PRMT1 and ADMA content. Moreover, EGCG-treated rats showed an improvement in smooth muscle expression, the ratio of smooth muscle cell/collagen fibril, SOD activity, and MDA levels when compared with untreated aged rats.

The stem cell growth factor receptor KIT is not expressed on interstitial cells in bladder.

The mast/stem cell growth factor receptor KIT has long been assumed to be a specific marker for interstitial cells of Cajal (ICC) in the bladder, with possible druggable perspectives. However, several authors have challenged the presence of KIT+ ICC in recent years. The aim of this study was therefore to attempt to clarify the conflicting reports on KIT expression in the bladder of human beings, rat, mouse and guinea pig and to elucidate the possible role of antibody-related issues and interspecies differences in this matter. Fresh samples were obtained from human, rat, mouse and guinea pig cystectomies and processed for single/double immunohistochemistry/immunofluorescence. Specific antibodies against KIT, mast cell tryptase (MCT), anoctamin-1 (ANO1) and vimentin were used to characterize the cell types expressing KIT. Gut (jejunum) tissue was used as an external antibody control. Our results revealed KIT expression on mast cells but not on ICC in human, rat, mouse and guinea pig bladder. Parallel immunohistochemistry showed KIT expression on ICC in human, rat, mouse and guinea pig gut, which confirmed the selectivity of the KIT antibody clones. In conclusion, we have shown that KIT+ cells in human, rat, mouse and guinea pig bladder are mast cells and not ICC. The present report is important as it opposes the idea that KIT+ ICC are present in bladder. In this perspective, functional concepts of KIT+ ICC being involved in sensory and/or motor aspects of bladder physiology should be revised.

Embracing change: striated-for-smooth muscle replacement in esophagus development.

The esophagus functions to transport food from the oropharyngeal region to the stomach via waves of peristalsis and transient relaxation of the lower esophageal sphincter. The gastrointestinal tract, including the esophagus, is ensheathed by the muscularis externa (ME). However, while the ME of the gastrointestinal tract distal to the esophagus is exclusively smooth muscle, the esophageal ME of many vertebrate species comprises a variable amount of striated muscle. The esophageal ME is initially composed only of smooth muscle, but its developmental maturation involves proximal-to-distal replacement of smooth muscle with striated muscle. This fascinating phenomenon raises two important questions: what is the developmental origin of the striated muscle precursor cells, and what are the cellular and morphogenetic mechanisms underlying the process? Studies addressing these questions have provided controversial answers. In this review, we discuss the development of ideas in this area and recent work that has shed light on these issues. A working model has emerged that should permit deeper understanding of the role of ME development and maturation in esophageal disorders and in the functional and evolutionary underpinnings of the variable degree of esophageal striated myogenesis in vertebrate species.

Gastric and pyloric sphincter muscle function and the developmental-dependent regulation of gastric content emptying in the rat.

Feeding intolerance is a common issue in the care of preterm neonates. The condition manifests as delayed emptying of gastric contents and represents a therapeutic challenge, since the factors accounting for its manifestations are unknown. The main goal of this study was to comparatively investigate the age-related function of rat gastric and pyloric smooth muscle and their putative regulators. We hypothesized that a reduced gastric muscle contraction potential early in life contributes to the delayed gastric emptying of the newborn. Newborn and adult rat gastric (fundus) and pyloric sphincter tissues were comparatively studied in vitro. Shortening of the tissue-specific dissociated smooth muscle cell was evaluated, and expression of the key regulatory proteins Rho-associated kinase 2 and myosin light chain kinase was determined. Gastric and pyloric smooth muscle cell shortening was significantly greater in the adult than the respective newborn counterpart. Expression of myosin light chain kinase and Rho-associated kinase 2 was developmentally regulated and increased with age. Pyloric sphincter muscle expresses a higher neuronal nitric oxide synthase and phosphorylated vasodilator-stimulated phosphoprotein content in newborn than adult tissue. Compared with later in life, the newborn rat gastropyloric muscle has a Ca(2+)-related reduced potential for contraction and the pyloric sphincter relaxation-dependent modulators are overexpressed. To the extent that these rodent data can be extrapolated to humans, the delayed gastric emptying in the newborn reflects reduced stomach muscle contraction potential, as opposed to increased pyloric sphincter tone.