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This review will detail the embryology and morphology of the heart. We will begin with the heart's origin in the lateral plate mesoderm of the early embryo, review its fusion into a linear heart tube, followed by looping and remodeling to create a four-chambered organ with pulmonary and systemic venous inflow and pulmonary and systemic arterial outflow. We will discuss important concepts in heart development at genetic, cellular, and physiologic levels and review some key experimental models and methods important to the study of cardiac development. This review will also serve to introduce some of the congenital heart defects that can result from abnormalities in development and some of the genetic syndromes that are linked to congenital heart defects.

Summary Lateral plate mesoderm (LPM) cells differentiate into various cell types including endothelial and hematopoietic cells. In zebrafish embryos, LPM cells migrate toward the midline along the ventral surfaces of somites during which their cell fate specification depends upon efficient integrin-mediated cell adhesion and migration. Herein, we present a protocol for analysis of integrin-mediated cell adhesion of LPM cells isolated from zebrafish embryos. This allows the study of the molecular mechanisms underlying integrin activation required for LPM cell fate specification. For complete details on the use and execution of this protocol, please refer to Rho et al. (2019)., Graphical abstract, Highlights • Zebrafish breeding and preparation of the LPM cells dissociated from the embryos • Protocol for LPM cell adhesion to the fibronectin-coated dish • Imaging the LPM cells adhering to the fibronectin-coated dish • Analysis of cell spreading and focal adhesion formations in the LPM cells, Lateral plate mesoderm (LPM) cells differentiate into various cell types including endothelial and hematopoietic cells. In zebrafish embryos, LPM cells migrate toward the midline along the ventral surfaces of somites during which their cell fate specification depends upon efficient integrin-mediated cell adhesion and migration. Herein, we present a protocol for analysis of integrin-mediated cell adhesion of LPM cells isolated from zebrafish embryos. This allows the study of the molecular mechanisms underlying integrin activation required for LPM cell fate specification.

Caudal-type homeobox 2 (.Cdx2) transcription factor is an essential regulator of differentiation to the intestinal epithelium, somatic mesoderm and trophectoderm function in the mouse. However, the regulation of Cdx2 in these processes is poorly understood. Separation of viable Cdx2 expressing cells during differentiation for downstream experiments is not possible due to its nuclear localization, limiting experimental possibilities and studying Cdx2 regulation. Here, we report generation of a Cdx2-mCherry knock-in reporter mouse embryonic stem cell line (TCMC), for modeling and studying in vitro differentiation of mESCs to intestinal epithelia, somatic mesoderm, and trophectoderm.

Summary The branchiomeric skeletal muscles co-evolved with new chambers of the heart to enable predatory feeding in chordates. These co-evolved tissues develop from a common population in anterior splanchnic mesoderm, referred to as cardiopharyngeal mesoderm (CPM). The regulation and development of CPM are poorly understood. We describe an embryonic stem cell-based system in which MESP1 drives a PDGFRA+ population with dual cardiac and skeletal muscle differentiation potential, and gene expression resembling CPM. Using this system, we investigate the regulation of these bipotent progenitors, and find that cardiac specification is governed by an antagonistic TGFβ-BMP axis, while skeletal muscle specification is enhanced by Rho kinase inhibition. We define transcriptional signatures of the first committed CPM-derived cardiac and skeletal myogenic progenitors, and discover surface markers to distinguish cardiac (PODXL+) from the skeletal muscle (CDH4+) CPM derivatives. These tools open an accessible window on this developmentally and evolutionarily important population., Graphical Abstract, Highlights • MESP1 induces bipotent PDGFRA+ cardiac/skeletal myogenic progenitors • MESP1+ PDGFRA+ cells functionally resemble cardiopharyngeal mesoderm (CPM) • TGFβ-BMP and Rho kinase signaling regulate CPM lineage choice • PODXL and CDH4 mark early cardiac and skeletal myogenic-committed progenitors, In this article, Kyba and colleagues demonstrate that a bipotent PDGFRA+ cardiac/skeletal myogenic progenitor population functionally resembling cardiopharyngeal mesoderm (CPM) can be generated from ES cells differentiated in vitro. They find that TGFβ-BMP and Rho kinase signaling regulate CPM differentiation, and identify PODXL and CDH4 as surface markers for the earliest cardiac and skeletal myogenic-committed progenitors, respectively.

Pentalogy of Cantrell is a rare syndrome consisting of a spectrum of five structural ventral midline defects, with the combination of an omphalocele and ectopia cordis being the hallmark of the disorder. Although the pathogenesis of this syndrome is not well understood, early embryologic developmental failure of a segment of the lateral mesoderm is a prevailing theory. Prenatal diagnosis relies on sonographic and echocardiographic findings and can be suspected as early as 12–13 weeks' gestation. A responsible gene has not yet been identified; therefore no genetic testing is available to provide a definitive prenatal diagnosis. The overall survival rate is poor and is directly correlated to the type and complexity of the thoracoabdominal defects and any associated intracardiac abnormalities. A multidisciplinary team approach is essential for accurate diagnosis as well as prenatal and postnatal care.

The embryonic heart forms from multipotent cardiovascular progenitor cells that migrate to anterior lateral splanchnic mesoderm. The cardiac transcriptional program is activated by the combinatorial activities of multiple intercellular signaling pathways and transcription factors. Early differentiating cardiomyocytes converge at the ventral midline to give rise to the cardiac crescent and linear heart tube (HT). The HT subsequently loops to the right and extends in length by addition of cardiac progenitor cells to the cardiac poles from contiguous pharyngeal mesoderm. The embryonic heart is the template for cardiac growth and septation during the transition to the definitive four-chambered heart.

Scimitar syndrome (SS) encompasses a constellation of malformations with as a most prominent feature the anomalous “scimitar” like drainage of the inferior right pulmonary vein into the inferior caval vein. In addition, the syndrome may encompass systemic-to-pulmonary artery connections, hypoplasia of the right pulmonary artery and lung, as well as airway and diaphragmatic abnormalities. In this chapter it is aimed to approach the anatomical spectrum of malformations observed in SS in the perspective of embryonic development. The splanchnic mesoderm, giving rise to the early splanchnic plexus, with initially both pulmonary venous-to-systemic connections and pulmonary arterial-to-systemic connections, will play a pivotal role. A working model for a developmental approach of SS is provided.

Hematopoietic stem cells (HSCs) sustain blood production throughout life and are of pivotal importance in regenerative medicine. Although HSC generation from pluripotent stem cells would resolve their shortage for clinical applications, this has not yet been achieved mainly because of the poor mechanistic understanding of their programming. Bone marrow HSCs are first created during embryogenesis in the dorsal aorta (DA) of the midgestation conceptus, from where they migrate to the fetal liver and, eventually, the bone marrow. It is currently accepted that HSCs emerge from specialized endothelium, the hemogenic endothelium, localized in the ventral wall of the DA through an evolutionarily conserved process called the endothelial-to-hematopoietic transition. However, the endothelial-to-hematopoietic transition represents one of the last steps in HSC creation, and an understanding of earlier events in the specification of their progenitors is required if we are to create them from naïve pluripotent cells. Because of their ready availability and external development, zebrafish and Xenopus embryos have enormously facilitated our understanding of the early developmental processes leading to the programming of HSCs from nascent lateral plate mesoderm to hemogenic endothelium in the DA. The amenity of the Xenopus model to lineage tracing experiments has also contributed to the establishment of the distinct origins of embryonic (yolk sac) and adult (HSC) hematopoiesis, whereas the transparency of the zebrafish has allowed in vivo imaging of developing blood cells, particularly during and after the emergence of HSCs in the DA. Here, we discuss the key contributions of these model organisms to our understanding of developmental hematopoiesis.

In progressing from a simple tube to mature digestive organs, the alimentary tract is patterned along distinct axes. Transcription factors pattern the gut tube along the rostrocaudal axis, delineating a foregut, a midgut, and a hindgut. Reciprocal signals along the radial axis then enable differentiation of the mesoderm-derived mesenchyme and muscle cells from diverse endoderm-derived epithelia. Inductive interactions further specify discrete organs such as the stomach, liver, and pancreas, which show dorsoventral and left–right asymmetries. Finally, patterning of the radially symmetric small bowel along a luminal–mural axis separates progenitors in submucosal crypts from differentiated cells restricted to the villi. Diverse forces drive morphogenesis: radial cell intercalation, cell shape changes, asymmetric tissue growth, and tension from surrounding structures. Much remains unclear about how positional and inductive cues activate particular genes to implement this developmental program.

In the developing kidney, the intermediate mesoderm must be specified along the anteroposterior and mediolateral axes of the embryo, such that the kidney progenitor field is separated from the paraxial and lateral plate mesoderm shortly after gastrulation. This region of mesoderm generates both the metanephric mesenchyme and the ureteric bud epithelium, the two populations of progenitor cells that differentiate into nephrons and collecting ducts, respectively. Cell signaling pathways, nuclear DNA biding proteins, and epigenetic modifiers act in concert to imprint a kidney specific fate upon the intermediate mesoderm and its derivatives. This chapter describes the process of early kidney development and introduces the latest advancements in the understanding of how epigenetic modifications affect renal lineage specification and the maintenance of the renal cell phenotype in the adult.

The bones and connective tissues of the body are derived from two embryonic origins, the mesoderm and neural crest. The appendicular skeleton arises from the lateral plate mesoderm, the axial skeleton from the paraxial mesoderm, and the craniofacial skeleton from a combination of cephalic paraxial mesoderm and neural crest cells. Bones are formed through one of two processes: Endochondral ossification involves the formation of primordial cartilaginous anlagen that are subsequently replaced by bone via the growth plates. Intramembranous ossification is characterized by the direct differentiation of mesenchymal cells to osteogenic tissue. The growth plate can be divided into a resting zone, a proliferating zone, a pre-hypertrophic zone, and a hypertrophic zone, reflecting a gradient of chondrocyte differentiation that is regulated by antagonism between bone morphogenetic protein (BMP) and fibroblast growth factor (FGF) signaling pathways and gradients of IHH and PTHrP. Replacement of cartilage by osteoblasts is dependent upon prior mineralization of the cartilage matrix and vascular invasion. The activities of the hypertrophic chondrocytes and of the osteoblasts are regulated by RUNX2. Osteoclasts are of hematopoietic origin (monocyte), and with osteoblasts, they remodel bone throughout an individual’s lifetime to adapt to mechanical demands. Joints and intra-articular structures arise from a cohort of progenitor cells distinct from those of the bones and are labeled by GDF5 and TGFBRII expression. Somites are formed from the segmental plate through the interaction of a cell-autonomous oscillator and a morphogenetic wave that coordinates somite boundary formation via the Notch-Delta juxtacrine system and epithelialization via WNT paracrine system and cadherin activity. Dysregulation of this developmental process leads to congenital scoliosis. The axial skeleton is derived from the ventro-medial sclerotome of somites. Differentiation of the somite is regulated by paracrine factors from surrounding tissues and the sclerotome in particular is induced by SHH. Tendons and ligaments develop from the interaction of mesodermal mesenchyme with myoblasts. The nucleus pulposus of the intervertebral disc is initially derived from the notochord. The gradual loss of “embryonic cells” from the disc may be related to disc degeneration and back pain.

This chapter treats one of the two major sources of skeletal cells, the somitic mesoderm. Skeletal elements of somitic mesoderm origin are the vertebrae, ribs, pectoral and pelvic girdles. The muscles but not the skeletal elements of limbs also are of somitic origin. Vertebral origins differ between teleost fish and tetrapods. Vertebrae initially form by mineralisation of perinotochordal sheath in fish and from condensations of sclerotomal (somitic) mesenchyme in tetrapods. Segmentation and resegmentation of somitic mesoderm are discussed, as are the origins of the intervertebral discs and the nucleus pulposus. Whether clavicles and furculae are homologous rounds out the chapter.

Chondrogenesis, the initial development of skeletal bones, starts during embryonic development in the early stages of somitogenesis and is considered to be the earliest phase of skeletal development. It involves progenitor cell specification, cell migration, epithelial-to-mesenchymal transition, and differentiation and maturation of chondrocytes. Mesenchymal cells that undergo chondrogenesis derive from neural crest cells, which originate from the neural crest ridge of the neural ectoderm; the presegmented paraxial mesoderm that differentiates into somites to elicit the sclerotome compartment, which leads to formation of the axis skeleton; and the somatopleure of the lateral plate mesoderm, which initiates limb buds for the appendage bones. Economically, the major skeletal disorders are associated with the rapid increase in growth rate achieved during the last few decades, and understanding of the relevant mechanisms will help in developing treatments for bone-related diseases. In this chapter, we portray the events leading chondrogenesis at the cellular and molecular levels and describe the various poultry bone disorders. The aim is to provide an introduction to the field, but by no means to cover all aspects of poultry bone development and disorders.

Publisher Summary This chapter reviews mechanisms that influence left–right asymmetry in vertebrate heart development. In the formation of the parallel systemic and pulmonary circulatory systems of the heart, left–right asymmetry is an essential aspect in determining both structure and function. It is becoming clear that traditional examples such as dextrocardia, the misplacement of the heart to the right, or heterotaxia, the alteration of heart orientation with respect to other organs, are only a subset of left–right cardiac defects. More commonly, subtle defects result in misalignment of the inflow and outflow tracts of the heart, resulting in a significant spectrum of congenital defects. Left–right patterning involves over 100 genes that encode components of several major cell–cell signaling pathways, as well as transcription factors and proteins required for formation of cilia. Strikingly, several embryonic tissues outside the cardiogenic fields are essential for normal cardiac left–right patterning, including cells in the embryonic node, caudal mesoderm, lateral plate mesoderm, notochord, and ventral floorplate of the neural tube. Thus, defects in cardiac left–right development can often be part of a syndrome that also presents with cilia defects or subclinical defects in midline or neural tube patterning. The understanding of the molecular basis of left–right development has greatly expanded in the last decade. The mechanisms that generate left–right asymmetry are actively studied in several vertebrate model systems, including embryos of mice, zebrafish, frogs, medaka fish, chick and rabbit, and by genetic analysis for inherited laterality defects in humans. In addition, the noncardiac cells that are essential for cardiac left–right patterning, such as ciliated cells, midline cells, and lateral plate mesoderm are becoming better understood.

Publisher Summary This chapter describes the current understanding of the mechanism of left–right patterning during mouse development. The breaking of left–right symmetry, at least in the mouse, is thought to be achieved by the unidirectional flow of extraembryonic fluid in the node, an embryonic midline structure located at the anterior tip of the primitive streak in mouse embryos. This leftward laminar flow of extraembryonic fluid in the node cavity, referred to as nodal flow, occurs at a speed of ∼15 to 20 μm/s and is generated by the rotational movement of 9 + 0 monocilia (cilia with nine outer doublets of microtubules but no central pair) that protrude from cells located on the ventral side of the node into the node cavity (Sulik et al., 1994). These 200 to 300 cilia rotate in the same direction (clockwise as viewed from the ventral side) at a speed of 600 rpm (Nonaka et al., 1998). Nodal flow takes place for only a short period of time: it is first apparent at the one- to two-somite stage, persists for several hours and ends by the six-somite stage. The asymmetric expression of Nodal begins in the lateral plate mesoderm at the two-somite stage and disappears by the six-somite stage. Nodal flow may therefore occur specifically to initiate Nodal expression on the left side of the lateral plate mesoderm.

Publisher Summary This chapter discusses the developmental physiology of the zebrafish cardiovascular system. The strength of zebrafish as a genetic model system enables the systematic dissection of interactions among genes in an intact vertebrate organism and bolsters its potential for screens of genetic or chemical modifiers of cardiovascular physiology. Phenotype-driven screens for genes that disrupt normal cardiovascular growth or function in the zebrafish offer an opportunity to understand the biology of heart and vascular physiology and pathways that may be disrupted in disease. Cardiac function is essential to the vascular flow, which in turn determines aspects of cardiac morphology. The cardiomyocytes of the zebrafish heart originate in the ventral–marginal zone, involute, and migrate through the lateral plate mesoderm and generate a heart tube precursor at the midline, a cone with the pre-ventricular cells pointing apically. The posterior lateral mesoderm forms vessel and blood precursors. It is adjacent to intermediate mesoderm that forms the kidney.

Cardiogenesis is an active and complex process with multiple and precisely coordinated phases of pattern formation and morphogenesis. Gastrulation, with its series of cell movements, transforms the bilaminar germ disc (epiblast and hypoblast) into a three-layered embryo—ectoderm, mesoderm, and endoderm. These three germ layers and cells also become committed to endodermal or mesodermal lineages during this process. The different destinies of the mesodermal cell populations are determined by the point of entrance of the epiblast cells into the primitive streak and the direction of their subsequent migration. On the basis of these two events, mesodermal cells can form tissues as varied as muscle, heart, kidney, or bone. The heart originates from the splanchnic mesoderm. It is formed from two crescent-like plates that, through a process of differential growth, are transformed into a tubular structure. During embryonic development, the genesis of somatic, visceral, and heart muscle are all derived from mesoderm progenitor cells, and they require the coordinated participation of multiple genes and signaling pathways. Subsequently, several distinctive transcriptional programs, as observed in the patterning of expression of a number of genes, will direct the compartmentalization of the forming heart.

Left–right (LR) patterning of the vertebrate embryo regulates heart, gut, and brain development. Three transient signaling centers in the embryo contribute to LR patterning: ciliated cells in the node/organizer/shield, the floorplate and notochord in the embryonic midline, and the lateral plate mesoderm. A cassette of asymmetrically expressed genes in the left lateral plate mesoderm is evolutionarily conserved and upstream of organ asymmetry. This cassette includes members of the TGF‐β signaling pathway family (Nodal and Lefty) and the transcription factor Pitx2. Here, we review two other signaling pathways hedgehog and fibroblast growth factor, which have been implicated in LR patterning but appear to have divergent roles in distinct classes of vertebrates.

This chapter discusses the segmentation of the paraxial mesoderm and the delineation of tissue compartments. Segmentation is a fundamental developmental process that subdivides the body, or parts thereof, into a series of serially repeated subunits and thereby generates a segmental pattern. In the mouse embryo, like other vertebrate embryos, the earliest manifestation of tissue segmentation is the formation of somites during organogenesis. Somites are blocks of mesodermal cells located on either side of the neural tube and the notochord. Together with the mesenchyme that envelops the cephalic neural tube, they constitute the paraxial mesoderm of the embryo. The segmental arrangement of the somites along the anterior–posterior body axis prefigures and underlies the metarnerism of the somite derived vertebral column and epaxial muscles and also determines the segmented arrangement of parts of the peripheral nervous system.

This chapter discusses the development of the excretory system. In all mammals, the excretory system begins as a single epithelial duct formed from the intermediate mesoderm, a region between the paraxial or sorrfitic mesoderm and the lateral plate mesoderm. As this nephric duct extends caudally, it induces a linear array of epithelial tubules, which extend medioventrally and are thought to derive from periductal mesenchyme. In mammals, a graded evolution of renal tubule development occurs with the most anterior, or pronephric tubules, being very rudimentary, and the more posterior, mesonephric tubules becoming well developed. In contrast, the pronephros of the zebrafish larvae is a fully developed, functional filtration unit with a single midline glomerulus. In fact, it is not altogether obvious in mammals where to draw the distinction between pronephric tubules and mesonephric tubules.

The vertebrate body plan has bilateral symmetry and left-right asymmetries that are highly conserved. The molecular pathways for left-right development are beginning to be elucidated. Several distinct mechanisms to initiate the vertebrate left-right axis have been proposed. These mechanisms appear to converge on highly conserved expression patterns of genes in the transforming growth factor-beta (TGFbeta) family of cell-cell signaling factors, nodal and lefty-2, and subsequently the expression of the transcription regulator Pitx2, in left lateral plate mesoderm. It is possible that downstream signaling pathways diverge in distinct classes of vertebrates.

Publisher Summary This chapter critically analyzes the current understanding of the transcriptional programs that control smooth muscle cells (SMCs) development and differentiation. One approach to understand the molecular mechanisms that regulate SMC differentiation is to identify the transcriptional regulatory elements and transcription factors that regulate SMC-specific gene expression. This approach has fundamentally increased the understanding of skeletal and cardiac myocyte-specific gene expression and differentiation. Vascular smooth muscle cells (VSMCs) arise from two embryological origins, lateral mesoderm and neural crest. However, it remains uncertain whether SMCs represent a single cell lineage that adapts its phenotype to local environmental cues, or whether SMCs are genetically preprogrammed to distinct tissue-restricted sublineages. Subtle differences in phenotype are observed between mesenchymal and neural crest–derived VSMCs in the proximal aorta. Current paradigms suggest that SMCs reversibly modulate their phenotype from a contractile cell to a synthetic cell. This model is based on the behavior of vascular smooth muscle cells in culture.

Publisher Summary This chapter focuses on the molecules involved in regulation of embryonic vascular development. Molecule involved in early mesodermal differentiation of endothelial cells include fibroblast growth factor (FGF), which plays a critical role in the induction of the mesodermal germ layer during the earliest stages of embryogenesis. FGFs are also required for the development of certain dorsal types of mesoderm, suggesting a synergism between FGF and the transforming growth factor-β family of growth factors, which are required for dorsal mesoderm formation. Angioblast differentiation in the mesoderm requires the activity of vascular endothelial growth factor (VEGF) and its receptor, Flk-1. It seems possible that the directed migration of flk-1 expressing endothelial cells from the lateral plate mesoderm toward the midline of the frog embryo occurs in response to a VEGF signal gradient. In vitro experiments have demonstrated that fibronectin can stimulate the migration of vascular endothelial cells. Second, when the distribution of fibronectin is analyzed in the chick embryo or the chick chorioallantoic membrane, it is associated with both migrating endothelial cells, prior to their coalescence into vessels, and with the early steps of angiogenesis, when capillaries are extending and invading avascular tissue. Further, platelet-derived growth factor is important for the recruitment of vascular wall components. It is found that Tie-2 is required for vascular remodeling. In addition to a role in vasculogenesis, angiogenesis, and endothelial cell migration, in vitro studies suggest that the extracellular matrix also influences the differentiation of endothelial cells.

Publisher Summary This chapter explores the similarities and differences between the development of cardiac and skeletal muscle. Vertebrate skeletal and cardiac muscle share a similar sarcomeric organization and function and express many common protein products, they have different embryonic origins arising from spatially separated populations of mesodermal progenitors. The cardiac cell lineage can be traced to a region of anterior lateral plate mesoderm, whereas skeletal muscle cells in the body are derived from cells within the segmented somites, which in turn arise from the paraxial mesoderm. The timing of terminal differentiation during embryogenesis is another feature that dramatically distinguishes the two muscle systems. Cardiac precursors express differentiation markers long before they complete their morphogenetic program by migrating to their final locations in the primordial heart tube. Skeletal myoblasts, in contrast, either differentiate in situ during somite maturation or undergo differentiation only after they have migrated to their final destination in the embryo. However, the most striking difference between the two muscle cell precursor populations lies in the fact that cardiac precursors differentiate and proliferate at the same time, whereas skeletal myoblasts have to withdraw from the cell cycle in order to differentiate. The discussion concludes that molecular programs governing skeletal muscle myogenesis are likely to be paradigmatic of other developmental processes in the embryonic mesoderm. Delineation of similar genetic pathways in cardiac development may prove to be more challenging, since master gene regulators of cardiac differentiation or commitment have yet to be identified in myocardial or vascular precursors.

Skeletal muscle development requires the formation of myoblasts that can fuse with each other to form multinucleate myofibers. Distinct primary and secondary, slow and fast, populations of myofibers form by the time of birth. At embryonic, fetal, and perinatal stages of development, temporally distinct lineages of myogenic cells arise and contribute to the formation of these multiple types of myofibers. In addition, spatially distinct lineages of myogenic cells arise and form the anterior head muscles, limb (hypaxial) muscles, and dorsal (epaxial) muscles. There is strong evidence that myoblasts are produced from muscle stem cells, which are self-renewing cells that do not themselves terminally differentiate but produce progeny that are capable of becoming myoblasts and myofibers. Muscle stem cells, which may be multipotent, appear to be distinguishable from myoblasts by a number of indirect and direct criteria. Muscle stem cells arise either in unsegmented paraxial mesoderm (anterior head muscle progenitors) or in segmented mesoderm of the somites (epaxial and hypaxial muscle progenitors). These initial stages of myogenesis are regulated by positive and negative signals, including Wnt, BMP, and Shh family members, from nearby notochord, neural tube, ectoderm, and lateral mesoderm tissues. The formation of skeletal muscles, therefore, depends on the generation of spatially and temporally distinct lineages of myogenic cells. Myogenic cell lineages begin with muscle stem cells which produce the myoblasts that fuse to form myofibers.

Publisher Summary Initial steps in smooth-muscle-cell differentiation start in very early stages. The different types of cardiovascular cells originate from the ventrolateral mesoderm (Xenopus) or the splanchnic mesoderm (Aminotes). This ventrolateral mesoderm can be regionalized using smooth muscle α-actin as a marker. Unlike the endothelial cells and cardiomyocytes, smooth muscle cells remain positive for smooth muscle α-actin expression. Smooth muscle cells are not unique in expressing smooth muscle α-actin. Other cells, most probably also from ventrolateral mesoderm origin, express smooth muscle α-actin as well, suggesting that smooth muscle α-actin marks a certain differentiation in state of cells of ventrolateral mesoderm origin or perhaps a functional state required in many cells during development. Actin expression in relation to the developing tunica media is associated with remodeling of the vascular system. There are, however, still actin patterns that are not well understood. They may be related to different influences on smooth-muscle differentiation in the vascular wall because until now, it is not known exactly which cells contribute to the development of vascular smooth muscle cells. In this respect, the role of neural crest cells is not clear as well. Some of the neural crest cells differentiate into smooth muscle cells, but others may have an inductive role.

This chapter describes the urinogenital system. The rudiment of the excretory system in the tadpole arises well before hatching as a longitudinal thickening of the somatic mesoderm on each side of the coelom, below the myotomes. Near the mouths of the funnels, a sacculated outgrowth of splanchnic layer appears. It is known as the glomus and becomes filled with blood from the dorsal aorta. The frog kidney, mesonephros, consists of a series of paired masses of cells in the nephrotomes in these middle segments, each of which develops into one of the kidney tubules, having at one end an opening into the segmental duct and at the other a Malpighian capsule with a glomerulus. In frog, and in amphibians generally, the segmental duct tends to lose its organizing capacity after it has traversed about two-thirds of its journey to the cloaca; no more mesonephros results behind this as the tube passes under or through the remaining posterior nephrotomes. The functional kidney of mammals, birds, and reptiles is called metanephric and its development is more complex.

Publisher Summary This chapter discusses the heart and vascular system. In the mesoderm of all vertebrates, there are blood vessels. In almost all forms, the arrangement of these vessels is related very closely to the transverse septum. The mesoderm of all vertebrates splits very soon after its movement over the edge of the blastopore. After the production of about a third of the mesoderm, splanchnopleure and somatopleure separate more completely; after this, coelom becomes more obvious between these layers posteriorly. The frog has no definable AIP (anterior intestinal portal) because its gut is an enclosed tube, with a floor, from its formation as the enteron. In most vertebrates, the heart is formed from a pair of rudiments, one each side of the midventral line under the foregut. They seem to be oriented in their paths by differential stickiness of the upper surface of the hypoblast, so that the individual cells finally come to lie as a group on either side of the forming AIP. Meanwhile, areas of splanchnic mesoderm have been breaking down to give blood spaces. The cells lose cell contacts with their neighbors and soon begin to synthesize hemoglobin and become blood cells lying in tissue fluids. Arteries run down the dorsal mesentery from the aorta to supply the gut and some of these survive in the adult as the anterior mesenteric artery, the lienogastric artery, the coeliac artery, and the posterior mesenteric and rectal arteries.

The immune system of anuran amphibians is established during the first 2–3 weeks postfertilization. The lymphoid organs are populated by cells of lateral plate mesoderm. Around day 12 the larval immune system starts to function and exhibits the following characteristics: low titers of antibody, poor IgG analog production, an antibody heterogeneity lower and different from the adult, presence of class II MHC molecules, lack of class I MHC molecule expression, and a not fully differentiated capacity of allograft rejection. The transition to the adult system occurs at metamorphosis when a second histogenesis of the lymphoid system occurs.

This chapter provides an overview on the muscular system. This system of vertebrates arises partly from the segmental somatic mesoderm, partly from the more ventral unsegmented mesoderm and partly from ectomesenchyme derived from neural crest cells. Along each side of the spinal cord, and usually of the brain, is a row of mesodermal somites. As these develop each becomes divided, probably under the inductive influence of neural tissue medially, skin laterally, and perhaps gut ventrally, into four regions. These are myotome, sclerotome, dermatome, an intermediate cell mass soon becoming nephrotome and gonotome. All of the segmental muscles of vertebrates arise in the embryo as paired concentrations of somatopleure on either side of the nerve cord and notochord, myotomes. Myotomes of the three most anterior segments provide the extrinsic eye musculature, myotomes four and five degenerate more or less completely; six, seven, and eight contribute to the gill musculature and may perhaps grow ventrally and contribute to the muscles of the lower jaw and even the tongue.