Chapter 9 Heart Development - early stages PDF

2024 Chapter 9 Chapter 9: Formation and turning of the heart tube Key terms and concepts: Cardiogenic field Ectopia Cordis, Cardia Bifida Heart tube, endocardial tube...

2024 Chapter 9 Chapter 9: Formation and turning of the heart tube Key terms and concepts: Cardiogenic field Ectopia Cordis, Cardia Bifida Heart tube, endocardial tube Truncus arteriosus Myocardium Bulbus cordis Intraembryonic coelom Primitive atrium Cardinal veins Primitive ventricle Vitelline vessels Sinus venosus Umbilical vessels Pericardial cavity Septum transversum Bulboventricular loop Dorsal mesocardium Learning objectives: By the end of this unit, you should be able to: 1. Describe the formation of intraembryonic coelom and primitive heart tube. How does the primitive heart tube become positioned in the embryo? Explain the physical location of the primitive heart tube and septum transversum during heart development. 2. Describe the location of the pericardial cavity and the relative position of the heart tube in the pericardial cavity during development. 3. Explain the role of folding and differential growth in the positioning of the heart. 4. Describe the layers of primitive heart. What tissue gives rise to the myocardium, cardiac jelly and epicardium of the primitive heart? 5. Be able to describe in 3D, preferably using props, the process of heart looping and how this process contributes to the adult configuration of the heart. 6. Know the names for the primitive subdivisions of the embryonic heart and how these regions will arrange themselves in space to form the mature heart. 7. Identify which “overarching themes in developmental biology” are relevant to this chapter. 8. Describe how any of the “general goals” of this course (listed in the syllabus) have been addressed in this unit. Watch the following demo depicting heart fields, Intraembryonic coelom, and Folding of the Embryo. https://mediaspace.wisc.edu/media/Demo+1- Formation+of+heart+tubes/1_dfqwi8wj Watch the following movie depicting heart development. It is a good idea to watch this movie several times over the course of the heart development unit. At first, just watch the movie for an overview of the process of heart development. Later, watch it with an eye for the details. Use the video to test your knowledge of the process of heart development at the end of the unit. http://www.youtube.com/watch?v=5DIUk9IXUaI&feature=related 1 2024 Chapter 9 I. Introduction A. The cardiovascular system is the first organ system to start developing and reach a functional state, even before its own development is complete. Cardiovascular development occurs during the 3rd to 5th week of embryonic life in dogs. B. Here is the problem: Up until the second week, diffusion is sufficient for the embryo to receive oxygen and nutrition and to get rid of waste products. However, by the third week, as the size and complexity of the embryo increase, diffusion alone is not sufficient to meet the needs of the growing embryo. The cardiovascular or circulatory system, including the heart, arteries, veins, and blood, develops as early as the third week of gestation to meet this need. C. We will start with the development of the heart. Then, we will concentrate on the formation of blood and blood vessels, including major arteries and veins. II. Heart fields A. The heart fields are of intra-embryonic mesodermal origin and are established at the cranial-most end of the embryonic disc, ahead (or “anterior”) of the future brain. B. The heart fields, also known as the cardiac crescent or cardiogenic field, will develop into heart tubes and eventually mature into a fully developed heart. C. Mesodermal cells proliferate in a position anterior to the cardiogenic field and form the septum transversum. III. Formation of the Heart Tube and Pericardial Cavity A. The intra-embryonic mesoderm on each side forms longitudinal columns, known as the paraxial (somite) mesoderm. Each column is continuous laterally with the intermediate mesoderm, which gradually thins out further laterally into the lateral plate mesoderm. B. The lateral plate mesoderm splits into two layers: the splanchnopleure and the somatopleure. C. The space between the splanchnic and somatic layers is the intra-embryonic coelom. The coelom on the right and the coelom on the left fuse, which later divides into the body cavities, namely the pericardial cavity, pleural cavities, and abdominal (peritoneal) cavity. D. The underlying endoderm layer secretes VEGF and induces the adjacent splanchnic layer to form blood islands. These islands coalesce to form a single large heart tube or endocardial tube on either side. The intra-embryonic coelom in this region will become the pericardial cavity. IV. Folding of the Embryo A. Up until now, the embryo has been a flattened disc. Notice that the heart tube at first is very near (anterior) to the brain and oral cavity (remember that the buccopharyngeal membrane will be located in the mouth). How does the heart get to its adult location in 2 2024 Chapter 9 the thorax? This occurs as a result of cranio-caudal and lateral folding. These foldings occur simultaneously and are not separate sequential events. The folding happens because certain tissues, such as the brain, are growing faster than others. B. Cranio-caudal folding: The forebrain grows cranially beyond the oropharyngeal membrane and overhangs the primitive heart. At the same time, the septum transversum, the heart tubes, and the pericardial coelom turn under onto the ventral surface. Cranio-caudal folding swings the heart tube into a position just ventral to the foregut pocket in the neck of the embryo, with the inflow oriented toward the tail of the embryo and the outflow oriented toward the head. The heart tube is suspended from the body wall by a sling of connective tissue called the dorsal mesocardium. After folding, the septum transversum lies caudal to the heart and develops into a major portion of the diaphragm. C. Lateral folding: Each lateral body wall (somatopleure) folds toward the midline, rolling the edges of the embryonic disk ventrally to form a cylindrical embryo. This lateral folding brings the forming heart tubes to the midline, where they fuse into a single tube. 3 2024 Chapter 9 D. Differential growth: The differential growth of the embryo causes the heart to be displaced toward the tail of the embryo, ultimately positioning the heart in the chest. CONCEPT: Differential Growth Tissues do not always grow at the same rate. Some will spurt ahead of others, just as the brain is doing here. If two structures start next to each other, then one slows its growth, or the other speeds up, can you see that their relative positions will change? We will see lots of examples of this later on. E. Formation of the Myocardium and Epicardium: The splanchnic mesoderm contributes to a second layer of cells that forms the myocardium around the primordial heart tubes. This newly formed myocardium begins secreting connective tissue components known as cardiac jelly, which will eventually become the connective tissue of the heart. The caudal or inflow part of the heart tube, the Sinus Venosus, provides cells that form the inner layer of the pericardium, also called the epicardium. These primordial heart tubes are surrounded by the pericardial cavity. F. Heart Malformations: Ectopia Cordis: A very rare condition where the heart is located outside the thoracic cavity. This can occur due to: a. Failure of the heart to descend into the thorax. b. Failure of the ribs and sternum to close around the heart. Cardia Bifida: Another rare condition where the bilateral heart fields remain separate, resulting in two separate hearts forming in the embryo, one on each side of the body. V. Regions of the "Primitive" heart tube, looping and derivatives A. Following fusion of the endocardial tubes and the degeneration of the dorsal mesocardium, the single cardiac tube is suspended in the pericardial cavity. The caudal and cranial ends never completely fuse, forming the inflow and outflow tracts of the heart. Caudally, the solid chunk of mesoderm is called the septum transversum, which will become the future diaphragm. B. Differential Growth: The heart tube undergoes differential growth, causing it to bulge at several points along its length.. C. Dextral Looping: The heart tube then undergoes right-sided bending or rotation, known as dextral looping or bulboventricular looping. The truncus arteriosus and bulbous cordis grow more rapidly, folding downwards, forwards, and to the right. Meanwhile, the lower parts of the tube, including the primitive atria and sinus venosus, fold upwards, backwards, and to the left. This dextral looping positions the heart chambers in their adult anatomical locations, with the atria anteriorly and the ventricles more posteriorly. Failure of the heart to loop properly results in dextrocardia, where the heart lies on the right side. 4 2024 Chapter 9 The twisting of the bulboventricular loop to the right is one of the first visible left-right asymmetrical events in the body. From this point, several organs will form asymmetrically with respect to the body axis. What causes this predictable, consistent breakdown in the early bilateral symmetry of the embryo? There is still much to learn about this process. As you learned in the gastrulation chapter (Chapter 5), cilia located at the primitive node are crucial in establishing signaling cascades that determine left-right positional identity in the embryo. Left-right axis specification by the node results in asymmetrical patterns of gene expression that direct asymmetrical events in the body. Abnormal left- right axis specification can lead to abnormal formation of the heart and improper positioning of asymmetrical organs in the body. This condition is known as Situs Inversus, where organs are positioned in a mirror-image to their usual locations. For more information about left-right axis specification, see a review article at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2742083/?tool=pubmed. D. Five Subdivisions of the Heart: Five subdivisions can be identified, each contributing to a different region of the definitive heart. Let’s follow the path of blood, starting from the caudal end of the primitive heart, and identify their adult derivatives (primitive = embryonic, definitive = mature form). 1. Right and Left Sinus Venosus a. The sinus venosus remains paired. Each half receives a set of three veins that travel through the septum transversum to reach the heart. We will discuss each of them in more detail later. 1) Vitelline veins - These veins bring blood back from the yolk sac. 2) Common cardinal veins - These veins drain the body of the embryo. 3) Umbilical veins - These veins are of extreme importance to the embryo since they carry blood back from the placenta, and so contain the highest level of oxygen and nutrients of any vessels in the embryo. b. Sinus Venosus: On the right side, it forms the Sinus Venarum (the smooth part of the right atrium), the cranial vena cava, and the caudal vena cava. On the left side, it forms the coronary sinus. 2. Primitive Atrium a. The primitive atrium is a single chamber that will later divide into the definitive right and left atria. As the heart curves, the cranial and caudal ends are brought closer together, positioning the atria dorsal to the rest of the heart. 5 2024 Chapter 9 b. The primitive atrium eventually develops into the trabeculated (rough) parts of the right and left atria, known as the pectinate muscles. 3. Primitive Ventricle and Bulbus Cordis a. Blood moves from the primitive atrium into the primitive single ventricle. b. We will describe in the next section how the Primitive Ventricle and Bulbus Cordis become the left and right ventricles. 4. Truncus Arteriosus a. This cranial portion of the tube is the “outflow” region of the heart and will form both the aorta and the pulmonary trunk. TRY THIS ONE! If you want to visualize how the heart you see in gross lab originated from a straight tube, try the following: With your right hand, grab the pulmonary trunk and aorta. This is the truncus arteriosus. With your left hand, grab the caudal and cranial vena cavae and the pulmonary veins (it's a stretch!). Think of this as the sinus venosus. Imagine that you pull your hands apart, straightening out the heart. Blood would go into the sinus venosus, through the atria and ventricles, then into the truncus arteriosus. That's the basic plan. What we will do in the next section is to find out how these single chambers become double. If you're confused, try this again at the end of the unit on heart development. Thought questions: Prior to the development of the vascular system, what are the mechanisms and sources of nutrition and gas exchange for the embryo? If you were designing it, when would you start the heart beating? Why? What effect will the development of the heart and blood vessels have on the growth of the embryo? 6