Cardiovascular System Histology & Embryology PDF

CARDIOVASCULAR SYSTEM DEVELOPMENT OF THE CARDIOVASCULAR SYSTEM The cardiovascular system (CVS) is the first system to function in the embryo. Until the 3rd week, it is nourished by diffusion. Starting from the 3rd week, the cardiovascular system begins to develop. DEVELOPMEN...

CARDIOVASCULAR SYSTEM DEVELOPMENT OF THE CARDIOVASCULAR SYSTEM The cardiovascular system (CVS) is the first system to function in the embryo. Until the 3rd week, it is nourished by diffusion. Starting from the 3rd week, the cardiovascular system begins to develop. DEVELOPMENT OF THE HEART The first stage of development begins with the settlement of epiblastic cells located on either side of the primitive streak into the cranial region of the lateral splanchnic mesoderm. The cranial-lateral endodermal cells of the embryo induce the formation of the cardiogenic mesoderm. In the early stages of heart development, angioblastic cords are formed in the 3rd week. o The angioblastic cords within the mesoderm develop into solid heart tubes. These tubes unite to form a single endocardial heart tube. o The mesenchymal cells surrounding the endocardial tube and the extracellular matrix between the myocardial and endocardial layers synthesize cardiac jelly. At the cranial end of the heart tube, the aortic sac is formed. At the caudal end, the heart tube joins to form the sinus venosus. The angioblastic cords at the intraembryonic coelom lead to the creation of the intraembryonic pericardial cavity. Mesothelial cells originating from the somatic and splanchnic mesoderm layers sequentially form the pericardium. The heart tube begins beating at 22–23 days. With the beginning of the heartbeat, blood flow starts in the embryo by the 4th week. DEVELOPMENT OF BLOOD VESSELS Within the mesoderm surrounding the heart region, epiblastic cells migrate and form separate vascular precursors (angioblastic cords) on the dorsal aorta and cardinal veins. Angioblastic cords are responsible for new vessel formation (vasculogenesis). The branching of the newly formed vessels is known as angiogenesis. During embryonic development, vessels forming on both the right and left sides of the cranial region are initially symmetrical. The aorta caudally merges and forms a single vessel. However, the cranial region remains symmetrical for a longer period. From the cranial region of the heart tube, the aortic sac forms. Branches extending from the aortic sac create the pharyngeal (aortic) arches. These arches initially form symmetrically on both sides. The dorsal aortae running from the cranial region to the caudal region form the main artery system in the embryo. Among the six pairs of pharyngeal arches that develop, the 5th arch does not develop or remains rudimentary. Vessels Developing from Pharyngeal (Aortic) Arches Aortic Arch Developed Structure 1st aortic arch Maxillary artery, part of the external carotid artery 2nd aortic arch Stapedial artery, hyoid artery Common carotid artery, part of the internal carotid 3rd aortic arch artery 4th aortic arch (right) Proximal part of the right subclavian artery 4th aortic arch (left) Contributes to the structure of the aortic arch 6th aortic arch (right) Right pulmonary artery 6th aortic arch (left) Left pulmonary artery, ductus arteriosus By the 4th week, the aorta and the developing embryonic veins establish three paired veins, forming connections with the sinus venosus of the heart: Vitelline vein system Umbilical vein system Cardinal vein system 1. Vitelline Vein System Location: These veins originate from the yolk sac and enter the embryo through the omphalomesenteric canal. Function: They drain blood from the yolk sac into the sinus venosus. Fate: o The right vitelline vein: ▪ Develops into the hepatic portal system, which carries blood from the gastrointestinal tract to the liver for processing. ▪ Forms part of the inferior vena cava (IVC), contributing to venous return from the lower body to the heart. o The left vitelline vein: ▪ Becomes rudimentary (non-functional) and eventually disappears. 2. Umbilical Vein System Location: These veins carry oxygenated blood from the placenta to the developing embryo. Fate: o The right umbilical vein: ▪ Disappears by the 7th week. o The left umbilical vein: ▪ Forms the ductus venosus, a critical structure that connects the umbilical vein to the inferior vena cava, allowing oxygenated blood to bypass the liver and flow directly to the heart. 3. Cardinal Vein System Location: These veins drain blood from the embryo's body and are divided into: o Anterior cardinal veins: Drain blood from the cranial (head) region. o Posterior cardinal veins: Drain blood from the caudal (lower) region. Fate: o The cardinal vein system evolves into the caval system, which includes the superior vena cava (SVC) and parts of the inferior vena cava (IVC). PHYSIOLOGY - HISTOLOGY AND EMBRYOLOGY The umbilical veins progress to the inner part of the liver. The umbilical vein, which carries highly oxygenated blood from the placenta, empties into the sinus venosus. However, the connection of the left umbilical vein to the heart quickly diminishes. The right umbilical vein disappears in the 7th week of development. The left umbilical vein continues to develop and forms the ductus venosus, which connects the liver to the inferior vena cava during further stages of development. The cardinal venous system forms with the merging of anterior and posterior cardinal veins. o The anterior cardinal vein carries venous blood from the cranial region to the sinus venosus. o The posterior cardinal vein carries venous blood from the caudal region to the sinus venosus. o Later, the cardinal venous system evolves into the caval system. SEPARATION OF HEART CHAMBERS The primordial heart tube is divided into separate chambers that form atria and ventricles. This separation process also results in the development of valves. The division process begins in the 4th week and continues until the 8th week. Division of the Atrioventricular Canal At the end of the 4th week, neural crest cells in the cardiac jelly form endocardial cushions. The endocardial cushions grow and merge in the ventral and dorsal regions, creating a right and left atrioventricular canal. During this process, the endocardial epithelial cells acquire mesenchymal characteristics and contribute to forming valves and the membranous septum. Division of the Atria Towards the end of the 4th week, the septum primum and septum secundum begin to form, dividing the atria into left and right chambers. Initially, a crescent-shaped tissue grows downward from the roof of the atrium, reaching the endocardial cushions, known as the septum primum. The gap between the septum primum and the endocardial cushions is called the ostium primum. This gap gradually narrows and eventually closes completely, forming the ostium primum closure. Following the closure of the ostium primum, apoptosis occurs near the roof of the atrium, creating a new opening called the ostium secundum. During this process, a second crescent-shaped tissue grows parallel to the septum primum. This tissue is known as the septum secundum. The septum secundum, along with the septum primum, forms the foramen ovale. The septum primum acts as a valve for the foramen ovale on the left atrium side. Division of the Ventricles A bulge begins to grow upward from the apex of the ventricles. This bulge merges with the membranous septum, formed by the endocardial cushion cells. Until the 7th week, before full fusion occurs, there is a gap known as the interventricular foramen. By the end of development, this foramen is completely closed.  CARDIOVASCULAR SYSTEM HISTOLOGY The cardiovascular system is a tubular structure composed of the heart, blood vessels, and lymphatic vessels. Blood leaving the heart flows through arteries to the body. Arteries branch into arterioles and reach tissues. Exchange occurs in capillaries, and blood returns to the heart through venules and veins. The cardiovascular system contains immune system-associated cells, and these cells can pass from venules to tissues. There are two main circulatory systems in the body: o Systemic circulation: Begins in the left ventricle, delivers blood to tissues, and ends when the blood returns to the right atrium of the heart. o Pulmonary circulation: Begins in the right ventricle, sends blood to the lungs, and ends when it returns to the left atrium of the heart. Sometimes, atypical divisions of the circulatory system are observed. These include the portal system. o A known example is the hepatic portal system and the hypothalamic-pituitary portal system. HISTOLOGICAL STRUCTURE OF THE HEART The heart functions as a muscle pump, ensuring the unidirectional flow of blood. It consists of three layers from the inside out: 1. Endocardium 2. Myocardium 3. Epicardium Endocardium Myocardium Epicardium Composed of endothelial and Composed of striated muscle The serous pericardium's visceral layer subendothelial connective tissue. originating from the heart muscle. is referred to as the epicardium. The outermost layer of the heart is The endothelium is made up of It is the thickest layer of the heart. composed of a single layer of mesothelial single-layer squamous epithelial cells and underlying adipose and cells and has the same cells as The atrial myocardium is notably connective tissue. vascular endothelium. thinner than the ventricular Between the epicardium and the parietal myocardium due to differences in pericardium lies the pericardial cavity, The subendothelial connective workload and pressure levels. which contains pericardial fluid that tissue contains smooth muscle facilitates smooth movements of the cells. The myocardium of the atria and heart. ventricles attaches to the fibrous Rapid accumulation of blood or fluid in the Between the endocardium and the layer located in the pericardial cavity is called cardiac myocardium lies the atrioventricular septum. This tamponade. This can lead to life- subendocardial layer, which fibrous layer isolates electrical threatening diastolic heart failure. In contains the cardiac conduction signals between the atria and emergency cases, fluid is drained via pericardiocentesis. system ventricles. In some books, it is noted that the. vascular and nerve network that supplies the heart directly resides within the epicardium. Other Parts of the Heart Interventricular septum: Separates the right and left ventricles with connective tissue and the membranous septum. It is where the conduction system of the heart is located. Interatrial septum: Separates the right and left atria, consisting of connective tissue. Atrioventricular septum: Consists of irregular, dense connective tissue formed by the fibrous ring and the fibrous trigone. It prevents electrical conduction between atria and ventricles. Coronary vessels: A pair of arteries separate from the proximal aorta, nourishing the heart. These terminate in coronary veins and are emptied into the right atrium. Heart valves: Formed by endothelial layers and connective tissue. They lack vascularization and contain no muscle tissue. o The heart valves have no blood vessels. Instead, the valves receive nutrients through diffusion from the blood flowing across their surfaces. o Endothelium on the atrioventricular valves continues toward the papillary muscles attached to the chordae tendineae HISTOLOGICAL STRUCTURE OF BLOOD VESSELS Blood vessels, arteries, capillaries, veins, and lymphatic vessels are classified into four types. Each vessel wall has three layers, from inside to out: 1. Tunica intima 2. Tunica media 3. Tunica adventitia CARDIOVASCULAR SYSTEM HISTOLOGY AND PHYSIOLOGY Tunica Intima The innermost layer of blood vessels. It is composed of endothelium, basal lamina, and loose connective tissue (subendothelial connective tissue). Endothelium is composed of single-layer squamous epithelial cells. Produces substances like nitric oxide and endothelin. Contains the angiotensin-converting enzyme (ACE), which converts angiotensin I into angiotensin II. Includes Weibel-Palade bodies, which contain P-selectin and von Willebrand factor (VWF): o Von Willebrand factor: Plays a role in platelet adhesion. o P-selectin: Plays a role in neutrophil transmigration. o Endothelin: Causes vasoconstriction. The subendothelial connective tissue of the tunica intima contains fenestrated elastic lamina (internal elastic lamina). Facilitates diffusion of materials between the blood and the vessel wall. Tunica Media A layer composed of smooth muscle cells. The thickest layer in arteries compared to veins. Contains elastic fibers (like type III collagen) and proteoglycans and glycoproteins. The external elastic lamina separates the tunica media from the tunica adventitia. Tunica Adventitia (externa) A layer containing type I collagen and elastic fibers in small amounts. This layer is thicker in large veins compared to arteries. In large vessels, the vasa vasorum (small vessels supplying the vessel wall) are found in the tunica adventitia and tunica media. It also houses nervi vascularis, (perivascular nerves) autonomic nerves regulating the smooth muscle cells of the vessels. PHYSIOLOGY - HISTOLOGY AND EMBRYOLOGY Arteries Arteries are classified into four main categories: 1. Large arteries (elastic arteries) 2. Medium-sized arteries (muscular arteries) 3. Small arteries 4. Arterioles Large (Elastic) Arteries Includes the aorta and its major branches. These vessels are responsible for maintaining blood flow from the heart and handling the formation of diastolic pressure. Their elastic walls store energy during systole and ensure continuous blood flow during diastole. Contain a significant amount of elastic fibers in their tunica media, forming elastic lamellae. The tunica media is the thickest layer and contains many elastic lamellae and smooth muscle cells. Smooth muscle cells are interconnected by gap junctions and synthesize connective tissue components. Tunica adventitia is relatively thin and composed of connective tissue. It also contains vasa vasorum and nervi vascularis. Medium-Sized (Muscular) Arteries These arteries distribute blood to the organs. Characterized by a prominent internal elastic lamina. The tunica media contains up to 40 layers of smooth muscle cells. Elastic fibers are present but not as prominent as in elastic arteries. The fibroblast layer is located between the elastic lamina and adventitia. Small Arteries and Arterioles The main difference between small arteries and arterioles is the number of smooth muscle layers: o Small arteries have 8–10 layers of smooth muscle cells. o Arterioles have only 1–2 layers of smooth muscle cells. Arterioles lack the external elastic lamina present in larger arteries. The most notable feature of arterioles is their ability to control blood flow to capillaries by regulating vascular resistance. For this reason, arterioles are referred to as resistance vessels. Arterioles lead to metarterioles and precapillary sphincters: o Metarterioles: Transitional vessels between arterioles and capillaries with scattered smooth muscle cells. o Precapillary sphincters: Control the flow of blood into capillaries. CARDIOVASCULAR SYSTEM HISTOLOGY AND PHYSIOLOGY Capillary Vessels The smallest blood vessels in terms of diameter but have the largest total cross-sectional area. Vessels responsible for gas (O₂, CO₂) and nutrient-exchange between blood and tissues. Most capillaries are smaller in diameter than a single red blood cell. Composed only of endothelium and basal lamina, with no smooth muscle or adventitia. The thin walls facilitate diffusion, and blood flow is controlled by vasomotion (constriction and dilation of upstream vessels). Morphological Classification of Capillaries Capillaries are divided into three types based on their structure: 1. Continuous (Non-fenestrated) Capillaries – Type 1 Capillaries 2. Fenestrated Capillaries – Type 2 Capillaries 3. Discontinuous (Sinusoidal) Capillaries – Type 3 Capillaries Continuous (Non-Fenestrated) Capillaries – Type 1 Capillaries Found in the nervous system, muscles, and lungs. Endothelial cells are connected by tight junctions (zonula occludens), which reduce permeability and limit diffusion. Substances pass through via pinocytosis. The basal lamina is continuous. Surrounded by pericytes (Rouget cells): o These are contractile cells that support the capillaries. o Pericytes are present around capillaries and post-capillary venules. Have embryonic characteristics resembling mesenchymal cells. Play a role in wound healing, contributing to the formation of new endothelium and smooth muscle cells. Fenestrated Capillaries – Type 2 Capillaries Found in tissues involved in active filtration or secretion: o Digestive organs, kidney peritubular capillaries, endocrine glands, and choroid plexus. The basal lamina is continuous. Endothelial cells contain fenestrations (pores) that allow the rapid exchange of substances. o These fenestrations are covered by a thin diaphragm (except in certain locations). Kidney glomeruli are composed of highly fenestrated capillaries, but the fenestrations lack diaphragms. o These are referred to as non-diaphragmed fenestrated capillaries. Discontinuous (Sinusoidal) Capillaries – Type 3 Capillaries Typically found in the liver, bone marrow, lymph nodes, and spleen. distinguished from other capillaries by the lack of a continuous basal lamina. Larger in diameter compared to other capillaries. Their walls may contain phagocytic cells to facilitate immune functions. Veins Veins are classified into four main categories: 1. Large veins 2. Medium-sized veins 3. Small veins 4. Venules Venules Divided into postcapillary venules and muscular venules: o Postcapillary venules: ▪ Lack tunica media and contain only a thin tunica adventitia. ▪ Surrounding them are pericytes. o Muscular venules: ▪ Have a tunica media containing 1–2 layers of smooth muscle cells. ▪ Pericytes are absent. Postcapillary venules are the primary site for fluid and leukocyte exchange. Small and Medium-Sized Veins Characterized by the presence of valves (valvulae) to prevent backflow. Tunica intima: Contains valves. Tunica media: Thin, with a few layers of smooth muscle cells. Tunica adventitia: Thick and well-developed. Large Veins Also known as reservoir veins due to their high compliance. Tunica intima: Thin, with an elastic membrane visible. Tunica media: Thin, containing a few layers of smooth muscle cells. Tunica adventitia: Thick and well-developed. o Contains vasa vasorum and longitudinal smooth muscle bundles, which provide additional structural support. Arteriovenous Anastomoses Direct connections between arteries and veins. All arteriovenous anastomoses contain nerve endings. Play a critical role in thermoregulation by controlling blood flow. In specialized areas like fingers, toes, and ears, complex anastomoses are found, called glomerular structures (glomera). Lymphatic Vessels Lymphatic vessels are blind-ended tubules. Their primary function is to transport tissue fluid to the blood vessels. Lined with a thin layer of endothelium. Possess a discontinuous basal lamina. Do not have a distinct separation between the layers of their walls. Contain more valves compared to veins to ensure unidirectional lymph flow.