Category: Embryology

Sinus venosus forms the inflow tract of the primitive heart tube. On each side of the sinus venosus, the common cardinal veins open into it. Anterior and posterior cardinal veins combine to form the common cardinal vein on each side of the sinus venosus. The common cardinal vein on the right side forms part of the superior vena cava. Besides the common cardinal veins, the umbilical and the vitelline veins also drain into the sinus venosus. On the left side, these veins undergo specific remodelling. This results in formation of specific anastamosis on the left side, giving rise to left to right shunts thereby causing most of the blood to be received by the right side of the sinus venosus. Consequently, the veins of the right side start maturing and increase in size relative to their left side counterparts. As the sinus venosus also grows and matures, it becomes incorporated into the right side of the primitive atrium. Eventually the vitelline, common cardinal and the umbilical veins of the left side degenerate. The blood received by the left side of sinus venosus is greatly reduced and as a result the left side of the sinus venosus shrinks.  Inferiorly the posterior cardinal veins anastamose together and form the iliac veins.

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The formation of the inferior vena cava is contributed by three set of veins:

  1. Anterior & posterior cardinal veins

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This lecture discusses about the structure, function and the differences in between the different components of the vasculature (arteries, arterioles, capillaries, venules, veins). Before delving into the details about individual components of the vasculature, an understanding about the general structure of the blood vessel will be discussed. A standard blood vessel is composed of three layers, which are discussed as following:

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  1. TUNICA INTIMA forms the innermost layer of any vessel. Tunica intima itself is composed of three component layers. Endothelium forms the innermost aspect of the tunica intima. Surrounding the endothelium is a connective tissue layer called the subendothelial layer. Subendothelial layer has a varying amount of elastic and smooth muscle fiber depending on the type of vessel. In elastic arteries the density of elastic and smooth muscle fibers is higher compared to smaller arteries. Outermost component of the tunica intima is the internal elastic lamina.
  2. TUNICA MEDIA

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DEVELOPMENT OF INTERVENTRICULAR SEPTUM & PARTITION OF PRIMITIVE VENTRICLES

Embryonic time line of IV Septum Development: begins in the late 4th week and commences by the end of 7th week.  

By the end of 4th week, the primitive ventricle is one single chamber which receives blood from the atria via the divided atrioventricular canal. Then there’s also a single aorticopulmonary trunk opening into primitive ventricles which forms the outflow tract. At this point aorticopulmonary trunk is formed of bulbus cordis and truncus arteriosus and it provides the outflow tract for the primitive ventricles. During later parts of the development the bulbus cordis gets incorporated into the right and left ventricles. Upon incorporation, on the right side the bulbus cordis forms the infundibulum of the right verntricle, and on the left side it forms the vestibule of the left ventricle. Hence, bulbus cordis forms the smooth outflow tracts of the ventricles on either side. Truncus arteriosus on the other hand develops into forming aorta and the pulmonary trunk on the left and the right sides respectively.

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Adult IV septum consists of two parts, a muscular part (derived from the myocardial cells) which forms the majority of the septum, and a relatively thin membranous part which forms the superior aspect of the IV septum which is part of the outflow tract.

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The aorta arises from the left ventricle as its outflow tract. Aorta forms an arch before descending downwards as the thoracic/descending aorta. This arch of aorta gives rise to three arteries; from right to left these arteries are the brachiocephaic trunk, left common carotid and the left subclavian artery. On the right side the brachiocephalic trunk gives rise to right subclavian artery and then continues as the right common carotid artery which eventually divides into internal and external carotid arteries of the right side. The left common carotid artery is a direct branch of arch of aorta and it ends up dividing into external carotid and internal carotid arteries of the left side. Finally the left subclavian artery arises from the arch of aorta prior to the ductus arteriosus, after which the arch curves downawards and continues as descending or thoracic aorta.

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As the embryo is developing, two dorsal aortae develop on either side and give off branches to oxygenate ventral and dorsal parts of the embryo. Simultaneously the pharyngeal arches develop along with their pharyngeal arteries. There are six pharyngeal arteries that develop.

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Atrial & Ventricular septum formation process begins at the same time by the start of the 4th week and wraps up by the 8th week. Though occurring simultaneously, however, atrial septation is a little ahead of ventricular septation. Common congenital cardiac anomalies mostly occur due to defects in the formation of these septae. In this lecture and its subsequent review, we discuss the high yield topic of atrial septation and development plus relevant congenital defect which can occur during this process.

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FUNCTION OF INTERATRIAL SPETUM:

  1. It divides the primitive atria into a right and left atria.

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INTRODUCTION: Cardiovascular system is the first organ system to start developing and reach a functional state; which is even before its own development is complete. Cardiovascular development occurs during the 3rd to 5th week of intraembryonic life. Up until the second week, diffusion is enough for embryo to receive the oxygen, nutrition and to get rid of the waste products. The lacunae of maternal blood filled spaces and embryonic villi of the syncytio-trophoblast are involved in an intimate relationship, which allows gaseous exchange via diffusion in between the developing embryo and the maternal blood. However, into the third week, diffusion alone is not sufficient to match the needs of the growing embryo and it needs a circulatory system, hence heart has to develop.

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GASTRULA  STAGE:Gastrula is a stage of development of embryo, when it is in the form of tri-laminar germ disc including ectoderm, mesoderm and endoderm. Endoderm is associated with umbilical vesicle; ectoderm is associated with Amniotic cavity. Cytotrophoblast around the gastrula develops multiple cavities. The cavities merge to form one big cavity called extraembryonic coelom. The cytotrophoblast is connected to embryo through the connecting stalk. The cellular layer from cytotrophoblast which covers the gastrula is called extraembryonic mesoderm

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