LEGACY UNIVERSITY, THE GAMBIA Knowledge and Research for Integrity

Embryology Of The Heart The development of the heart. The primitive heart is a single tube which soon shows grooves demarcating the sinus venosus, atrium, ventricle and bulbus cordis from behind forwards. As this tube enlarges it kinks so that its caudal end, receiving venous blood, comes to lie behind its cephalic end with its emerging arteries.

Embryology Of The Heart Legend Figure For The coiling of the Primitive heart tube into Its definitive form.

Embryology Of The Heart The sinus venosus later absorbs into the atrium and the bulbus becomes incorporated into the ventricle so that, in the fully developed heart, the atria and great veins come to lie posterior to the ventricles and the roots of the great arteries. The boundary tissue between the primitive single atrial cavity and single ventricle grows out as a dorsal and a ventral endocardial cushion which meet in the midline, thus dividing the common atrio-ventricular orifice into a right (tricuspid) and left (mitral) orifice. The division of the primitive atrium into two is a complicated process but an important one in the understanding of congenital septal defects

Legend For The development of the chambers of the heart. (Note the septum primum and septum secundum which form the interatrial septum, leaving the foramen ovale as a valve- like opening passing between them.)

The Embryology Of The Heart. A partition, the Septum Primum, grows downwards from the posterior and superior walls of the primitive common atrium to fuse with the endocardial cushions. Before fusion is complete, however, a hole appears in the upper part of this septum which is termed the Foramen Secundum in the Septum Primum. A second membrane, the Septum Secundum, then develops to the right of the primum but this is never complete; it has a free lower edge which does, (Note the Septum Primum and Septum Secundum which form the Interatrial Septum, leaving the Foramen Ovale as a valve-like opening passing between them.)however, extend low enough for this new Septum to overlap the foramen secundum in the Septum Primum and hence to close it.

The Embryology Of The Heart. The two overlapping defects in the septa form the valve-like foramen ovale which shunts blood from the right to left heart in the fetus. After birth, this foramen usually becomes completely fused leaving only the fossa ovalis on the septal wall of the right atrium as its memorial. In about 10% of adult subjects, however, a probe can still be insinuated through an anatomically patent, although functionally sealed foramen. Division of the ventricle is commenced by the up growth of a fleshy septum from the apex of the heart towards the endocardial cushions. This stops short of dividing the ventricle completely and thus it has an upper free border, forming a temporary interventricular foramen.

Embryology Of The Heart At the same time, the single truncus arteriosus is divided into aorta and pulmonary trunk by a spiral septum (hence the spiral relations of these two vessels), which grows downwards to the ventricle and fuses accurately with the upper free border of the ventricular septum. This contributes the small pars membranacea septi, which completes the separation of the ventricle in such a way that blood on the left of the septum flows into the aorta and on the right into the pulmonary trunk. The primitive sinus venosus absorbs into the right atrium so that the venae cavae draining into the sinus come to open separately into this atrium. The smooth-walled part of the adult atrium represents the contribution of the sinus venosus, the pectinate part represents the portion derived from the primitive atrium. Rather similarly, the adult left atrium has a double origin. The original single pulmonary venous trunk entering the left atrium becomes absorbed into it, and donates the smooth- walled part of this chamber with the pulmonary veins entering as four separate openings; the trabeculated part of the definitive left atrium is the remains of the original atrial wall.

The Development Of The Aortic Arches And Their Derivatives.

The Development Of The Aortic Arches And Their Derivatives: Cellular Development. Cellular Two types of vascular cells derive from mural cells. The first is the pericyte, which affects more small vessels, while the second type, vascular smooth muscle cells, are found, especially in large vessels. Pericytes play immune functions and can behave like stem cells. Vascular smooth muscle cells are involved in the function of vasoconstriction and vasodilation of the vessels (blood pressure and distribution of blood to the tissues). These cells also have the task of maintaining the morphology of the vessel.

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Biochemical Many molecules stimulate the maturation of mural cells (angiogenic factors and cytokines). For example, fibroblast growth factor (FGF) and beta-FGF, transforming growth factor of alpha type (TGF-alpha), angiogenin, interleukin type 8 (IL-8), angiopoietins. Molecular Bone morphogenetic protein (BMP), Notch (receptor affine to integrin proteins) and Gata (transcription factors) families, T-box (a group of transcription factors); myocardin, semaphorin family are protein molecules capable of inducing a correct embryological development of the aortic arch. Hoxa3 is a protein-coding gene involved in the regulation of the third pharyngeal arch, which molecule comes from the neural crests. This molecule conditions the physiological development of the carotid artery system..

Molecular Development Of The Arterial and Venous Systems. Molecular Development Of The arterial and venous. Angiopoietins (Ang1–Ang4) Notch probably mediates choice of fate between arterial and venous. Prox1 Prospero-related Homeobox 1 - expressed in a subpopulation of blood endothelial cells that then generate, by both budding and sprouting, cells of the lymphatic vascular system. Triggers the molecular program leading to the formation of the lymphatic system. (OMIM - PROSPERO-RELATED HOMEOBOX 1; PROX1) Tie (Tie1 and Tie2) tyrosine kinase receptors. Vascular endothelial growth factor (VEGF) family of proteins and angiopoietin/Tie, Notch, and ephrin/Eph pathways play major roles in eary vessel development. (VEGFR-3) LYVE-1 Podoplanin

Genetic Diagnostic Considerations. There were several article on genetic tests published by several groups dealing with cardiac and arterial diseases: Genetic testing could help prevent diseases and provide adequate treatment in clinical practice in the near future. However, further advances in research are necessary before endorsing genetic testing as a first approach for human embryological assessment

The Development Of The Aortic Arches And Their Derivatives. Emerging from the bulbus cordis is a common arterial trunk termed the truncus arteriosus, from which arise six pairs of aortic arches, equivalent to the arteries supplying the gill clefts of the fish. These arteries curve dorsally around the pharynx on either side and join to form two longitudinally placed dorsal aortae which fuse distally into the descending aorta. The 1st and 2nd arches disappear; the 3rd arches become the carotids. The 4th arch on the right becomes the brachiocephalic and right subclavian artery; on the left, it differentiates into the definitive aortic arch, gives off the left subclavian artery and links up distally with the descending aorta.

The Development Of The Aortic Arches And Their Derivatives.

The Development of The Arterial System. The arteries arise from the combination of the ectoderm (cells from the neural crests) and the mesoderm (pharyngeal mesoderm). The first arteries that develop are the right and left primitive aortae, which are a continuation of endocardial cardiac tubes. These primitive aortae curve posteriorly in the first pharyngeal arch, around the anterior part of foregut and then continue posteriorly as two dorsal aortae. These two aortae also fuse cranially close to the heart, forming the aortic sac. The aortic sac continues caudally as truncus arteriosus and lies ventral to the pharynx. The two dorsal aortae lie dorsal to the primitive gut and pass caudally and fuse at the distal end to form a common aorta while the cranial part remains separate.

The Development of The Arterial System. Development The aortic sac and the two dorsal aortae connect ventrally with aortic arches that are six pairs of arteries. The aortic arches run in the pharyngeal arches along the pharyngeal wall. During the development of the pharyngeal arches, the aortic sac sends a pair of branches to each pharyngeal arch which curves around the pharynx in the corresponding pharyngeal arch and eventually ends in the dorsal aorta. This artery is called an aortic arch artery, and these six pairs of arteries are never present at the same time in an embryo; by the time the third pair develops, the first pair has regressed. The aortic arches decrease in number and undergo rearrangement, and each arch then gives rise to a structure vital for adult life.

The structures arising from the aortic arches are as follows. First aortic arch: It regresses except for a very small part that gives rise to the maxillary artery. Second aortic arch: It regresses except for a very small part giving rise to the stapedial artery. Third aortic arch: This arch is the source of the common carotid artery and the proximal part of the internal carotid artery, and the external carotid which arises as a bud from this arch. Right Fourth aortic arch: Is the genesis of the proximal part of the right subclavian artery. Left Fourth aortic arch: Gives rise to the medial portion of the arch of the aorta. Fifth aortic arch: The fifth aortic arch regresses completely and very early in the development. Sixth aortic arch: Either of the sixth aortic arches divides into ventral and dorsal segments, and therefore, their derivatives also divide into these two segments. Right Sixth Arch: Ventral: Gives rise to the right pulmonary artery. Dorsal: It degenerates completely and loses its connection with the dorsal aorta. Left Sixth Arch Ventral: It gives rise to the left pulmonary artery that goes to the left pulmonary bud. Dorsal: It forms a vital connection during intrauterine life between the left pulmonary artery and the arch of the aorta.

The Development Of The Aortic Arches And Their Derivatives.

This structure is called ductus arteriosus. The aortic sac has two horns; right and left. The right horn gives rise to the brachiocephalic artery, which is continuous with the right subclavian artery stem and the right common carotid artery. The left horn and the stem of the aortic sac give rise to the proximal part of the arch of the aorta. The 5th arch artery is rudimentary and disappears. When the truncus arteriosus splits longitudinally to form the ascending aorta and pulmonary trunk, the 6th arch, unlike the others, remains linked with the latter and forms the right and left pulmonary arteries. On the left side this arch retains its connection with the dorsal aorta to form the ductus arteriosus (the ligamentum arteriosum of adult anatomy). This asymmetrical development of the aortic arches accounts for the different course taken by the recurrent laryngeal nerve on each side. In the early fetus the vagus nerve lies lateral to the primitive pharynx, separated from it by the aortic arches.

The Development Of The Aortic Arches And Their Derivatives.

Development of the Aortic Arch: The arch of the aorta has origins from multiple structures. The part of the aortic arch proximal to the origin of the innominate artery is the proximal part of the aorta, and it arises from the stem of the aortic sac. The area between the innominate artery and the left common carotid is called the middle region of the aortic arch and develops from the left horn of the aortic sac. The part of the aortic arch distal to the left common carotid artery forms from the left fourth aortic arch and the lower part of the left dorsal aorta. Both the dorsal aortae also give rise to seven cervical intersegmental arteries, and upper six of those anastomose vertically which eventually gives rise to the second part of the vertebral artery, superior intercostal artery, and the deep cervical artery. The last seventh segmental artery gives rise to the subclavian artery.

The Development Of The Aortic Arches And Their Derivatives.

What are to become the recurrent laryngeal nerves pass medially, caudal to the aortic arches, to supply the developing larynx. With elongation of the neck and caudal migration of the heart, the recurrent nerves are caught up and dragged down by the descending aortic arches. On the right side the 5th and distal part of the 6th arch absorb, leaving the nerve to hook round the 4th arch (i.e. the right subclavian artery). On the left side, the nerve remains looped around the persisting distal part the 6th arch (the ligamentum arteriosum) which is overlapped and dwarfed by the arch of the aorta.

The Development Of The Aortic Arches And Their Derivatives.

The Common Dorsal Aorta: It forms when the right and left dorsal aorta merge at the level of fourth thoracic to fourth lumbar somite segment. It gives off several branches which later become blood supply to vital organs in the adult life: Splanchnic arteries: Ventral splanchnic arteries: Arise from the ventral aspect. Supply the blood for the foregut, the midgut, and the hindgut. Its branches anastomose with each other to form a network that supplies the gut.

Lateral splanchnic arteries,The Intersegmental arteries &Umbilical arteries. Lateral Splanchnic Arteries: These arteries arise from the lateral aspect of the dorsal aorta in a pair, i.e., right and left. Supply structures arising from the intermediate mesoderm The middle suprarenal, gonadal, and renal arteries are its branches. Inter-Segmental Arteries: These arteries arise from the posterolateral aspect of the common dorsal aorta in pairs passing laterally between the somites. In adult life, it manifests as the posterior intercostal, lumbar and subcostal arteries. The fifth lumbar intersegmental artery gives rise to the external iliac artery They anastomose ventrally to form the internal thoracic, inferior, and superior epigastric arteries. Umbilical Arteries. They arise from the dorsal aorta, but once they anastomose and communicate with the intersegmental artery, they disconnect with the dorsal aorta. The fifth lumbar intersegmental artery gives rise to the external iliac artery while the umbilical artery is attached to its distal end, which later transforms into the internal iliac artery. The proximal part of the parent artery then forms the common iliac artery.

Development Of The Arteries In The Extremities: The arteries in the upper extremity develop from the axial artery. The axial artery comes from the seventh intersegmental artery and continues distally to form axillary, subclavian, and anterior interosseous arteries. The axial artery terminates in the hand and forms the deep palmar arch of the hand in the adult life. The anterior interosseous artery gives out a branch called the median artery, which runs alongside the median nerve and merges with the capillary plexus in the hand. The axial artery also gives off two more branches in the region of the elbow: the radial and ulnar arteries.

Development Of The Arteries In The Extremities:

Similarly, the axial artery in the lower limb is the continuation of the fifth lumbar intersegmental artery and follows the course of and with the sciatic nerve. Therefore, it is named as the sciatic artery as it travels downwards from the gluteal region traversing the back of the thigh, back of the leg and running deep to the popliteus muscle and the calf muscles ending in the sole of the foot after forming a deep vascular plexus. The external iliac artery continues in the lower limb to become the femoral artery and travels in the front of the thigh but bends posteriorly to join the axial artery in the popliteal fossa forming the popliteal artery just above the popliteal muscle. The tibial arteries originate from the local vascular network in the anterior and posterior portion of the leg and communicate with the popliteal artery. In adult life, the axial artery degenerates and leaves behind the inferior gluteal artery, peroneal artery, and the companion artery of the sciatic nerve as remnants.

The Development Of The Aortic Arches And Their Derivatives. The fetal circulation The circulation of the blood in the embryo is a remarkable example of economy in nature and results in the shunting of well-oxygenated blood from the placenta to the brain and the heart, leaving relatively desaturated blood for less essential structures. Blood is returned from the placenta by the umbilical vein to the inferior vena cava and thence the right atrium, most of it by-passing the liver in the

The Development Of The Aortic Arches And Their Derivatives.

Relatively little mixing of oxygenated and deoxygenated blood occurs in the right atrium since the valve overlying the orifice of the inferior vena cava serves to direct the flow of oxygenated blood from that vessel through the foramen ovale into the left atrium, while the deoxygenated stream from the superior vena cava is directed through the tricuspid valve into the right ventricle. From the left atrium the oxygenated blood (together with a small amount of deoxygenated blood from the lungs) passes into the left ventricle and hence into the ascending aorta for the supply of the brain and heart via the vertebral, carotid and coronary arteries.

The Development Of The Aortic Arches And Their Derivatives.

As the lungs of the fetus are inactive, most of the deoxygenated blood from the right ventricle is short-circuited by way of the ductus arteriosus from the pulmonary trunk into the descending aorta. This blood supplies the abdominal viscera and the lower limbs and is shunted to the placenta, for oxygenation, along the umbilical arteries arising from the internal iliac arteries. At birth, expansion of the lungs leads to an increased blood flow in the pulmonary arteries; the resulting pressure changes in the two atria bring the overlapping septum primum and septum secundum into apposition which effectively closes off the foramen ovale. At the same time active contraction of the muscular wall of the ductus arteriosus results in a functional closure

The Development Of The Aortic Arches And Their Derivatives.

Congenital abnormalities of the heart and great vessels Congenital Abnormalities Of The Heart & Great Vessels The complex development of the heart and major arteries accounts for the multitude of congenital abnormalities which may affect these structures, either alone or in combination. Dextro-rotation of the heart means that this organ and its emerging vessels lie as a mirror-image to the normal anatomy. It may be associated with reversal of all the intra-abdominal organs(situs Inversus)

The Pathophysiology &Molecular Pathology Of The Embryological Defects Pathophysiology &Molecular Pathology Of The Embryological Defects Notch pathway is essential for the construction of the aorta and aortic arch. We know several receptors like Notch1–4, and the related ligands (Jagged1– 2, Delta-like1-4); these molecules are found on the surface of the cell and are defined as transmembrane proteins. The activation of these molecules produces a cascade of metabolic reactions that come to influence DNA. Alagille syndrome is a complex pathology, which carries links to an alteration of Jagged's response1; the disease alters, among other pathological signs, the function and morphology of large vessels. A new protein-coding, the HECTD1 ubiquitin ligase, has been shown to be essential in the development of the aortic arch, influencing the function of retinoic acid. Its functional alteration causes hypoplasia or pathological changes of the aortic arch. Inosine 5 'monophosphate dehydrogenase (IMPDH) of type 2, deriving from the neural crests is fundamental for the development of large vessels. Its importance derives from its ability to influence the guanine nucleotide synthesis correctly; its dysfunction causes aberrations of the large vessels. In the Human Gene Mutation Database, it is possible to find all the mutations known about humans and large vessels.

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Clinical Significance Of Studying The Embryological Origin Of The Heart&Blood Vessels Clinical Significance: The in-depth and thorough knowledge regarding the embryological origin and evolution in adulthood is pivotal in understanding the pathophysiology of various congenital and acquired anomalies and enables physicians to initiate the treatment in the most accurate fashion. The integration of embryology in the medical curricula with the assistance of cutting-edge state of the art technology is also essential as it is associated with better patient outcomes.

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Figure Legend For A Computerized angio-tomographic showing coarctation of the aorta. The alteration of the mitochondrial DNA (mtDNA), in particular hypervariable segments 1-4 (HV1-3) can cause alterations in the development of the aortic arch, leading to an aortic coarctation.

Congenital abnormalities of the heart and great vessels

Congenital abnormalities of the heart and great vessels Septal Defects Septal Defects At birth, the septum primum and septum secundum are forced together, closing the flap valve of the foramen ovale. Fusion usually takes place about three(3) months after birth. In about 10% of subjects, this fusion may be incomplete. However, the two septa overlap and this patency of the foramen ovale is of no functional significance. If the septum secundum is too short to cover the foramen secundum in the septum primum, an atrial septal defect persists after the septum primum and septum secundum are pressed together at birth. This results in an ostium secundum defect, which allows shunting of blood from the left to the right atrium. This defect lies high up in the atrial wall and is relatively easy to close surgically.

Legend For The development of the chambers of the heart. (Note the septum primum and septum secundum which form the interatrial septum, leaving the foramen ovale as a valve-like opening passing between them.)

Congenital abnormalities of the heart and great vessels Septal Defects A more serious atrial septal defect results if the septum primum fails to fuse with the endocardial cushions. This ostium primum defect lies immediately above the atrioventricular boundary and may be associated with a defect of the pars membranacea septi of the ventricular septum. In such a case, the child is born with both an atrial and ventricular septal defect. Occasionally the ventricular septal defect is so huge that the ventricles form a single cavity, giving a trilocular heart. Congenital pulmonary stenosis may affect the trunk of the pulmonary artery, its valve or the infundibulum of the right ventricle. If stenosis occurs in conjunction with a septal defect, the compensatory hypertrophy of the right ventricle (developed to force blood through the pulmonary obstruction) develops a sufficiently high pressure to shunt blood through the defect into the left heart; this mixing of the deoxygenated right heart blood with the oxygenated left-sided blood results in the child being cyanosed at birth.

Congenital Heart Defects

The commonest combination of congenital abnormalities causing cyanosis is Fallots tetralogy. This results from unequal division of the truncus arteriosus by the spinal septum, resulting in a stenosed pulmonary trunk and a wide aorta which overrides the orifices of both the ventricles. The displaced septum is unable to close the interventricular septum, which results in a ventricular septal defect. Right ventricular hypertrophy develops as a consequence of the pulmonary stenosis. Cyanosis results from the shunting of large amounts of unsaturated blood from the right ventricle through the ventricular septal defect into the left ventricle and also directly into the aorta.

Congenital abnormalities of the heart and great vessels Septal Defects-- Legend Figure For The Tetralogy of Fallot.

Legend Figure For (a) Persistent Ductus Arteriosus Demonstrating Its Close Relationship To The Left Recurrent Laryngeal Nerve. (b) Coarctation Of The Aorta.

Congenital Abnormalities Of The Heart & Great Vessels Persistent Ductus Arteriosus &Aortic Coarctation A Persistent Ductus Arteriosus is a relatively common congenital defect. If left uncorrected, it causes progressive work hypertrophy of the left heart and pulmonary hypertension. Aortic Coarctation is thought to be due to an abnormality of the obliterative process which normally occludes the ductus arteriosus. There may be an extensive obstruction of the aorta from the left subclavian artery to the ductus, which is widely patent and maintains the circulation to the lower parts of the body; often there are multiple other defects and frequently infants so afflicted die at an early age. More commonly there is a short segment involved in the region of the ligamentum arteriosum or still patent ductus. In these cases, circulation to the lower limb is maintained via collateral arteries around the scapula anastomosing with the intercostal arteries, and via the link-up between the internal thoracic and inferior epigastric arteries. Clinically, this circulation may be manifest by enlarged vessels being palpable around the scapular margins; radiologically, dilatation of the engorged intercostal arteries results in notching of the inferior borders of the ribs.

FigureLegend For A Computerized angio-tomographic showing coarctation of the aorta. The alteration of the mitochondrial DNA (mtDNA), in particular hypervariable segments 1-4 (HV1-3) can cause alterations in the development of the aortic arch, leading to an aortic coarctation.

Congenital Defects Of The Heart&Great Vessels

Legend Figure For (a) Persistent Ductus Arteriosus Demonstrating Its Close Relationship To The Left Recurrent Laryngeal Nerve. (b) Coarctation Of The Aorta. Abnormal development of the primitive aortic arches may result in the aortic arch being on the right or actually being double. An abnormal right subclavian artery may arise from the dorsal aorta and pass behind the oesophagusa rare cause of difficulty in swallowing (dysphagia lusoria). Rarely, the division of the truncus into aorta and pulmonary artery is incomplete, leaving an aorta–pulmonary window, the most unusual congenital fistula between the two sides of the heart.

Legend Figure For (a) Persistent Ductus Arteriosus Demonstrating Its Close Relationship To The Left Recurrent Laryngeal Nerve. (b) Coarctation Of The Aorta.

The Embryological Development of The Lymphatic System. Lymphatic Tissue Development Lymphatic tissue development begins by the end of the fifth week of embryonic development. LEARNING OBJECTIVES Describe lymphatic tissue development Essense The first lymph sacs to appear are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins. The next lymph sac to appear is the unpaired retroperitoneal lymph sac at the root of the mesentery of the intestine. It develops from the primitive vena cava and mesonephric veins. The last of the lymph sacs, the paired posterior lymph sacs, develop from the iliac veins. The posterior lymph sacs produce capillary plexuses and lymphatic vessels of the abdominal wall, pelvic region, and lower limbs. With the exception of the anterior part of the sac from which the cisterna chyli develops, all lymph sacs become invaded by mesenchymal cells and converted into groups of lymph nodes.

The spleen develops from mesenchymal cells between layers of the dorsal mesentery of the stomach.

Relevant Terminology: Mesoderm: One of the three tissue layers in the embryo of a metazoan animal. Through embryonic development, it produces many internal organs of the adult, e.g. muscles, spine, and circulatory system. Cisterna chyli: A dilated sac at the lower end of the thoracic duct into which lymph from the intestinal trunk and two lumbar lymphatic trunks flows. Lymph node: Small oval bodies of the lymphatic system, distributed along the lymphatic vessels, that are clustered in the armpits, groin, neck, chest, and abdomen. They act as filters, with an internal honeycomb of connective tissue filled with lymphocytes and macrophages that collect and destroy bacteria, viruses, and foreign matter from lymph. Lymph sac: Precursors of the lymph vessels.

The Embryology Of The Lymphatic System The Context Of The Embryology Of The Lymphatic System The lymphatic system is an endothelium-lined network of blind-ended capillaries found in nearly all tissues, draining via collecting vessels into large vascular trunks that eventually empty via an evolutionarily conserved drainage point into the blood circulatory system. Other than these conserved drainage connections, the lymphatic system is entirely separate and anatomically distinct from the blood circulatory system, although lymphatic vessels are frequently found in close juxtaposition to veins and arteries. Lymphatic tissues begin to develop by the end of the fifth week of embryonic development. Lymphatic vessels develop from lymph sacs that arise from developing veins, which are derived from mesoderm. The first lymph sacs to appear are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins. From the jugular lymph sacs, lymphatic capillary plexuses spread to the thorax, upper limbs, neck, and head. Some of the plexuses enlarge and form lymphatic vessels in their respective regions. Each jugular lymph sac retains at least one connection with its jugular vein, the left one developing into the superior portion of the thoracic duct.

The Context Of The Embryology Of The Lymphatic System The next lymph sac to appear is the unpaired retroperitoneal lymph sac at the root of the mesentery of the intestine. It develops from the primitive vena cava and mesonephric veins. Capillary plexuses and lymphatic vessels spread from the retroperitoneal lymph sac to the abdominal viscera and diaphragm. The sac establishes connections with the cisterna chyli, but loses connections with neighbouring veins. The last of the lymph sacs, the paired posterior lymph sacs, develop from the iliac veins. The posterior lymph sacs produce capillary plexuses and lymphatic vessels of the abdominal wall, pelvic region, and lower limbs. The posterior lymph sacs join the cisterna chyli and lose their connections with adjacent veins. With the exception of the anterior part of the sac from which the cisterna chyli develops, all lymph sacs become invaded by mesenchymal cells and are converted into groups of lymph nodes. The spleen develops from mesenchymal cells between layers of the dorsal mesentery of the stomach. The thymus arises as an outgrowth of the third pharyngeal pouch. An important part of the cardiovascular system is the lymphatic vasculature, which functions to both return interstitial fluid (lymph) to the bloodstream and also as part of the immune system. In the embryo, lymphatic development begins at the cardinal vein, where venous endothelial cells differentiate (express Prox1) to form lymphatic endothelial cells that out-pocket and bud to form lymph sacs. During development these lymph sacs remodel to form both the lymphatic space within future nodes, formed by engulfed connective tissue, and the associated afferent and efferent vessel network.

Legend Figure For The Lymph glands of the head.

The Development of the Human&mammalian lymphatic vasculature. The Development of the Human&mammalian lymphatic vasculature" The two vascular systems of our body are the blood and lymphatic vasculature. Our understanding of the cellular and molecular processes controlling the development of the lymphatic vasculature has progressed significantly in the last decade. In mammals, this is a stepwise process that starts in the embryonic veins, where lymphatic EC (LEC) progenitors are initially specified. The differentiation and maturation of these progenitors continues as they bud from the veins to produce scattered primitive lymph sacs, from which most of the lymphatic vasculature is derived. Here, The current understanding of the key steps leading to the formation of a functional lymphatic vasculature is summarized

Legend Figure For The Primary lymph sacs.

Figure Legend For Lymphatics of pharynx.

Contributions From historical Data. In the Understanding OF The Embryology Of The Lymphatic System Contributions From historical Data. Although this system was first identified by Aselli G. (1627) in a paper "De Lacteibus sive Lacteis Venis", Quarto Vasorum Mesarai corum Genere novo invento. Milan: Mediolani. The existence of the lymphatic system has been appreciated since antiquity. Hippocrates first described vessels containing white blood around 400 B.C., and Gasparo Aselli re-identified lymphatic vessels in the 1600s, noting the presence of lipid-filled milky veins in the gut of a well-fed dog (Aselli, 1627). In the early 20th century, classical vascular anatomists such as Florence Sabin used dye injection and histological methods to characterize the anatomy of the lymphatic system in detail, demonstrating that it innervates and ramifies throughout nearly every part of the body, forming a secondary vascular system arguably as complex as the circulatory system (Sabin, 1902; Sabin, 1909). Then postulated by Sabin (1902) as venous in origin, it required a recent 2007 lineage tracing study to confirm this theory. Only vertebrates possess a true lymphatic vascular system, with primitive fish possessing a lymphatic-like secondary vascular system that also contains blood. Clinically, important for roles in immune surveillance and oncogenic (cancerous) processes.

Legend For Lymphatics of The Face.

Figure Legend For Lymphatics of The Tongue.

Legend For The Thoracic and right lymphatic ducts

Figure Legend For The Tracheobronchial Lymph Glands

Paraxial Mesoderm Is the Major Source of Lymphatic Endothelium Endothelial cells (ECs), which line blood and lymphatic vessels, are generally described to come from the lateral plate mesoderm despite experimental evidence for a broader source of origin, including the paraxial mesoderm (PXM). Key molecules in lymphatic development, function, and identification" While both blood and lymphatic vessels transport fluids and thus share many similarities, they also show functional and structural differences, which can be used to differentiate them. Specific visualization of lymphatic vessels has historically been and still is a pivot point in lymphatic research. Many of the proteins that are investigated by molecular biologists in lymphatic research have been defined as marker molecules, i.e. to visualize and distinguish lymphatic endothelial cells (LECs) from other cell types, most notably from blood vascular endothelial cells (BECs) and cells of the hematopoietic lineage. Prox1 is necessary for lymphatic cells to specify. It is believed that Prox1 is the single most important transcription factor that programs the fate of endothelial cells becoming lymphatic components. It has also been found that ectopic expression of Prox1 in blood vascular epithelium can force vascular endothelial cells to convert into lymphatic cells.

Legend For Human embryo (23 mm) mesenteric sac and cisterna chyli

Lymph capillaries - begin as blind-ending tubes in connective tissue, larger than blood capillaries, very irregularly shaped. Lymph collecting vessels - larger and form valves, morphology similar to lymph capillaries. Lymph ducts - 1 or 2 layers of smooth muscle cells in wall. (Remember the anatomy acronym NAVEL = Nerve, Artery, Vein, Empty space and Lymph)The empty space is to allow for venous expansion

Legend For The Lymph Capillary

Histological Features of The Lymphatic Capillaries Single-cell layer of overlapping endothelial cells Lack a basement membrane Lack smooth muscle cells or pericytes (pre-collecting and collecting trunks contain both) Linked by discontinuous endothelial cell-cell junctions (button-like). Junctions open in response to increased interstitial fluid pressure.

Legend Figure For The Lymphatic microvasculature model

Legend Figure For The Lymph capillaries of the human conjunctiva.

Legend Figure For The Lymph Capillaries From The Human Scrotum.

Legend Figure For The Lymph capillaries of the sole of the human foot.

Embryology Of The Lymphatic System

The model illustrated here is from a recent paper and is based on ultrastructural observations performed in in vitro and in vivo models of lymphangiogenesis. Lymphatic endothelial cells (LEC) display tight junctions and interdigitations, and are connected to the surrounding collagen fibers by anchoring filaments. Note that this postnatal model may differ from developmental lymphatic vessel development. A - LEC alignment. Elongated LEC migrate and extend long cytoplasmic protrusions. B - Vacuolization and matrix degradation. The continuity of LEC lining is mediated by interdigitations (i). Vesicle invaginations lead to the formation of intracellular vacuoles (v) in the cytoplasm and in protrusions. Matrix degradation (d) occurs intracellularly and extracellularly generating space between cells. C-Luminogenesis. The lumen (lu) is formed de novo in the intercellular space. The intracellular vacuoles coalesce (cv) and likely fuse with the cytoplasmic membrane to increase the lumen.

Legend Figure For The Tunneling model of lymphatic vessel formation Described Above

Functional Histology Of The Lymphatic System- Lymphatic Vessel Contraction Lymphatic Vessel Contraction Lymphatic vessels undergo spontaneous rhythmic contractions which aid lymph flow. This is most easily demonstrated in models based upon mesentery lymphatics of the gastrointestinal tract. Contractile activity is regulated by physical factors (transmural pressure) and neurological (alpha-adrenergic, histamine, bradykinin) acting on lymphatic smooth muscle. Contractility and receptor expression may also be different in different parts of the lymphatic system. Alpha-adrenergic - alpha 1- and not alpha 2-adrenoceptors. Histamine - lymphatic smooth muscle via stimulation of H(1) (and in some vessels H(2)) receptors. Bradykinin - chronotropic but not inotropic effects on lymphatic pump activity via stimulation of B1 receptors.

Legend Figure For Lymphatic Vasculature Organization

Abnormalities Of The Lymphatic System-Lymphangioma Abnormalities Of The Lymphatic System. Lymphangioma Dysplasia of childhood form lymphatic capillaries or collectors, which form fluid-filled cysts. Lymphatic spaces lined by endothelium Smooth muscle fascicles in the septa between the lymphatic spaces Lymphoid aggregates in the delicate collagenous stroma

Legend For The Lymphatics Of The Human embryo measuring 8 mm and 9 mm

Legend For The Human Embryo Measuring 10.5 mm

Legend For The Lymphatics Of The Human embryo measuring 11 mm

Legend Figure For The Human embryo measuring 22 mm

Legend Figure For The Human embryo measuring 30 mm

Legend Figure For A Lymph gland (Node).

Legend Figure For A Lymph gland tissue.

Legend Figure For The Thoracic and right lymphatic ducts.

Legend Figure For The Modes of origin of thoracic duct.

Legend Figure For The Terminal collecting trunks of right side.

Figure Legends For The Lymph glands of the upper extremity.

Figure Legends For Lymphatics of the mammary Gland.(The Female Breast)

Figure Legends For The Lymphatic vessels of the dorsal hand surface.

Figure Legend For The Lymph glands of popliteal fossa.

Figure Legend For The Superficial lymph glands and vessels of the lower extremity.

Figure Legend For The Parietal lymph glands of the pelvis.

Figure Legend For The Iliopelvic glands.

Figure Legend For The Lymphatics of stomach.

Figure Legend For The Lymphatics of cecum and vermiform Appendix

Figure Legend For The Lymphatics of cecum and vermiform process.

Figure Legend For The Lymphatics of Colon.

Figure Legend For The Lymphatic of the Bladder.

Figure Legend For The Lymphatics of the Prostate.

Figure Legend For The Lymphatics of the Uterus.

Figure Legend For The Lymphatics of the thorax and abdomen.

Embryology Of The Lymphnodes The lymphatic vessels form, like the blood vessels, from hemangioblastic stem cells which aggregate to form fine tubular vessels. The first signs of developing lymph nodes are found already in the 5th week 15 (36th day) as so-called lymph sacs near where the inferior cardinal vein and superior cardinal vein flow together and form the common cardinal vein. Bilateral evaginations of the venous system above the later subclavian vein arise as the jugular lymph sac and somewhat later below as the axillar lymph sac. In other regions of the body further such evaginations arise: mesenteric, lumbar, iliac and retroperitoneal sacs.

Legend For Lymphatic plexus in stage 18(ca. the 44th day)

The Lymphatic plexus in the fetal period Legend For The Lymphatic plexus at the end of the embryonic period (ca. the 44th day)Illustrated Above 1-Superior cardinal vein(jugular vein) 2-Jugular lymphatic sacs 3-Right subclavian vein 4-Axillary lymphatic sacs 6-5-Left brachiocephalic vein 7-Thoracic duct (bilateral) 8--Lumbar lymphatic sacs 9--Iliac lymphatic sacs ______________________________________

Embryology Of The Lymphatic System

Legend For The Lymphatic plexus at the end of the embryonic period (ca. the 56th day)

Legend For The Lymphatic plexus at the end of the embryonic period (ca. the 56th day) Illustrated Above 1-Right jugular vein 2-Right jugular and axillary lymphatic plexus 3-Subclavian vein 4-Superior vena cava 5-Right thoracic duct 6-Left jugular vein 7-Left jugular and axillary 8-lymphatic plexus 9-Left subclavian vein 10-Left thoracic duct 11-Cysterna chyli Inguinal lymphatic plexus __________________________

Embryology Of The Lymphatic System As in the venous system the lymphatic vessels also atrophy selectively and unilaterally. At the thoracic and abdominal level only a thoracic duct remains to drain the lymph of the entire lower part of the body and the left head and arm region. This lymph empties into the venous system at the junction of the jugular and left subclavian vein. Dashed lines indicate the atrophied portion of the lymphatic system.

The development of the lymphatic systems at the Embryonic Period Of Life Three bilateral lymphatic sacs (jugular, axillary and lumbo-iliac) as well as the incompletely formed lymphatic vessel plexus are shown. The most important lymphatic vessels arise bilaterally and form a fine network. The site where plexuses are located transform later into lymph nodes, the lymph filtering stations. At the end of the embryonic period 3 bilateral systems have formed: jugular-axillar lymphatic sac mesenterial lymphatic plexus lumbo-inguinal lymphatic plexus They are connected through a very variable lymphatic vessel system. The thoracic duct drains the lymph from the lower as well as the left upper half of the body into the left venous angle between the jugular vein and the left subclavian vein. The right arm and the right half of the head are drained by the jugular and axillary lymphatic plexus, respectively, which empties into the right venous angle. The lymph nodes develop in the early fetal period through a septation of the lymph sacs by mesenchymal cells. The spaces thus delimited become the sinus of the adult lymph nodes. Other mesenchymal cells build the connective tissue framework of the lymph node and its capsule.

Legend Figure For The Basic Structure of a Lymph Node.

Legend For The Development Of The Venous System

1-Right jugular vein 2-Right jugular and axillary 3-Lymphatic duct 4-Subclavian vein 5-Superior vena cava 6-Left jugular vein 7-Left jugular and axillary 8-Lymphatic duct 9-Left subclavian vein 10-Cysterna chyli 11-Inguinal lymphatic nodes 12-Thoracic duct

Development Of The Venous System Three systems could be recognised; The Vitelline Venous System which develops into the Portal Venous System. The Cardinal System which Forms The Caval System. The Umbilical Venous System which disappears at Birth; The complicated Venous System in particular is characterized by by many abnormalities such as double inferior and superior venae cavae and The Left Superior Venae Cavae. The Anterior Cardinal veins drain the cephalic part of the embryo. The Posterior Cardinal vein drains the remaining part of the embryo. During the 4 th week The Anterior and posterior cardinal veins unite to form a short common Cardinal before entering the the horn of the Sinus Venosus.At this time the left and right venous systems ARE SYMMETRICAL.

Development Of The Venous System During the 5 th to the 7 th week a number of additional veins are formed. The subcardinal veins which mainly drain the kidneys. The Sacrocardinal veins which drain the Lower extremities. The SUPRACardinal veins which which drain the body wall by way of the intercostal veins thereby taking over the function of the from the Posterior CaRDINAL Veins The Formation of the vena cava is charcterised by the appearnace of an anastomosis between the Left and Right Cardinal venous sytems in such a way that bloof from the Left Cardinal systems is channelled to the Right side. The anastomosis between the anterior cardinal veins develops into the Left Brachiocephalic vein,most of the blood from the Left side of the head and upper extremity is then channelled to the Right.

Development Of The Venous System The Superior Vena cava is is formed by the Right common cardinal vein and the proximal portion of the right anterior cardinal vein. The anastomosis between the subcardinal vein forms the Left Renal vein; The proximal portion of the Left subcardinal vein disappears and its distal portion of the remains as the Left Gonadal vein Hence the Right Subcardinal vein becomes the main drainage channel and dfevelops into the renal segment of the inferior vena cavae.

Development Of The Venous System The anastomosis between the Sacrocardinal veins forms the Left common Iliac vein. The Right Sacrocardinal vein finally becomes the SacroCardinal segment of the Inferior Vena Cavae. When the Renal Segment of the inferior vena cavae connects with the hepatic segment,which is derived from the Right Vitelline vein,the inferior vena cava is complete. It consists then of a: (a)- hepatic segment (b)- renal Segment. ©-& Sacro-Cardinal Segment. With Obliteration of the Major part of the Posterior Cardinal Veins,the SupraCardinal Veins gain importance. The 4 th to 11 th intercostal veins empty into the Right Supracardinal vein,which together with a portion of the posterior cardinal vein,forms the azygos vein. On the Left the 4 th to 7 th intercostal veins enter into the Left supracradinal vein. After the development of a communicating vessel between the two supracardinals,the Left supracardinal vein empties into the azygos vein and is then known as the hemiazygous vein.

Functions Of The Lymphatic System. Functions Of The Lymphatic System. The lymphatic system has been shown to have a number of important functions. It transports fluids, plasma macromolecules, and cells extravasated from blood vessels, returning them back into the blood circulation and preventing their build- up in tissues throughout the body. Defects in the lymphatic system, whether congenital (primary lymphedema, relatively uncommon) or acquired (secondary lymphedema, a common complication of surgery and certain parasitic infections), can result in severe, disfiguring edema of affected tissues. The lymphatic system is also a major route for absorption of lipids from the gut (hence the milky appearance of the vessels that led to their early identification). Lymphatics are a critical component of the immune system, transporting white blood cells and antigens from distant sites to lymphoid organs. Recent evidence also indicates that the lymphatic system is a major pathway for the dissemination of metastatic cells, making them an important new focus of efforts to develop effective cancer therapies. Despite its importance, the formation of the lymphatic system has remained relatively obscure in comparison to the blood vasculature. This is partly due to the intense research focus recently placed on blood vessels, but also due to a number of technical challenges in studying lymphatics.

Molecular identification of has only recently become possible Lymphatic vessels are frequently difficult to visualize in histology or electron micrographs because, unlike blood vessels, they are often irregular and collapsed. When they are observed, lymphatic vessels are frequently difficult to distinguish with precise certainty from blood vessels. Defects in the lymphatic system often occur relatively late in postnatal life and are slow in onset, making it challenging to assess the proximal etiology of lymphatic disorders. Molecular identification of has only recently become possible. Although most molecular markers of lymphatic endothelial cells are shared with blood endothelial cells, in the past few years a small number of genes have been identified whose expression within lymphatic endothelium is diagnostic for these cells (although none of the genes are expressed exclusively within the lymphatic endothelium). Some of these genes have also been shown to be required for proper formation of the lymphatic system, mostly through murine knockout studies. This Lecture will also focus on the mechanisms that regulate the development of the lymphatic vasculature during embryogenesis. Recently, there has been a relative explosion in the number of studies that focus on various aspects of this process. This is not surprising, given that lymphatics play an integral role in tissue homeostasis, immunity, and cancer metastasis. This Lecture will attempt to examine and highlight some of the most recent advances in understanding the mechanisms that govern lymphatic development.

The Structure of The Lymphatics. The lymphatic system is a vascular network comprised primarily of thin-walled, blind-ended capillaries. These capillaries are made up of a single-cell layer of extensively overlapping endothelial cells with endothelial cell leaflets linked by discontinuous button-like endothelial cell-cell junctions which open in response to increased interstitial fluid pressure. This structure, together with the lack of a basement membrane and supporting smooth muscle cells or pericytes, makes lymphatic capillaries highly permeable to the protein-rich lymph fluid. Lymphatic capillaries also possess specialized structures called anchoring filaments, extracellular fibrillar structures linking lymphatic endothelial cells to surrounding matrix and tissues These anchoring filaments also help to keep the lymphatic capillaries open and increase their permeability as interstitial pressure rises. The lymphatic capillaries converge into precollecting lymphatic vessels, which carry lymph to the main collecting trunks (e.g., thoracic duct) for return to the venous circulation via the anastomosis with the cardinal vein. Unlike lymphatic capillaries, pre-collecting and collecting trunks contain smooth muscle cells and pericytes. Collecting lymphatics also have internal valves to prevent retrograde flow of lymph fluid. Lymphatics are generally found in most tissues innervated by the blood vascular system, often in very close association with blood vessels. One notable exception to this is the central nervous system, which lacks lymphatic vessels. Although lymphatic vessels do not contain red blood cells, they do contain lymphoid cells, and depending on the species, the lymphatic system also includes lymphoid organs important for immune responses, such as lymph nodes, tonsils, Peyers patches, spleen, and thymus.

Legend Figure Demonstrating The Characteristics of the lymphatic vasculature.(Illustrated Above) Legend Figure Demonstrating The Characteristics of the lymphatic vasculature. (A) An overview of the human lymphatic system, including lymphatic vessels, lymph nodes, and lymphoid tissue (s-spleen, t-thymus). Major veins into which the lymphatics drain are shown in blue. (B) The lymphatic endothelial cells attach directly to the extracellular matrix and surrounding cells via anchoring filaments (red). Valves (blue) prevent lymph reflux to promote unidirectional lymph propulsion. Note the extensive overlap of adjacent endothelial cells in lymphatic capillaries. (C) Surface view of a lymphatic capillary emphasizing the loose, button-like intercellular junctions (blue).

Data From Comparative Anatomical Studies. Data From Comparative Anatomical Data. Comparative phylogenetic analysis has shown that a true lymphatic vascular system is present only in the vertebrates. A lymphatic-like secondary vascular system is found in primitive fish, but it contains blood and is still considered to be part of the circulatory system. Teleost fish are arguably the first vertebrates with an anatomically distinct lymphatic system. The lymphatics of amphibians and reptiles are similar to those of fish, although they often have specialized propulsive organs for the lymph called lymph hearts, small contractile structures containing striated muscle fibers. Lymph hearts are absent in mammals and adult birds, and are only exceptionally found in fish. The lymphatic system of amphibians, reptiles, and fish often appears as a large system of interconnected sacs or sinuses, rather than a defined network of tubular vessels. Unlike fish and frogs, lymphoid tissues are connected to the lymphatic vascular system in birds, although there are only a very small number of lymph nodes. Unlike most lower vertebrates, birds and mammals have lymphatic networks consisting of distinct, narrow, thin-walled tubular lymphatic vessels. Mammalian lymphatics generally contain large numbers of valves and many lymph nodes.

ORIGINS OF LYMPHATIC ENDOTHELIAL CELLS The ontogeny of the lymphatic vascular system and the source of the lymphatic endothelial cells (LEC) of which it is comprised have been controversial. Classical pre-molecular era studies examining the origins of the lymphatics relied primarily on either injection (dyes, plastic resins) or serial sectioning methods to visualize and characterize the emergence of the earliest lymphatic vessels and lymph sacs. Use of these alternate methods early in the past century led to the formulation of two major types of models to explain the origins of lymphatics. Anatomists and embryologists using injection methods favored the view that the primary lymph sacs bud off from primitive veins, and that the lymphatic vessels grow out centrifugally from these sacs by endothelial budding and lymphangiogenesis. Other investigators objected to these findings, noting that injections can only be made into an uninterrupted system of tubes, so that failure to demonstrate lymph vessels by way of injections does not necessarily mean that the injected area is devoid of lymphatics existing in the form of mesodermal clefts. In contrast, researchers using serial sectioning methods developed the alternative view that lymphatic vessels arise from mesenchymal spaces. The primary sacs arise, and the general systemic lymphatic vessels initially develop, along the course of the embryonic veins. Centripetal extensions subsequently make connections with the lymph sacs, which make secondary connections with the venous system. The centripetal model was championed by Huntington and McClure (1910). The methods of investigation leading to this concept were also criticized, on the grounds that any study of growing lymphatics in serial sections is unreliable because not all of the lymphatic endothelium present at any period of development can be seen in stained cross-sections and the identification of lymphatics is problematic.

ORIGINS OF LYMPHATIC ENDOTHELIAL CELLS The recent advent of molecular markers (see below) and methods for analyzing the formation of the lymphatics has permitted a more conclusive re-examination of the origins of LEC in a number of different developmental model organisms. The origins of avian lymphatics were studied using quail-chick embryo chimera grafting experiments(Wilting et al., 2006; Wilting et al., 2000). Thus, it seems likely that if there is a hematopoietic contribution to LEC during murine development, it is a minor component that appears relatively late and/or peripherally in the lymphatics. However, other reports do suggest that alternative sources of LEC can contribute to normal or pathologic lymphangiogenesis during postnatal life. Perhaps the most definitive evidence for a venous origin for early lymphatic endothelial cells has come from the zebrafish. As noted above, recent studies have demonstrated that the zebrafish possesses a lymphatic vascular system with many of the morphological, molecular, and functional characteristics of the lymphatics of other vertebrates. All of the cells examined arose from progenitors in the parachordal vessel, itself a derivative of the posterior cardinal vein. Results from ongoing experimental studies provide direct, conclusive evidence that the vast majority of cells contributing to the LEC of thoracic duct in the zebrafish arise from primitive veins.

MOLECULAR REGULATION OF LYMPHATIC DEVELOPMENT As alluded to above, application of molecular methods to lymphangiogenesis research has resulted in dramatic progress in the study of this traditionally somewhat elusive tissue. Many new molecular markers of the lymphatic endothelium have been identified (although few if any are absolutely specific to lymphatic vessels), and these markers have greatly facilitated experimental analysis of lymphatic vessel formation. Functional analysis of some of these genes has led to important new insights into the molecular regulation of lymphatic endothelial specification and lymphangiogenesis, some of which are described below. The realization that some of these functionally important genes correspond to human lymphatic disease loci has begun to lead to advances in our understanding of and ability to treat lymphatic disorders.

Regulation of lymphatic endothelial cell specification As described above, the use of lymphatic endothelial-specific markers has permitted a detailed assessment of the emergence of the initial LECs in murine embryos. The application of modern molecular techniques has reinvigorated the investigation of the lymphatic system. Classic studies of the ontogenesis of lymphatic vessels proposed the centrifugal and centripetal models. Roughly a century later, the use of lymphatic-specific markers in model organisms has provided evidence for both models. Definitive evidence for the centrifugal model has been provided by the study of Prox1, as well as live imaging in zebrafish of the establishment of the thoracic duct by lymphatic endothelial precursors from the cardinal vein. Studies of lymphatic development in chick and frog support both the centrifugal and centripetal models. Likewise, the study of human genetic disease and murine knockout models has provided insight into some of the molecular mechanisms required for lymphatic specification, lymphangiogenesis, and lymphatic maturation. The establishment of zebrafish as a model for lymphatic development will allow forward genetic screens to identify additional genes involved in each of these processes. This will increase the number of candidate genes for genetic testing in cases of idiopathic forms of lymphedema, such as Meige disease, in addition to sporadic cases of lymphedema. Similarly, a better understanding of each of these processes will eventually lead to effective treatments for disorders of the lymphatic system.

Lymphangiogenesis-The Development Of The lymph vessels. Lymph sacs are a part of the development of the human lymphatic system, known as lymphangiogenesis. The lymph sacs are precursors of the lymph vessels. These sacs develop through the processes of vasculogenesis and angiogenesis. However, there is evidence of both of these processes in different organisms. In mice, it is thought that the lymphatic components form through an angiogenic process. But, there is evidence from bird embryos that gives rise to the idea that lymphatic vessels arise in the embryos through a vasculogenesis-like process from the lymphangioblastic endothelial precursor cells. The development of the lymphatic system has been a highly debated topic in developmental biology for a long time. Previously, it was debated whether the lymph sacs developed from the venous system, or if they came from spaces in the mesenchyme, which come together in a centripetal direction and secondarily opened into the veins. However, more recent research has shown that the formation of the lymphatic system begins when a subset of endothelial cells from the previously formed jugular vein sprout off to form the lymphatic sacs. Because lymph sacs form from the venous system, they typically contain red blood cells.

Lymphangiogenesis-The Development Of The lymph vessels. It is believed that the lymph sacs are directly connected to the venous system and that the venous components and lymphatic components communicate through a small hole. Studies have demonstrated that the development of lymph sacs occurs through swelling and outgrowth of pre-lymphatic clusters from the cardinal vein, in a process termed ballooning. Following ballooning, there is the process of pinching, which separates the lymph sacs from the venous system. These processes begin forming the lymph sacs during the 5th week of fetal development. At this time, the jugular lymph sacs develop. These are a pair of enlargements that function in collecting fluid from the lymphatics of the upper limbs, upper trunk, head, and neck.

Figure Legend For The Lymphatic plexus in the fetal period Legend

Figure Legend For The Lymphatic plexus in the fetal period Illustrated Above 1-Right jugular vein 2-Right jugular and axillary 3-Lymphatic duct 4-Subclavian vein 5-Superior vena cava 6-Left jugular vein 7-Left jugular and axillary 8-Lymphatic duct 9-Left subclavian vein 10-Cysterna chyli 11-Inguinal lymphatic nodes 12-Thoracic duct

Embryology Of The Lymphatic System The lymph nodes eventually develop at the place of the jugular lymphatic sacs. From the left jugular lymphatic sac, the cervical part of the thoracic duct forms. From the right jugular lymphatic sac, the right lymphatic duct and the jugular and the subclavicular lymphatic trunks form. One week later, during the 6th week of fetal development, four more lymph sacs form. These are the retroperitoneal lymph sac, the cysterna chyli, and paired posterior lymph sacs. The posterior lymph sacs are associated with the junctions of the external and internal iliac veins. These four new lymph sacs function in the collection of lymph from the trunk and lower extremities of the body. The cysterna chyli drains into a pair of thoracic lymphatic ducts initially. These ducts drain into the venous junctions of the internal jugular and subclavian veins. However, these ducts eventually become one thoracic duct that is derived from the caudal portion of the right duct, the cranial portion of the left duct, and median anastomosis.

The Embryology Of The Venous System As in the venous system the lymphatic vessels also atrophy selectively and unilaterally. At the thoracic and abdominal level only a thoracic duct remains to drain the lymph of the entire lower part of the body and the left head and arm region. This lymph empties into the venous system at the junction of the jugular and left subclavian vein. Dashed lines indicate the atrophied portion of the lymphatic system.